Chassis design for Formula SAE: SAE 971584 3

http://web.umr.edu/~formula/library/sae_paper/saepaper.pdf

This is a reasonable introduction to designing an SAE car. I think they made a few bad choices, but the process they recommend is sensible.

I'd have to say the process they went through is towards the MINIMUM I'd expect to see for a sensibly designed car, generally. In particular I'm surprised the SAE regs allow different camber settings for the different events.

Bad choices, in my opinion - inboard suspension, rather heavy spaceframe, still got the tire temperatures wrong, roll centre heights (but that is arguable).

Things they got right - no anti-roll bars (in an open ended design you shouldn't need them), almost fully triangulated spaceframe, recognition that hardpoint design/detailing is FUNDAMENTAL to durability, awareness that you design to measurable performance, some respect for driver safety.

All in all a worthwhile paper. Cheers

Greg Locock

 

[CoryPad]

What do you recommend for FSAE instead of inboard suspension?

I think you are being overly critical of the mass - the vehicle was designed 7 years ago, it was representative of the average vehicle mass at the time, and mass has dropped considerably since then.

I agree with you that they presented a reasonable approach, and that it is a worthwhile paper.

 

[GregLocock]

If I sounded too critical, I didn't mean to be. I was assessing the design, more than the paper.

Well, outboard coil over shock is easier to engineer, and package, has half the number of hardpoints, half the force at the fulcrum (roughly) and is therefore lighter and cheaper. Spend a bit of the costsave on anodising and/or painting the shock. The judges must be bored witless by seeing so many inboard suspensions (I'll admit to only having seen 3 FSAE cars, this may be an exaggeration). Another option that would be harder to engineer is torsion springs, they are sexy and work well with SLAs, and spaceframes.

I reckon by eyeball 20 kg would be a more reasonable target for the spaceframe mass, I'm really not convinced that materials or analysis techniques for spaceframes have changed much since 1955, never mind 1995. Frankly the whole spaceframe just looks too complex. A genetic search for an optimum structure would be an interesting project that could be done now, but not then, I suppose. I'd certainly expect to see other design criteria than just torsional rigidity, and would be interested to see a justification for the figure chosen. (I think I can guess, and disagree with the choice).

My comment on the RCH was a bit harsh. By doing what they did they will have slowed the weight transfer side to side on initial turn in. This means they can tune it with shock valving, which is easy. Also the car will be less jittery. This could be very important for a car driven by students.

On the other hand the scrub radius seems needlessly high, which is bad for bump steer. They'd be better off using more trail and less scrub to get their steering feel, if the tyres are at all like a road car's tyres. I doubt the test speeds are high enough for the pneumatic trail effect to be important.

OK, I'll stop poking holes, after all there aren't many student projects that even raise interesting questions like this. I wish we'd had FSAE at uni, but then we'd have had to get our hands dirty, oh the horror!
Cheers

Greg Locock

 

[NeilRoshier]

Greg perhaps a simper spaceframe with aluminium panels as reinforcement would also be a solution to incorporating crash protection and limiting weight? Say 20mm thick/.3mm skins? Just a thought.

 

[NeilRoshier]

whoops, meant ali honeycomb panels (recent interest hence query)

 

[MoreWing]

Just to add a little closure. This car is still in existance. It must be about the most robustly designed FSAE car on the planet. It's seen nearly a decade of ham-fisted drivers and still comes back for more. Probably overbuilt for such a short competition, but even at that, it wasn't overly heavy.

The inboard suspension is pretty much a requirement of the judges. They say that an outboard suspension has too much aero drag. Ridiculous at the speeds they run, yes, but you have to give the customer what they want. The roll axis inclination and roll center height choices were specifically to reduce the understeer of previous designs. The nearly non-existant KPI coupled with about 4 degrees of caster made for a little too much inside front tire wear, but steering effort and feel was actually pretty good.

I know the 2 guys who wrote the paper. I drove the car once. My big complaint about the car was corner entry oversteer, but in general, it was a very nice piece. They said that characteristic helped during the SAE competition because the course was so tight, but on a higher speed course, it was a little hairy.

 

[sacem1]

Hi Greg:

I've been out of touch and did not know about this FSAE design, as you know I'm involved in a super 7 design right now but I am going to give it a good study and try to learn more about it.

Thanks for the info

SACEM1

 

[NeilDS]

I worked on the FSAE cars at WVU from '92 to '94. I remember in '92 that we got dinged for not having anti-roll bars on the car. It finished 4th overall anyways. Our '94 car had a 19.5 kg space frame and an overall dry weight of about 173 kg. It finished top ten design and 13th overall. Good wirte up, make me want to build another

Neil