A new free vehicle dynamics resource - Dan's Vehicle Dynamics Corner [Archive]

ChassisSim

06-13-2012, 05:15 PM

Hey Guys,

Just one thing I want to clarify. The lap time simulation in ChassisSim is fully transient. There are no pseudo static calcs in there what so ever. It's what makes ChassisSim unique. You might find the following link useful,

http://www.chassissim.com/blog...r-laptime-simulation (http://www.chassissim.com/blog/chassissim-news/what-makes-chassissim-unique-for-laptime-simulation)

Moop - Let me answer your questions one by one

1) You specify a path for ChassisSim to follow. We went down this road because by the time the car's have hit the track and laid down rubber etc you pretty much have one racing line per category of car. I can tell you from experience I've done the other way and it will drive you mad for very little reward.

2) In terms of the sates of the ChassisSim vehicle model these are,

Forward Speed (Vx)
Sideways Speed (Vy)
Yaw rate (r)
Sprung mass heave and heave velocity
Sprung mass pitch angle and pitch angle velocity
Sprung mass roll angle and roll angle velocity
All 4 wheel displacements and velocity
All 4 surface Tyre temps

and watch this space
All 4 internal tyre temps - but this is under development.

Also guys while I'm on the topic let me pass on observation/word of advice. I know a lot of people on this forum like to play with advanced analysis etc. The key to using simulation properly is you start simple and then get detailed later. If you go the other way you'll spend months going no where.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

06-13-2012, 06:21 PM

Guys,

By the way I was actually going through the ChassisSim tutorials and found this,

http://www.youtube.com/watch?v...index=9&feature=plcp (http://www.youtube.com/watch?v=ajIckKSYv_o&list=UU4W7Lqv3XrQps-RlcR4fEOA&index=9&feature=plcp)

It's about to how to classify wing and aero changes in ChassisSim. I show you some handy shortcuts to economise the number of aeromaps you need.

I know strictly speaking in FSAE there are alot of cars out there that don't run a lot of downforce. That being said if you want to move on into motorsport you might find this helpful.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

06-13-2012, 09:16 PM

Guys,

Just a quick clarification by motorsport I mean professional motorsport where you get paid to engineer and play with cars for a living and or where you need to adjust downforce! Sorry guys I hit the send button too soon!

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

06-20-2012, 08:33 PM

Hey Guys,

This is the latest episode of Dan's Vehicle Dynamics corner. It's about slip angles, neutral steer and ackermann. I leave you with a challenge at the end.

http://www.youtube.com/watch?v...DqA&feature=youtu.be (http://www.youtube.com/watch?v=6CNT-N9bDqA&feature=youtu.be)

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-18-2012, 10:42 PM

Hey Guys,

I've seen some interesting stuff crop up in Racecar Engineering on tyre modelling/simulation.

Not that I don't think many of you need this, but you might find the following interesting as a counter point,

http://www.youtube.com/watch?v...index=1&feature=plcp (http://www.youtube.com/watch?v=0voOPwddOZg&list=UU4W7Lqv3XrQps-RlcR4fEOA&index=1&feature=plcp)

Enjoy Guys

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-07-2012, 03:12 AM

Hey Guy's. I've been hearing some interesting stuff with regards to using simple parameters to simulate a race car. In particular lap time simulation. Have a look at this because I think this provides an interesting counter point,

http://www.youtube.com/watch?v...Z-Q&feature=youtu.be (http://www.youtube.com/watch?v=AJE5gX-6Z-Q&feature=youtu.be)

Food for thought guys.

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-07-2012, 03:31 AM

Guy's I'd also add to that list, vehicle mass, front and rear track and wheelbase.

There's a lot more than you think!

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-15-2012, 08:44 PM

Hey Guys,

A few of you have been talking to me about car modelling and correlation. An over looked area is the measurement and importance of motion ratios. You might find the following helpful,

http://www.chassissim.com/blog...nce-of-motion-ratios (http://www.chassissim.com/blog/chassissim-news/dans-vehicle-dynamics-corner-the-importance-of-motion-ratios)

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-29-2012, 08:59 PM

Hey Guys,

I've just uploaded a new episode of Dan's Vehicle Dynamics corner,

http://www.youtube.com/watch?v...index=1&feature=plcp (http://www.youtube.com/watch?v=H7yt2L2dzd4&list=UU4W7Lqv3XrQps-RlcR4fEOA&index=1&feature=plcp)

Given the recent interest in simulation, it's about why we at ChassisSim so what we do.

Also following on from this for those of you who will be in Cologne (Koln) Germany on Thursday the 15th of November 2012 (same week as Professional Motorsport World Expo) I'm hosting a simulation bootcamp,

http://www.chassissim.com/blog...-in-germany-nov-2012 (http://www.chassissim.com/blog/chassissim-news/racecar-simulation-bootcamp-in-germany-nov-2012)

For those of you who want to get up to speed on simulation, this bootcamp would be perfect for you.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-05-2012, 07:32 PM

Hey Guys,

Just to give everyone the heads up I've been getting a few emails about the simulation bootcamp. In particular what it's about how does it slot in to what you guys are doing.

To fill everyone in I thought I'd give you a few teaser's about what we are covering,

Interfacing with data

http://www.youtube.com/watch?v...ndex=37&feature=plcp (http://www.youtube.com/watch?v=rvtLfpqLnJU&list=UU4W7Lqv3XrQps-RlcR4fEOA&index=37&feature=plcp)

Creating circuit models from data

http://www.youtube.com/watch?v...index=4&feature=plcp (http://www.youtube.com/watch?v=1ROgqDSousQ&list=UU4W7Lqv3XrQps-RlcR4fEOA&index=4&feature=plcp)

If you want to get on top of simulation, then this is the event for you. You can find out more info about it at this link,

http://www.chassissim.com/blog...-in-germany-nov-2012 (http://www.chassissim.com/blog/chassissim-news/racecar-simulation-bootcamp-in-germany-nov-2012)

Hopefully see a few of you there.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-12-2012, 08:57 PM

Hey Guys,

Jost posted this video about some advanced techniques for tyre modelling. If you want to have really sensitive tyre models this might be of good use,

http://www.chassissim.com/blog...exponential-function (http://www.chassissim.com/blog/chassissim-news/refining-a-temperature-based-tyre-model-using-the-exponential-function)

Also for those of you who will be at PRI this year by all means come in and say hello. We are at booth 5716. You'll find the details on the ChassisSim blog,

http://www.chassissim.com/blog/

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-12-2012, 09:00 PM

Hey Guys,

Jost, should be Just! I hit the reply button too soon!

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-19-2012, 06:31 PM

Hey Guys,

Given the interest in simulation recently I've just uploaded this video that might be helpful,

http://www.youtube.com/watch?v...index=1&feature=plcp (http://www.youtube.com/watch?v=XjsVi5KYG8w&list=UU4W7Lqv3XrQps-RlcR4fEOA&index=1&feature=plcp)

It's about ChassisSim v3.19. However the thing to note here is the tree structure. This tree structure describes the simulation work flow we find works best.

I hope this is of use to everyone.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-04-2012, 12:38 AM

Hey Guys,

In the spirit of my last post on simulation work flow I've just uploaded this series of articles to the ChassisSim blog,

http://www.chassissim.com/blog...onnecting-the-dots-2 (http://www.chassissim.com/blog/chassissim-news/racecar-simulation-connecting-the-dots-2)

It's about connecting the dots when you setting up racecar simulation. It outlines the work flow we have found in the ChassisSim community that delivers results. Consequently it will be a valuable resource for you.

Also on another note I've been reading with great interest recently on various threads in this forum about simplicity vs detail in engineering analysis. Let me state from the get go I actually think it's a good idea. Indeed the mark of true intelligence is the ability to take something very complicated and break it down simply so anyone can understand. That being said be very wary of watering down stuff so much you lower your standards. You'll see what I mean in this video link,

http://www.youtube.com/watch?v=4oLSYSJO5Ik

It's the Top Gear segment on Ayrton Senna. I've always used this as inspiration in what it takes to win.

Enjoy Guys

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-10-2012, 10:30 PM

Hey Guys,

I've just uploaded a video giving you a taste of what will be covered at the ChassisSim simulation seminars at the end of the year,

http://www.chassissim.com/blog...he-u-s-a-novdec-2012 (http://www.chassissim.com/blog/chassissim-news/simulation-seminars-in-germany-and-the-u-s-a-novdec-2012)

Hopefully I'll see a few of you there.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-11-2012, 06:04 PM

Hey Guys,

I thought I'd give everyone the heads up that we at ChassisSim are looking for Interns. You'll find the details here,

http://www.chassissim.com/blog...-looking-for-interns (http://www.chassissim.com/blog/chassissim-news/chassissim-is-looking-for-interns)

This is ideal if you want to move into motorsports but you need some hands on experience. If this sounds like you read the link above and send your resumes to info@chassissim.com .

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-17-2012, 08:47 PM

Hey Guys,

I realise I'm highlighting an old topic but the following might come in useful,

http://www.chassissim.com/blog...he-chassissim-system (http://www.chassissim.com/blog/chassissim-news/lap-time-simulation-made-easy-using-the-chassissim-system)

It's about systematising what you need to do to run race car simulation.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-24-2012, 10:22 PM

Hey Guys,

Just a couple of things for this week.

Firstly for those of you who are going to be in Germany on Nov 15, you might want to check this out,

http://www.chassissim.com/blog...he-u-s-a-novdec-2012 (http://www.chassissim.com/blog/chassissim-news/simulation-seminars-in-germany-and-the-u-s-a-novdec-2012)

I've had another 3 registrations today for the simulation bootcamp. Places are filling fast. If you want to go you might want to do something about this sooner rather than later.

Also another thing is to give you a thought for the week,

http://www.chassissim.com/blog...measure-up-a-racecar (http://www.chassissim.com/blog/chassissim-news/how-to-measure-up-a-racecar)

This was an article I wrote for Racecar Engineering on how to measure up a race car. I've been talking to a lot of students lately and based on my discussions I think you'll get a lot out of this.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-31-2012, 05:39 PM

Hey Guys,

I was going through some of my old Racecar Engineering articles and I figured you might get alot out of this,

http://www.chassissim.com/blog...-models-from-nothing (http://www.chassissim.com/blog/chassissim-news/creating-tyre-models-from-nothing)

It's about creating tyre models from nothing. This is a good primer for getting ready for simulation.

Also places are filling fast for the simulation bootcamp which is now only two weeks away. For those of you in Europe who want to go, email me at info@chassissim.com

All the best

Danny Nowlan
Director
ChassisSim Technologies

ben

11-01-2012, 06:50 AM

Danny,

Have you done any correlation of your model against a flat-track tyre test of the same tyre?

Regards,

Ben

ChassisSim

11-07-2012, 07:28 PM

Ben,

My apologies for the late reply. I've been a bit hammered this week.

The short answer with correlating to flat track testing is no because we have never needed to. I say this because the Tyre force modelling toolbox works straight from race data so we have a lot of customers who can usually bypass tyre rig test results and go straight off race data.

Two classic cases in point are V8 Supercars and the ALMS LMPC category. What both of these cars had in common was the tyre test rig results for both of these categories was either not released or had not been done. The correlation we have achieved with both of these cars speaks volumes for the ChassisSim tyre model and the use of the tyre force modelling toolbox. You might find the following link useful,

http://www.chassissim.com/blog...illing-in-the-blanks (http://www.chassissim.com/blog/chassissim-news/modelling-tyres-using-chassissim-filling-in-the-blanks)

It’s the how to guide about how to do tyre modelling.

That being said fitting results to flat track data shouldn’t be an issue, I just haven’t done it. However this is just my opinion, if I am creating a tyre model that I want to use in anger, particular in simulation I like to go off race data. The reason I say this is my customers and I have been led up the garden path many times with erroneous rig results.

Don’t get my wrong, I think rig testing and flat track testing have their place and they are valuable tools. I think any tyre company worth their salt would be crazy not to use them.

That being said there is no replacement for looking at race data when you have to use a tyre model in anger. This is no different to validating your engine and aero results on track.

If you have any more questions please don’t hesitate in asking me.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

11-07-2012, 07:34 PM

Guys,

On another note - for those of you how are based in Europe and want to attend the simulation bootcamp in Cologne/Koln Germany next week I need to hear from you very shortly.

The details for the seminar are below,

http://www.chassissim.com/blog...he-u-s-a-novdec-2012 (http://www.chassissim.com/blog/chassissim-news/simulation-seminars-in-germany-and-the-u-s-a-novdec-2012)

The venue is starting to press me for numbers. If your interested click on the link and get into contact with me in the next 24 hours.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ben

11-09-2012, 12:47 AM

Originally posted by ChassisSim:
Ben,

My apologies for the late reply. I've been a bit hammered this week.

The short answer with correlating to flat track testing is no because we have never needed to. I say this because the Tyre force modelling toolbox works straight from race data so we have a lot of customers who can usually bypass tyre rig test results and go straight off race data.

Two classic cases in point are V8 Supercars and the ALMS LMPC category. What both of these cars had in common was the tyre test rig results for both of these categories was either not released or had not been done. The correlation we have achieved with both of these cars speaks volumes for the ChassisSim tyre model and the use of the tyre force modelling toolbox. You might find the following link useful,

http://www.chassissim.com/blog...illing-in-the-blanks (http://www.chassissim.com/blog/chassissim-news/modelling-tyres-using-chassissim-filling-in-the-blanks)

It’s the how to guide about how to do tyre modelling.

That being said fitting results to flat track data shouldn’t be an issue, I just haven’t done it. However this is just my opinion, if I am creating a tyre model that I want to use in anger, particular in simulation I like to go off race data. The reason I say this is my customers and I have been led up the garden path many times with erroneous rig results.

Don’t get my wrong, I think rig testing and flat track testing have their place and they are valuable tools. I think any tyre company worth their salt would be crazy not to use them.

That being said there is no replacement for looking at race data when you have to use a tyre model in anger. This is no different to validating your engine and aero results on track.

If you have any more questions please don’t hesitate in asking me.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Thanks for the reply. My concern with this approach is that you're assuming the model is correct therefore the tyre model that fills in the gaps must represent the tyre.

Presumably if you flat-track tested a tyre and it was different to your ChassisSim tyre model, that would presumably indicate a gap somewhere else in the model?

Ben

ChassisSim

11-24-2012, 02:46 AM

Ben,

As always my friend my apologies for the late reply.

Just to clarify my friend, in reality no tyre model is perfect, particularly the empirical ones. The Pacjeka model and the ChassisSim tyre model are no exceptions to this. A classic case in point with the Pacjeka model is it will always tell you to add negative camber.

Empirical tyre models are approximations and once you make your peace with that you can use it as a tool to help you fill in the blanks of your knowledge.

Alot of what is in the ChassisSim tyre model has been as a direct result of trying to fill in the blanks of stuff I've seen at the race track. I don't claim I get it right all the time but people still find it a good tool so I have to be doing something right! Also if I observe a discrepancy from flat track testing this will tell me something to improve things.

That being said the tyre force modelling toolbox combined with using actual data is arguably one of the most powerful tools ChassisSim has on offer. I say that because not only does ChassisSim give great correlation it gives you the appropriate trends. I've done this without tyre rig results in categories as diverse as A1GP, F3, GP2, ALMS and V8 Supercars to name a few and it has always been my first line of defence at the race track. Not just for me but more importantly for other members of the ChassisSim community.

I will close though with this thought. Yesterday I was having a discussion with a colleague of mine from Cranfield University. We got talking about tyre testing and he made this observation. The results from either rig testing or flat track testing aren't wrong. The problem is there is no guarantee the thermal conditions in the test will match those when you get on track. This is why I like to go from track data. However this is just my opinion.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

12-17-2012, 04:53 PM

Hey Guys,

I just thought I'd give everyone the heads up that ChassisSim Professional Online is now live,

https://www.chassissim.com/onlinesimulation/index.php

What this allows you to do is to run ChassisSim Standard, the bump profiling toolbox and the tyre force modelling toolbox on a pay by the simulation basis. It's a great tool to use advanced simulations on a limited budget. The online simulation is ideal for freelance race engineers working in professional categories.

That being said I still think the FSAE community will get alot out of it. If it is something you want to make use of shoot me an email to info@chassissim.com and I'm sure we can figure something out.

Enjoy Guys and Merry Christmas.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

01-16-2013, 06:43 PM

Hey Guys,

I thought I'd give everyone a break down about how straightforward it is to get going with in depth race car simulation,

http://www.chassissim.com/blog...-process-made-simple (http://www.chassissim.com/blog/chassissim-news/the-chassissim-process-made-simple)

Enjoy Guys

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

01-23-2013, 03:27 PM

Hey Guys,

I realise I posted this a while a go, but recently I've been seeing some traffic on ackermann steering. This will provide some food for thought,

http://www.chassissim.com/blog...and-food-for-thought (http://www.chassissim.com/blog/chassissim-news/slip-angles-neutral-steer-ackermann-and-food-for-thought)

In the end I do set a challenge for you. Let me give you a hint. Take the car speed at say 30 km/h, 60 km/h and 100 km/h. Assume lateral acceleration at say 1g. Crunch the numbers at those speeds and see where it works out.

Enjoy Guys

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

01-30-2013, 05:11 PM

Hey Guys,

This week I had the pleasure of being schooled in one of my interests by the best in the world at what they do.

It really rammed home the importance of knowing your basics in order to succeed. In that vein I realise this is a retread but I think you guys might get some use out of this,

http://www.chassissim.com/blog...onnecting-the-dots-2 (http://www.chassissim.com/blog/chassissim-news/racecar-simulation-connecting-the-dots-2)

It's about connecting the dots with regards to simulation.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

monoposto

01-30-2013, 11:08 PM

Awesome! I love learning, please don't stop making these!

mono

ChassisSim

02-06-2013, 05:29 PM

Hey Guys,

Mono - many thanks for the compliment. I'll keep up the good work. Over the last couple of years I have been writing for Racecar Engineering and contributing to this forum. I have written on many aspects of race car vehicle dynamics and how you can use it. It's also something I have genuinely enjoyed doing.

Guys I have some exciting news for you. It's time for you to apply this knowledge and what better way of doing it then through a competition,

http://www.chassissim.com/blog...l-online-competition (http://www.chassissim.com/blog/chassissim-news/the-chassissim-professional-online-competition)

We at ChassisSim are running a competition using ChassisSim Professional Online to model an F3 car. Also there's a pretty big carrot on offer here. You'll find more information on the link.

Registrations are due by March 1, Final submissions March 21. Knock yourselves out and I look forward to your entries.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

02-13-2013, 07:27 PM

Hey Guys,

From time to time you'll be presented with a scenario when you have incomplete data. This video tutorial offers some tips and tricks about what to do about it,

http://www.chassissim.com/blog...ings-dont-go-to-plan (http://www.chassissim.com/blog/chassissim-news/dans-vehicle-dynamics-corner-when-things-dont-go-to-plan)

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

02-20-2013, 07:43 PM

Hey Guys,

I'm just letting you all know that ChassisSim v3.20 has just been released. Click here to find out more,

http://www.chassissim.com/blog...-in-chassissim-v3-20 (http://www.chassissim.com/blog/chassissim-news/new-features-in-chassissim-v3-20)

Also just a note. Quite a few of you have registered for the ChassisSim professional Online competition. Just to let everyone know I am cutting of entries after March 1. Consequently if you want to participate in the competition register sooner rather than later. Forgive me for the repetition but the details can be found here,

http://www.chassissim.com/blog...l-online-competition (http://www.chassissim.com/blog/chassissim-news/the-chassissim-professional-online-competition)

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

02-27-2013, 04:39 PM

Hey Guys,

I've just posted this video on tips and tricks on how to create aeromaps from race data,

http://www.chassissim.com/blog...ling-tips-and-tricks (http://www.chassissim.com/blog/chassissim-news/race-car-aero-modelling-tips-and-tricks)

It encapsulates what we at ChassisSim have learnt about aero modelling over the last couple of years. You'll get a lot out of this so enjoy.

Also just to give everyone a reminder, I'm cutting off entries for the ChassisSim competition on March 1! That is tomorrow aka Friday afternoon! However I will make Friday afternoon March 1 5:00pm Californian time. I look forward to your entries.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-06-2013, 04:24 PM

Hey Guys,

Recently I've been getting alot of questions on aeromapping and how you represent the aero of a race car. I realise I've posted this before, however this is something you can still get quite a lot out of,

http://www.chassissim.com/blog...d-aero-in-chassissim (http://www.chassissim.com/blog/chassissim-news/classifying-and-changing-wings-downforce-and-aero-in-chassissim)

Also for those of you competing in the ChassisSim Online competition it might be of interest to you.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-13-2013, 06:21 PM

Hey Guys,

This was an article I wrote for Racecar Engineering a few months a go,

http://www.chassissim.com/blog...of-hand-calculations (http://www.chassissim.com/blog/chassissim-news/the-joy-of-hand-calculations)

It's about how to do hand calculations and I have some worked examples. I wrote it for all of you so I hope you get something out of it.

Enjoy,

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-20-2013, 08:24 PM

Hey Guys,

Over the last couple of weeks I've been getting a lot of questions on how to get the most out of tyre rig test results. I hope the following is of some help,

http://www.chassissim.com/blog...yre-test-rig-results (http://www.chassissim.com/blog/chassissim-news/interpreting-tyre-test-rig-results)

Also I just wanted to take this opportunity to issue a congratulations to ChassisSim customers PR1 motorsports on their class victory at Sebring. You can find more details about this here,

http://www.chassissim.com/blog...gory-at-sebring-2013 (http://www.chassissim.com/blog/chassissim-news/pr1-motorsports-win-the-lmpc-category-at-sebring-2013)

Enjoy Guys

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Silente

03-25-2013, 02:56 PM

Hi Danny,

first of al thanks for your posts and the material you publish.

Would be nice to have something new more often, but i understand it's not good for your business to show your competitors what you do and how.

I have a question about your article on hand calculations, in particular regarding the approach you propose to build a first aproximated tire model.

After you have calculted vertical loads based on Downforce and Lateral Load Transfer, you increase that load by 20% to save for situation where you could potentially have more donwnforce. But the maximum load value is influencing also the shape of the parabola you build up based on ka and kb, right? So increasing it (or anyway changing the MAX LOAD value) lead to a different tire model...in particular to different intial Coefficient of Grip and load sensitivity.

So this way of proceeding seems to me a bit arbitrary...or maybe i am loosing something?

Thanks!

ChassisSim

03-27-2013, 06:00 PM

Silente,

Thank you for the compliment. It gives me a real thrill to see guys like you getting something out of this thread.

My apologies for the delay in answering your questions. I've been flat out developing new features for ChassisSim.

In terms of the extra 20% of the load that's there to cover you for bump load variation and as a safety margin. To put this in perspective look at a plot of tyre loads at a place like Sebring!

However the whole point of this is to create something to get you started. It's also a great educational tool because it gets you prepared for the loads you can expect. Then you use data and ChassisSim's toolboxes to refine the results.

I trust this makes things clear. If you have any more questions please don't hesitate in contacting me and sorry for the hold up.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-27-2013, 06:08 PM

Hey Guys,

I've just posted this video on how to make best use of the ChassisSim quick start features,

http://www.chassissim.com/blog...o-modelling-features (http://www.chassissim.com/blog/chassissim-news/tips-on-using-chassissims-auto-modelling-features)

I've posted this because it puts into perspective how to use computational tools such as ChassisSim as calculators as opposed to blindly relying on the results.

This is something all engineers need to keep in mind and I hope you get something out of it.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Silente

03-28-2013, 12:54 AM

Danny,

thanks for your reply.

i understand your point and this explanation was already pretty clear from your paper.

Anyway, what i meant is that, if you change the max load value, the complete Lateral (Longitudinal) Force vs Vertical Load curve is changing, leading at the end to a completely different tyre model.

So ok, we than take into account for bigger vertical loads, but which is then the "better" tire model to represents our tire?

I would post a picture of the two different curves i get changing maximum load, but i am not able to in this forum...

ChassisSim

04-03-2013, 08:11 PM

Silente,

Again my friend the apologies for the delay.

When starting a tyre model I always advocate a larger tyre load range. You lose sensitivity but it's the safer road to take. I've seen tyre loads from racecars in the order of 1000kgf! This is the key reason I put a safety factor on my tyre modelling.

Just remember this hand calculation gets you going. The key thing is you start on the hand calculation then you use tools such as the ChassisSim tyre force modelling toolbox to finish the job.

I hope this helps.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

04-03-2013, 08:14 PM

Hey Guys,

I've just put this post on my blog,

http://www.chassissim.com/blog...d-to-do-to-get-going (http://www.chassissim.com/blog/chassissim-news/chassissim-what-you-need-to-do-to-get-going)

It's about how to get going when using simulation.

What this does is it articulates the process you need to get going with simulation. If you want to get started with simulation this is a great start point.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

04-10-2013, 09:06 PM

Hey Guys,

I was going through some of my old Racecar Engineering articles and I found this,

http://www.chassissim.com/blog.../the-damper-workbook (http://www.chassissim.com/blog/chassissim-news/the-damper-workbook)

It's the damper workbook which outlines the basic calculations you need to get on top of your dampers. The equations and approaches in this article have saved my neck at the race track more times than I can remember. You'll find it a very useful tool.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

04-17-2013, 09:23 PM

Hey Guys,

I just posted this tutorial about the ChassisSim optimisation toolbox,

http://www.chassissim.com/blog...optimisation-toolbox (http://www.chassissim.com/blog/chassissim-news/optimising-setups-using-the-chassissim-optimisation-toolbox)

The reason I bring it up is I think it's a good lesson about how to use optimisation techniques. However I also mention when not to use it and that is something to keep in mind.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-01-2013, 08:24 PM

Hey Guys,

I've just released ChassisSim v3.21. You can find out more details here,

http://www.chassissim.com/blog...m-v3-21-new-features (http://www.chassissim.com/blog/chassissim-news/chassissim-v3-21-new-features)

What is of particular interest to the FSAE community is the ability to run open loop simulation, being able to predict hot pressures from cold conditions and revised algorithms to deal with structural flexibility.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-08-2013, 07:00 PM

Hey Guys,

I just posted this video on circuit modelling,

http://www.chassissim.com/blog...ing-chassissim-v3-21 (http://www.chassissim.com/blog/chassissim-news/creating-circuit-models-using-chassissim-v3-21)

It shows you how to create high fidelity circuit models that can really nail down bumps and other nuances of the circuit to really represent what is going on with the car.

I think you'll get a lot out of it.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-16-2013, 06:22 PM

Hey Guys,

For those of you using ChassisSim the following is worth a look,

http://www.chassissim.com/blog...-lap-time-simulation (http://www.chassissim.com/blog/chassissim-news/dialing-in-grip-and-bump-factors-for-lap-time-simulation)

It's about how to dial in grip and bump scale factors manually. It's a really good guide to what to adjust and when to adjust it. It also walks you through the mechanics of how to do it.

I realise this tutorial is a bit long in the tooth and has been pushed to the side by the advances we have made with auto grip and the advances in bump profiling. That being said if you have a circuit that is difficult to model this tutorial will be of great help and there are some good insights you can learn from.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-22-2013, 10:58 PM

Hey Guys,

I just posted this article on my blog,

http://www.chassissim.com/blog...a-to-refine-aeromaps (http://www.chassissim.com/blog/chassissim-news/analysing-pitch-data-to-refine-aeromaps)

It's about how to refine aeromaps from pitch data from the racecar.

I realise this isn't ultra applicable for a FSAE car but its still a useful resource to have. I know some of you might be considering jobs in motor racing and this is a good thing to have in your back pocket.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-29-2013, 09:29 PM

Hey Guys,

For the ChassisSim users in the FSAE community I've just posted this video on how to consolidate your circuit models,

http://www.chassissim.com/blog...-master-circuit-file (http://www.chassissim.com/blog/chassissim-news/simplifying-chassissim-circuit-models-using-the-master-circuit-file)

It's about how to use the Master circuit file which we have found to be a great tool.

I also realise that I've posted this before but this is a great guide to getting going with lap time simulation,

http://www.chassissim.com/blog...onnecting-the-dots-2 (http://www.chassissim.com/blog/chassissim-news/racecar-simulation-connecting-the-dots-2)

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

06-05-2013, 06:18 PM

Hey Guys,

I've just posted this article on how to use virtual shaker rigs,

http://www.chassissim.com/blog...lbox-tips-and-tricks (http://www.chassissim.com/blog/chassissim-news/chassissim-shaker-rig-toolbox-tips-and-tricks)

This has proven to be a very powerful tool in the ChassisSim community. In particular I think it is something the FSAE community could get quite a bit out.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

06-12-2013, 10:01 PM

Hey Guys,

Over the last couple of weeks I've had a few people in the ChassisSim community asking me questions about the order in which they should dial in bump and grip scale factors when creating their circuit models.

I've seen the same theme's recurring and this tutorial addresses this,

http://www.chassissim.com/blog...ating-circuit-models (http://www.chassissim.com/blog/chassissim-news/dialling-in-bump-and-grip-factors-when-creating-circuit-models)

Think of it as the follow up to my post from a couple of weeks a go on the mechanics of dialling in bump and grip scale factors.

For those of you who are ChassisSim users you'll find this very useful.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

BillCobb

06-13-2013, 08:29 PM

I'm obliged to tell you that your simple model equations are missig the 4 tire aligning moments. The net effect is a very large rigid body yaw moment as well as a likely understeering effect from steering compliance.

The net effect is the predicted total vehicle understeer will be far different than actual and the predicted system (steering) gain will be too high.

You can spend a lot of money buying or renting MTS or Michigan Scientific wheel force transducers to validate these conclusions. The parametric models provided with nonlinear tire data MUST include the MZ functions. MX functions can be important, too, if the car reacts badly to aligning torque camber inducements.

Since a 'race' car has low, but measureable total vehicle understeer (SAE definition as the difference between the front and rear axle sideslip gradients), the tire MZ terms are a very large fraction of this value. Getting that right will alter any simulated track or test results, as you can imagine...

ChassisSim

06-16-2013, 06:10 PM

Hey Guys,

Just fixed the link for the latest video tutorial. I hit the publish too soon! Sorry about that.

Bill - with regards to the concern that you raised, self aligning torque primarily makes its presence felt with regards to steering feed back. In particular it's really important for driver in the loop simulation.

However with regards to vehicle performance prediction it's impact is minor. Hence why I didn't include it. This allows the reader to get straight to what really counts. This isn't just based on hearsay. The ChassisSim community covers categories as diverse as GP2, F3, V8 Supercars, DTM, Sports prototypes, GT and the FSAE community to name a few. These cars are very heavily instrumented and all of them have outstanding correlation. Bottom line if this was an issue I would have heard about it a very long time a go.

However don't take my word for it, do some numbers on it. Take a FSAE car weighing 250kg in a 1g turn with a 50/50 weight distribution and a 1.7m wheelbase. Also assume the peak trailing arm of the self aligning torque is say 50mm. Calculate the moment arm from the front axle to the c.g and do the same for the self aligning torque. The numbers are very revealing.

Enjoy Guys

Danny Nowlan
Director
ChassisSim Technologies

06-18-2013, 07:45 PM

Of the above two posts (Bill's and Danny's), I feel Bill's is closer to the truth, but I don't know by how much.

So, for any FSAE-VDer's who might be interested...

Under/oversteer can be thought of in terms of "static margin". Roughly speaking, this is the plan-view-longitudinal-distance between the car's CG and the (roughly lateral) Line-of-Action of the resultant force vector from all four wheels (Edit! -> ), when this force vector is that which gives neutral steer (<- EndEdit). If the CG is in front of neutral-steer-force-LoA, then understeer. If CG behind LoA, then oversteer. If CG right on LoA, then neutral steer. (MoreEdit. See Tim's post below for better definition...)

Anyway, for a reasonably handling car the static-margin is only a few inches/cm. This is a similar distance to the "Pneumatic Trail" of typical tyres. And since Mz of a tyre can be thought of as the Fy force multiplied by the PT distance (ie. Mz = Fy x PT = Fy force acting PT behind tyre centre), then ignoring Mz can give an error that has similar magnitude to the static-margin.

BUT! Mz is typically zero at zero slip-angle, rises to its peak at about half peak-Fy-slip-angle, then drops back to about zero at peak-Fy-slip-angle (roughly speaking). This is because the pneumatic trail starts at maximum at zero slip-angle and ~zero Fy (so Mz = 0 x PT = 0), and PT then drops to ~zero at peak-Fy (so Mz = Peak-Fy x 0 = 0), but inbetween, at ~half-peak-slip-angle, there is some Fy and some PT, so some Mz. So the above paragraph makes sense at low and half-peak cornering forces when the PT is of similar magnitude to the static margin. But at peak cornering forces the Mz effect may be negligible, because PT = ~0???

I haven't seen enough of the curves of real tyres to know just how big Mz is at typical peak-Fys. My guess is that the Mz curves move about quite a bit depending on all the other factors (longitudinal forces, camber, tyre pressure, etc...).

So, for the numbers, it is over to you guys, Bill and Danny..... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

ChassisSim

06-18-2013, 11:01 PM

Hey Guys,

The number's speaks for themselves,

*Moment arm from tyre to c.g 800mm - 1500mm depending on the car.
*Self aligning torque moment arm in the tyres are in the order of 20mm - 50mm. Also this bleeds off as we hit peak slip angle.

Self aligning torque can be visualised as a result of the lateral force applying a moment arm about the centre of the contact patch. Consequently when you do the numbers for the static margin it's effectively swamped by the moment arm of the axle to the c.g. Also this moment arm bleeds off as we hit peak slip angle.

Consequently in terms of car handling and static margins it's a mild second order effect. However it makes its presence felt in the torque the driver feels through the steering wheel. This is where the good drivers earn their money.

However guys - work through the example I presented above in my previous post. To really ram home the point calculate the typical contact patch length for a given load, tyre pressure and area. The conclusions are pretty obvious.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

06-19-2013, 03:52 AM

Danny,

The large moments about the CG from the front and rear tyre forces largely cancel out (hopefully!), leaving only the small moment of the resultant force from all four tyres multiplied by <STRIKE>the static margin</STRIKE> its offset from the CG. ( <- Edit.)

If the effect of pneumatic trail (or Mz) moves all four tyre forces backwards by your quoted "20mm - 50mm", then that is a significant first-order effect, because that distance is similar to the static margin.

So that leaves the question of how small does Mz/pneumatic-trail become as Fy reaches peak? Bill??

Z

PS. The decreasing pneumatic trail (or Mz) as Fy approaches peak moves the handling balance towards more oversteer, so it is worth keeping in mind...

Tim.Wright

06-19-2013, 05:07 AM

Originally posted by Z:
Under/oversteer can be thought of in terms of "static margin". Roughly speaking, this is the plan-view-longitudinal-distance between the car's CG and the (roughly lateral) Line-of-Action of the resultant force vector from all four wheels. If the CG is in front of wheel-force-LoA, then understeer. If CG behind LoA, then oversteer. If CG right on LoA, then neutral.

Not sure that's right. In a steady corner, the tyre resultant force is going to go through the CG isn't it?

My understanding of the static margin is that its the distance from the CG to the neutral steer point. The neutral steer point is the longitudinal location which will give a neutral response if you apply a lateral force there. I.e. the slip angle response will be the same front and rear.

I have just done a quick calc on a bicycle model with 50% mass dist and an understeer (measured as delta of front to rear slip angles) of 3deg/g which I think is probably closer to a road car but anyway... I've calculated the neutral steer point and found that the static margin is in the order of 400mm. Obviously, the more neutral the car is, the smaller the static margin is and then a higher percentage of the balance is coming from the tyre Mz. However, at this point you are really trying to pick very small changes in balance so it doesn't really make sense to be speaking in terms of percent.

So, my feeling then is that the Mz would be interesting if you are really trying to accurately represent your slip angles to within a few percent. But for sure I wouldn't go as far as saying its a first order effect. At least not in steady state.

I think it would be important when assessing the response of road cars because people are interested in small changes of balance which are giving tactile feedback to the driver. But in this case, you are talking very small values.

ChassisSim

06-19-2013, 06:03 AM

Hey Guys,

Tim - your on the right track on that one - 400mm for a static margin is a tad high but in terms of some rough rules of thumb it's not that far out. That being said you've got the heart of this nailed.

Z - Here's some food for though my friend. The self aligning torque is intrinsically linked to the force being generated by the tyre. If we are going to do some rough rules of thumb of calculation the static margin we have,

SM = (mom_arm_f*C_f - mom_arm_r*C_r)/(C_f + C_r)

Here we have,

SM = static margin
mom_arm_f = Front moment arm to the c.g
mom_arm_r = Rear moment arm to the c.g
C_f = Change in front tyre force/change in slip angle
C_r = Change in rear tyre force/change in slip angle

Giving the self aligning torque the considerable benefit of the doubt we can write for the moment arms front and rear

mom_arm_f = a + self_aligning_torque_arm_f
mom_arm_r = b + self_aligning_torque_arm_r

Remember the numbers we discussed before, on a FSAE car, 800mm plays 50mm, and that's being pretty generous. Also remember as we get close to the peak grip of the tyre the self aligning torque moment arm falls away. Consequently as I stated before the moment arm effect of the axle blows away the self aligning torque.

To put this in perspective let's revisit some moment arms from our previous example.

Axle moment from lateral force to c.g = 125*9.8*0.8 = 980 Nm
Moment arm self aligning torque = 125*9.8*50e-3 =61.25 Nm

The difference in the moments speaks for themselves. Also that self aligning torque will drop away as we hit the maximum force of the tyre.

Where the self aligning torque will make its presence felt as the car gets close to a static margin of zero. A static margin close to 0 should not be confused with neutral steer. When we hit a static margin of 0, you’re right on the borderline of being able to control the car. The concept of static margin was originally developed by the aerospace industry to quantify pitch stability. I can tell you from experience that long before you hit a static margin of 0 the driver will be totally spooked, and the effects of self aligning torque is the equivalent of shuffling the deck chairs on the Titanic. I learnt this one the hard way.

Anyway Guys I hope this clears a few things up.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Tim.Wright

06-19-2013, 06:55 AM

Danny, I agree regarding the drop off in controlability, though I would have thought that a static margin of zero is theoretically neutral steer.

It might not be what a driver would feel or describe as neutral, but from a vehicle dynamics point of view, at a static margin of zero, the front and rear slip angles are the same. to me thats a neutral car.

06-19-2013, 05:38 PM

Originally posted by Tim.Wright:
Not sure that's right. In a steady corner, the tyre resultant force is going to go through the CG isn't it?

My understanding of the static margin is that its the distance from the CG to the neutral steer point. The neutral steer point is the longitudinal location which will give a neutral response if you apply a lateral force there. I.e. the slip angle response will be the same front and rear.
Tim,

Oops! Yes, I worded that poorly (should have checked my definitions!). I have editted my above posts to make more sense.

Also, I prefer to think in terms of unsteady conditions. So if a car is cornering neutrally around a skid-pad (say), then it has the inwardly directed resultant tyre force LoA (from all four tyres) passing through the NSP, and this is in balance with the outwardly directed centrifugal force from the CG, which is also at the NSP. Now if the driver gradually increases speed (with same steer angle), then both the centrifugal and tyre forces also increase. But if, as the tyres approach "saturation", the pneumatic trail decreases, then the resultant of the tyre forces moves forward, and this together with the centrifugal force forms an over-steering yaw couple. So the tail starts sliding outward.
~~~~~o0o~~~~~

Danny,

You have;
"mom_arm_f = a + self_aligning_torque_arm_f
mom_arm_r = b + self_aligning_torque_arm_r"

I would say; mom_arm_f = a minus self_aligning_torque_arm_f.

Putting this into your equation for Static Margin has the a and b subtracting, while the two self-aligning-torque-arms add together.

Also, there is nothing impossible about a CG behind the NSP. In fact, can be common on holidays when the boot is fully loaded! Thankfully, yaw-damping and low speeds can make it controllable.

Z

ChassisSim

06-19-2013, 06:40 PM

Z,

My friend a couple of things,

*The moment arm definition I've just put in as a general plus. It's up to the end user to put in the sign they want. If I was going to be fussy about it I would have it in as a minus but I was using it as a point of illustration. Anyway the numbers due to self aligning torque are still very small.

If anything to give it a proper treatment I'd have to include the self aligning torque as its own derivative in the Static Margin equation. This is where things get real interesting. Looking at say some F3 tyre results for various cambers you are looking at peak tyre forces of 5000 N with the corresponding self aligning torque peaks being at about 100 Nm. Where things take a particular twist is after the slip angles have passed the peak self aligning torque. The lateral forces still have a bit of gradient left in them, but the self aligning torque curves have flattened out. This in concert with the small numbers of the self aligning torque pretty much renders the self aligning torque as a passenger for the Static Margin particularly as we get close to the peak grip of the tyre.

*The yaw damping is actually part of the Static Margin. Both Milliken and I gave very detailed treatments of this in our respective books. This also has its background in aviation stability and control which is our respective backgrounds.

When you’re close to the static margin being 0 you’re at the cusp of unstable behaviour. I've flown aircraft with low and positive values of static margin and as a race/data engineer I've seen cars with low values of static margin and it is ugly. For the automotive case when you at this point as I stated in my last post the self aligning torque will have some impact but it's like shuffling deck chairs on the titanic.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

06-19-2013, 08:51 PM

Hey Guys,

In light of what has been discussed here recently I think you should get an awful lot out of this,

http://www.chassissim.com/blog...c-race-car-stability (http://www.chassissim.com/blog/chassissim-news/dans-vehicle-dynamics-corner-an-introduction-to-dynamic-race-car-stability)

It's about transient racecar stability and the static margin in particular.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

Tim.Wright

06-20-2013, 01:36 AM

Originally posted by Z:
Also, I prefer to think in terms of unsteady conditions. So if a car is cornering neutrally around a skid-pad (say), then it has the inwardly directed resultant tyre force LoA (from all four tyres) passing through the NSP, and this is in balance with the outwardly directed centrifugal force from the CG, which is also at the NSP. Now if the driver gradually increases speed (with same steer angle), then both the centrifugal and tyre forces also increase. But if, as the tyres approach "saturation", the pneumatic trail decreases, then the resultant of the tyre forces moves forward, and this together with the centrifugal force forms an over-steering yaw couple. So the tail starts sliding outward.

I follow you, and I do agree that the Mz would make up a large percentage of the understeer for a neutral or near neutral car, but my argument (and I think also Danny's) is that a large percentage of (practically) nothing is still nothing. Or at least safe to neglect.

Granted that neglecting or including Mz for a neutral tending car might change the sign of the calculated understeer, but if you think what this means in terms of alfa_front - alfa_rear, you are talking decimals of a degree and probably not detectable by the driver.

To come back to Danny's point too, you are already up shit creek if you are operating in this region anyway so to me its not important to know if your balance is +0.2deg/g (calculated with Mz) or -0.2deg/g (calculated without Mz). Either calculation (and probably your driver too) is going to tell you that you are too close to neutral steer.

This is also true in your example of the road car with a lot of rear mass. There is not going to be a lot of difference in the vehicle response if the neutral steer point is 20mm in front of the CG or 20mm behind it. Only that one is mathematically understeering, the other is mathematically oversteering. If you were to drive or simulate both cases I wouldn't expect you to see much of a difference.

ChassisSim

06-26-2013, 09:22 PM

Hey Guys,

A couple of things for this week.

First things first I'd like to acknowledge the life of Allan Simonsen who was killed at the LeMans 24 Hour race on Saturday. I knew Allan, but more importantly other members of the ChassisSim community knew him better than I. Not only did they rate him as a driver, but he was well respected as a man. This is something that should be remembered and celebrated.

Also I think it is a timely reminder, that even though what killed him was a bad combination of circumstances it really re-iterates the point of safety when designing your cars. Ladies and Gentleman the price your pay for driving at the edge of a vehicle's performance envelope is that occasionally you'll fall off and someone may get hurt or even killed. If you can't make you peace with this you are in the wrong business. That being said as engineers we must take every precaution to ensure we have done everything we can to protect the drivers. There are no shortcuts in this business.

The other thing I wanted to touch is I've just put a post on my blog that consolidates the ChassisSim circuit modelling process,

http://www.chassissim.com/blog...s-the-complete-guide (http://www.chassissim.com/blog/chassissim-news/creating-chassissim-circuit-models-the-complete-guide)

What this post does is it puts under one roof all the video tutorials and documents you'll need to create a high fidelity circuit model for lap time simulation. It's all based on what has been done in the field so it should be a great resource for all of you.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-03-2013, 10:28 PM

Hey Guys,

I've been seeing a lot of traffic recently about dampers and how to crunch the numbers. The following is good food for thought,

http://www.chassissim.com/blog...using-damping-ratios (http://www.chassissim.com/blog/chassissim-news/the-damper-guide-part-1-using-damping-ratios)

It was a series of articles that was originally published in Racecar Engineering. It was about what to look for in damping rates and how to calculate them.

Next week I'll post some state space analysis stuff.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-10-2013, 09:14 PM

Hey Guys,

I realise I'm preaching to the choir on this one, but check this out,

http://www.chassissim.com/blog...-you-need-simulation (http://www.chassissim.com/blog/chassissim-news/dans-vehicle-dynamics-corner-why-you-need-simulation)

It outlines why you need to use simulation.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-17-2013, 10:58 PM

Hey Guys,

This is part 2 of my damper analysis guide,

http://www.chassissim.com/blog...state-space-analysis (http://www.chassissim.com/blog/chassissim-news/the-damper-guide-part-2-using-state-space-analysis)

It's about using state space analysis to look at dampers. I realise this is high level stuff but you should get something out of it.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-31-2013, 10:59 PM

Hey Guys,

Just posted this article about how to use Racecar simulation as a weapon,

http://www.chassissim.com/blog...mulation-as-a-weapon (http://www.chassissim.com/blog/chassissim-news/using-racecar-simulation-as-a-weapon)

Sorry about the delay in the updates. I've been working on some new stuff that I'll let you know about in the next couple of days or so.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-07-2013, 10:42 PM

Hey Guys,

From time to time in your careers you are going to have to face generating an engine curve from data. This is particular apparent if you don't get a dyno curve. This article shows you how,

http://www.chassissim.com/blog...-curves-from-nothing (http://www.chassissim.com/blog/chassissim-news/creating-engine-curves-from-nothing)

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-21-2013, 08:25 PM

Hey Guys,

Great to see the newer and better version of the FSAE forum! I posted this tutorial last week about classifying camber,

http://www.chassissim.com/blog/chassissim-news/modelling-tyre-camber-sensitivity-with-chassissim

Enjoy

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-29-2013, 12:42 AM

Hey Guys,

I've just released ChassisSim v3.22,

http://www.chassissim.com/blog/chassissim-news/new-features-in-chassissim-v3-22

There's some good stuff in there.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-04-2013, 08:51 PM

Hey Guys,

ChassisSim is exhibiting at PRI in Indy this year,

http://www.chassissim.com/blog/chassissim-news/chassissim-will-be-at-pri-in-2013

For those of you who haven't been to PRI it is definitely worth the trip. For you guys on this forum based in North America it would be a great to meet you face to face.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-11-2013, 08:59 PM

Hey Guys,

We are running the Lap time simulation 101 seminar at PRI this year. You can find out more details here,

http://www.chassissim.com/blog/chassissim-news/lap-time-simulation-101-seminar-at-pri-2013

We have a few special things planned this year, but if your serious about knowing how to use simulation as a weapon, this seminar is a must attend event.

I look forward to seeing you all there.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-18-2013, 11:19 PM

Hey Guys,

For those of you in Europe the simulation bootcamp is back on. You can find out more here,

http://www.chassissim.com/blog/chassissim-news/the-simulation-bootcamp-is-back-in-europe

This is a hands on seminar that is specifically been designed to get you up and running with race car simulation. We are hosting it in France this year and more details will follow. We have some exciting stuff planned so stay tuned.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-25-2013, 08:51 PM

Hey Guys,

Just posted this video giving you a teaser about the simulation bootcamp in Europe and Lap time simulation 101 at PRI in Indianapolis this year,

http://www.chassissim.com/blog/chassissim-news/chassissim-seminar-teaser-2013#!

Both of these are fantastic tools to use lap time simulations at it's full potential and I look forward to seeing you all there.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-03-2013, 06:40 AM

Hey Guys,

Just thought I'd let everyone know that ChassisSim has just become a technical partner of the sportscar manufacturer ORECA. You can find out more here,

http://www.chassissim.com/blog/chassissim-news/chassissim-has-just-become-an-oreca-technical-partner

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

Tim.Wright

10-07-2013, 03:28 PM

Hi Danny,

You up for a discussion about the stability index you mentioned in your video blog? Just to clarify, I'm speaking of the equation used by yourself and Milliken:
SI = (dN/aAy)(1/mL)

I've been stuffing about with the stability index calculations with a mix of FSAE and non FSAE data in both simulation and from track data. I thought it would be appropriate to discuss here and hopefully it can develop into a useful discussion for everyone.

So... Milliken (if I understand correctly) describes the SI as a representation of the static stability moment (dN/dß)ß and the yaw damping moment (N_r x r) for a constant control moment (dN/dδ)δ.

Myself and another aquaintance are trying to get our heads around two things:

1. Theory: how applicable is this linear, steady state calculation to non linear/transient track data
2. Practice: how do you filter/calculate this without ending up with nothing but noise?

On my first point, I'm a bit skeptical about how valid it is to calculate dN/dAy from track data considering that its definition comes from a condition of constant steering angle (not to mention trimmed steady state).

For me, the fact that the driver is continuously correcting the steering on track means that the yaw acceleration (and therefore yaw torque) is not a function only of the static stability moment and yaw damping moment but also the control moment. In fact I'd guess that most of the yaw dynamics of the car in a turn are coming from the driver's steering inputs rather than the yaw damping and static stability moment.

My second point is about implementing this on track data. With dAy in the denominator of the formula, the resulting signal is constantly going to +-infinity. When I've put the first equation above into my track data, the result looks like white noise. I don't want to filter out high frequencies because I feel that these higher frequency (ca. 10Hz) movements are exactly what we are trying to measure. Am I going about this the right way?

I hope I've explained myself properly. Would be interested to hear anyone elses comments on this...

T

PS, this forum needs an equation editor!!

BillCobb

10-07-2013, 06:45 PM

In deference to Doug Milliken's observation on my presence here, I will say that the best answer for you has to do with holding circus tents up and also famous Japanese WWII warplanes...

I will explain once all the verbosity is infiltrated...

Silente

10-08-2013, 01:16 AM

i am totally with Tim...

I tried to take out the same channels both on sim and real data, but no success on really using it for stability evaluation. Sometimes, a part from the noise mentioned by Tim (which makes very difficult to use this metrics), i also noticed that results from SI calculations seem to be contrary to what is shown by comparing "actual Steering angle" to "Ideal Steering angle".

ChassisSim

10-08-2013, 06:54 AM

Hey Guys,

It's fantastic to see a genuine high level technical discussion on this forum. Tim many thanks for kicking this off.

Firstly I think it would be wise to step back a bit and discuss where the stability index comes from. The stability index has it's origins in Longitudinal aircraft stability analysis. Aeronautical engineers came up with something called the static margin. What this represents is a non dimensionalised moment arm between the centre of aerodynamic forces and where the centre of gravity is. Effectively the highlights are,

*When the static margin is less than 0 the aircraft is stable. You give it a disturbance be it a gust or control input and it will stabilise itself.
*When the static margin is 0 the aircraft is Neutral. That is given an input it will just keep on going.
*When the static margin is greater than 0 the aircraft is unstable. That is you give it an input and it will swap ends.

Also the magnitude of this number gives you a direct measure of the ability of the pilot. Bottomline the closer they get to 0 the better the pilot is. Mathematically the definition of this is,

SM = (dCm/dCL)/mean chord

Effectively it's measuring the change in ptich moment vs change in lift co-efficient.

The motorsport incarnation of this is,

SI = (dN/day)/mt*wb

The derivation for this is covered in both Milliken's book and my book the Dynamics of the RaceCar. I actually dedicated a whole chapter to this. As per the aerospace definition we are measuring the change in Moment about the centre of gravity as a function of Side force. A very effective way of measuring this is lateral acceleration. Tim I trust this resolves your question because what we are measuring here is not just a control slop for a given input. The Stability Index is a leaving breathing animal that is a direct measure of the stability or otherwise of the race car.

Let me show you a little party trick that will measure the stability index. Tim and Silente should answer your question. What you need to do is simply put an x-y plot of Yaw Moment vs lateral acceleration. Yaw moment is in N, lateral acceleration in g. To instrument this to your car is easy. You put lateral accelerometers on the front and rear axles and the Yaw Moment you are plotting becomes,

N = wdf*mt*ay_front*a - wdr*mt*ay_rear*b

We have

N = Yaw moment (Nm)
wdf = Weight distribution at the front (%age/100)
ay_front = Front lateral acceleration (g)
wdr = Weight distribution at the front (%age/100)
ay_rear = Rear lateral acceleration (g)
a = moment arm from front axle to c.g (m)
b = moment arm from rear axle to c.g (m)

Here's the trick, make the vertical axes Yaw Moment (N) and the horizontal axis lateral acceleration. Filter the signals and the slope is the stability index. On this one you might have to do some work. Just don't do a curve fit. Look at what it is telling you. Silente in answer to your question I gave an analysis in a racecar article and my book comparing actual vs neutral steer and correlating that to the stability index.

I trust this gets you all thinking. By all means keep the comments and questions rolling in, but I'll give you all the heads up about something. At the end of the year you can ask me this in person. If your in Europe I'm giving a simulation bootcamp on Nov 20 at the ORECA factory in Signes France. If your in the U.S We'll be at PRI booth no 137 on Dec 12-14 in Indianapolis and I'm given a series of free seminars at the show.

I trust this has given everyone some good food for thought.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Tim.Wright

10-08-2013, 05:01 PM

Hi Danny, thanks a lot for the answer. I will need a bit of time to digest the theoretical part...

On the practical side, I've messed around with some track data and its looks a bit more useable when I represent it like you said. Still not perfect but I will experiment with the filtering when I get some spare time this week.

I have seen already that long steady state corners will give you a trendline gradient of zero because you have a lot of points in the steady region and not so many in the transient region. I'm beginning to think maybe the useful information here is only in the transient regions.

I will come back when I've got something useful to share.

Tim

ChassisSim

10-08-2013, 06:01 PM

Tim,

Good hunting my friend. One thing I will say it is best used as a transient tool which is what it was set up for in the first place.

Good Luck!

Danny Nowlan
Director
ChassisSim Technologies

10-08-2013, 08:45 PM

In fact I'd guess that most of the yaw dynamics of the [FSAE] car in a turn are coming from the driver's steering inputs rather than the yaw damping and static stability moment.
Tim,

Not enough time for details, but I agree with your above quote. With the low speeds and constant changes of direction in FSAE I reckon a car could be highly UNstable, but nevertheless be quite fast around a typical FSAE track.

I reckon all 2nd year+ teams should be testing these sorts of things with their previous year's car. Take ~50 kg of barbell weights and bolt either to nose of car, or under driver's seat, or to rear jacking bar (or rearward extension). Bolt to car at same height, so same total mass and same CG height, but different "Static Margin". Then do some hot laps to find out just how important is a stable, or neutral, SM. Or not...?

(Splitting the 50 kg so it is half-at-front + half-at-rear (for bigger Yaw MoI), or all under driver's seat (for original Y MoI), and then some hot laps gives an indication of how important is a low Y MoI. Or not...?)

Z

ChassisSim

10-09-2013, 07:39 PM

Hey Guys,

Just some updates on what is going on with our seminars.

Firstly the bootcamp in Europe on Wednesday Nov 20, will be at the ORECA factory in Signes France,

http://www.chassissim.com/blog/chassissim-news/the-european-simulation-bootcamp-2013-is-at-the-oreca-factory

Also for those of you who are based in the U.S we have some format changes coming to the Lap Time Simulation 101 seminar at PRI. You can find out the details here,

http://www.chassissim.com/blog/chassissim-news/exciting-news-for-laptime-simulation-101-at-pri-2013

Also any sensible/in-depth questions about the Stability Index keep them coming in. Given the interest this has generated I might run a mini seminar about this at the Booth. We are at Booth no 137. However we'll see how busy we get!

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Tim.Wright

10-10-2013, 06:59 AM

I intend to keep the discussion going. I'm trying to get a hold of some data that I'm allowed to share to use in the discussion.

Tim

Moop

10-11-2013, 08:10 PM

Let me show you a little party trick that will measure the stability index. Tim and Silente should answer your question. What you need to do is simply put an x-y plot of Yaw Moment vs lateral acceleration. Yaw moment is in N, lateral acceleration in g. To instrument this to your car is easy. You put lateral accelerometers on the front and rear axles and the Yaw Moment you are plotting becomes,

N = wdf*mt*ay_front*a - wdr*mt*ay_rear*b

I sat down and worked out that calculating the yaw moment on the car this way is equivalent to calculating the yaw acceleration and assuming the car's yaw inertia is total mass * a * b. This may be a reasonable yaw inertia estimate for a production car or cars where most of the mass is sitting far from the CG and in between the wheels. But it's not a very good estimate for an FSAE car, where the two heaviest parts of the car, the driver and the engine, are sitting right next to the CG.

To check this, I dug up a published FSAE car yaw inertia and also calculated the yaw inertia with this estimate. The estimate was nearly twice as big as the published value.

Claude Rouelle

10-11-2013, 09:02 PM

I agree with Moop. The whole front (rear) mass is not located on one point at the center of the front (rear) axle. k2/ab is one of the "magic number " to master in transient.

Tim.Wright

10-13-2013, 03:11 PM

I've had a look at Danny's suggestion at looking at the data in the CN-AY plane and extracting a trend from that. What I noticed immediately is that you get a completely different looking response for a hairpin, esses and a long sweeping bend. Some had positive trends, some negative and I thought that this isn't telling me what I want.

So, I've done a bit of research on this. In the absence of any track data thats shareable, I made a 2dof bicycle model in excel and ran it through some tests. The advantage of this approach is that I can calculate the stability index exactly using the model's mass, tyre and geometric data. The disadvantage is obviously that its not real data and for now it doesn't include tyre lag effects. Still, what I have seen is interesting.

Using the bicycle model, I've done 3 operations which use a single nominal steering angle (37deg giving 1g) and speed (50kmh):
1. In the time domain I've done a few maneuvers (step steer, track corner, sinusoid 2Hz, frequency sweep all at 37deg swa) and had a look at the response in the N-Ay plane to see if I can extract a trend to call the stability index. I'm only looking at transients here.
2. I have created a reduced MMM diagram from the model showing a single line of const steering angle (37deg). The slope of this line is the stability index and it is -0.3188 for my model
3. I have calculated the stability index using a single formula. Again, I get -0.3188 for this model.

Basically what I found, which was my feeling from the start, is that different steering inputs from the driver give different responses on the N-Ay plane. So with a single model, with a single defined SI, I have seen that fitting a line to the CN-AY plane reponse doesn't give you what you want. The following examples will show this:

The step steer gives the best approximation of the stability index according to the calculation and in terms of the response in the CN-AY plane. I'd say this is because its the closest condition to a constrained MMM test. That is constant steering angle, so the yaw response comes primarily from the yawrate and sideslip effects. When I fit a line to the linear part of the CN-AY response, I get a number of -0.2939 which is reasonably close to the calculated SI of -0.3188
https://lh5.googleusercontent.com/-uoVLs_ql7YA/Ulr2lsQUQEI/AAAAAAAAAYI/PBA7IJprMAg/s640/bici_step_response.jpg

Next I gave it a steering input like you might see through a corner. So there is a transient section on corner entry, a steady section in the middle and a transient section on corner exit. In the response below I fitted a curve to both of the transient sections independantly and for both I curiously got identical numbers of +0.0075. Obviously not very well related to the stability index, but interestingly, the CN-AY reponse remains bounded by the single MMM line.
https://lh4.googleusercontent.com/-K6eZ4awQNIY/Ulr2mgYH-II/AAAAAAAAAYQ/Pkd9OT7nNMg/s640/bici_vconst_corner.jpg

Next test, a sinusoidal input at constant velocity. Someting like you might see in silverstone through Maggots, Becketts but at a higher frequency (2Hz). Here its possible to appreciate some trend (trendline slope of -0.0193). Again its not equal to the stability index, BUT it does run tangent to it. So like the previous test, it is bounded by the single MMM line.
https://lh3.googleusercontent.com/-6pV6nfo1iCg/Ulr2nhaaC3I/AAAAAAAAAYg/qxxSmbKKLI8/s640/bici_const_sine.jpg

Finally I did a frequency sweep. This gave the coolest looking plot which had no trend at all but again seems perfectly bounded by the MMM line.
https://lh3.googleusercontent.com/-vHd7TRcjnK4/Ulr2oguWX7I/AAAAAAAAAYo/viPf-f5BBRY/s640/bici_sin_sweep.jpg

So, my conclusion so far is that the transient response you see on the CN-AY plane IS related to the stability index, BUT its not as simple as fitting a line to the data. I think I've shown here that with one car, one steering angle and one speed, you get totally different CN-AY reponses depending on what kind of steering input you give.

I think once you add in other realities such as braking/acceleration effects on speed and load transfer as well as tyre lag and non linear effects it would be pretty dangerous to fit a curve to the CN-AY plot and call it the stability index.

Tim

P.S. Sorry for not labelling my plot axes.
P.P.S Actually I'm not really sorry...
Tim

BillCobb

10-13-2013, 07:11 PM

Take the results from the chirp steer input to your model and process the Ay by steer and the Yaw Velocity by steer data to produce the transfer function summary in Bode form (Gain and Phase vs frequency). Then compute the phase margins for each and you will have a reliable stability metric because THAT'S WHAT A PHASE MARGIN IS).

Now change the model parameters and produce families of vehicles which have the same gain (ay by steer), the same understeer, the same rear cornering compliance and even 3 different values of rear roll-steer (steer by roll: deg/deg) [as in +.05, 0.0 and -.05]. The different roll steers will require you to recomputed the effective tire cornering stiffnesses in order to maintain the same rear cornering compliance for that family of runs.

When you compute the phase information for these combinations , the results will all be in Control System Engineering sense.

The only trick is to lump the front and rear tire and suspension parameters into cornering compliance units (I could post some of these for you if you can't figure it out).

Its so much easier and elegant when you use engineering tactics instead of IT or here-say methods.

To tackle the nonlinear effects, there are several alternative procedures:

1) Localized linearity: run the same play as for the linear model, except use the chirp steer signal running between 34 to 39 degrees of steer.

2) Us the Bendat and Piersol technique of running the results thru an inverse filter having the reverse characteristics of your nonlinearity (most probably them there tars). Since they are softening springs, the inverse tire model method is a classroom course first homework assignment. They wrote an entire book on this and I was fortunate to attend a class taught by these two guys on how to it works. I've enclosed the book details for those of you who can only text or tweet thus far in your educational resume'.

3) Run the fully nonlinear model results data through the FR test post processing and find out how disappointing the difference is between linear vehicles and non-linear vehicles in the frequency domain (except for one special detail). Again, all the nonlinearities in vehicle handling are (or better well should be) stiffening or softening springs. Anything else produces angry tires, angry drivers and angry owners (but happy fans and spectators).

If you say you are an Engineer, but use religion to explain your results, make sure you blow your whistle at every intersection crossing.

Tim.Wright

10-15-2013, 02:56 AM

Thanks Bill, I will look into this when I get a few more spare minutes.

I have done something like the local linearised method already with a thesis student at my old uni. It was not in a dynamic model but it had nonlinear tyres. From what I had seen so far (its still in progress) there can be a reasonable difference between the behaviour in the linear range and at the limit.

Also, I had started the discussion because Im interested to see if there is a way to quantify stability using only track data. Here you generally don't have a tyre model to use as reference and no slip angle measurement. And your input is not a perfect chirp signal so you cant post process it using fft methods.

So for me there is still a gap for a lot more discussion.

ChassisSim

10-15-2013, 07:29 PM

Hey Guys,

Many thanks for contributing to this excellent discussion. There are few things I'd like to throw in and clarify,

First things first Tim - excellent work. It's really good to see some good analysis here. Let me throw in some observations and clarifications. Firstly the calculation of Stability Index via tyre data is an excellent start. It doesn't give you the exact answer but it tells you what to expect and this in of itself is a good thing.

Just with regards to what you said about Yaw moment vs lateral acceleration the overall curve fit is a tool to get you going but in reality you do have to dig a bit deeper which is precisely what you have done. As I stated in my last post about the Stability Index it is a measure of car stability that will change with the car conditions. The variation you got in the chicane result didn't surprise me at all. The thing that makes Race car vehicle dynamics so tricky is that your inputs have a fundamental impact on the characteristics of the car. Hence why they are going to vary. Let me give you a good case in point. When I was analysing some F3 data a while ago when I plotted Yaw moment vs lateral acceleration. When the g was low the slope was nice and linear indicating the car had good stability. As we approached peak cornering conditions the plot started to go all over the place since this was a particular bumpy circuit. This was a strong indication we had hit unstable behaviour. The give away was comparing actual to Neutral steer because as we got closer to the limit the neutral and steer lines where crossing in and out indicating unstable behaviour. Consequently I always use these two tools in concert with each other and I use the initial curve fit to get me going. I trust this makes sense.

Also just for my reference could you just clarify what your axes are. I found that bit just a bit confusing. Also too feel free to email me your results so we may have a more detailed discussion. I won't be able to respond this week because I'm tied up at time attack at Eastern Creek.

Moop - I would strongly suggest you double check your numbers. That little equation I put forward to approximate Yaw moment I've seen in use on many formula from V8 Supercars, GP2 and F3. I have lost count of the number of race engineers I know who have used this in anger as a sanity check to tell them what the car is doing . Granted this would break down in extreme cases of structural flexibility but if your at this point your throwing out the car anyway so it's a moot point.

Claude - My friend this is no magic number. The stability index is a measure of car stability at a particular point in time. It's up to the end user to determine what to do with it.

Keep up the good work guys. For those of you in the U.S I'm tending towards giving a mini seminar at this at PRI.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-15-2013, 07:32 PM

Hey Guys,

On another note, I just thought I'd give you another taster about what I'll be talking about at the simulation bootcamp in Europe and PRI in December at Lap Time Simulation 101,

http://www.chassissim.com/blog/chassissim-news/modelling-tyre-camber-sensitivity-with-chassissim

I'm looking forward to seeing you all there.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

BillCobb

10-15-2013, 08:04 PM

Tim. A Datron optical sideslip sensor is the most widely used xducer in the U.S. field. I have seen the telltale Datron lamp beam on the pavement during races when such measurements are forbidden. Oops.

Given a sideslip reading at centerline of the rear axle (best place to put it), this along with corrected lateral acceleration directly measures the vehicle's rear cornering compliance. You can also create a Math channel for the c.g. sideslip angle and a situation for which this signal is zero is a special case in vehicle dynamics. Called the Tangent Speed (because the vehicle is tangent to the turning circle path), the higher the Tangent Speed, the lower the rear cornering compliance. And, knowing this speed allows you to compute the rear cornering compliance just from vehicle geometric parameters. From this you can ascertain the tire stiffness at that trim by knowing a bit about the rear chassis compliances. In a racing vehicle, there ought not to be very much deflection steer or rear roll steer so the tire properties can be easily obtained or verified. Nothing like a K&C test to fill in these missing suspension parameters.

The goal of your test session would be to measure the tangent speed for every change in trial settings. The highest number speed is the best value. Of course the can be ugly, but if the number is high and the controllability is low (measure its Yaw rate by Steer Coherence). then you also know that the 'problems is at the front end. Best results come from a steering torque sensor because a 'feeling' driver will be in a moment control dialog with the car. Poor yaw velocity by steer torque coherence with also point out that friction is not your friend any more.

I believe I pointed out earlier that the FFT of Lateral Acceleration - Yaw Velocity phase information ( I call it the "RAY function") graphed vs. frequency will lie on a curve whose initial slope can also produce the vehicle's rear cornering compliance (rear axle axle sideslip angle) metric. Since there may be scale and other instruments snafus in your D.A. box, its pretty hard to screw up the time base, so this phase metric will be pretty freakin' accurate.

Now all you need is a constant radius portion of the track or test area and and a driver able to run a reasonable speed sweep through it. BTW: this is an I.S.O. test procedure. The Math functions for it are printed in the I.S.O. document as I recall.

You need to pat attention to more than yaw velocity because a high speed vehicle should be sidesliping a lot more than turning. And, turning more at low speeds. Even the Zamboni driver at my hockey rink knows this....

10-15-2013, 09:59 PM

Originally posted by Tim:
I made a 2dof bicycle model in excel and ran it through some tests. The advantage of this approach is that I can calculate the stability index exactly using the model's mass, tyre and geometric data. The disadvantage is obviously that its not real data and for now it doesn't include tyre lag effects. Still, what I have seen is interesting.

Using the bicycle model, I've done 3 operations which use a single nominal steering angle (37deg giving 1g) and speed (50kmh):
1. In the time domain I've done a few maneuvers (step steer, track corner, sinusoid 2Hz, frequency sweep all at 37deg swa) and had a look at the response in the N-Ay plane to see if I can extract a trend to call the stability index. I'm only looking at transients here.
2. I have created a reduced MMM diagram from the model showing a single line of const steering angle (37deg). The slope of this line is the stability index and it is -0.3188 for my model
3. I have calculated the stability index using a single formula. Again, I get -0.3188 for this model.

Tim,

I have had a quick look through the MMM section in RCVD to remind myself about this stuff. (Doug, if anything wrong below, then please correct.)

1. It seems that this MMM approach is quite simplified (nothing wrong with that!) in that it is a bicycle model in pseudo-steady-state cornering (err..., at very large radius). This means that any Yaw Moment (= Yaw Couple!!!) "N" acting on the car DOES NOT cause a Yaw acceleration. So no Yaw MoI is included in this model, since it does not matter for this type of analysis. So not really good for "looking at transients"...

2. Minor point, but Milliken's "CN" is a non-dimensionalised "coefficient" of the Yaw force (= N/Wl), and their AY is also non-dimensionalised (= Ay/g, so in units of "G" = ~0-2 for typical FSAE cars). However, even in this one chapter there seemed to be some variability on the labelling of axes.

3. Milliken's "stability index" seems to be dCN/dAY for FIXED front steer angle, but varying car side-slip angle (= beta), measured at CN = 0.

4. The whole MMM map, similar to your last picture of a frequency sweep, gives the maximum "performance envelope" of the car (given the limits of this simple model). The baseline "stability index" at steer-angle = 0, side-slip-angle = 0, CN = 0, and AY = 0, is a diagonal line with similar slope to the one you show, but passing through the origin. The "limit performance" of the car is indicated by the right-hand corner of the diamond shaped map. If this corner is below the AY-axis, then understeer at the limit. If above AY-axis, then oversteer at the limit.
~~~o0o~~~

Anyway, I am curious to know more details about your analysis.
1. Do you include Yaw MoI, or are you as per the MMM?
2. What are the labels of your axes?
3. What formula did you use to calculate the stability index exactly (I suspect this should give the same as 3 above)?

Finally, I remind anyone doing this MMM sort of analysis that its pseudo-static nature, with no account of Yaw MoI, gives it VERY LIMITED APPLICABILITY in FSAE conditions (which are mostly "transients").

Z

DougMilliken

10-15-2013, 11:11 PM

1. It seems that this MMM approach is quite simplified (nothing wrong with that!) in that it is a bicycle model in pseudo-steady-state cornering (err..., at very large radius). This means that any Yaw Moment (= Yaw Couple!!!) "N" acting on the car DOES NOT cause a Yaw acceleration. So no Yaw MoI is included in this model, since it does not matter for this type of analysis. So not really good for "looking at transients"...
Z -- The MMM analysis can be arbitrarily detailed: Some of the early examples in Chapter 8 use a bicycle model, possibly using tire data based on pair analysis (Chapter 7). The later examples in C.8 include detailed tire data, aero data and measured K&C. MMM is a simulation of a constrained test (like wind tunnel tests and aircraft "statics") so there is no yaw acceleration, but, the N (or CN) would most certainly produce a yaw acceleration if the constraint were removed.

...in this one chapter there seemed to be some variability on the labelling of axes.
You might want to look at Table 8.4 which lists some of the different diagrams (plotted on different axes) and how they relate to different kinds of maneuvers.

Like any tool, it has strengths and weaknesses. One strength is the ability (after some practice by the user) to see the whole performance envelope of the car, which makes it easy to look at many of the big questions in preliminary design. A major weakness (in terms of commercial acceptance!) is that it typically takes a month or two of use and training to get familiar with the concepts and learn to interpret the results...

Tim.Wright

10-16-2013, 04:45 AM

Erik,

I will give a more detailed answer later when I'm back home but I agree with your points above. The MMM line was generated using the basic sum of forces and sum of moments equations (using the control derivatives) and with a sweep of slip angle (I think I did a range of =-3deg). BUT the one manipulation is that in these equations I have replaced yawrate with Ay/v. So yes, this does make is pseudo steady state like you mentioned. In fact, you can see in the step steer results, the vehicle response asymptotes towards the MMM line as it reaches steady state (i.e. when the condition r = Ay/v actually becomes true).

Im not sure its valid to talk about limits here because I'm using a linear tyre model.

The axes are:
Vert: CN = (yaw_moment/(wheelbase x mass))
Hor: AY = lateral_acceleration/9.81

murpia

10-16-2013, 11:16 AM

Tim.Wright

10-16-2013, 03:32 PM

I'll answer point by point, but I'll start with your questions about my model:

Anyway, I am curious to know more details about your analysis.
1. Do you include Yaw MoI, or are you as per the MMM?
2. What are the labels of your axes?
3. What formula did you use to calculate the stability index exactly (I suspect this should give the same as 3 above)?

My model is a time domain bicycle model. So there is yaw inertia . The tyres are linear and there are no tyre lag effects included at the moment. My prameters are:
mass = 335 at 49%F
wheelbase = 1.535
Izz = 150kgm^2

The formula for stability index was one I derived myself from other equations in RCVD:
SI = ( Nb(mV - Yr) + Nr x Yb ) / vYb

Where:
m = mass
V = velocity
Nb = Yaw moment due to sideslip
Nr = Yaw moment due to yawrate
Yb = lateral force due to sideslip
Yr = lateral force due to yawrate

Its possibly written in RCVD as well but I wanted to derive it to understand it better.

If anyone want the excel file its here. Its a bit of a mess but I think its error free now. Basically, column J is a time varying number between 0 and 1 which specifies the steering angle as a ratio of the nominal angle in cell B16. I set it to equal column F for a step steer, col G for a constant sine steer, col G for a sin sweep and col I for the demo corner.

https://skydrive.live.com/redir?resid=52F68D345B88042A!117&authkey=!AOJ8LzxvRWG934I

1. It seems that this MMM approach is quite simplified (nothing wrong with that!) in that it is a bicycle model in pseudo-steady-state cornering (err..., at very large radius). This means that any Yaw Moment (= Yaw Couple!!!) "N" acting on the car DOES NOT cause a Yaw acceleration. So no Yaw MoI is included in this model, since it does not matter for this type of analysis. So not really good for "looking at transients"...
I'd disagree that its at only showing cornering at a very large radius. What its showing is the behaviour when r = Ay/v which is steady state. However, the fact that the MMM diagram also represents untrimmed conditions (N =/= 0) means that it does have some transient information inside it.

In fact I've shown that even with very dynamic maneuvers, the CN-AY response is bounded by the MMM line.

3. Milliken's "stability index" seems to be dCN/dAY for FIXED front steer angle, but varying car side-slip angle (= beta), measured at CN = 0.
Yep, this is how I came up the SI calculation and also how I generated the MMM line. This is also the main reason why I have reservations about extracting a slope of N vs AY from track data because the steering angle isnt constant.

4. The whole MMM map, similar to your last picture of a frequency sweep, gives the maximum "performance envelope" of the car (given the limits of this simple model). The baseline "stability index" at steer-angle = 0, side-slip-angle = 0, CN = 0, and AY = 0, is a diagonal line with similar slope to the one you show, but passing through the origin. The "limit performance" of the car is indicated by the right-hand corner of the diamond shaped map. If this corner is below the AY-axis, then understeer at the limit. If above AY-axis, then oversteer at the limit.
I touched on this before, but my linear model has no limits. I just set the steering angle so it reaches 1g in steady state. I'm not sure the plot made from the frequency response is comparable to the MMM diagram (but at least it looks cool). I think the best way to quantify stabiliy from this simulation is by post processing the data like Bill suggested.

Finally, I remind anyone doing this MMM sort of analysis that its pseudo-static nature, with no account of Yaw MoI, gives it VERY LIMITED APPLICABILITY in FSAE conditions (which are mostly "transients").
I think it gives quite a lot of information about the transient. Ok, not the full picture but I've at least shown that it has some relationship to a real (ok simulated) transient response.

Tim

PS Are there really no buttons to include links or pictures in this forum?? I might be fluent with Matlab, but I had to login to another forum to remember the BBcode to include a link!

Moop

10-16-2013, 11:13 PM

Moop - I would strongly suggest you double check your numbers. That little equation I put forward to approximate Yaw moment I've seen in use on many formula from V8 Supercars, GP2 and F3. I have lost count of the number of race engineers I know who have used this in anger as a sanity check to tell them what the car is doing . Granted this would break down in extreme cases of structural flexibility but if your at this point your throwing out the car anyway so it's a moot point.
Like I said, it probably works well for bigger cars where most of the mass is located between the wheels, but this is not the case for an FSAE car.

The goal of your test session would be to measure the tangent speed for every change in trial settings. The highest number speed is the best value.
Is the goal of reducing tangent speed/rear cornering compliance to shorten the yaw rate/lateral acceleration response time or what? Is there a point at which it would maybe be not beneficial to have the car responding faster(say, if the driver couldn't handle it?)?

Also, I thought that cornering compliances were driven by tire stiffnesses, chassis/suspension compliances and geometric steer. Aside from maybe roll steer/camber, what would you be changing at the track that would impact the rear cornering compliance?

I believe I pointed out earlier that the FFT of Lateral Acceleration - Yaw Velocity phase information ( I call it the "RAY function") graphed vs. frequency will lie on a curve whose initial slope can also produce the vehicle's rear cornering compliance (rear axle axle sideslip angle) metric. Since there may be scale and other instruments snafus in your D.A. box, its pretty hard to screw up the time base, so this phase metric will be pretty freakin' accurate.

Now all you need is a constant radius portion of the track or test area and and a driver able to run a reasonable speed sweep through it. BTW: this is an I.S.O. test procedure. The Math functions for it are printed in the I.S.O. document as I recall.
I thought that this was measured as the difference between the slopes of phase vs frequency at zero frequency for lateral acceleration by steer and yaw rate by steer? What's the FFT for? Unless you're using that to get phase vs frequency instead of calculating the transfer functions from the PSDs and CSDs?

Also, I thought this metric was usually measured in a steering frequency response test? Have you ever seen it attempted with a driver instead of a steering robot? I'm not sure how confident I am in my driver to produce a consistent sinusoid at a given frequency, although I guess I haven't given him a chance yet.

Silente

10-17-2013, 06:02 AM

Tim

regarding the feasibility of Stability index for on-track studies (data engineer typical work, race engineer as well in many teams), i think it doesn't work as it is.

The assumptions made to produce a MMM diagram make them absolutely fantastic in a design phase and probably also for setup investigations, but as you said, since driver inputs are not constant (above all steering) and are sometimes evolving quite quickly, it could be tricky to apply them for a track side study.

To me, the same is true also for the SI itself. I have tried to calculate it with math channels, but it doesn't show many useful info. To use a X-Y plot with lateral acceleration and yaw acceleration is already better, but still you have to be very careful with the track section you select and it is also difficult to extract a precise number to quantify any tendency.

I normally looked to a slightly modified version of what Danny would call Actual Steer vs Ideal Steer comparison, normalized vs lateral acceleration. I find it useful to quantify understeering tendencies, above all in corner-center (quasi steady state) conditions and it is very repeatable.
It falls down, of course, in corner entry, where it really doesn't give many information about what the car is really doing (or at least less info than in corner center)

I have seen that some info about stability in transients could come anyway from the yaw rate channel itself, which is anyway the one you would use to calculate yaw acceleration. Actually, i guess you could potentially compare the actual yaw rate sensor reading vs an ideal yaw rate value that you could calculate through the other channels you have (see speed, lateral G), above all if you use GPS (although the GPS receivers i have used were a bit slow and they normally showed some delay against the other "in-car measured" channels).

If your measured yaw rate is bigger (at least in its absolute value) than the calculated one, then you should be in an oversteering situation.

Although this method doesn't tell much about the Yaw Moment it could still helps to quantify car stability in transients, at least when used together with the ideal steer vs actual steer one.

I will try to look at it and post something, if it comes out to be a feasible way.

Tim.Wright

10-18-2013, 12:19 AM

I have seen that some info about stability in transients could come anyway from the yaw rate channel itself, which is anyway the one you would use to calculate yaw acceleration. Actually, i guess you could potentially compare the actual yaw rate sensor reading vs an ideal yaw rate value that you could calculate through the other channels you have (see speed, lateral G), above all if you use GPS (although the GPS receivers i have used were a bit slow and they normally showed some delay against the other "in-car measured" channels).

If your measured yaw rate is bigger (at least in its absolute value) than the calculated one, then you should be in an oversteering situation.

I think that using yawrate is the right way to go because it contains a component of sideslip velocity.

Slightly off topic, but in my calcs for natural frequency and yaw damping ratio seem to indicate that an FSAE car is highly damped and has a very low natural frequency. Not a good combo for a car that needs a very good yawresponse. This was also backed up in my bicycle model where I saw the yawrate didnt follow the typical magnitude response where from 0Hz it increases in magnitude until it reaches a natural frequency somewhere in the range of 1-2Hz, and then drops down again.

I'd be interested to know if anyone else can confirm this behaviour or if its an error somewhere in my working. I suspect it could be that the cornering stiffness of these tyres (Hoosiers in my case) are in the wrong range for cars of this mass and wheelbase.

If its true, I think there could be some scope to design in some instability to the car to at least give the yaw response a bit of a boost.

BillCobb

10-18-2013, 04:58 PM

Controls Engineers (and Vehicle Dynamics fishing addons recognize that it is NOT necessary to use a swept frequency steer input (often called a 'chirp') to generate adequate transfer functions for relatively simple systems such as a vehicle. All that is needed is a pulse of duration width Tf/2, where Tf is the total time response from input to settling out time. The pulse does not have to be perfect, just sufficient to cover the range of interest in the transfer function and its coherence for multiple run segments. The only limitation in this technique is that the system is stationary (That means time invariant). It does NOT mean it must be linear, just consistent with parts and components whose properties do not change over the measurement timespan (like temperature, pressure, wear state and figmosity.

Also, a good FR chirp need not be accomplished with a robot. A human driver properly trained in the methodology can easily produce excellent chirp steer inputs from 0 to 4 Hz if the steering effort is light and the required steer angle amplitude is not too high. A good crutch to train a driver is the use of a CD player with a chirp tone MPEG file. I produced several versions for our test drivers using Matlab. After a few runs with the CD, they no longer need the crutch.

One more thing: As is often the case, a FSAE car can be very low in understeer which is OK for the limited speed range of the tasks, unless it leads to loss of control and major embarrassment(s) involving loss of bodily fluids. Checking your model for a designed in level of understeer or oversteer is one of the first things a design engineer must do. Most of the time, models do not include the elastic chassis compliances in steer or camber either because they can't believe they could be possible or they have no idea their magnitude. That's the eye opening feature of a K&C test. As a result, the low understeer models produce nearly first order FR transfer functions while the road tests produce whooping amounts of peak to steady state yawrate response.

I point out the pulse steer inputs because a 'feeling' driver makes use of this style to read the steered tires aligning moment response to this input type. And driving by moment control is a lot more successful than driving by displacement control IMHO.

ChassisSim

10-23-2013, 07:14 PM

Hey Guys,

Many thanks for contributing to this excellent discussion on the stability index. A colleague of mine was gracious enough to provide me with some race data for a Ford GT-40 with accelerometers on both axles of the car. This will be the subject of my next Racecar Engineering article. Suffice to say the methods I outlined have for calculating the stability index worked very well in this case. However I will post this in the next couple of days since it deserves a proper treatment and I have some pressing development work I need to be getting on with. There are some big things coming in the pipeline for ChassisSim but I'll let you know about this at the appropriate time.

In other news for those of you in Europe if you want to attend the simulation bootcamp on Nov 20 email me know at info@chassissim.com. Places are starting to fill fast.

In the meantime this is an oldie but a goodie - modern approaches to tyre modelling.

http://www.chassissim.com/blog/chassissim-news/modern-approaches-to-tyre-modelling

Enjoy and I'll post that stability index article as soon as I can.

All the Best

Danny Nowlan
Director
ChassisSim Technology

ChassisSim

10-31-2013, 12:15 AM

Hey Guys,

Here's the latest episode of Dan's Vehicle Dynamics corner outlining what I found with those accelerometers fitted to both axles of the GT-40 racecar.

http://www.chassissim.com/blog/chassissim-news/dans-vehicle-dynamics-corner-evaluating-racecar-stability-with-accelerometers

I won't claim this is the last word on the stability index but there is some really good food for thought. I learned a lot in the process and I hope you get something out of it.

Also for those of you in Europe, if you want to come to the simulation bootcamp contact me soon. Places are filling very fast. email at info@chassissim.com . Otherwise if you are based in the U.S I'm looking forward to seeing you at Booth 137 and our Lap time simulation 101 seminars at PRI in Indianapolis.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

11-06-2013, 09:23 PM

Hey Guys,

I'm about to get on a plane for Europe and the U.S for tradeshow and seminar commitments. Here's a reminder of where to catch us.

http://www.chassissim.com/blog/chassissim-news/key-dates-for-chassissim-for-novemberdecember-2013

I look forward to meeting quite a few of you face to face.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

11-27-2013, 12:56 PM

Hey Guys,

Sorry for the lack of contact recently. I've had my hands full travelling in Europe and the U.S on business. Here's a quick summary of what we have been up to,

http://www.chassissim.com/blog/chassissim-news/chassissim-seminar-updates-and-article

Also I've seen a lot of traffic recently on the magic number and lateral load transfer. I've enclosed an old race car article where tyre loads are derived from so I think that will be a great reference for everyone.

Looking forward to meeting a few of year at PRI in Indianapolis on Dec 12-14.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

01-22-2014, 07:07 PM

Hey Guys,

First things first I trust everyone had a great Christmas and a good new year's celebration.

Now that we are getting back into it I just wanted to kick of 2014 by posting this tutorial on our online simulation,

http://www.chassissim.com/blog/chassissim-news/getting-started-with-chassissim-online

When I was in Europe and the U.S I was getting a lot of questions about this and this is your best way of getting going with ChassisSim. If you want to get going follow the instructions in the video and drop me a line at info@chassissim.com and I'll set you up with some free simulations so you can take it for a spin.

Looking forward to a great 2014 and good luck for all your competitions this year.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

01-29-2014, 09:13 PM

Hey Guys,

Just following up from last week's video I've just posted this video about how to get a model going in ChassisSim Online,

http://www.chassissim.com/blog/chassissim-news/creating-and-using-a-model-in-minutes-using-chassissim-online

I've posted it here because a really important principal in simulation is to start simple and get complicated later. Even though ChassisSim can go into fine detail you can start simple which is what I show you in this tutorial.

Anyway some good food for thought.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

02-06-2014, 06:22 PM

Hey Guys,

I've got a few things for you this week. Firstly I have a video of one of my customers in action in the Daytona 24 hour. It's a little showy but it's good fun anyway,

http://www.chassissim.com/blog/chassissim-news/chassissim-customer-pr1-motorsports-in-action-at-daytona

However on a more serious note I was informed this week of the passing of Bill Mitchell, the man behind the suspension geometry software WinGeo. One of the things that we don't do particularly well in motorsports is to acknowledge when distinguished engineering figures pass away. Bill was a giant. He has left many legacies but to me his biggest legacies was his work on Force Based Roll centres which I will always be in his debt for. It is definitely worth your while chasing down his SAE Paper on the subject. Here is the link -

http://papers.sae.org/2006-01-3617/

It's really good food for thought.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

02-12-2014, 04:55 PM

Hey Guys,

I have a real treat for you this week. ChassisSim customer Maranello Motorsport won the Bathurst 12 hour on the 9th of February 2014. Here is the link with more details,

http://www.chassissim.com/blog/chassissim-news/chassissim-customer-maranello-motorsport-win-the-2014-bathurst-12-hour

There is a link to a video for the last 12 minutes of the race. Enjoy it will have you on the edge of your seat.

On a more serious note though the race engineer of that car is my Australian Dealer. We had a really good debrief about what happened at the event and how ChassisSim was used. Stand by for some really good tutorials that will spin off from this. I'll get to these when I can I'm just hammered on a few projects right now.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

02-19-2014, 06:36 PM

Hey Guys,

I just posted the latest episode of Dan's Vehicle Dynamics corner,

http://www.chassissim.com/blog/chassissim-news/evaluating-racecar-stability-using-accelerometers-part-2

It's a follow on from a tutorial I did on evaluating racecar stability using accelerometers. It has a correction to the first tutorial I did. However more importantly it contains a much improved technique that you'll get a lot out of. I also did a Racecar Engineering article about this as well.

Enjoy and have fun trying things out.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

02-26-2014, 06:36 PM

Hey Guys,

We have just released ChassisSim v3.24. The details can be found here,

http://www.chassissim.com/blog/chassissim-news/new-features-in-chassissim-v3-24-and-the-chassissim-tutorial

One of the things that we have done with this release is that we have enclosed a tool called the ChassisSim Tutorial. What it does is it takes you step by step through everything you need to do to get going with ChassisSim. It's designed to get you through the beginner hump.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-05-2014, 08:12 PM

Hey Guys,

I've got a real treat for you today. Remember a couple of weeks a go I mentioned one of my customers, Maranello motorsport won the Bathurst 12 hour. Here is one of the tools they used,

http://www.chassissim.com/blog/chassissim-news/using-chassissim-track-replays-to-determine-tyre-pressures-and-temperatures

This is a tutorial about determining hot tire pressures from a cold start condition using the ChassisSim track replay feature. You'll get a lot out of this.

Enjoy!

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-12-2014, 06:42 PM

Hey Guys,

I'm bringing up a link to an older post that I put up over 18 months ago. It's about how to create a tyre model from nothing using ChassisSim,

http://www.chassissim.com/blog/chassissim-news/modelling-tyres-using-chassissim-filling-in-the-blanks

Over the last couple of weeks I've had a lot of questions about tyre modelling so this post is particular relevant. What you have in this post is a complete battle plan to how to do tyre modelling from race data so you have a tyre model you can use in anger.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

Goost

03-19-2014, 09:30 AM

Danny,

I have been planning to buy your book - it's the next on my list now that I recently got Seger's 2nd Edition Data Aq book.
Jorge quotes you on a tire modeling technique and I have a question about the equation he lists. You may explain this better in your book (I couldn't find it in the first chapter preview) but I can't make sense of it as given.

~~~

To find a first approximation Fy vs Fz relationship from tire data, we use a 2nd order relationship (linear decrease in friction coefficient) that can be written as

Fy_max = a1*Fz^2+a2*Fz (called 'D' c.f. Pacejka '89 etc.) (1)

or you give it as

Fy_max = ka(1-kb*Fz)*Fz = (-ka*kb)*Fz^2+ka*Fz (2)

this form being convenient since

Lp = 1/(2*kb) -> kb = 1/(2*Lp) where Lp is the 'load at which the tire peaks' (3)

I follow this so far - does it seem correct?

~~~

In the book, it seems that the approximation is that Lp is approximately the maximum normal load on a tire after sufficient track data has been gathered. We can then directly solve for kb.

This is fair for some tires I think. Do I understand this correctly?

~~~

Here is where I am confused: the next section reads (pp 414-415):

"Equations 15.20 and 15.21 estimate the total lateral tire forces ... by applying a force equilibrium around the ... axles."

The equation listed is:
Fn_LF + Fn_RF = ka * ( (1-kb*Fn_LF)*Fn_LF + (1-kb*Fn_RF)*Fn_RF ) (4)

First, this sentence (Seger's wording I suppose?) is odd because to my knowledge only [I]moments are in equilibrium around anything - and I suppose this implies Static equilibrium too though not stated.

Anyway, I cannot come up with the above equation from a FBD or any train of thought thus far. I think it could be modified to include the relevant distances to the CG as follows (assuming a right-hand turn):

(Fn_LF - Fn_RF)*(t/2) = ka * ( (1-kb*Fn_LF)*Fn_LF + (1-kb*Fn_RF)*Fn_RF )*(h) (5)

Does this new formulation seem correct or was the original right? If so, what am I missing?
Sorry if this is not your concept - though you are quoted as being the source for the idea maybe you know what I miss?

notes:
a2 = initial coefficient of friction (1/1)
ka = 'initial coefficient of friction' (1/1)
a1 = load saturation parameter (<0) (1/N)
kb = 'drop off of coefficient with load' (>0) (1/N)
Fz = load on the tire (N)
Fn_LF = load on the left front (outside) tire (N)
Fn_RF = load on the right front (inside) tire (N)
t = trackwidth (m)
h = CG height above the ground (m)

Thank you!

ChassisSim

03-19-2014, 07:28 PM

Goost,

My friend this is an excellent question and my apologies for the late reply. I have been hammered on a lot of projects.

The key that you are missing here is when you do this force balance you do it based on the weight distribution of each axle. Consequently it should read something like this,

wdf*mt*ay = ka * ( (1-kb*Fn_LF)*Fn_LF + (1-kb*Fn_RF)*Fn_RF )

Where,

wdf = weight distribution at the front
mt = total mass of the vehicle in kg
ay = lateral acceleration in m/s^2

To fill in the blanks a bit the following will help,

http://www.chassissim.com/blog/chassissim-news/creating-tyre-models-from-nothing

This takes up where Jorge left of. Also when you are ready to buy the dynamics of the race car shot me an email to info@chassissim.com

Also one other thing I'll say to you and anyone reading this. Jorge Segers is a good friend of mine and one of the most switched on data and race engineers I know. Pay very close attention to his data logging book. You would be crazy not to have it.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-19-2014, 07:30 PM

Hey Guys,

Just on another note for any of your familiar with the Engineering software package Altair we have just joined forces,

http://www.chassissim.com/blog/chassissim-news/chassissim-is-now-part-of-the-altair-partner-alliance

Just for everyone's reference, However pay attention to that post above. It was a very good question.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Goost

03-20-2014, 09:53 AM

Danny,

Thank you; the 'tires from nothing' paper was exactly what I was looking for and your own explanation clears it up a lot.
Especially tying it back to the neutral steer channel / equilibrium concept.
I will probably be contacting you about your book soon!

ChassisSim

03-23-2014, 09:04 PM

Austin,

No worries my friend. Glad it helped.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-26-2014, 09:49 PM

Hey Guys,

I've been working with a few of my customers this week and a recurring theme that has come up is how do you employ simulation techniques. The following should help,

http://www.chassissim.com/blog/chassissim-news/using-racecar-simulation-as-a-weapon

It's one of my racecar articles about how to use racecar simulation as a weapon.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

04-02-2014, 10:22 PM

Hey Guys,

Just a few housekeeping things. A few of you have been asking about getting started with the online simulation. This should help,

http://www.chassissim.com/blog/chassissim-news/getting-started-with-chassissim-online-using-the-chassissim-tutorial

Also just giving everyone the heads up the ChassisSim/Altair Webinar is on the 17th of April 2014. You can find more details here,

http://www.altair.com/(S(2e5lqjqqhqqxdu2q2iogvurl))/EventDetail.aspx?event_id=4576&event_culture=Global&region=Global&date_location_id=3294

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

04-09-2014, 10:40 PM

Hey Guys,

Quite a few things going on this week. Let me break it down for you 1 by 1,

1) We are about to launch the ChassisSim newsletter. It will keep you up to date on things with race car simulation and vehicle dynamics. It's also a really good to review some older tutorials and Dan's vehicle dynamics corner episodes that have dropped of the radar screen. Anyway the first one is heading out next week. If you want to subscribe here is the link,

https://confirmsubscription.com/h/r/41758C420BC5FEAA

2) I've just posted the latest video tutorial on the blog. It's about one of the key things to race car simulation. It blows me away how overlooked this gets. Any way here is the link,

http://www.chassissim.com/blog/chassissim-news/the-key-to-race-car-simulation

3) Lastly a reminder about the Altair webinar which is next Thursday. The link is above.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

04-16-2014, 10:17 PM

Hey Guys,

Just a reminder that tonight I'm giving a Webinar about how ChassisSim fits into the Altair alliance. It will be a good overview of ChassisSim. Here is the link,

http://www.chassissim.com/blog/chassissim-news/chassissim-altair-webinar-in-april-2014

Also keep tuned next week. I have something special planned for everyone.

Hopefully I'll see a few of you tonight my time.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Luniz

04-17-2014, 04:28 AM

Hi Danny,

sorry I missed the early session for the europeans and I will not be able to attend the late one... Do you know if there will be a recording of the webinar?

Btw. I'm working at Altair in Germany ;-)

Cheers, Lutz

ChassisSim

04-21-2014, 08:45 PM

Lutz,

No worries my friend and sorry for the late reply. There should be a link on the Altair website where you can view a recording of the webinar. If it hasn't been put up already I'll chase down the link for you.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

04-23-2014, 09:12 PM

Hey Guys,

Today you guys are in for a real treat. This is the latest episode of Dan's Vehicle Dynamics corner where I talk about how to use ChassisSim in the race car design process,

http://www.chassissim.com/blog/chassissim-news/using-chassissim-to-design-a-race-car

If truth be told what I've just shared with you here is the tip of the iceberg. I just got of the phone with a customer and they are using ChassisSim in the design process of their race car. There using it in a way I wouldn't have thought of in a million years. This is what you have got at your fingertips.

Enjoy this is a good one.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-01-2014, 12:44 AM

Hey Guys,

This is what happens when you use simulation properly,

http://www.chassissim.com/blog/chassissim-news/chassissim-customer-tds-racing-wins-at-silverstone-in-the-elms

A very recent ChassisSim user just took out the opening round of the European LeMans Series.

Enjoy

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-07-2014, 08:46 PM

Hey Guys,

I was dealing with a few FSAE students as well as veteran engineers this week. A few things came to light and the following might be useful,

http://www.chassissim.com/blog/chassissim-news/the-joy-of-hand-calculations

It's one of my old racecar articles about the importance of doing hand calculations. I beg of each and everyone of you who views this post this week. Download the article, read it cover to cover and practice it. Hand calculations are the basis of engineering. It tells you what to look for and it builds up your instincts. It's also the ultimate BS detector. Unfortunately it is a dying art.

Enjoy guys.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

mech5496

05-08-2014, 02:47 AM

This back to basics was much needed, thanks Danny!

ChassisSim

05-14-2014, 08:40 PM

Hey Guys,

First things first - Harry you are very welcome. I'm glad you got a lot out of it because this is a skill that every practising engineer needs to master.

On another note the second ChassisSim newsletter is going out tomorrow. If you haven't already subscribed here is the link,

https://confirmsubscription.com/h/r/41758C420BC5FEAA

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-29-2014, 01:09 AM

Hey Guys,

I've just been getting a lot of questions over the last couple of weeks about circuit modelling. In particularly getting started and refining the results. The following should be a great help,

http://www.chassissim.com/blog/chassissim-news/chassissim-circuit-model-creation-tips-and-tricks

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

06-11-2014, 08:44 PM

Hey Guys,

On the the 4th of June 2014 we ran a joint webinar with Altair about vehicle simulation. We discussed tyre modelling and Altair discussed their Motion Solve software using the F-tire plugin. Here's the link for the recording,

http://www.chassissim.com/blog/chassissim-news/chassissim-altair-vehicle-simulation-webinar

I certainly got a lot out of it and I reckon you'll get something out of it too.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-02-2014, 08:03 PM

Hey Guys,

Sorry for the lack of content recently. I was in Europe speaking at a conference and chasing up a few loose ends.

I'll be speaking about what I got up to in Europe later. However just taking the pulse of the forum it looks as though the whole theme of tyre test rig results and quantifying tyre performance has raised it's head. The link below is an oldie but a goodie. This is the game plan from deriving tyre models from race data,

http://www.chassissim.com/blog/chassissim-news/modelling-tyres-using-chassissim-filling-in-the-blanks

Many senior members of the ChassisSim community use this as their go to reference when they are faced with a tyre they have never seen before and have achieved good repeatable results for their vehicle simulations. Given the current discussions I have seen I reckon everyone will get a lot out of this. It's long but bare with it. The results are worth it.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-23-2014, 08:45 PM

Hey Guys,

Sorry for being off the radar screen. I've been fighting of the flu that has been circulating my part of the world.

A couple of big ticket items that are coming up.

Firstly Altair and ChassisSim are running a ChassisSim training session on Tuesday the 29th of July 2014. You can find more information here,

http://www.chassissim.com/blog/chassissim-news/chassissim-training-presented-by-altair

Secondly and the big news is the ChassisSim simulation bootcamp is on for 2014. It will be hosted in Cologne Germany on Wednesday the 12th of November 2014. More details can be found here,

http://www.chassissim.com/blog/chassissim-news/the-chassissim-simulation-bootcamp-is-on-for-nov-12-2014

These are going to be great hands on opportunities to learn about ChassisSim. In particular the bootcamp always books out so book in early to avoid disappointment.

Enjoy Guys

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-30-2014, 10:20 PM

Hey Guys,

Usually in this forum I share tidbits of vehicle dynamics knowledge. However I know many of you reading this forum are young engineers. A good friend of mine sent me this,

http://www.goodreads.com/author/quotes/1479495.Hyman_G_Rickover

Admiral Hyman G Rickover is acknowledged as the father of the US Nuclear Navy. Regardless of what you think of nuclear power and the military as all of you work through your designs and projects, and as you move on to your professional lives you'll be responsible for projects of all shapes and sizes. I trust the following will give you some good food for thought.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-06-2014, 10:41 PM

Hey Guys,

Just to give our North American friends the heads up ChassisSim is exhibiting at PRI,

http://www.chassissim.com/blog/chassissim-news/chassissim-is-exhibiting-at-pri-in-2014

Also we are running the Lap Time Simulation 101 Seminar at PRI as well. In addition to covering the basics of lap time simulation we have some special things planned. However I'll keep you informed closer to the date.

Anyway for those of you based in North America I'm looking forward to seeing you all there.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-13-2014, 10:32 PM

Hey Guys,

Just a reminder about our face to face seminars at the end of the year,

http://www.chassissim.com/blog/chassissim-news/chassissim-face-to-face-training-seminars-for-2014

This is a great opportunity to get up to speed face to face with ChassisSim. Both the bootcamp and Lap Time Simulation 101 will cover some great stuff about how to get going with simulation and how to get the most out of it.

Also we are working on some really exciting long term stuff. However I'll bring you up to speed on that another time.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-20-2014, 10:04 PM

Hey Guys,

Just a couple of things,

Firstly I had a question during the week about the Lap Time Simulation 101 seminars in Indianapolis at PRI. You don't need to register for these seminars. They are free. Just turn up!

Secondly this week I ran a training session for a colleague of mine about one of my latest articles in Racecar Engineering. It was about tying together damping ratios and state space analysis to specify a damping curve. Any way we filled in a lot blanks and on the back of that here is the latest episode of Dan's Vehicle Dynamics Corner that ties these two techniques together.

http://www.chassissim.com/blog/chassissim-news/using-damping-ratios-and-eigenvalues-to-specify-racecar-damping

Enjoy

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-03-2014, 11:01 PM

Hey Guys,

Sorry for the lack of recent activity. I have some big ticket developments coming down the pipe and that has been dominating my radar screen.

That being said I have an oldie but a goodie for you,

http://www.chassissim.com/blog/chassissim-news/creating-engine-curves-from-nothing

It's about creating engine curves from nothing. From time to time you'll be plunged into this situation. These are some approaches and techniques I have found very helpful. I trust you can all make good use of them.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-17-2014, 11:18 PM

Hey Guys,

I realise the following is a year old but here is a really good taster of what is coming at the bootcamp this year and Lap Time Simulation 101 at PRI,

http://www.chassissim.com/blog/chassissim-news/chassissim-seminar-teaser-2013

Forgive the references to last year's seminar. For those of you in Europe register quick the bootcamp is filling up.

Enjoy Guys

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-02-2014, 02:14 AM

Hey Guys,

I have a real treat for you today. We have been hard at work at ChassisSim developing an Electric power train module. Here is the latest episode of Dan's Vehicle Dynamics Corner that is about Electric power train basics,

http://www.chassissim.com/blog/chassissim-news/electric-power-trains-for-motorsport-part-1

With the advent of Formula E and with the electric component of FSAE it's a future we need to consider. Anyway you should find it a really good resource.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-08-2014, 09:27 PM

Hey Guys,

A couple of community service announcements.

Firstly for all of you based in the U.S a good friend of mine Jorge Segers is running one of his data logging seminars in the U.S,

http://js-engineering.webnode.nl/data-acquisition-seminars-2014/

Jorge is one of the sharpest minds in this business I've worked and personally for all you in FSAE I think you would be mad not to give this a good hard look.

Also places for the Simulation bootcamp in Europe are filling fast and Lap time simulation 101 is coming to Indy,

http://www.chassissim.com/blog/chassissim-news/chassissim-face-to-face-training-seminars-for-2014

If you are in Europe and you want to come to the bootcamp let me know quickly.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

mech5496

10-09-2014, 04:18 AM

Hello Danny,

the electric powertrain simulation part was really interesting! I have some questions regarding the bootcamp in Germany, specifically about the agenda/topics to be discussed and registration fees for FS/FSAE teams.

Thanks,
Harry

ChassisSim

10-15-2014, 08:08 PM

Hey Guys,

A couple of quick updates.

Firstly Harry my apologies in the delay in getting back to you. Things in ChassisSim world have been outrageously busy. The bootcamp will cover the basics of getting going with race car simulation and then introducing you to the more advanced features of ChassisSim. We'll be covering simulation work flow, creating a car and a circuit model, creating advanced tyre and aero models and then introducing to you to advanced features like shaker rig simulation. It is a very hands on seminar so you'll be driving ChassisSim quite a bit. The nominal rate for students is Australian $150 but I can discount for larger groups. If anyone is interested shot me an email at info@chassissim.com

In other news guys we will be exhibiting at Professional Motorsport World in Germany in November of this year. You can find the details here,

http://www.chassissim.com/blog/chassissim-news/chassissim-is-exhibiting-and-giving-a-seminar-at-pmw-expo-2014

We are going to be show casing a lot of the new features of ChassisSim that includes our electric power train module and something else that I'm keeping my powder dry for now. If your based in Europe it would be great to see you face to face. We are at stand 5036.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-20-2014, 06:07 PM

Hey Guys,

I've got some big news. The ChassisSim simulation bootcamp is going to be run in the U.S for the first time on Thursday Dec 4, 2014 at Altair HQ at Troy Michigan. The details can be found here,

http://www.chassissim.com/blog/chassissim-news/the-chassissim-bootcamp-is-on-for-the-usa-in-2014

Technically this is an Altair event so I don't know what they will charge. However for those of you based in the U.S it's a fantastic opportunity to get up to speed with ChassisSim.

I look forward to seeing you there and if you can't make that I'll see you at PRI.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-22-2014, 08:01 PM

Hey Guys,

Just a quick one - ChassisSim v3.25 has just been released. Here are the details,

http://www.chassissim.com/blog/chassissim-news/chassissim-v3-25-has-just-been-released

The big thing with this release is the electric power train module. Also a reminder you can see it yourself at both PMW in Germany and PRI in the USA.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

12-09-2014, 04:35 PM

Hey Guys,

Sorry it's been a while. I've been on the road swamped with work.

Just a quick reminder I'll be at PRI in Indianapolis this week at Booth #137. We are also running our lap time simulation 101 seminar as well in room 201. If your based in the US I'm looking forward to seeing you all there.

In the mean time I have some food for thought for you all. Recently I have been reading about the life of Col John Boyd. He is the father of Energy Management methodology for fighter aircraft and some have called him the Father of the F-15 and F-16. One of his greatest contribution was the specific power equation. This is,

Ps = V*(T-D)/W

Where

Ps = Specific power (m/s)
V = Vehicle velocity (m/s)
T = Thrust or applied longitudinal forces (N)
D = Drag (N)
W = Weight of the vehicle (N)

Anyway I have a little twister for you all. Why is this so significant and for those of you how choose to accept this mission see if you can apply it to an analysis of a FSAE car.

See you at PRI guys.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChristianChalliner

12-09-2014, 08:49 PM

I'll give this a shot! :)

It's so significant because in air combat if you have a higher specific power you can out manoeuvre your opponent whether that be in a straight line or in turns. So you play to your strengths, if you have higher specific power in a turn and low specific power in a climb (compared to them) you engage in a turning battle rather than trying to out climb them because you have a greater chance of victory in that situation. The same applies to two cars battling on track (or against a stop watch).

It can be applied to FSAE in all cases, in fact, it's probably an interesting thing to compare between cars, acceleration is simpler because you can estimate rough drag co-efficients and frontal areas for the cars and the forward thrust based on engine data, weight can come from something like racecar engineering.

Cornering is a little more tricky but i think you could apply it still by making some approximations. Maybe consider all cars cornering at 1g, find Fy and Fz forces, then effective mu values and compare to tyre data peaks. Extrapolate out what your peak Fy values could be, assume these values, calculate cornering g and then this is your T value in the equation where your 1g situation is the D value. Not quite sure how the Velocity would factor in but that's just an idea from me.

It's probably all wrong :) but i think if i understand it correctly you can still apply it to all FSAE scenarios (straight line or cornering).

Certainly worth thinking about in any case!

Christian

ChassisSim

12-21-2014, 06:26 PM

Christian,

My apologies for the delay on this. I've just been hammered on a few things.

Your observations are spot on. The only thing I would add to this is that specific excess power applies everywhere through out the flight continuum. The holy grail of fighter design is to be able to pull maximum g while losing minimum energy. The achilles heel of the current generation of western fighters such as the F-15 and F-16 while they can pull high sustained energy to do so they have the after burners plugged in making it a nice juicy target for an IR missile. It's not that often that I'll mix what I do professional to what I do privately but here is a video of the SAAB Gripen that illustrates the holy grail,

http://youtu.be/t2PPS_g3Yvw

Note for half the flight it's pulling high g with the afterburner off.

At the other end of the spectrum is the F-35 Joint strike fighter

Tim.Wright

12-22-2014, 02:12 AM

I'd also add that any power/energy calculations can be used, after removing the aero component, as an input to a tyre thermal model.

ChassisSim

01-21-2015, 09:56 PM

Hey Guys,

First things first I trust everyone had a great Christmas and a Happy New year. I also trust you are raring to go on your new projects.

I've had a few requests for the presentation that I gave at the Professional Motorsport World Expo in Cologne/Germany on November 15 2014. I figured I could do one better. Here is the tutorial/episode of Dan's Vehicle Dynamics Corner,

http://www.chassissim.com/blog/chassissim-news/chassissim-pmw-2014-presentation

Enjoy

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

01-28-2015, 10:08 PM

Hey Guys,

I've just posted this video on the Shaker rig toolbox,

http://www.chassissim.com/blog/chassissim-news/how-to-use-the-chassissim-shaker-rig-toolbox

It's a really powerful tool for understanding the frequency behaviour of the car. It's also been used to tune mechanical grip in applications such as IndyCar, FIA GT and V8 Supercars hence why it would be a useful tool for everyone to consider.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

02-05-2015, 12:14 AM

Hey Guys,

Some exciting news for everyone. We are know offering unlimited use of ChassisSim Online per month. You can find out more here,

http://www.chassissim.com/blog/chassissim-news/chassissim-online-an-affordable-professional-simulation-tool

Obviously I have quoted professional prices. However if you guys want to make use of this (in particular the shaker rig toolbox we discussed last week is part of ChassisSim Elite Online) then get into contact with me and I'm sure we can work something out.

Enjoy Guys

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

02-18-2015, 08:35 PM

Hey Guys,

From tuning in to some of the discussions over the last couple of weeks this is worth another look,

http://www.chassissim.com/blog/chassissim-news/why-we-at-chassissim-do-what-we-do

It gives a background of why ChassisSim went down the road it did. Some good food for thought.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-05-2015, 01:54 AM

Hey Guys,

On March 3 2015 I ran a webinar for Altair Engineering on how to use ChassisSim for both automotive and motorsport use. Here is the link for the recording,

http://www.chassissim.com/blog/chassissim-news/altair-webinar-motorsport-to-automotive-using-chassissim

I used a couple of case studies that showed how ChassisSim can be used in road car design. In particular I covered suspension geometry changes using the lap time simulation.

I also touched upon the shaker rig toolbox and track replay simulation. All important elements that can apply to Formula SAE.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-18-2015, 11:38 PM

Hey Guys,

One of the questions I get asked a lot is can ChassisSim be used on a FSAE car? The answer is a big yes and this latest episode of Dan's Vehicle Dynamics Corner shows you how,

http://www.chassissim.com/blog/chassissim-news/chassissim-for-fsaeformula-student

I should also add that everything here is not theory. ChassisSim has been used in anger in FSAE. Some valuable lessons have been learned and it would only be appropriate to share
it with everyone so you can all get the benefit in using a tool like ChassisSim.

Enjoy.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Dylan Edmiston

03-19-2015, 11:00 PM

Once you get past the learning curve of the software, ChassisSim can be a very easy and useful tool for FSAE. Despite the low curvatures of the tracks we run on, ChassisSim is still capable of simulating the vehicle accurately. My team has used ChassisSim as a tool to influence some of our changes for competition which we have been able to validate at home.

ChassisSim

04-08-2015, 10:44 PM

Hey Guys,

I just posted the latest episode of Dan's Vehicle Dynamics Corner. Here is the link,

http://www.chassissim.com/blog/chassissim-news/calling-chassissim-from-an-external-program

It's about how to call ChassisSim from an external program such as Excel, Matlab or optimisation programs such as Mode Frontier or Altair's HyperStudy.

A very advanced feature of ChassisSim but well worth getting your head around. Enjoy

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Tim.Wright

04-10-2015, 07:24 AM

Slightly unrelated - but have you considered adding the possibility to model the suspension using K&C lookup tables instead of (or as well as) hardpoint definitions? This way you aren't locked into a pre-defined set of set of suspension layouts.

ChassisSim

04-13-2015, 07:25 PM

Tim,

Sorry for the late reply - Work in progress on that one. I spoke to Bob Simons from Morse Measurements on that one. So stay tuned.

In terms of lookup tables for Roll centres it's one of these ideas that are great in theory but not really practical. It we take a typical suspension geometry layout you have 4 input variables, wheel movement, vertical movement and roll.
If you have a lookup table for 10 points for each of those variables you need 10^4 or 10000 points. This is a coarse lookup table. I've run it back to back with the hard point method and it was a night and day difference.

The lookup table simply couldn't compete.

However where the results from the K&C rig come into their own is validation. I had a GT customer with a really weird front end and where the K&C rig came into it's own was validating the results from ChassisSim.

Excellent question and point though.

All the Best

Danny Nowlan
Director
ChassisSim Technologis

Tim.Wright

04-14-2015, 05:40 PM

Yea, the point densities should be "reasonable" but these days I don't see any problem using a 100+ element lookup table given the available computing speed we have at our disposal.

If you coordinate the point density so you have a low density in the linear range of the suspension and then a higher density when the bump/rebound stops are touched then you don't end up with unreasonably sized maps.

If the maps are made properly I don't see why there should be any difference between that and the hardpoints method. Save to say that any K&C data needs to be very carefully filtered...

ChassisSim

04-14-2015, 07:45 PM

Tim,

The thing that hurts you is run time and memory usage by using a lookup table. To do it properly you are north of 10000 elements per variable. It's something I've played with on multiple occasions and I've always gone back to using the hard points. Also memory management is a nightmare.

All the Best

Danny

ChassisSim

04-16-2015, 02:53 AM

Hey Guys,

We've just released ChassisSim v3.26. More details can be found here,

http://www.chassissim.com/blog/chassissim-news/chassissim-v3-26-has-just-been-released

For those of you involved in Electric vehicles there is some really good stuff in there. There is also some really cool new features as well.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

BillCobb

04-16-2015, 11:20 PM

I'm not a supporter of tabular data lookup for several reasons. Besides the inventory issue, tables introduce quite a few problems related to how they are generated, how they are loaded, how they perform and how often they are fully utilized.

Instead, I'm a fan (and proponent) of parametric relationships and have used them in virtually all simulations and data processing situations where applicable. A parametric relationships is a function/equation that relates output(s) to input(s). It's continuous, differentiable, easily linearized, handily and efficiently stored in a database and introduces metrics to the Development World (the people who 'touch the car') that becomes part of the chassis vernacular (like stiffness or position, or range, or falloff, or ratio, or percent, etc.

For example, here's a parametric formulation of a steering stiffness laboratory test that is non-linear, could be tabularized, can have the hysteresis added as a spatial relaxation, can be simply treated as a linear relationship and can be idealized or optimized to produce the 'best' vehicle performance. Data was processed in 1999 by a Fortran program (with Fortran graphics, no less, 12 colors, baby !) with results stashed in a Lotus 1-2-3 spreadsheet database. Just imagine how this is done in today's Big Science department ! If you insist, I will show some more examples).

In fact, this data 'model' and its sisters and brothers covers quite a few good, bad and ugly chassis K&C type relationships. Sometimes you will discover your vehicle property is so 'bad' that the fitting function does a poor job. Trust me, that's a very poorly designed or build vehicle and should not be taken out of the garage. BTW: Polynomials are generally not wanted for these functions. Just a few terms, please. BTW, if you are using Pacejka tire models, you are already using a parametric equation for the tire responses for MIMO results.

If 'think time' is an important part of your engineering process, this approach is valuable. Note that the bridge from kinematic or multibody models to the parametric data format is also easily built and makes a splendid interface for either math or laboratory component test results.

You will find in the end that the vehicle you should build is the vehicle the data models told you would be best to build. Tables will make this very much harder. Most will probably have only 4 points ( 2 pair) in them anyways.

Tim.Wright

04-17-2015, 03:02 AM

Yes, I agree that parameterising the curves is a better way to go, especially in the design development stage but having a tabular definition allows you have your own parameterisation. I have used various models with large tables defining K&C characteristics as a function of wheel forces, vertical travel and steering input and the resulting models are generally able to run between 5 to 10 times faster than realtime.

dynatune

04-21-2015, 05:56 PM

I agree with Tim. Look up tables can be fast, but to be honest one only needs 5 or 6 compliance characteristics to cover the majority of compliance effects. At least that is the experience I have made.
And many of the "compliance gradients" as they are measured on a K&C rig can be allocated to "on center" handling or high-g handling. With a little bit of understanding of the matter "simple" tools can get you very close.

Cheers,
dynatune

exFSAE

04-22-2015, 02:54 AM

In terms of lookup tables for Roll centres it's one of these ideas that are great in theory but not really practical. It we take a typical suspension geometry layout you have 4 input variables, wheel movement, vertical movement and roll.
If you have a lookup table for 10 points for each of those variables you need 10^4 or 10000 points. This is a coarse lookup table. I've run it back to back with the hard point method and it was a night and day difference.

The lookup table simply couldn't compete.

Hm, can't say I agree with this. To calculate your jacking forces all you need are 1D look ups at each corner (for independent front and rear susp). I'd agree that 10 points per curve *sounds* coarse, but particularly for most racecars with relatively small suspension travel, it's not unreasonable. Kinematics tend to not be wildly nonlinear or extreme curvature in your working range, too.

In any event that means 10 [data points] * 5 [kinematic look up curves per corner] * 4 [corners] = 200 points, not 10000.

I'd say it's typical in commercial solvers that the lookup approach is considerably faster than full multi body kinematics.

ChassisSim

04-22-2015, 06:26 PM

Flight909

04-22-2015, 07:01 PM

exFSAE,

This is roll angle, heave and the two wheel displacements. All of these effect the roll centre which in turn impact the jacking forces and Force application points. Even on a course grid of 10 points you need to store it in a 4d matix and this is where the 10^4 comes from. In terms of software implementation you have big time memory management issues. Also in terms of running this in real time you have just handicapped yourself. It gets worse when you have to return cambers as well..

To be fair the roll angle not really a suspension parameter, it is a vehicle parameter. If you read 10 mm displacement on your front right suspension, you don't know if that is coming from roll or heave. So, you can describe the different kinematic characteristics as an function of single wheel displacements. You can then transform these values from the chassis coordinate system to the world coordinate system to account for roll and pitch (and then solve your jacking forces and so on).

I agree with exFSAE, in my experience this gives well-behaved curves. I would however argue that you might want to have a 2D-lookup for the front, where steer input affects the kinematics as well.

04-22-2015, 08:44 PM

exFSAE,

As a word of warning, whenever Danny starts his reply to you with "My friend...", I suggest you brace yourself for the inevitable tsunami of BS coming your way! :)
~~~o0o~~~

If we look at a typical suspension geometry you have 4 input variables per half of the car. ... This is roll angle, heave and the two wheel displacements ... and this is where the 10^4 [numbers] comes from.

Danny,

You are not advancing the credibility of your racecar consulting business, software, or maths skills, with the above comments (repeated several times now!).

Or, if you still insist you are right...

Can you explain how, for a "half-car" and ignoring tyre-squash, "roll-angle" and "heave" can change INDEPENDENTLY of the "two wheel displacements"?

Z

Claude Rouelle

04-22-2015, 09:47 PM

One comment of one of our engineers who do work on creating and using simulation tools: Danny leaves out a crucial part, and that is what kind of datatypes you work with. The memory requirements for holding double precision numbers versus half precision numbers are quite drastically different, so simply talking about how many points you have without talking about the size of each point is a bit useless..

Even taking his "worst case" example of 10,000 datapoints in a table, each at double precision (64 bits) - that is 0.08 megabytes of total memory. Let's say this number is "not large".

Goost

04-22-2015, 09:51 PM

Can you explain how, for a "half-car" and ignoring tyre-squash, "roll-angle" and "heave" can change INDEPENDENTLY of the "two wheel displacements"?

Z

Tire deflection isn't ignored in his model. Shouldn't be in any proper dynamics model as you surely know.
This is off topic to an already off-topic discussion...

~~~

Running sims at 5 times real speed isn't Danny's business - have you watched the videos?
He can simulate a lap in less than seconds.
He can run optimization loops and loops on your setup before the car can make it around the track and back to the pits.

Sure lookup tables have their place, this discussion is just missing the point of the computationally lightweight and surprisingly accurate modeling technique Danny uses.

Would be neat to hear some objective comparison of the two. Accuracy vs speed vs complexity vs how fast did it make your car in the end?

04-22-2015, 10:59 PM

Tire deflection isn't ignored in his model.

Goost,

The point is, Kinematically speaking (and for a "half-car", etc.), "roll-angle" and "heave" are DEPENDENT VARIABLES on the "two wheel displacements" (as pointed out by Flight909).

Danny is suggesting the use of 10,000 numbers, when 9,900 of them are redundant!
~~~o0o~~~

As for the "...surprisingly accurate modeling technique" of Danny's, and all the other racecar simulators, I wonder just how accurate the predicted lap-times would be without their use of a "global fudge factor". That is, to my knowledge, ALL these simulators require the input of some sort of "global grip factor". This ARBITRARY input parameter then directly scales the output laptime. So, get this "fudge factor" right, ... and the laptime is spot-on.

But, of course, this requires you to know the right answer (= real laptime) BEFORE you do the simulation! I would like to see how accurate these simulations are when you DO NOT have a good guess of the "global-tyre-road-Mu" beforehand. For example, when a lot of dust has blown onto the track... Or, when an FSAE Team uses the TTC data directly (ie. with some Mu = 2++!), with no scaling...

Always easy to accurately "predict" a number, when you already know that number...

Z

(PS. Can anyone give their typical "scaling factor" for TTC-Mu (ie. x 90%?, x 80%?...)?)

Tim.Wright

04-23-2015, 04:36 AM

Erik, actually I'm with you on this, the amount of fudging that goes on in the world of lap simulation is depressingly mind boggling... I've seen many "validated" lapsim models running on trajectories derived from telemetry data which neglect track banking, gradient and kerbs which which have massive first order effects on the vehicle behaviour.

This is why I always start with open loop handling responses to quantify a design or setup change in the first instance.

If you do a decent enough job of the fudging and you don't completely destroy the model - you will at least have something to give you some idea of laptime sensitivies to various high level design decisions.

exFSAE

04-23-2015, 06:50 AM

exFSAE,

My friend go through and redo you maths. Let's take a 2 x 2 matrix. You have 4 elements and or 2^2. If we look at a typical suspension geometry you have 4 input variables per half of the car.

This is roll angle, heave and the two wheel displacements. All of these effect the roll centre which in turn impact the jacking forces and Force application points.

My friend, let's redo your assumptions ;)

I alluded to this before, but this guy's follow-up post hits the nail on the head (emphasis added):

To be fair the roll angle not really a suspension parameter, it is a vehicle parameter. If you read 10 mm displacement on your front right suspension, you don't know if that is coming from roll or heave. So, you can describe the different kinematic characteristics as an function of single wheel displacements.

(And yes, I for sure agree to his point you can add fidelity but including steer on the front to make for a 2D table at each corner, and then 1D still on both rears)

You don't need to know where the roll center is to find the jacking force on any one corner. In an independent suspension, you have your IC and that's all you need to know. Or really, the more complete picture to me is to forget about tracking the IC location and just work jacking force out of an energy balance at each hub. All you need then is the kinematic derivative at any suspension position (e.g. dCamber/dZ) This is covered in either Matchinsky or Blundell. I know it's one of the blue books on my desk :)

In any event this reduces the 4D lookup to 1D.

Running sims at 5 times real speed isn't Danny's business - have you watched the videos?
He can simulate a lap in less than seconds.
He can run optimization loops and loops on your setup before the car can make it around the track and back to the pits. Sure lookup tables have their place, this discussion is just missing the point of the computationally lightweight and surprisingly accurate modeling technique Danny uses.

If a full vehicle simulation is doing a complete kinematic solution at each point, I'm fairly certain you will find considerable speed gains when using a lookup. The math and implementation are trivial by comparison.

In the case of full dynamic multibody simulations, you can go from struggling to hit realtime performance.. to running several times faster than realtime. If you're not resolving the full dynamics, sure you may be starting faster than realtime as it is.. but simplifying the calculation of wheel position and orientation from multibody to lookup table is for sure going to be a speed gain.

Tim.Wright

04-23-2015, 08:11 AM

Just to share my experiences on using lookup tables in a number of vehicle simulation models.

I use quite coarse curves to define the kinematics (max 20pts). For compliance maps I generally use less because I use a linear approximation where possible (because summing multiple non-linear compliances responses guarantees a wrong result).

Usually the front axle has 5 maps to describe each wheel centre coordinate (X, Y, Camber, Spin, Toe) as a function of wheel vertical (Z) travel and steering rack travel for each wheel. So if your steer/vertical space is defined with 20 points, you have:
2 (wheels per axle) x 5 (dependent d.o.f. per wheel) x 20 (steering positions) x 20 (damper positions) so 4000 points for the front axle wheel movements.

Then you could add another 3 maps for spring, damper and ARB deflections which is another:
2 (wheels per axle) x 3 (dependent d.o.fs. spring, damper, bar) x 20 (steering positions) x 20 (damper positions) so another 2400 points.

The rear axle is the same but without the steer input so 320 points.

That gives a total of 6720 points for the kinematic definition of the full vehicle including all your steering geometry, kingpin orientation, jacking effects and motion ratios.

For compliances you can define each dependent degree of freedom of the wheel as a function of an applied force (braking, acceleration, cornering, aligning torque). This would give you an extra:
4 (wheels) x 4 (loadcases) x 5 (dependent d.o.fs) x 5 (load values) = 400 points.

If you want to have the compliance of the left wheel dependent on forces applied to the right wheel (i.e. to model steering or subframe compliance) then you can double that to get 800 points.

If you want to make the compliance dependent on vertical wheel travel and steering position your point count goes up dramatically. Less density is required for this so consider 5 points for vertical and steer inputs this requires a point count of:
4 (wheels) x 4 (loadcases) x 5 (dependent d.o.fs) x 5 (wheel travel pts) x 5 (steering positions) x 5 (load values) = 10000 points.

Again you double this to add in assymetrical effects coming from forces acting on the opposite wheel - so 20000pts

So if you have all of the above options ticked you have:
6720 pts for the kinematics
20000 pts for the compliance
26720 pts in total

With this level of complexity you are able to solve at 5-10 times faster than real time from what I have seen.

Goost

04-23-2015, 08:21 AM

The point is, Kinematically speaking (and for a "half-car", etc.), "roll-angle" and "heave" are DEPENDENT VARIABLES on the "two wheel displacements" (as pointed out by Flight909).

Danny is suggesting the use of 10,000 numbers, when 9,900 of them are redundant!

Yes that's fair for this example. In regards to the original question 'tables vs math' the size still depends on which tables you use, resolution, their coordinate system etc.

global fudge factor

The literature suggest that this is actually a decent assumption until you get into tire/ground interaction.
Couple sources that come to mind:
Georg Rill, SAE Paper 860575
Halliday, SAE Paper 983030

...for sure going to be a speed gain.

OK, I just want someone to show objective proof of this.

Here's my two cents for the morning:

Made a little dynamic simulation in MATLAB. 2nd order SMD with a nonlinear (quadratic) spring.
Using the direct math solution F=k*x*x is 100 times faster than a lookup table, even for low resolution.

(Yes it's not compiled and yes trig calculations use something close to lookup tables + polynomials...
I don't have a lap simulator to test this, maybe someone who does could compare and show their objective proof?)

~~~

Code attached; as per usual, change '.txt' to '.m' to run.

exFSAE

04-23-2015, 09:04 AM

Gonna have to leave the "objective proof" as an exercise to the reader - and of course it's a bit more involved than f = kx^2. Just saying from my own experience is that full vehicle MBD in pro motorsport applications imposes significant runtime (and solver) challenges. Simplifying it to look-ups is a big gain.

On a side note, I would recommend avoiding Matlab for these type of exercises. It is generally horribly slow, though there are some things that get redirected to a compiled Fortran linear algebra library (LAPACK), and then their JIT'er does some things better than others. It's a good sandbox application and does some things quite well, but I would not use it for any sort of algorithm or numerical benchmarking - you can easily fool yourself.

BillCobb

04-23-2015, 11:58 AM

Couple of comments from my direct experiences with all of this:

Compiled Matlab is not 'compiled' in the traditional sense. It's processed for distribution to limit circulation of the libraries. I have and use the Matlab compiler. It's executables are no faster than the editable scripts. (Geez, I almost said edible ???)!

I use the Watcom 32 bit Fortran compiler to produce double precision executable engines that are stuffed, loaded and post-processed by Matlab GUI's built with the Guide tool. Yes, they are really that fast 100x real time. With the flick of a LinkEdit switch, you can produce a .dll callable from Excel. So much for Invisible Basic.

Actually, heave and roll should be treated as separate reactions because of the effects of in-phase and out-of-phase side forces. (Don't let this phase you when you are not cornered). Steering system in front is to blame (net vs. difference tie-rod loads), and asymmetric rear suspensions (Panhard bars).

Next time you get onto a K&C machine (perhaps a virtual one), take along a complete set of tire patch loads (vertical, lateral, and aligning moments to start). These loads probably came from or could come from a cornering and braking simulation. Then recover your suspension parameters (tables or chairs) and recomputed another set of input loads. After 1 or 2 iterations of this technique (called "bootstrapping"), you will be able to judge for yourself the values of multi-dimensional tables, superposition, linearization, effects and manifestation (infestation ?) of structural compliance and compliance model equations.

The real value from the parametric equations comes from the identification of trends: Am I getting better or worse ?) because the structural (I.E. your ass-you-me d ) infinitely stiff unobtainium frames and cross-members just ain't so.

The problem you still face is the unnatural effect(s) of the clamping method used during the K&C test. Vehicles really use a process called "Inertia Relief". as in the NASTRAN Solution type. Most K&C clamping methods restrict the body/frame modes that contribute to bending, torsion and match-boxing. Sometimes you might want to eliminate 1 of the 4 corner clamps or replace them with a couple of front centric mounts using Cardan joints. If your results are much different, all your 20,000 data points are probably worth less than the ink cartridge they could be printed with (~ $35.00 U.S.) .

BTW: Some of you (like myself) still keep 16bit apps around that served us well for vehicle dynamics studies but no longer run directly under 64 bit Windows. (Like Pac-Man). To keep them alive, I am running a 32bit Windows for Workgroups app under a DosBox virtual machine and this combo keeps the 20th Century alive at my house for suspension analysis, etc. Rumor has it that the next version of Windows will have this ability directly by popular demand.

Play the Song, Jack ...

ChassisSim

04-23-2015, 08:56 PM

Hello everybody,

It might be worth at this point of the discussion to take a deep breath and remember what we are doing in the first place.

Firstly the crux of this debate is do you use lookup tables to generate roll centres and other suspension geometry parameters or do you calculate them from first principles. Firstly breaking this down by half of the car I am referring to the front end or rear end of the car. To this end there are 4 input variables that will effect roll centres, cambers and other suspension geometry variables. These are,

*Roll angle
*Heave displacement of the front and rear end.
*Individual wheel movements dictated by tyre squish/movement of the hubs on the tyre springs.

Also make no mistake the roll centres have a fundamental effect on the jacking forces/forces applied to the sprung mass. A cursory Free Body Diagram will show this conclusively. Consequently this isn't just a 1 D lookup table. You might get away with a 2D table resolving wheel and chassis movement. However this gets destroyed by roll angle so know we have a 3D table. However where this runs into trouble is the effect the roll centre locations are fundamentally effected by what both the left and right hand sides are doing. So we are back to 4 variables.

So that means we have four variables in play. So if we want to represent this at with 10 intervals per variable you are going to need 10^4 or 10000 points. Let me also state that this is a very course grid. If you doubt the numbers look at a typical 10 x 10 aeromap this is 100 points. It's what you need to carry around bear minimum so if you want to carry this around for suspension geometry there is no getting around the 10000 points. You also need this for cambers as well. On top of this you need to do it for the front and the rear. It doesn't take very long for all this to add up. Don't believe me download a C Compiler and start adding in the numbers and look at what happens. You also think you can cheat using lower memory variables but if you go down this road round off error will kill you.

The other thing you'll run into is the algorithms and processing times you'll need. The book in Numerical Recipes in C has an excellent multi dimension lookup table algorithm. They display it in two variables. Once this is expanded to 4 variables or 4 dimensions the equations get big, and the finer your grid gets the more searching you have to do. This is where the sting in the tail is for the lookup table method is for suspension geometry and you pay for it in run time.

The advantage of doing this from first principle is that the values are totally continuous and the run times where much faster. I also backed to backed the values so there was no point in using lookup tables. Also the values in ChassisSim have been validated on K&C rigs and other 3rd party packages so this method is on solid ground.

I'm not saying you can't use lookup tables to represent this. If you have the computing power and memory storage knock yourself out. However a first principles method works just as effectively with a fraction of the memory input and run time.

The ultimate payoff of this is the results. Check out the image in this link,

http://www.chassissim.com/blog/chassissim-news/chassissim-online-an-affordable-professional-simulation-tool

Look at the image - coloured is real, simulated is black. The steer trace, throttle and steer and dampers are almost indistinguishable. Also there are no grip or fudge factors here. This is what happens when you develop tyre models from real data and you use full multi body transient simulation. Just for the record the numerical engine runs about 80 times faster than real time on a ho hum laptop.

The validity of this speaks for itself.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

Tim.Wright

04-24-2015, 03:32 AM

The reason I brought up the case of the lookup tables was purely to allow arbitrary suspension to be modelled in the software rather than being restricted to the built in types that you have built in (if I remember is double wishbone but I don't remember which others there are). How can you currently model 5 link, 4.5 link (Audi, BMW, Ferrari), 4 link (Ford) & McPherson type suspensions?

Regarding the fudge factor. Correct me if I am wrong but if I remember correct, the process of making a Chassissim model is that you parameterise everything on the car except for the tyres. Then you provide the software a lap of logged data from the actual car and from this Chassissim reverse engineers the tyre model. Is this correct?

In this case the fudge factor is inside this reverse engineering process. Basically, in this way you will always have a match between your data acquisition and your model right? An example would be your tyre Fy will always be underestimated on the front but overestimated on the rear because you don't have the Mz in your tyre model.

Silente

04-24-2015, 08:39 AM

From direct experience, reverse engineering the tire is a nice technique if you don't have any data at all, but it is at the same time very dangerous, above all if your driver is, for any reason, not close enough to the car limit (see for example FSAE, most probably, or amateur racing, which is now up to LMP2 sometimes).
Beside it, you are always ignoring some effects, like camber.

As i said, it is in my opinion a chance if you don't have any data (assuming you know your car very well and you trust completely your data), but still i would always use a tire model as a base if i had one.

Claude Rouelle

04-24-2015, 09:58 AM

Silente wrote " From direct experience, reverse engineering the tire is a nice technique if you don't have any data at all, but it is at the same time very dangerous, above all if your driver is, for any reason, not close enough to the car limit (see for example FSAE, most probably, or amateur racing, which is now up to LMP2 sometimes"

Interestingly this is related to one last Z’s question: “(PS. Can anyone give their typical "scaling factor" for TTC-Mu (ie. x 90%?, x 80%?...)?)”

Tire model made on flat track test are always a bit too optimistic because the A) belt grip (and temperature) is not necessarily representative of a given track asphalt B) moreover; there is no bumps on a flat track machine (unless you recreate different vertical tire movement frequency and amplitudes but not a lot of tire test programs include that feature)

So my answer would be 85 % of scaling factor for a Senna-like driver and 70 to 80 % for other ones, depending on their skills. Not very accurate, isn’t it?

I have been in situations where after “calibrating” the model (or fudge the data) you could see the real data of the steering, throttle, brake, suspension, linear potentiometers etc.. close to perfectly overlaying with the simulation. Then you put another quicker or slower driver or 2 drivers with the same lap time but with different driving styles and suddenly the data and the simulation traces do not overlay anymore. Same thing if the next day with the same car, same setup, same tire, same driver ….and the data and the simulation do not overlay because the track temperature changed 5 degrees C.

So if reverse engineering the tires is questionable and if tire model from flat track test are not precise who and what do you trust?

That is where my statement “You work in Delta” comes in.

I still remember vividly my first test on a tire testing machine many years ago where the manager of the tire testing facility told me: “You do not simulate here; you model”

I have been in a situation where we predicted that the lap time a given car on a new circuit was going to be 1.40.5 but the driver made the pole in 1:39.9. So we were 6/10” wrong. Yep but at the time we ran the simulation I did not know which way the wind was going to blow or what the tarmac temperature was going to be.

The goal of simulation is not so much spot on lap time prediction but calculation of the ratio of second of lap time per Kg of fuel or per degree of rear wing or per degree of front camber of per mm of front ride height. In other words I am more interested of the SLOPE of a given parameter influence on the lap time than its absolute value. Similarly when testing tires on a tire testing bench (flat track) I don't worry about the tire grip measured as much as I focus on a the tire grip VARIATION per KN of vertical load, per degree of camber, per degree of slip angle per psi of pressure. THAT is the kind of DELTA information that help us to build a winning car design and/or setup.

Just wanted to share this perspective in this (very good) debate.

exFSAE

04-24-2015, 11:01 AM

Firstly the crux of this debate is do you use lookup tables to generate roll centres and other suspension geometry parameters or do you calculate them from first principles.

To do one or the other is merely a choice - no right or wrong answer there. And I don't think anyone would argue that as you add dimensions to a look-up table it gets more computationally expensive.

The crux of the debate is if you do choose the lookup approach, whether you need to tabulate kinematics against 4 input variables. Beyond that, it could be asked whether you need to know spatial location of roll centers to calculate jacking forces.

The answer to both those is no.

Going in reverse order - there are different ways to compute jacking forces. Personally I've always been a fan of energy methods in various facets of engineering, and they can be applied to this case. Using the energy balance approach you can calculate the jacking force on any corner of the car knowing only (a) the tire forces and moments, and (b) the motion derivatives. You will find the 'theorem of virtual work' appear repeatedly in "Road Vehicle Suspensions" (Matchinsky). This is a very powerful method as it works for any type of suspension topology, and includes the effects of all tire forces and moments. With one implementation of tables you can support SLA or multi-link or whatever you want, transparently.

As an aside, that is an easy trap to fall into when just using spatial IC's or RC's, just about everyone seems oblivious to the fact that you will have jacking from Mx, My, Mz. Depending on your geometry, you can of course choose to ignore those effects if you know them to be small - but you can't just be ignorant of their existence.

In any event, let's come back to lookup tables. For an independent rear, the kinematic motion (and derivatives) are a function of wheel displacement only. As someone mentioned earlier, if you read 10mm of displacement it's of no consequence whether it came from heave or roll or pitch. All then that's needed is a 1D lookup - easy and fast. Even if you were to still use the "spatial" approach you still don't need to know where the axle "roll center" is. Let's say we want to work out the jacking at the LR. If you have knowledge of the LR's IC and tire forces then that's all you need (the IC of course just being another way of capturing information about the motion derivatives). For that time step the displacement or forces of the RR have no consequence to what's happening at the LR.

On the front, you have a choice. To be complete, the kinematics (most importantly the derivatives) are obviously a function of both wheel and steer displacement. At most you only need a 2D table at each corner. However, if you know that your motion derivatives don't change much with steer, then you may simplify these down to 1D lookup as well.

I think you will find then that this approach can have considerably faster runtime performance than doing full kinematic solutions. Your mileage may vary, but I would expect you will find this is typical among the major, industry-standard commercial solvers out there - written in C, Fortran, or hand-tuned assembly language by experienced specialists.

DougMilliken

04-24-2015, 12:27 PM

... The crux of the debate is if you do choose the lookup approach, whether you need to tabulate kinematics against 4 input variables. Beyond that, it could be asked whether you need to know spatial location of roll centers to calculate jacking forces.

The answer to both those is no. ...
After adapting our software for several different large customers (through the 1980s and 1990s), we gave the user several choices for defining the suspension. The documentation to cover these different methods grew considerably!

The original method was curve fits to K&C data (noted earlier by Bill Cobb), which was available from a few sources back then. This included the results of Fx, Fy tests at multiple ride heights, which gave jacking forces as noted by exFSAE -- a very general and clean representation. This made model building very quick, working directly from K&C plots that could be generated in a day or two on the test rig. I don't recall if we ever added curve fits for jacking from Mz input, but conceptually it would have been easy. This method requires that you have a car (and access to K&C) -- which was a limitation for some customers.

Later, tables were added, I think there were at least two types, one was a model of 2D instant center location and movement. These types of kinematic description are easier to populate if you are designing the suspension and don't have a car to measure on a K&C rig.

During that time period, multibody models began to become popular, but a full compliant suspension model really slowed down calculations for a full vehicle. Building a full multibody model was also very time consuming and error prone (I've seen some wild errors). Some users characterized the suspension by multibody simulation, then converted to a curve fit representation to calculate vehicle responses in a reasonable amount of time.

Bits of this history are documented in RCVD, for example, lap time (going back to slide rule days) is discussed starting on page 340.

Tim.Wright

04-24-2015, 02:56 PM

Hi Bill, can you elaborate a bit on this?

Actually, heave and roll should be treated as separate reactions because of the effects of in-phase and out-of-phase side forces. (Don't let this phase you when you are not cornered). Steering system in front is to blame (net vs. difference tie-rod loads), and asymmetric rear suspensions (Panhard bars).

I'm familiar with the need to treat in phase and out-of-phase forces seperately when defining compliances (as I mentioned in a previous post) but I don't follow why you would need the same also for the kinematics.

ChassisSim

04-24-2015, 09:43 PM

It's good to see some sensible points have been raised in this discussion.

Tim to your question on suspension configurations that aren't modelled in ChassisSim. Definitely the lookup table is certainly an option and I certainly haven't ruled out but given the past experience I have discussed I've been very gun shy about implementing about it. That being said in the code in ChassisSim it's really easy to add new suspension geometry configurations and I've done this for suspension configurations such as the F458 rear end and the BMW Z4 link. In this regards a K&C rig is worth it's weight in gold because it allows you to validate what you have come up with. This was really useful when I was developing the F458 rear end.

One thing I do need to touch upon is ChassisSim modelling process. Tim the technique you discussed is pretty much on the money except for the whole fudge factor thing. Yes you do enter parameters such as spring rates, damper curves, pick up points etc to construct the vehicle model. Once you have your data from a flying lap that's where the tyre force modelling comes into play.

The basis of the tyre force modelling toolbox is that using the multi body vehicle dynamics model in ChassisSim to do a whole bunch of track replays to minimise the difference between simulated and actual lateral acceleration. This is where the transient vehicle model comes into play because you can now do the bumps you can get a much better idea of what the loads are. This is where the reliance on fudge factors falls away.

A good question that was raised is how good does the data need to be? Obviously the better the driver the less work you need to do. However given the nature of the track replays if you do have a so so driver you do have something to work with. Let me give you a war story. A couple of years I was doing some work in a junior formula in the states. When I went through the tyre modelling process it was predicting lap times 1s a lap quicker then what the drivers where doing on the home circuit. I made a mistake and trusted the race car drivers. When they went to Sebring they get blown away. After finishing tearing my hair out I reset the grip factors to where they should have been and all of a sudden the car was doing lap times in comparison to the front runners. The moral of the tale is you can get significantly down the road with race data. Also here is your game plan on how to do it,

http://www.chassissim.com/blog/chassissim-news/modelling-tyres-using-chassissim-filling-in-the-blanks

The senior members of the ChassisSim community have found this to be a really useful resource and I recommend it to you.

Also make no mistake this is an iterative process. It will take a couple of goes to get it right. Also everything we have discussed here has been used in formulas as diverse as GP2, Sportscars and V8 Supercars to name a few. Given the results my customers get I think the validity of this approach speaks for itself.

Also just a word on accuracy and lap time simulation. I totally concur with Claude that your goal in simulation is not the correlation game but ensuring what the car does the simulator does. That being said one of the things we have found in the ChassisSim community is that the better the tyre model the accuracy tends to look after itself.

Great food for thought.

BTW - For the Australians and New Zealanders reading this - Lest we forget have a good ANZAC day.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

04-24-2015, 10:51 PM

Danny,

Given that this thread is an advertisement for your wares (namely racecar-consultant, software-guru, etc.), I suggest you think more carefully before pressing "Submit". Your last post on page 20 is doing you no favours. (Edit: You just posted (top p21) as I was writing this...)

For example:

To this end there are 4 input variables that will effect roll centres, cambers and other suspension geometry variables. These are,

*Roll angle
*Heave displacement of the front and rear end.
*Individual wheel movements dictated by tyre squish/movement of the hubs on the tyre springs.

Firstly, your explanation of all this is very poor (ie. NO CLEAR DEFINITIONS!), which is not what is expected from a good consultant.

Secondly, and more importantly, it has been explained several times now that your 4-D tables are NOT necessary. See exFSAE at top of p19, and again with Tim near bottom p19, and exFSAE's quite clear explanation near bottom p20. Again, your seeming lack of ability to grasp these things does not reflect well on your VD knowledge or software skills.

(BTW, to me, "tyre squish" is NOT so much an input variable, but more of an output, namely the result of computed Fz tyre-load. At most there might need to be a very small 1-D look-up table for the tyre's (non-linear) spring-rate curve. One such table for each different tyre. So only two small tables if different tyres F&R.)
~~~o0o~~~

... the roll centres ...
You might get away with a 2D table resolving wheel and chassis movement. However this gets destroyed by roll angle so know we have a 3D table. However where this runs into trouble is the effect the roll centre locations are fundamentally effected by what both the left and right hand sides are doing. So we are back to 4 variables.

Again, meaningless gibberish! And again, see exFSAE's most recent post.
~~~o0o~~~

... [programming language] C ... multi dimension lookup table algorithm...
... the finer your grid gets the more searching you have to do ... and you pay for it in run time.

Piffle! The whole point of look-up tables is that while they might take up more memory, their time-cost is tiny and constant. Regardless of how many table entries you have, each one is accessed by the same simple "address call". Computing the address is likewise a trivially simple and fast operation, and equal for all entries. (Note that some "high-level" languages like Matlab might make this whole process ridiculously slow, but not so in the lower-level languages like C.)
~~~o0o~~~

The ultimate payoff of this is the results. Check out the image in this link.
...
... there are no grip or fudge factors here. This is what happens when you develop tyre models from real data...

Huh? NO FUDGE FACTORS???

As pointed out by Tim (middle p20), your method starts with the "real data" (ie. the 100% correct answer!), then you "develop tyre models" which are fed back into your Simulation, which finally gives you a result which is not quite the same as the "real data" that you started with...

(Edit: Danny's latest post (top p21) seems to confirm that it is all a huge fudge!)
~~~o0o~~~

Now, even though your Simulations do not output the same correct answers that are being fed into them, they still have some potential to be useful predictive tools.

But, IMO, a Simulation is ONLY USEFUL IF you very clearly state all the simplifying ASSUMPTIONS you make, and also give a very clear estimate of the size of ALL POTENTIAL ERRORS. (Claude, I suggest Optimum-G also do this. There is much more to this than just "deltas".)

To paraphrase Prof. Walter Lewin's much repeated comment on these matters,

"Any measurement, or Simulation, is MEANINGLESS (!!!!!), without knowledge of its UNCERTAINTY!!!"

(And, again, apologies to Prof. Lewy if I didn't quite catch the right tone of his shouting here. His "meaningless" is very high-pitched, and much repeated! :))
~~~o0o~~~

Here are just two potential sources of error that should be clearly accounted for, beyond the most obvious one of (track-surface NOT-EQUAL test-belt-surface), so (road-tyre-Mu = roll-the-dice).

1. Does your (or Optimum-G's, ++) Dynamic-Simulation include information about the directions of the mass-distribution "Principal-Axes" of the various bodies? That is, do you realise that a car in very simplified Steady-State cornering (ie. flat-road, constant radius/speed/beta, zero-aero-forces, etc.) has Rearward-Load-Transfer when its longitudinal PA slopes down-to-front, but Forward-LT when the PA slopes up-to-front? Do you know the size of these potential errors (ie. changes to F/R tyre Fzs, and hence also Fxs and Fys)?

2. Does your Dynamic-Sim include the effects of all the gyroscopic-couples acting on the car from all its rotating components (ie. 4 x wheels, the engine and driveline components, energy-storage-flywheels, etc.)? Again, how big are these potential errors (ie. changes to ALL tyre Fzs/Fxs/Fys)?

I have calculated the potential size of these errors. Which is why I have not bothered buying any of the Sims out there!
~~~o0o~~~

exFSAE,

The "Principle of Virtual Work" is indeed a powerful thing. As a historic note, it was used at least as far back as the third century BC, by Archimedes in his proof of "The Law of The Levers" (where it amounted to a purely mechanical version of the First Law of Thermodynamics, namely "there is no free lunch").

Classical Mechanics has deep roots! :)

Z

DougMilliken

04-25-2015, 09:27 AM

..."tyre squish" is NOT so much an input variable, but more of an output, namely the result of computed Fz tyre-load. ...
It is considerably more complex if the goal is to predict ride height with some accuracy (part of simulating ground effects aero). Tires are rarely standing up perfectly straight, and loaded radius is also a function of (to name a few):
+ Inclination angle, or camber in car axes
+ Fy distorts and typically shortens the tire sidewalls
+ Fx has a small effect for normal tires but is a major factor for drag race tires with "wrinkle walls"
+ Tire pressure

dynatune

04-27-2015, 05:09 AM

Since we are talking about Software & Dynamics let me add my 1p.

I have spent a long time of my professional life working with Multi-Body-Analysis Tools (like ADAMS, DADS etc.) an developing cars. In these complex tools all of the
above described "problems" in the discussions are "automatically" implemented. The only - but huge - disadvantage that it is fairly difficult to filter out any
specific root causes for any specific effects. This "desire" of coming to a better understanding has created many tools - also commercially available - that each have
their claim to fame. Yet, all do fail to some extend more or less with respect to the "big" ones. To me it seems also quite clear that it is difficult for David to
beat Goliath, especially since Goliath is not sitting on his butt doing nothing at all. Even so, these models are still in many ways not as close to reality as I had
expected or would like to have.

So I came to the conclusion - and some others with me - that you either go for the full blown multibody car model and accept it's limitations (and with all risks
included if "driven" by a novice) or one goes for a as simple as possible model that allows a quick verification of most basic principles of Vehicle Dynamics. All other
software "in between" is just an attempt - each of them valid for some degree and probably very well suited for a specific objective but overall still an attempt that
comes only close to a cigar.

Furthermore reading the discussion on what parameters / effects should be considered in the software - and what effects they would/could have on the results - I have
been consistently surprised that the most important of them all are just not considered in many software.

The last 25 years of suspension design have not really brought mind blowing new insights in kinematics but certainly did teach us a lot on suspension compliance. Yet
many of the commercially available tools for suspension design do not consider them nor provide a way of estimating them at least to some degree. Whereas moving
pickup-points on the axis of rotation on for instance a lower wishbone does not affect kinematics it can most definitely change your compliant behavior of the
suspension quite drastically. Often the effects of compliance can be more than the effects of kinematics. Of course arguments are made that racing suspensions are
"stiff" meaning that there would be no need for looking at it, but at the end of the day even the most "rigid" suspension link is to some degree flexible. The huge
progress that has been made in the last 25 years is to start making these ever presented compliance work in favor for you (for instance classic example create Toe-In
under braking on a rear suspension).

With respect to Suspension Stiffness if for instance we take a chain of compliance in a non-bushed solid track-rod link with:

Upright Bracket Stiffness = 25000 N/mm
Rose Joint Stiffness = 25000 N/mm
Link Stiffness = 100000 N/mm
Rose Joint Stiffness = 25000 N/mm
Chassis Bracket = 25000 N/m

The total stiffness adds up - as springs in series - to 5882 N/mm. I might say nly 5882 N/mm. Getting to a number above 10000 N/mm is already quite challenging as one
can see quite easily.

Based on my experience with compliance I think that any vehicle dynamics tool that is not considering the effect of suspension compliance (in one way or another) on
the tire angles is simply not sufficiently correct. Especially on race cars with slick tires, that work with small slip angles, compliance can have a huge impact.
And as we know the tire slip angle is the mother of all forces on the car. So, if the mother of all forces is calculated by not considering the major part of all
suspension parameters that lead to it's creation ....

Cheers,
dynatune, www.dynatune-xl.com

DougMilliken

04-27-2015, 08:12 AM

... The last 25 years of suspension design have not really brought mind blowing new insights in kinematics but certainly did teach us a lot on suspension compliance. Yet
many of the commercially available tools for suspension design do not consider them nor provide a way of estimating them at least to some degree.

Nice example, but your timing is off by a couple of generations... Olley and his co-workers recognized that compliance was important in the 1950's or earlier. They called it "deflection steer", so you might not find it in the literature under compliance.

Tom Bundorf gave a nice overview of the work at GM in his presentation at the SAE Automotive Dynamics and Stability Conference in 2000. A pdf version can be downloaded from the bottom of http://www.millikenresearch.com/olley.html or a direct link http://www.millikenresearch.com/VehicleDynamicsAtGMByRTBundorf.pdf

Deflection steer and rigs to measure it are noted on pages 15 and 35 and page 36 features a rendering of the Vehicle Handling Facility (VHF), perhaps the first unified rig for measuring suspension K&C parameters. It's described in Nedley, A. L. and W. J. Wilson, "A New Laboratory Facility for Measuring Vehicle Parameters Affecting Understeer and Brake Steer." SAE Paper 720473, 1972.

Summary -- understanding compliance (deflection steer) is one area where passenger car development was well ahead of race car development. I'm sure there are a few exceptions--for example, Chaparral may have tested on a compliance rig at GM in the 1960s? It seems possible although I do not know for sure either way.

Tim.Wright

04-27-2015, 09:09 AM

I think he meant it (MBS simulation) give you a better understanding on how the compliance occurs within a given suspension - not the effects on vehicle dynamics. Prior to full 3D MBS models, I'd assume any tuning using compliance was based largely on trial and error.

Regarding the importance of compliance in racecars. I did a hand calc on the axial stiffness of a 300mm long steel tube, 19mm diameter x 1mm wall thickness (something FSAE sized) and found its the same stiffness as the rubber bushings used on the road car I have been working with in the last few years.

Though I would concede - it practically impossible to have a decent estimate on the compliance of a racecar without a full assembly FEM study (including compliant joints) or by actually building and testing the car (by which point its too late to simulate it).

DougMilliken

04-27-2015, 10:07 AM

... Prior to full 3D MBS models, I'd assume any tuning using compliance was based largely on trial and error. ...
Olley and his associates started out using roll steer (and later, I think they decided it was a bad idea) , but were also using deflection steer.

"Deflection Steer Due to Lateral Forces" and "Deflection Steer Due to Aligning Torques" are included in the simple understeer budget calculations in Bundorf, R. T. "The Effect of Vehicle Design Parameters on Characteristic Speed and Understeer", SAE paper 670078, 1967. While this presents a very simple analysis method, it does hit the important points and is still a nice quick way to get a feel for a car.

The pioneers had to think...because they didn't have powerful computers!

dynatune

04-27-2015, 01:26 PM

And another paper from Bundorf & Leffert in 1976 was called "The Cornering Compliance Concept for Description of Vehicle Directional Control Properties" but I can equally well live
with deflection steer. The only thing I was and I am still surprised about is that - since we all seem agree to agree that compliance/deflection is a very important contributor to vehicle
behavior - many of current commercially available software tools either for Suspension Design or for Vehicle Dynamics Analysis do not consider this phenomenon. I fully subscribe to Tim's
experience that a full steel tube/joint suspension link is hardly any stiffer than a bushing from a road-car. That by itself should make people wonder.

Cheers,
dynatune, www.dynatune-xl.com

BillCobb

04-27-2015, 05:20 PM

In case you didn't know, there was this paper: SAE(840561) "Typical Vehicle Parameters for Dynamic Studies: Revised for the 1980's" that gives good information about kinematic and compliance suspension parameters (among other things necessary for good simulation). In case you need it for the 21st Century (not that much has changed), take a look at these pictures. Sign convention: + is understeer effect regardless of axle. Minus is the "O" effect word.

These days, these compliances (and 3 others) are managed by design, specification or audit. It's part of a chassis design 'recipe' that originates from competitive vehicle assessment or from packaging or suspension synthesis tools (synthesis meaning I want this value, what does it take to get it). In pawing thru these and other suspension design factors (like roll steer or stiffness or roll axis heights [determined from FY side-loading and FZ constraint loads) in cloud form, you can/could see the results from some perhaps prestigious car makers that would or should bother you, especially if you have measurements from multiple samples of the very same model. I might also point out that when you see a car with very different tire and or rim sizes and pressures front to rear, you know something has gone hay wire in their handling objectives. (I use plastic twine myself in my John Deere baler for this reason).

Anyways, when I hired in as a junior co-op student, the K&C machine was in Areaway 5 building, GM Milford Proving Grounds with air bearings under each when and with 2 air cylinders either in phase or out of phase supplying FY or MZ inputs. Ride and roll inputs were formed by pumping water into tanks hung from a support frame under the car. Outputs from HP XY pen recorders ! (on pre printed and labeled paper, thank-you). This was early 60's technology and could easily be duplicated by a creative FSAE team and stationed on a trailer at a track to make some money with. Shark Tank candidate.

Still, today, SAE and Vehicle Dynamics International papers are being published which completely ignore the roll, camber, and deflection steer wheel reactions. As a result, their predictions for vehicle understeer levels are totally ridiculous.

I also go along with the race car having the same or worse compliances. Measurements clearly show where things go wrong. Yeah, that's a splendid looking control arm you have there Laddy, too bad you mounted it to and undercarriage softer than yesterday's spaghetti. Power steering gears are the worst performers here, but their softness may be part of the 'insufficient understeer' gimmick. Surprizing amounts of MZ and MX deflection from wheel bearings and and camber deflections from strut rods.

Just about all of these compliances are linear with some hysteresis this my comments elsewhere that gradients are acceptable. However, the steering system can provide an ugly nonlinear plot that sometimes defies even the most sophisticated equation fitted model.

And THAT is the rest of the story. No reason not to apply this type of design recognition to a FSAE car. Cleanup in aisle 3 !!!

dynatune

04-27-2015, 05:38 PM

You mean something ugly non-linear like this ? 581
I could not follow completely the "where have you been" reasoning since I think we are fully agreeing that compliance induced steer effects should be analyzed on an FSAE car too.
Yet almost no one seems to offer the tools.

Cheers,
dynatune, www.dynatune-xl.com

PS: the graph explanation is meant for educational purposes to teach to "novice" students the principal difference between on-center behavior &
off-center handling. So don't even start ... :confused:

DougMilliken

04-27-2015, 07:20 PM

I think Bill had something like this in mind, but he's probably seen worse(?). I took off the identifying information, this is from an actual aligning torque compliance test, with the power steering operating. A degree (+/-) of steer hysteresis on center is pretty awful, more on the left front (dotted line) than the right.

This figure appears near the end of my lecture on Data for Vehicle Modeling, after a number of good looking K&C plots. I put this one up to see who is awake. I've given this talk to a number of FSAE teams over the last ~10 years.

ChassisSim

04-27-2015, 08:35 PM

Hello everybody,

Well this discussion has certainly taken on a life of it's own hasn't it!

Some really good food for thought about compliances. I've done a lot of work in two formula that brings this to the fore. These categories are German DTM and highly modified sports sedans. I've learnt a lot from both of them so let me share some lessons learnt because it is very easy to get lost.

The first principle about compliances is double check with a hand calc if you need to worry about it. Once you are starting to deal with outrageous spring rates and large downforce you need a simple model to break down what is doing what. It was really important when I was resolving some questions with DTM and was a great help and saved a lot of heartache.

Also have a look at your logged data and use that to calculate some loads and or perform rig tests. This is really important to nail down steering compliances as well.

Lastly you can learn a lot from a K&C rig. For those of you based in North America it's worth paying Morse Measurements a visit. Here is the link,

http://www.morsemeasurements.com/

All of the above will tell you what you need to worry about.

Also let me also illustrate a key principle about simulation. Start simple and get cute later. Don't do it the other way around you will get lost. I really needed to state this because I could see this discussion spinning off in that direction. The ultimate validation of this is one of my US customers who have won the LMPC category at the Sebring 12 hour two years running,

http://www.chassissim.com/blog/chassissim-news/pr1-motorsports-win-the-lmpc-category-of-the-2015-sebring-12-hour

Their model started simple and then got added to. Given I haven't had that much to do with that model their results and the fundamentals behind it speak for themselves.

Good food for thought.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

BillCobb

04-27-2015, 08:54 PM

Gee Doug, now I don't have to show and tell. The "Where have you been" remark pertains to a statement made previously that compliances have never been talked about in the industry or literature.

As for tools to accommodate bushing, Cardan joint, and flexible beam elements, We had SNAC (Static Nonlinear Analysis Code) running on mainframes from punched cards in the mid 60's, then from foreground TSO sessions on mainframes soon after.

Knable & Associates sold a version of this called SKAT (then to SSNAP in the late '90's and then on to SuspensionSim in the 2000's. SuspensionSim is available now thru the CarSim folks.

As you can see (first 2 pics are 1 file split into 2 images, control arm bushings were an optional element, as was frame or structural compliance (think twist axles) besides wheel bearing springs. Running with and without settled all bets on their necessity. 'Theoretically", you still need the nonlinear pressure - angle data from the steering valve to get the steer vs. Mz compliance done right. Even for manual steer cars the Cardan joint stiffness(s) can be a real source of error.

So now there is SimMechanics able to perform as much as you want, if you have the license and the savvy. Need a bushing press, though (supplier data) and maybe some fixturing to get the structural elasticity.

04-27-2015, 10:54 PM

I, too, fully agree that structural compliances should play a much greater part in VD simulations than is currently the case (ie. in some of the Simulators mentioned here).

But, big question again, HOW IMPORTANT IS ANY ONE "COMPLIANCE"???

My point is that until a thorough ERROR ANALYSIS is done on these things, it is all just a moot argument.

For example, a current fashion in the production car world (well, last few decades) is to give the wheels large longitudinal compliances, mainly for less NVH from low-profile tyres over small sharp bumps. Such large compliances, if done correctly, may NOT have any significant effects on laptimes, at all. In fact, they may improve laptimes? But as noted before, small "deflection steers" can have massively negative effects.

So, IMO, a good Simulator should first clearly state which compliances, kinematics, dynamics, etc., it is modelling. And then, most importantly, it should very clearly state the potential errors, or UNCERTAINTIES!!! (as per Prof. Lewy), that result from its choice of things to model, or not to model.
~~~o0o~~~

Perhaps the most effective way of driving this point home to the students is via Bill's suggestion:

... This [K&C rig] was early 60's technology and could easily be duplicated by a creative FSAE team and stationed on a trailer at a track to make some money with.

If not done by a student Team, then this is certainly something that the Organisers should think about.

I envisage something very simple mounted on a box-trailer, with the FSAE car tested up at table-top level so the students can easily see everything getting twisted out of whack. Simple ratchet-straps to apply loads, and fish-scales (0-300 kg) and string-lines/tape-measures to measure loads and deflections. This would give a high level of "believability", and guarantee of no fudging in magic boxes.

Output of this K&C rig could then be fed into a VD Simulator and compared with the "undeflected" car. Open-loop, of course, with no "global fudge factors" applied to either simulation.

The Team with the floppiest car should win some sort of trophy. Perhaps this? :)

http://wiefling.com/wp-content/uploads/2009/08/rubberchickenlarge1.jpeg

Z

Tim.Wright

04-28-2015, 03:04 AM

The problem with modelling compliances on a race car is that they come primarily from the gross deflection of otherwise rigid parts NOT the joints.

Road car compliance can be fairly accurately modelled using rigid bodies and compliant bushings because most of the compliance you see at the wheel (in terms of toe, camber etc) comes from the bushings.

Racecars replace these bushings with spherical bearings which (in my opinion) are MUCH stiffer than the suspension and upright members (because they are basically small blocks of solid steel - and stiffness is inversely proportional to size). Now your total compliance at the wheel is MAINLY coming from the larger, thin section parts, i.e. the links+upright+wheel. Now, only a small portion of the wheel compliance comes from the joints themselves.

Therefore, attempting to model them by introducing point elasticities at the suspension pickup points is not correct. It's not even conceptually correct. I've tried and seen other attempts to model rigid body compliances by introducing point compliances in an MBD model of a race suspension and its an abortion. It simply doesn't work.

So the only ways to characterise this properly, if you are still in the design phase, is to create a full assembly FEA of the suspension (this is a massive job). Thereafter you could introduce flexible bodies into your Adams model or characterise the curves for input to a lookup table.

Unfortunately its a bit of an impossible problem for a university FSAE team. Honestly, who in the academic world would have the tools, the expertise and the experience to be running analyses like this?? Where do you draw the line between cost (in time and effort) and benefit (sim accuracy)?

dynatune

04-28-2015, 04:01 AM

Again I am fully agreeing with Tim .. but since there is still a need for some well educated engineering guesses we need some tools.

And the question from "Z" How important is any one compliance ? That is indeed the key question and - forgive me to make some PR at this point - but

since I am utterly convinced that compliance should be considered in any suspension or vehicle dynamics analysis tool, I have to refer to some work that

has been done on creating tools that can be helpful.

I agree that a "K&C" rig would be a very nice thing to have but since it is not yet available to many teams and many teams do not have the knowledge to

run MBS / FEM tools accurately. Ergo, has to divert to some "simple" steps to get some indication. As has been announced in another thread here our

suspension design tool can handle - in an elegant way - suspension deformation (can be read about here http://www.dynatune-xl.com/modeling-sdm.html). One

can for instance simply turn on or off "compliance" on every link and start understanding what is actually happening in your suspension. The tool comes

with link loads in order to indicate where your chassis should not be made of spaghetti. Certainly the tool is not as fancy as MBS software or a

fullblown FEM will certainly be more accurate, but as a pocket calculator it does it's job very neatly. Quite a few SAE teams have started using it.

Coming back to the effect of compliance the handling characteristics: This is also included in all our R&H tools, or better said is actually the backbone

of the vehicle dynamics tools. It is possible to see what the effects of 5 selected "most important" compliance parameters are on handling (linear and

non-linear range). Besides that, their effect on maximum achievable lateral acceleration is instantly calculated allowing to get a direct feeling for

their impact on max. performance (always for a given tire of course). One can quickly turn on/off each compliance parameter and look at the consequences

and indicate to you where to pay attention.

I have seen that one can easily loose 15% or more of the cars maximum g capability by having the "wrong" compliance on the "wrong" place on the car and

that is why I keep on saying that the effects of compliance should be included in any kind of vehicle dynamics simulation software. In someway

somehow....

Excuses for the text formatting ... that happens lately somehow by itself after publishing ....

Cheers
dynatune, www.dynatune-xl.com

BillCobb

04-28-2015, 08:10 AM

Famous last words: "Say, that's not supposed to be moving. I did a kinematic analysis of the steering ...."

DougMilliken

04-28-2015, 04:51 PM

Hi Bill -- where did you attach (ground, reference) the plate and tubes that the magnetic base indicators sit on? With everything bending around I never was completely happy with any of the places that I've tried.

ChassisSim

05-13-2015, 08:55 PM

Hey Guys,

I just posted this latest video tutorial on the ChassisSim website,

http://www.chassissim.com/blog/chassissim-news/generating-aeromaps-from-track-replays-using-chassissim-v3-26

It's about using the Aero track replay feature in ChassisSim v3.26 to generate an aeromap. It's a feature we have just released that has proven very powerful in testing.

Strictly speaking I know it's highly unlikely you'll use it on a FSAE car but it nonetheless to have in your back pocket if you ever work in a race team.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-01-2015, 07:58 PM

Hey Guys,

Sorry for the lack of content. I've been on the road visiting customers and other engagements in Europe.

Just a quick community notice. A big congrats to ChassisSim customer ORECA for their ORECA 05 chassis winning the LMP2 category of the 2015 LeMans 24 hour. More details can be found here,

http://www.chassissim.com/blog/chassissim-news/chassissim-customer-oreca-win-the-lmp2-category-of-the-2015-lemans-24hr

A just reward for all their hard work.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-08-2015, 03:09 AM

Hey Guys,

I have a real treat for you today. You can use ChassisSim for circle track simulation and step input simulation. You can use this by using the track replay simulation feature. The how is outlined here,

http://www.chassissim.com/blog/chassissim-news/using-chassissim-to-simulate-step-inputs-and-circle-track-testing

This is particular relevant for everyone in the FSAE community. What this means is that you can now combine predictive lap time simulation with step steer and circle track simulation.

Enjoy this is a good one!

All the Best

Danny Nowlan
Director
ChassisSim Technologies

BillCobb

07-13-2015, 05:39 PM

So, therefore, one could input a System's Engineering 'chirp' (time dependent frequency responce input), run the play, recover the appropriate data logger channels for steer, yaw velocity, roll angle, lateral acceleration and sideslip (for example) and recover the system dynamics in Bode form to further analyze gain, damping, stability, understeer, and phase margin metrics. Since these tests are easily run with actual vehicles, a comparison would be worth seeing. Yes, the test is considered a 'linear range' performance region evaluation, but there are ways to go beyond this because car nonlinearities are usually just softening springs.

Here is your signal for Excel SWA: = SWGain * =SIN((0.5*(time-1))^2). (use a gain of 1.0 or 0.1 at the road wheels if you don't have SWA)for starters.

Start the sim with zero steer up to time of 1.00 seconds and zero steer 1.00 second before a 40.96 second run segment finish time. Output sampling at .01 seconds will produce 4096 data points per channel for an ideal constant PSD, no aliasing, no filter required FFT process input.

Then compute the transfer functions for AY by steer, Yawrate by steer, sideslip by AY and roll by AY. You can do this Matlab in just a few statements.

The Bode plots will indicate quite a few revelations including goodness of the model and the simulation. Its a reality check for sure.

Even better: fit the Bode responses to s-plane transfer function form and deliver system gain, damping stability margin and phase margin from Matlab's built-in Systems Aanalysis toolbox.

Get an Indian student to do this and get +10 extra points and a complment from Z !!

Good cars test well...

07-13-2015, 09:06 PM

So, therefore, one could input a System's Engineering 'chirp'...
...
Then compute the transfer functions for AY by steer, Yawrate by steer, ...
....
...and get +10 extra points and a complment from Z !!
Bill,

Or not...? :)

I am preparing some sketches/posts on this subject (= transient lateral accelerations) and will post them on the "MMM" thread. But still a few weeks away (other stuff to do)...

Z

(Hint: Good to compute/measure both Ay AND Yawrate by steer, but bad to measure only Ay x steer, as many do...)

MCoach

07-13-2015, 11:49 PM

Bill,

Or not...? :)

I am preparing some sketches/posts on this subject (= transient lateral accelerations) and will post them on the "MMM" thread. But still a few weeks away (other stuff to do)...

Z

(Hint: Good to compute/measure both Ay AND Yawrate by steer, but bad to measure only Ay x steer, as many do...)

Which yaw rate? Front axle, rear axle, or at the CG? (same for Ay) ;)
Hopefully the front yaw rate and and steer input roughly correlate.

07-14-2015, 12:11 AM

Which yaw rate? Front axle, rear axle, or at the CG? (same for Ay) ;)

MCoach,

This is exactly the sort of mistaken thinking I want to address.

Assuming "reasonably rigid bodies", which is a reasonable assumption in most VD, the Yaw-rate (= "rotational velocity") is ALWAYS THE SAME for all points on the body, at any instant in time. Similarly for Yaw-Acceleration. But most definitely NOT so for linear accelerations (such as Ay), which differ greatly at different points on the body, at the same instant in time.

More later...

Z

BillCobb

07-14-2015, 09:31 AM

Yawn rates decrease significantly as you move rearward in a moving bus, especially with comfy seats. Yaw rate however, stays the same unless you are in a trailer or unless your car body is made of gluten free pasta. Roll is often different in some special cases (as in trucks). Roll at the front can be much different than the roll at the rear and inside the cab. These are structure and cab mount design effects. Frame? we don't need no stinking frame...

The reasons to compute ay by steer are to obtain the linear range understeer of the vehicle (an important design, safety and legal system bait metric) and the lateral acceleration bandwidth (i.e. the 63% or 90% response time in the time domain. 90% correlates VERY strongly with customer (driver) perceptions of 'safe' and 'secure' handling predictability (and not just in Grandma's opinion, either. Yawrate by steer produces the ever so mystical 'yaw damping' metric for the vehicle dynamics religious followers, and the sideslip by ay transform produces the rear cornering compliance. That plus understeer gives you front cornering compliance so you are now off to the races. Roll by ay gives you, well, you should already know what that's about.

The challenge to this simulation is whether the results look anything like a real car. In all cases of this challenge put to any simulation tool, the results from road tests don't usually look anything like sim results because there's more to a 'car' than just weights and tires and a few parameters and relationships with cool sounding names and labels. However, the learnings from the comparison DO affect the evolution of the tool and significantly improve its value (.i.e. it's trust and applicability). Textbook equations using tire cornering stiffness as the only player in the dynamic relationships are not useful, productive or valuable in quantifying a vehicle, not even a go cart (IMHO of course, or off course, or maybe I'm too coarse).

This is New School engineering, not science or grease monkey work. You need ALL the old school tools (free body diagrams, napkin drawings, chalk board pictures, metal in bending theory and a differential equation course to wade into this pool. Its deeper than you may think.

Remember, I measured the gains and understeer of my BOAT. That's a vehicle, too, eh ? Propellers make a big difference.

Now show me the money...

MCoach

07-14-2015, 10:44 AM

Assuming "reasonably rigid bodies"....

...unless your car body is made of gluten free pasta.

Unfortunately, Kraft mac and fsae is still a thing. So, you need to check the front axle and rear axle to see if your frame compliance (or suspension mounting, or anything else in the corner compliances along the load paths) is acceptable and not having undesirable dynamic effects (frame stiffness =< roll stiffness, oh no!). There are still many cars built for this competition today that would have better luck with some lard, sticks of pasta, and sheets of lasagna to hold the damn thing together. Assuming rigid anything would mean that the those with a 0.5" rear toe base and 3" of caster trail would also be sound logic. Even at the top, there is a reason ETS named their 2013(?) car "La Fromage". In the chase for minimum weight, finding the limits of what is considered "not-reasonably-rigid-but-maybe-"acceptably"-rigid-but-even-then-it's-barely-acceptable-do-you-think-that's-too-much?" for every system becomes a game in itself.

"In the final analysis, every engineering material is rubber"
- Sir Henry Royce, Rolls Royce

Tim.Wright

07-14-2015, 10:59 AM

And you really think you are going to see compliance this in a pair of front/rear mounted yaw gyros?

CWA

07-14-2015, 12:48 PM

And you really think you are going to see compliance this in a pair of front/rear mounted yaw gyros?

My thoughts exactly. I think it is very reasonable to assume that any FSAE chassis capable of supporting its vehicle's own weight is rigid enough to not have to worry about phasing between front / rear yaw rate responses.. I've never heard of a lack of chassis stiffness causing a yaw rate measurement issue.

A single gyro and a single lat. accelerometer in an IMU at a known location in the car lets you derive your front / rear axle lateral responses well enough, I've never heard of anyone having to fit a gyro at each axle because their chassis might be too soft in this mode.

What would the implications to the driver / engineers even be if there was such a discrepency?? I would love to hear of a tale where this has been an issue in the past. Just how bad was that gluten free body you once came across Bill?

07-14-2015, 11:24 PM

Yawn rates decrease significantly as you move rearward in a moving bus, especially with comfy seats. Yaw rate however, stays the same unless you are in a trailer or unless your car body is made of gluten free pasta.
I fully agree with Bill, Tim, and CWA here.

Although last year I did see a car that had a significantly different Yaw-rate, front-to-rear, but only for a short time. The poor thing must have been a little tired and emotional coming home from the Bowling Club, and managed to slide off the road and into a gum tree (ahh, Eucalyptus - properly hard wood). Standing about 5 metres away you could see the front and rear number plates, at the same time!

Yep, the delta-F-R Yaw-rate integrated over maybe a tenth of a second came out at well over 90 degrees. But I don't think that counts as normal "VD". That is more "modern production car structural design".

Anyway, any difference in measured signals from front and rear gyros is much more likely to come from electrical noise (?), or the way they are mounted (some sort of rubbery foam can be a good idea, given that you don't want to pick up the high frequency vibrations of the flimsy bracket that the gyro is mounted to, which, again, is not "VD"!).

Z

BillCobb

07-15-2015, 10:33 AM

Z: Were they drifting ? Hard to do with a rate gyro.

Actually, its easy to advance the urban legend about yawrates being different front, rear, left right, top and bottom because of many reason:

Cheap gyros (they're relatively low cost anyways) are commonly solid state comprized of hundreds or even thousands of little bits that all vote for concensus on what the actual median turning rate is. As they wear out, blue states turn to red states. Having a set from different batches will indicate a severly bent up car. Of course, no-one would be so thoughtful to mount all two or three in the same spot and realize that there could be differences in a calibration test. Nah, that would require a note from a stockholder's meeting.

You can buy rate tables to calibrate your instrument(s). Even a WW-II bomber gyro will still do a good job but it requires 24V - 28V and will affect your effective weight distribution.

Some still subsrcibe to the multiple lateral accelerometers (front, rear and center) to use in their cleverly perceived method of integrating sum and difference signals to get yawrates and angles, etc. The thing worth noting is that you'll even have a hard time doing this with perfect simulation outputs, much less with the stuff from your rolling trash compactor.

My most memorable gluten free wiggly frame with several rigid body yawrates had bad breath and required a faceless filter to get a good signal to noise ratio. (Its called a pillowcase filter in Matlab).

Still waiting for the money shot....

DougMilliken

07-15-2015, 11:38 AM

You can buy rate tables to calibrate your instrument(s).
Or integrate yaw rate recorded while driving around a circle, and compare to the lap time (beacon signal in the same data set)? Noisy, but better than no calibration at all.

Even a WW-II bomber gyro will still do a good job but it requires 24V - 28V and will affect your effective weight distribution.
Ha! Thanks for the memory, we used one of those in the 1980s. Don't remember the weight, but it was heavy.

07-15-2015, 09:22 PM

Z: Were they drifting ?
Bill, Yes, the driver was drifting ... off to sleep. Fortunately there was no passenger, because that's where the gum tree ended up!

Honestly, the thickest sheet-metal on these modern cars is thinner than a cigarette paper...

Z

(PS. I advise all you students who have been driving for 3, 4+ years now (so you know everything there is to know about it), to go to the Police pound, or wherever they take crashed cars, and have a good look at them. Very sobering!)

ChassisSim

07-22-2015, 11:20 PM

Hey Guys,

Just a heads up that ChassisSim v3.27 has been released. You can find out more here,

http://www.chassissim.com/blog/chassissim-news/chassissim-v3-27-has-just-been-released

There are some things I've added to help with circle track and fixed input testing. Also I've given an excel example that calls ChassisSim. However I will go into more depth on this in another tutorial shortly.

Enjoy Guys

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-11-2015, 01:08 AM

Hey Guys,

The ChassisSim bootcamps are on for 2015. More details can be found here,

http://www.chassissim.com/blog/chassissim-news/the-chassissim-bootcamps-are-on-for-2015

These bootcamps are a great opportunity for novice simulation users to get up to speed as well as a brush up for intermediate users. The dates and venues are,

*Cologne Germany - 10th of November 2015
*Charlotte NC USA - 3rd of December 2015
*Indianapolis IN USA - 9th of December 2015

I fully anticipate they'll fill up quickly. If you want to come shot me an email to info@chassissim.com

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-12-2015, 09:09 PM

Hey Guys,

I have a real treat for you today. Here is a hands on example of how to run ChassisSim from an Excel sheet,

http://www.chassissim.com/blog/chassissim-news/integrating-chassissim-with-excel

Where this comes into it's own is if you have detailed setups in an excel sheet you can set this up so ChassisSim can run it so you don't have any loss of information. It's a powerful tool.

Enjoy

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

08-26-2015, 08:18 PM

Hey Guys,

A couple of things. First things first my thoughts and condolences go out to the friends and family of IndyCar Driver Justin Wilson,

http://www.chassissim.com/blog/chassissim-news/rip-justin-wilson

A very tragic event and it shows us in racing not to take anything for granted.

Also while this is not about vehicle dynamics I found this on YouTube and I had to share it,

https://youtu.be/sEsCh3bA6Xw

It's about the approach of one of the most skilled air show pilots in North America Sean D Tucker. Yep it has been dressed up and it has Hollywood stamped all over it. However it is a fantastic case study of dedication, professionalism and passion rolled into one. I have no idea of Mr Tucker lives by this off camera but it's a great thing to aspire to.

Enjoy

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-02-2015, 09:29 PM

Hey Guys,

Just thought I'd give you all a taster of what to expect at the forthcoming ChassisSim bootcamp,

http://www.chassissim.com/blog/chassissim-news/chassissim-bootcamp-teaser

We have three venues this year being Cologne in Germany, Charlotte in North Carolina and Indianapolis in Indiana in the US.

We are getting a lot of interest and the thing about the bootcamps is that you'll learn hands on how to get use racecar simulation to get the most out of your racecar. The techniques we'll discuss apply to both racecars and FSAE cars.

I don't want you guys to miss out.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-16-2015, 11:03 PM

Hey Guys,

Just a quick one today. If you going to be in Cologne Germany for PMW 2015 I am speaking on the Friday. Here are the details,

http://www.chassissim.com/blog/chassissim-news/chassissim-will-be-presenting-at-pmw-expo-2015

I'll be speaking on Friday the 13th at 11:30am. Feel free to pop in and say hello.

Also if you want to attend the European bootcamp get in touch with me fast. It is filling up quickly.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

09-24-2015, 01:16 AM

Hey Guys,

Just a quick one today guys. One of the biggest objection to getting going and using racecar simulation is a lack of tyre test rig data. This is how you deal with it,

http://www.chassissim.com/blog/chassissim-news/creating-tire-models-with-no-test-rig-data-using-chassissim

Enjoy

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

10-07-2015, 10:33 PM

Hey Guys,

Just another quick one for today. One of the biggest objections I see to using racecar simulation and the most asked question I get is do you have a circuit model for this track?

Well if you have some race car data this won't be a problem. I show you how to get going with this in 3 minutes here,

http://www.chassissim.com/blog/chassissim-news/chassissim-3-minute-winning-tips-circuit-model-creation

It's not the whole story but it shows you what you can do in 3 minutes.

Enjoy Guys

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

03-30-2016, 08:46 PM

Hey Guys,

Sorry about the lack of posting over the last couple of months. There have been some big projects I have been working on. The good news is there will be some great stuff coming down the pipe for all of you.

In the meantime though I have a treat for you all. Check this out,

http://www.chassissim.com/blog/chassissim-news/creating-tyre-models-from-nothing-2

This is all centres around using the second order approximation of the traction circle radius vs load characteristic to get going from a blank sheet of paper.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

04-27-2016, 09:43 PM

Hey Guys,

I've got a treat for you today. I was contacted recently about a FSAE team about tyre modelling. They where keen to get going with lap time simulation but they where perplexed about tyre data.

This got me to thinking. There is a lot more in ChassisSim than just lap time simulation. Here is a quick run down,

http://www.chassissim.com/blog/chassissim-news/chassissim-non-lap-time-simulation-features

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-04-2016, 08:54 PM

Hey Guys,

I've got something that you can make good use of. One of the untold stories of ChassisSim is how you can use it to evaluate suspension geometry,

http://www.chassissim.com/blog/chassissim-news/using-chassissim-to-evaluate-suspension-geometry

You can use the analysis and sim tools to get your head around what the geometry is actually doing and get an idea of what the roll centres, cambers and anti dive and squat are doing.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

05-26-2016, 12:36 AM

Hello everybody,

Just a quick community service announcement that ChassisSim v3.29 has been released. You can find out the details here,

http://www.chassissim.com/blog/chassissim-news/chassissim-v3-29-has-just-been-released

I'll be doing a more detailed tutorial about the rally aspects of it shortly but this should get you up to speed.

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

06-01-2016, 11:20 PM

Hello Everybody!

Today I've got a real treat for you. Find attached the latest episode of Dan's Vehicle Dynamics Corner,

http://www.chassissim.com/blog/chassissim-news/the-dynamics-of-rallyingdirt-and-using-this-in-chassissim

This is about the vehicle dynamics of rallying and running on dirt and ice. It's a good one so enjoy!

All the Best

Danny Nowlan
Director
ChassisSim Technologies

ChassisSim

07-14-2016, 01:18 AM

Hello Everybody,

I've just posted the latest episode of Dan's Vehicle Dynamics Corner,

http://www.chassissim.com/blog/chassissim-news/dans-vehicle-dynamics-corner-how-to-look-at-simulated-data

It's about the differences and what to look for when using simulated data and how it contrasts with actual data.

Enjoy

All the Best

Danny Nowlan
Director
ChassisSim Technologies

BillCobb

07-14-2016, 06:39 PM

The Vehicle Dynamics industry recognized method for correlating simulation to test is via open loop tests to establish steady state gains, response times, natural frequencies and damping in the linear and nonlinear regions of lateral and longintudinal acceleration. Open loop control inputs via ISO standard tests and computation of system response parameters can dismiss claims that often accompany simulation model demos. These tests only require a few performance signals that kits for these tests can provide.

Arguments for testing race cars for these very same factors are based on the reality of them having a wide linear range and few nonlinear components (except tires (which are just softening springs). Good simulations also reveal that urban legends and fears about using frequency response techniques on race cars are unfounded. They also tend to dismiss the common belief that most of your good cars are neutral steer or oversteer. There's just so much more than weight distribution and tires. Compliances, dynamic geometry, aero and 'large' (low load rating utilization) tires add plenty of complexity and amusement to any vehicle dynamics engineer to investigate. Then you can add a driver...

Now show me the money.