Vehicle Dynamics starting points and design process

[Mumpitz]

bonus points for Jawa content

http://www.thumpertalk.com/topic/101...ely-a-thumper/

[MCoach]

Matt,

My assumption is that the differential is never in front of the axle centerline which basically leads us to the same conclusion.

However, this should not be the focus of this thread, and not really Jawa motors either. This should be dedicated to entertaining the questions and progress of "vehicle dynamics starting points and design process" and not allowed to be spiraled into another bitch and moan thread about everyone's an idiot. Let's stay on track and encourage our newcomer along his objective process of designing and building this thing.

Christian,

I see you have your desired weight balance and some other parameters posted. While these are considered achievable, you can make a spreadsheet to track as much of the heavy items in the car as you can, (Engine, Driver, Tires, Wheels, Steering Rack, Bodywork, Chassis, Differential, etc) so that you can easily track your MoI, overall CG relative to wheelbase, and make decisions on discrete placement of packaging items beyond "hm, well, this 12lb battery fits nicely right here on the front bulkhead, ol' chap".

This can help with the accuracy of your calculations compared to what you actually build.

[ChristianChalliner]

Just a quick note from me (Too tired to write something in full now, that will come tomorrow).

Thanks MCoach for saying pretty much what I was going to say (and also giving me a good laugh with the battery comment!). I have no idea why the Jawa is relevant here and I don't wish for this to turn into another argument like most threads on here.

I have made a CofG spreadsheet though and I'm currently weighing all our fixed components to get an idea of F/R WD and CofG height. We also made the trip over to our neighbors Birmingham City University today (we are literally across the road from each other) really good people and had a great chat about things

EDIT: I did also try to change the title of this thread to "Aston University Design and Build Thread" or something similar but it doesn't appear to have worked.

[Z]

Christian,

I am going to digress from the topic of "Aston University Design and Build Thread", but I am sticking firmly to the topic of "Vehicle Dynamics Starting Points and Design Process". I will try to keep this digression to just this post.

I am doing this because of the huge resistance there still is here to any serious consideration of these "Starting-Points/Big-Picture" issues. Despite the importance of these issues being repeatedly discussed since this Forum began about twelve years ago, and despite Geoff putting in a mountain of work with his "Reasoning..." thread of over five years ago, by far the majority of Teams still TOTALLY IGNORE THIS "BIG-PICTURE". Worse yet, some actively fight against it (see below).

Much of the problem comes from the Team Supervisors, such as yours, who lock their Teams into an ineffective design by mandating the engine-type and wheel-sizes. There is not much you can do about this, but other Teams do have the option to move to better designs. They just have to have the big-picture issues explained to them. Again..., and again..., and again...
~~~o0o~~~

So, as a reminder to other Teams out there, here are the most important VD features that a good FS car should have, IMO.

Minimum wheelbase,
+ very low CG,
+ very low Yaw MoI,
+ best F:R% for RWD (~40:60+),
+ driver's CG close to car CG (for consistent F:R%, and implying driver entirely WITHIN wheelbase),
+ narrow fuselage for good aeroflows,...

And, once those are in place, the biggie to add is a shed-load of aero-downforce (ie. add mega-AD to above VD).

To restress this, these Overall-Size and Overall-Mass-Distribution issues are MUCH, MUCH more important to having a fast car than all the fussy little Suspension-Kinematic, and Spring-Damper-Rates, and whatever else..., little details that most Teams spend most of their time on. As noted before, cars have won FS/FSAE with effectively NO suspension movement at all. All those little details amount to squat.

Get these big-picture issues right first, then develop faster drivers, and BETTER AERO.
~~~o0o~~~

Whether you students realise it or not, the main thing preventing you from achieving the above big-picture goals is a deeply ingrained, and utterly irrational, belief that you MUST use engines taken from motorbikes. That is, ONLY engines from 600 cc four-cylinder sportsbikes, or 450 cc single MotoXers, or something very similar, seem to be considered OK. Well, there are a few exceptions (see below), but VERY FEW.

This irrationality is then supported by a never-ending stream of DEFEATIST arguments, such as Menisk's below.

"Very low CG means lying your driver (the heaviest component of your car) flat,...
... so ... you're not going to have a minimum wheelbase. ...
...You're asking a lot to fit the driver entirely within the wheelbase and be minimum wheelbase [because ... more excuses] ...
... Literally the only way to achieve that is to turn the engine around a have a mid driveline and dangle the engine off the back...
... Also last time I check engines aren't squishy. They ... have a set size...
... you can't just magically make the gearbox disappear and the engine shorten...
... a 40:60 bias ... no chance with an inline 4 with low CG...
... With a single you're not going to have to worry about putting power down, you don't have any...
... Long and short of it is you can't have all the things you want. They're all trade offs.
...
... Please stop chanting that people are mindless idiots on this forum. There are annoying realities ... that you completely ignore."

Contrary to "ignoring the realities", I thought through all these issues when I first became aware of FSAE back in the 1990s. The "realities" are very simple and straightforward, and my thinking on them has not changed in 20 odd years. Specifically:

The Rules regarding driver position, foot-box size, mandatory steering-wheel, and FRH above steering-wheel, all mean that a driver "lying flat" cannot see where he is going. So pointless. But lifting the driver's head high enough that he can see over the lowest legal and practical FRH means his head-rest can be less than 1.4 metres behind the pedals. This comfortably fits the driver entirely within the wheelbase!

Furthermore, the driver's raised head and shoulders give plenty of room for an engine and driveline ALSO entirely within the wheelbase. Namely, tucked under the driver's back.

The flaw in the usual student reasoning, as above, is the sub-conscious ASSUMPTION that said engine-driveline MUST come from a motorbike, and it MUST be mounted as in the motorbike, with its chain-drive going back to the diff-sprocket.
~~~o0o~~~

The quick dismissal of the Jawa in some posts above is an indication of just how strong this irrational thinking is.

Briefly, for those interested, a standard Jawa puts out more horses than the standard 450-singles. The Jawa is widely available all around the world. Jawa also make a neat, and separate, two-speed-gearbox, which is useful for different packaging options. And two speeds is enough at these power levels. (I suggest reverse-rotating-crank layout, head pointing to rear, and modified 2-speed-g'box integrated with diff.)

And again, for the record, air-cooled 450 cc turboed "singles" were putting out ~125 hp over 40 years ago. The limit through the FS restrictor is ~120 hp. So MAXIMUM POWER IS POSSIBLE FROM A COMPACT SINGLE! (This is another area where many lazy and "mindless" students were arguing that "it's all too hard, ... impossible...", until I spent 5 minutes finding the countless web-sites that show that this is all, indeed, very possible! Use Search!)
~~~o0o~~~

And there is the "Scat-Single" that I suggested recently. This would require making engine-cases, and possibly a crank, although a pre-existing crank could be used. With moderate boosting 100+ hp would be possible. At these power levels only one gear is needed, though a Jawa-style box could be used. Or a CVT. Or a IVT. Or a regular, small car, auto-box type torque-convertor, preferably with lock-up, but only single-speed. All this could be comfortably fitted, with the driver, entirely within the wheelbase.

The "extra work" required for these types of engines is little more than that usually done on many FS engines, with their intakes, exhausts, dry-sumps, and all the EFI stuff. Given that many Teams are now on their Nn-th iteration of billet-machined-uprights, none of which are any better than the folded-sheet-steel ones they built Nn+ years ago, imagine how far down the track to an ultimate FS-engine those Teams could be now if they decided to do billet-machined engine-cases instead!

It is quite bizarre that in this engineering competition there are so many student engineers who are so averse to engineering an engine!
~~~o0o~~~

Perhaps even more troubling is that even when some Teams are presented with a golden opportunity to do some of the above, they IGNORE THE BIG-PICTURE issues.

So, take for example RMIT at Oz-2013, who ran the Yamaha "Genesis" 500 cc engine. This is a vertical, parallel-twin engine, coming standard WITHOUT a gearbox. (IIRC, it is meant for snowmobiles, with CVT drive as standard? Corrections welcome.)

Anyway, a perfect opportunity to really squash the car up, as explained above. But the driver was sat in the same place as all standard cars! Namely, with knees over the front-axle. Then there was a significant gap behind the fire-wall back to the engine. Then another gap back to a bespoke 6-speed sequential gearbox. (Why 6-speed!? And why the really stupid carbonfibre case!!!?) Then another long gap back to the chain-driven diff.

In short, a great opportunity wasted, because NO THINKING at this important "VD-Starting-Points..." level.

Someone..., some day..., please.....

Z

(Edit: Here is another car with the Genesis/Phazer engine. Is all that space between engine and diff really necessary? Or is it there "...because that's how everyone else does it"?)

[Menisk]

The Rules regarding driver position, foot-box size, mandatory steering-wheel, and FRH above steering-wheel, all mean that a driver "lying flat" cannot see where he is going. So pointless. But lifting the driver's head high enough that he can see over the lowest legal and practical FRH means his head-rest can be less than 1.4 metres behind the pedals. This comfortably fits the driver entirely within the wheelbase!

Have you ever driven one of these car's Z? We have a 30 degree seating angle this year and you have plenty of visibility forward. You only need to see the top of the cone looking forward, the majority of your vision requirement is the front wheels so you can place them as close to the cones as possible. You can seat the driver nice and upright too, but he is the heaviest component and you're now contradicting your low CG requirement. Also how the heck do you expect to have your rear axle 125mm behind the headrest without an unachievable amount of driveshaft angle. I don't know what shape engines are in your part of the country, but motorcycle engine or not you're going to have trouble squeezing anything bigger than a 100cc kart engine behind a driver and have the driveline somewhere the allows the rear axle to be 125mm behind your headrest.

There are ways you could do it. You can have the fun of turning the engine around and running a mid driveline and working out an intermediate shaft to get some form of drive to it from your gearbox output. You could try and turn a single sideways and make a small transaxle for it to a mid driveline. But teams already struggle to get conventional cars with more simple setups built in a year. Who do you expect is building these cars? Fully qualified, full time engineers? Our team's philosophy is the simplest solution with the fewest parts because 9 times out of 10 it's the lightest and most reliable solution.

The "extra work" required for these types of engines is little more than that usually done on many FS engines, with their intakes, exhausts, dry-sumps, and all the EFI stuff. Given that many Teams are now on their Nn-th iteration of billet-machined-uprights, none of which are any better than the folded-sheet-steel ones they built Nn+ years ago, imagine how far down the track to an ultimate FS-engine those Teams could be now if they decided to do billet-machined engine-cases instead!

It is quite bizarre that in this engineering competition there are so many student engineers who are so averse to engineering an engine!

Have you ever built one of these cars Z? Let alone while also trying to study an engineering degree that already has most students forgoing sleep to complete assessment while doing it? There is a reason it costs Honda millions of dollars to have full time engineers design and build engines. To think that some sleep deprived student is going to be able to tear an engine apart to do work with internal mods, cranks, gearboxes etc. and end up with a better package that's lighter than what the motorcycle manufacturers produce is stupid. On top of that they still have to sort out an intake and exhaust and sump solution to suit their car and keep from starving engines of oil. Sorting out a good, efficient and driveable fuel map in itself takes days of time on an engine dyno. I'm lucky enough to have worked with a man that's being doing it for 40 years. To sort out all the setup, track down a few niggling issues and get the fuel and ignition map where we wanted it took 24 hours of engine dyno time. Not all teams are even lucky enough to have access to an engine dyno.

I don't disagree with you that gearboxes are unnecessary in this competition with a 600. We drive our car in 2nd gear everywhere. 2nd gear tops out at 105 and will happily light the rear wheels up at 20km/h. The problem is that engines capable of decent power without a gearbox - like a Yamaha Fazer - are not easy to come by in some places. Apparently when you live in a country that's filled with desert snow mobiles aren't so popular. Motorcycles however are.

This irrationality is then supported by a never-ending stream of DEFEATIST arguments, such as Menisk's below.

Not defeatist, realist. Have you ever built one of these cars? Have you ever built one with a team of student engineers? You've probably built things, how did your first attempt turn out? The key word is student. Engineering students already have a massive university load, and on top of this most need to work in order to feed and house themselves AND build an FSAE car. Most of these students have never built anything close to the complexity of a car and we need to learn all the manufacturing processes at the same time. Yet somehow you seem to think that doing all sorts of crazy engine work that students don't yet fully understand is perfectly achievable on top of the usual intake, exhaust, sump and EFI that we all have to do to begin with. We're students for a reason. We don't know how to do this stuff yet. Clearly we're all stupid and defeatist for choosing a cheap, available and reliable engine package to save resources and minimise the chance of engine trouble ruining the competition we've spent all year preparing for. Not to mention you somehow seem to think you can do all this to have an engine the size of an RC car motor and a driveline packaged under a driver to allow your rear axle to be 100mm behind the headrest. Also, for the record in choosing our engine package we didn't just assume it had to be a motorcycle engine. We looked at everything from snowmobiles and ATVs to microlight aircraft. Motorcycle engines are simply the most practical and affordable option.

So yes Z, you are ignoring realities. So I'll ask again? Have you ever built or driven one of these cars? How well did you first backyard project work? I would love for you to draw me some pictures (with dimensions) of how you intend to fit a 95 percentile driver within the wheelbase of a car and not have insane amounts of CG height. After that you should get out into your shed and build it, and then come beat us all at competition. Until then learn to encourage students without reminding them how stupid you think they are and how superior you think you are. You need to learn how to play nice with others.

[tromoly]

(Edit: Here is another car with the Genesis/Phazer engine. Is all that space between engine and diff really necessary? Or is it there "...because that's how everyone else does it"?)

Z,

That's my picture, the short answer is the space is needed to make everything fit, also that was the first year we used that engine so there were some uncertainties regarding center-to-center from engine PTO to secondary shaft, so this was left "stock", the next year pulled an inch or so out of the center distance and it worked alright. You do point out the second-biggest issue I have with using a CVT, having to use a secondary shaft increases the rear length of the chassis quite a bit, if a single-cylinder gearbox engine were used atleast 6-8+ inches of chassis could be eliminated.

In addition, IIRC the Phazer twin weighs about the same as a single-cylinder with gearbox, once the CVT pulleys and secondary are added onto the Phazer weight it's already quite a bit heavier and, I suspect, has quite a bit more rotational inertia.

So to answer your question, about 90% of that space is needed for everything to fit.

[ChristianChalliner]

Ok, so my thoughts (very basic but time is limited again),

I disagree that fitting the driver inside the wheelbase isn't possible, I think it is but that it does require a relatively small engine or extreme angling of driveshafts to do it. MCoach makes an interesting point in having the driver and percy sit differently in the car which is pretty much exactly how I put the basic sketch together, the driver would sit laid down but percy sat relatively upright, meaning that a 'real' driver sits with legs bent in the car and with the bulkhead moved much closer to the driver. We were also considering UJ'd half shafts as opposed to the current tripods we use, we think we can have the UJ's at greater angles and push the diff even further back but we'll see...

I agree with Z that putting the driver flat makes it impossible to see out (something we confirmed with our very basic cardboard 'screen' setup for a front roll bar to see how high was too high).

I agree with Menisk that it is quite difficult to find the time to actually work on very elaborate solutions whilst still working on other projects/coursework/etc.

I completely agree with the steel vs alloy upright issue Z raised, in fact, we dug out some of our old steel ones and commented on how light they were for their stiffness, so much so that it's something we've already committed to this year. Here's a picture of one of our old ones:



Not the best in terms of neatness (or design) but these are easily lighter than our alloy ones from last year and don't have the rigidity of a wet towel either.

[MCoach]

Christian,

I've sent you a PM.

[Forbes]

Christian,

NASA (USA's Space group) put together mass/inertia data for each major component of a human being. They published this information (I do not recall where, one of my team members found it in 2013) somewhere online, and that allowed us to look at seat back angle vs. CG height vs. Yaw Inertia for various seat back angles.

-Matt

mdavis, Christian

I believe this is the link to the NASA document: http://msis.jsc.nasa.gov/sections/se...NTHROPOMETRICS

Forbes

[Z]

So yes Z, you are ignoring realities. So I'll ask again? Have you ever built or driven one of these cars? How well did you first backyard project work?

Menisk,

Since you ask, no, there wasn't any FSAE when I was young.

But I did build something a bit bigger, at highschool, by myself. It went really well! There is a short thread about it (Gallery section, search "Young Z"), but it seems the photos disappeared when the Forum changed hosts. (I can dig up photos if anyone wants.)
~o0o~

Our team's philosophy is the simplest solution with the fewest parts because 9 times out of 10 it's the lightest and most reliable solution.

Me too. I am interested to see your version of "simplest". What Team are you, and will you be at FSAE-Oz-2014?
~o0o~

... it costs Honda millions of dollars ...
... To think that some sleep deprived student is going to ... end up with a better package ... is stupid.

Honda spend millions designing MOTORBIKE engines. Komatsu spend millions designing EARTHMOVING engines. Etc., etc. So what? You want an FSAE engine. Very different.

Look here to read about two trade school boys who wanted to build an AEROPLANE engine. So they did! Fortunately, they only talked to Engineers AFTER they built it, otherwise my guess is they would have been told "IMPOSSIBLE!!!".

(BTW, one cylinder+head off that engine would also be a great starting point for a cracker of a little FSAE engine. Just DON'T get any Engineers involved! Their new site here.)
~o0o~

... get out into your shed and build it, and then come beat us all at competition.

Since 2002 I have been lobbying the FSAE organisers to include at each event some "demonstration runs" from all sorts of non-FSAE, but small, racecars. So, different types of go-karts and other "mini-racecars". Let them clock Acceleration, Skid-Pad, and Autocross times, and let the students learn how simple it can be.

But, so far, NOTHING. (My guess is that the organisers fear the reality of it all.)

Should this ever happen, and especially if Prize Money is offered ($ = ), then I will certainly get busy in the shed...
~o0o~

... learn to encourage students ...
... learn how to play nice ...

The real world is NOT NICE!

This is perhaps the biggest problem with the Modern Education System. You will eventually learn this reality, but it would be nice if your teachers let you in on this secret a bit earlier (ie. the occasional "old-school kick-up-the-bum" can work a treat).
~~~~~o0o~~~~~

Tromoly,

I would have sent the CVT drive FORWARDS to a Secondary-Shaft under the seat-back. Plenty of room there (ie. move any junk there elsewhere). From the SS a chain-drive would go rearwards, just to right of engine, to a diff/sprocket mounted just behind the engine. Or a two-speed Jawa-style gearbox and clutch could be mounted in front of the engine (ie. as the "SS"), instead of the CVT.

From the photo I reckon the rear-wheels could move a good 0.4 metres (16") forward. So front-wheels also move forward, and driver's feet are now about on front-axle line.

Not sure what to do about the flooding though? Gumboots for all???
~~~~~o0o~~~~~

Christian,

Cardboard-Aided-Design works a treat with those uprights, too. Try "cornflakes packet cardboard" for your modelling. Cut with Mum's scissors, weld with masking tape or hot-melt glue. Think through the process of assembling it all, while doing this "CAD".

I suggest ~1 mm thick sheet steel, although slightly thicker (~1.6 mm) is easier to weld. Have the sheet completely surround the central bearing support tube, so only a circular weld at each end of this tube. Weld with "TIG" ("inverter" machines are dirt cheap these days, and you can buy "throw-away" gas bottles). Give several Team members plenty of practice welding scrap off-cuts. No heat-treat is necessary (other than maybe a lick of oxy-flame), but do final machining AFTER welding.

Importantly, try to find a "bloke-in-shed" who knows all this stuff, and ask for tips.

More importantly, there is not a lot you can do about "ultimate best weight distribution" now, so aim for "better than last year", and focus on EARLY BUILD FINISH.

Z