Question For The Aero Teams

[BenB]

I am working on a Formula car desing for SCCA (not FSAE), but I was wondering if anybody had input for a suspension/aero problem I have been working on:

The amount of downforce this car will make is enough to demand a 3rd spring and damper (which I would like to avoid doing)...To keep the car off the track the suspension springs would need to be so stiff that the roll stiffness would be about 3X what I want it to be.

Originally the idea was to use parallel A-Arms to make the roll center height 0 so there would be a large rolling moment, but it is becoming clear that the roll center would need to be pretty far below the ground to make the roll moment large enough that the roll stiffness would be low enough to be acceptable. (I think Toyota F1 was trying to do this last year if you look at their A-Arm geometry, but I believe they abandoned the design this year)

Does anybody know any other "tricks" that might solve this problem besides a 3rd spring? CART cars make a lot more downforce than my car and they do not seem to need a 3rd spring. In fact I think they have an anti-roll bar to increase the roll stiffness even further!

[BenB]

I am working on a Formula car desing for SCCA (not FSAE), but I was wondering if anybody had input for a suspension/aero problem I have been working on:

The amount of downforce this car will make is enough to demand a 3rd spring and damper (which I would like to avoid doing)...To keep the car off the track the suspension springs would need to be so stiff that the roll stiffness would be about 3X what I want it to be.

Originally the idea was to use parallel A-Arms to make the roll center height 0 so there would be a large rolling moment, but it is becoming clear that the roll center would need to be pretty far below the ground to make the roll moment large enough that the roll stiffness would be low enough to be acceptable. (I think Toyota F1 was trying to do this last year if you look at their A-Arm geometry, but I believe they abandoned the design this year)

Does anybody know any other "tricks" that might solve this problem besides a 3rd spring? CART cars make a lot more downforce than my car and they do not seem to need a 3rd spring. In fact I think they have an anti-roll bar to increase the roll stiffness even further!

[exFSAE]

What are you basing your roll stiffness targets on? Why are you afraid of too little roll?

Having visited a ChampCar team not too long ago (bout a year), I can tell you the Lolas definitely ran 3rd spring.

If you've got a downforce car, that's the way to go man.

This a FSCCA or F1000 car or somethin?
If you're running underbody, very stiff in roll is good. And you can do some trick stuff with the effective "AoA" of the underbody with 3rd damper rates.

[Alan]

Somebody already mentioned this in another thread but a lot of high downforce cars run some sort of ride control spring that only works in compression. It could be a coil spring that only engages under bump or some sort of bump rubber with a gap that you set with packers. If packaging makes a 3rd impractical, running bump rubbers on the corner springs is an option as well. Rising rate rockers could also help a bit but could potentially hurt other things.

[exFSAE]

Actually, I take back what I said. Was lookin at my wrong set of visit pics.. coulda sworn the Lola had a 3rd Spg setup. Maybe not! Nor the Panoz in this front photo.

[flavorPacket]

I was under the impression that the Lola chassis used the Penske interconnected pitch damper with no auxiliary spring. Didn't it have the one-off shuttle ARB as well?

[murpia]

Personally, I wouldn't attempt to design an aero car without the option of a 3rd spring or 3rd 'gapped' bump rubber. It's really not that hard to package in a push / pull rod car, if you consider it from the outset. What's put you off?

Whether or not certain Champ cars run a 3rd spring is not strictly relevent. All teams have their own ideas, not always good ones, and the Panoz is a spec car so maybe they deleted the 3rds for cost and to give the engineers a harder time? Same with GP2, world series, Lola F3000 etc. Free cars like LMPs, Nippons & F1 usually have 3rds.

For a rear-heavy car with some aero I've found the calcs typically suggest the need for a front ARB and rear 3rd, but that does depend on a lot of factors. The lighter the car and the more the aero, the more you head towards a front 3rd too.

Regards, Ian

[BenB]

The roll stiffness target is 0.8deg/g. 3.5g's is estimated to be the max lateral acceleration.

There is 3rd spring/damper on the rear of the car actually. It was pretty easy to package, but on the front it is pretty much impossible to fit a 3rd spring as currently designed. The body will need to be made a little bigger to fit the damper/spring assembly. I think this will have to be done, but I thought it was worth asking around if there might be another option not being considered.

Alan, A progressive rate spring or a packer was considered, but I was thinking of these more as band-aids. Maybe that idea should be given more consideration. Did that work pretty well for the champ car teams or would you always use a 3rd spring instead?

Murpia, This car probally would not fit your "rear heavy" car criteria. The weight bias is about 55% rear and I've been trying to shave weight out of the rear pretty aggressively to try to bring it as close to 50/50 as possible. The car will be a little under 800lbs without driver (class rules is for 1000lbs min weight) so there should be quite a bit of weight to move around the car for most drivers.

The car is being designed to race in F1000. Its kind of a small but growing class. I think its going to be a great class though. Kind of like DSR but for formula cars.

[flavorPacket]

.8?? that's a good number for a non-aero fsae car, not a real race car. Why do you want to be so soft? What's your goal pitch frequency? heave?

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BenB:
Alan, A progressive rate spring or a packer was considered, but I was thinking of these more as band-aids. Maybe that idea should be given more consideration. Did that work pretty well for the champ car teams or would you always use a 3rd spring instead? </div></BLOCKQUOTE>
I agree with the 'band-aid' assessment. Given the choice I'd always go with a 3rd.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BenB:
Murpia, This car probally would not fit your "rear heavy" car criteria. The weight bias is about 55% rear... </div></BLOCKQUOTE>
Sounds rear-heavy to me! To expand my comment, once you go beyond 50% rear, you start to need more roll moment distribution at the front. That puts front wheel rates &gt; rear wheel rates (including ARBs). But, with rear-biased aero, that means more aero induced suspension travel at the rear than the front. To maintain rake you need more stiffness in axle heave at the rear, hence the front ARB and / or rear 3rd recommendation. Also, if you want to maintain higher axle heave frequencies at the rear (so axle bump inputs get in phase and the car heaves rather than pitches) the same applies - go for a rear 3rd.

0.8deg/g sounds a bit soft for a 3.5g car, but good for low warp stiffness if you can run enough static camber without compromising straightline braking and accel too much. That'll depend a lot on your driving style...

Regards, Ian

[exFSAE]

Interesting. Buddy of mine and I were thinking about designing an F1000 car.

I'm also not sure why you want a 50/50 weight distribution.

[BenB]

0.7 Sorry I looked back at my numbers.

RCVD (pg 605) says good values for aero-cars are 0.25-0.5.

I wanted to go a little softer than that (or at least have the option) because it seems like you might be sacrificing mechanical grip if you are as stiff as 0.25.

Actually if I plug 0.25deg/g into my spreadsheet my calculated ride rate gives me very close to what I need for the roll rate (therefore no 3rd spring needed). I'm just concerned about for rough tracks that have a mix of low speed and high speed corners. I think it might be desirable to be a little softer than 0.25-0.5deg/g.

Maybe 0.7 is a little too conservative. Any thoughts?

[flavorPacket]

not including tires, we ran .4 last year. the car was fantastic on smooth surfaces.

.7-.8 times 3g's is a LOT of roll. If your kinematics are not spot-on, you will have serious problems. Furthermore, rolling from 0 to 2.5-3 degrees will take a lot of time, and thereby leave you with an unstable aero platform exactly when you need it, in the middle of the corner.

[Alan]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BenB:
Alan, A progressive rate spring or a packer was considered, but I was thinking of these more as band-aids. Maybe that idea should be given more consideration. Did that work pretty well for the champ car teams or would you always use a 3rd spring instead?
</div></BLOCKQUOTE>

For sure having a 3rd is much easier to characterize and tune. What sort of layout are you looking at as far as rockers and associated linkages?