over Ackerman?

[Bob Wright]

I want to know a bit more about the reasoning behind some of the ackerman set ups people run. Even with the tight radius corners I cant work out why 200 percent ackerman is working on these cars. If the outside is running at a reasonable slip angle on a 4 meter radius corner for instance, the inside tyre is at an angle that seems so extreme that it looks like its working by giving a huge drag component to the force at the contact patch to make it turn in. When our car corners (with no wings) it nearly fully unloads the inside rear. This gets the inside front working as much as possible. On our track width f/r ratio and at 1.5g lateral, the inside front still has at least 30% the load of the outside front. I have never seen tyre data that would work those loads efficiently with 200% Ackerman. So what is happening? Are teams deliberately draging inside fronts to get the thing turning in? I would like to know. We got our car working well with parrallel steer but I would like to know the difference.

Bob Wright
Monash University
Australia

[ben]

It has been mentioned on another thread that these cars run on tyres designed for cars twice the mass i,e, FFords and the like.

Now this suggests that relatively benign amounts of ackerman, camber and caster are going to see us with much lower tyre temps than the FFord boys, and lower tyre forces as a result.

It seems fair to say that FSAE cars go better when we get a lot of heat into the tyres with camber and dynamic toe-out (ackerman). We will hopefully be able to confirm this later in the year when we get some IR sensors pointed at the tyres and some slip angle data to go with it.

Ben

[Alan]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by ben:
We will hopefully be able to confirm this later in the year when we get some IR sensors pointed at the tyres and some slip angle data to go with it.

Ben<HR></BLOCKQUOTE>

I am curious how you are going to be measuring slip angle.

Kettering University FSAE

[-Gabriel]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>
I am curious how you are going to be measuring slip angle.
<HR></BLOCKQUOTE>

I have not tried this yet, but I think that with a data aquisition system calculating the cornering radius (speed and lateral acceleration) and the steering movement, you can approximate the front tyres slip angles by knowing your steering geometry.

Gabriel Denoury
Ecole Polytechnique de Montréal FSAE
www.fsae.polymtl.ca (http://www.fsae.polymtl.ca)

[ben]

We have recently agreed a deal with Datron Technology for the provision of a 2D datalogger and loan of sensors, which is planned to include the Correvit laser slip angle sensor.

The plan is to do this testing in the run up to the British competition. In advance of doing it, if anyone wants to contact me off the board with any suggestions as to things to measure and any useful post processing we could do, I think we could all learn something.

Ben Michell
UBRacing Vehicle Dynamics Group

[David Money]

Ummmmmmmm I don't think so. I recently attended the Claude Rouelle / Motec seminar on racecar vehicle dynamics and it is VERY hard to measure slip angles of the wheels. It is also VERY expensive. They have a sensor developed just for that purpose but it's looking at something like $20,000 USD to do it. From his experience it was argued for well over an hour that it could not be done with basic data acq.

As for why 200% ackerman vs 100% ackerman. It all boils down to slip angles and the tire data you have (or in most cases don't).

[ben]

The sensor we are likely to use is the $20,000 dollar unit that the F1 teams use. Are you saying that we'd still have problems interpreting individual wheel slip angles due to compliance, etc, even if we had this sensor on the body?

Incidentaly, I will be attending a Claude Roulle seminar in advance of using the sensor, so should be able to get some advice.

Ben

[David Money]

In my best Claude Rouelle French accennt "No no no no no....if you have z $20,000 zinzor zen it iz not a problem for you!" From what Claude said over the past 3 days, if you have the sensor and know how to use it, it shouldn't be a problem. I was just saying that as he pointed out you cannot get slip angles with typical accelerometers, or at least not that he has seen with any remote accuracy.

By the way, you will love the seminar. Try to convince EVERY member of your design team to attend the seminar. For the price it is a bargain. I cannot say enough good things about it. I am already planning on taking teh 2 week seminar he will be teaching in June/July

[Jeff Curtis]

Remember tire forces don't act in just lateral directions, increasing the drag force due to slip angle on the inside front more than the outside front will create a yaw moment in the turning direction.

[vinHonda]

Ben, are u from Brunell??

My big question is why does Cornell run ANTI ackermann?! about 50-75%.

Vinh

University of Toronto Formula SAE Racing Team
www.fsae.utoronto.ca (http://www.fsae.utoronto.ca)

[ben]

I'm from Birmingham (www.ubracing.co.uk) (http://www.ubracing.co.uk)).
The theoretical reason for anti-ackermann is that due to load transfer the outer wheel needs a larger slip angle to reach its peak cornering force (slip angle for maximum force increases with load).

That's obvious, the question is, is this a better solution than running lots of positive ackermann and getting the tyre drag forces to create a large yaw moment?

Ben

[vinHonda]

Do I remember you from FStudent? hmmm...

Yes, I know what anti-ackermann does.... but how does Cornell know to run it?? What tire data do they have to confirm that the tire works better at that larger slip angle?

I'm curious to know what other teams run anti ackermann.

Vinh

University of Toronto Formula SAE Racing Team
www.fsae.utoronto.ca (http://www.fsae.utoronto.ca)

[zorr0]

Tire data? Make some steering arms. Weld something to your uprights. Just try it.

http://www.auburn.edu/~zornmat/eagleside.jpg

http://www.auburn.edu/~zornmat/pics/img_0399.jpg]

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by vinHonda:
Do I remember you from FStudent? hmmm...

Yes, I know what anti-ackermann does.... but how does Cornell know to run it?? What tire data do they have to confirm that the tire works better at that larger slip angle?

I'm curious to know what other teams run anti ackermann.

Vinh

University of Toronto Formula SAE Racing Team
http://www.fsae.utoronto.ca<HR></BLOCKQUOTE>

I wasn't at FStudent last year (year in industry).

I remember hearing the FSAE tech debrief where Carroll Smith mentions Cornell's adjustable ackermann geometry and design of experiments testing methodology - I think that answers your questions. Back-to-back testing and statistical analysis to prove which geometry was quickest.

Ben

[Erich Leonard]

Vinh,

Cornell has never run anti-ackerman. Carroll Smith noted that we had adjustable ackerman steering arms on the 2002 car and that he found it interesting that we were not running tha amount of ackermann that he felt would be ideal. During the design event, we showed him testing data that concluded that there was little difference between track numbers in skidpad and autocross events and that the setting we were running was only marginally faster than our other available settings. A DOE mentality was used to come up with our results, but anti-ackerman was not the setting being run at the competition.

Erich

[vinHonda]

Really???

I could have sworn that the 02 car sitting in my garage when you guys came up was set up w/ anti-ackermann! Maybe my eyes were acting up........it was 2am!

Vinh

University of Toronto Formula SAE Racing Team
www.fsae.utoronto.ca (http://www.fsae.utoronto.ca)

[Bob Wright]

yeah, anti ackerman works with tyres that have a smaller optimum slip angle with decreasing load, which is typical of cross plies but not of radials. Our tyres are cross plies and work well around parrallel.

I would be interested to see what Cornell run in terms of % front roll resistance. Im guessing its pretty low- which would also help.

Interesting about that drag yaw moment thing, I see how it works but Ive also seen some really chopped up inside front tyres.

Bob Wright
Monash University
Australia

[Schumi_Jr]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> Remember tire forces don't act in just lateral directions, increasing the drag force due to slip angle on the inside front more than the outside front will create a yaw moment in the turning direction. <HR></BLOCKQUOTE>

true, but I can think of reasons why this is a bad idea. Your inside front slip angle is too high- this means that it is producing less than it's maximum possible force. The induced yawing moment from tire drag has less of a moment arm than a lateral force would(1/2 track vs. distance from front wheels to cg). Additionally the slipping tire produces less force (static vs dynamic coefficient of friction). If both your front tires are operating at their optimum slip angle for whatever their loading, you will see a higher steer-induced yaw moment than if you're dragging along your inside front. So... in my opinion too much ackerman= bad idea. But hey, if you can't get your car to turn-in...

[Bob Wright]

yes, that was exactly my point when I started the thread. I would still like to know if anyone has any testing data to back up any work on this though.

Bob Wright
Monash University
Australia

[Chris]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>When our car corners (with no wings) it nearly fully unloads the inside rear. This gets the inside front working as much as possible. <HR></BLOCKQUOTE>

I can understand the reasoning behind using lots of Ackerman to get the yaw moment working for you. But with your setup, would you want that? Seems to me that since the yaw induced by the inside front is counter-acted by the rear tires. Since the rear cornering force is all pretty much generated by your outside tire, and since that tire is already getting a workout from the purely lateral forces and (hopefully) fore/aft forces being put on it, wouldn't adding that yaw moment would just overload the tire? I guess what I'm getting at is this: on a car with a lot of lateral load transfer like yours, wouldn't lotsa Ackerman make for a severe lack of rear grip????