Racecar Vehicle Dynamics

[NJIT FSAE]

so ive been doing lots of research on suspension design for our teams first car. ive gone through carroll smiths books, and ive started in on Racecar Vehicle Dynamics..its alot, and i dont believe i will have the time to finish the whole book before i design the suspension, so i am in need of some advice on what sections i should be really focusing on for the car.

i want one of the goal of the suspension to be to optimize tire contact patch, i do not have data from the TTC yet, but i will be attending this upcoming one to try and learn and absorb as much as possible

thanks for your help
-Aaron

[NJIT FSAE]

so ive been doing lots of research on suspension design for our teams first car. ive gone through carroll smiths books, and ive started in on Racecar Vehicle Dynamics..its alot, and i dont believe i will have the time to finish the whole book before i design the suspension, so i am in need of some advice on what sections i should be really focusing on for the car.

i want one of the goal of the suspension to be to optimize tire contact patch, i do not have data from the TTC yet, but i will be attending this upcoming one to try and learn and absorb as much as possible

thanks for your help
-Aaron

[BenB]

I think there are a lot of different opinions on what the best procedure is to designing a suspension, but here is what I've done that works out pretty well for me:

1. Guess how much suspension travel you need and
how much body roll you will use. Normally about
1.5-2"+ a little extra for large bumps of bump travel
is enough and maybe 1" of rebound travel.

2. You need a design strategy for your A-Arm's. Normally you will want your lower A-Arm nearly horizontal and your upper A-Arm sloping up away from the chassis. Read "Tune to Win" chapters 3 and 4 some for explanations on what changing your A-Arm geometry will do. Normally the goal here is to keep your tire camber at specific angles to maximize grip (you'll probably want tire data that shows what camber angles the tires work best at for this), and to locate your roll center in an area that you determine to be "acceptable". I normally try to make high roll centers because you can use smaller anti-roll bars since your roll moment is reduced, but I hear that you can develop a "jacking effect" if you get too carried away with this. I think in general you will mess up your camber curves before you get your roll centers too high.

You will probably need some kind of software for moving the suspension through it's motion to make sure the tire camber stays where you want it through the suspension roll/ride ranges. I think there is some free software on the FSAE forums for this somewhere. If you have ~$500 Mitchell is a pretty good package. I think they even give deals to FSAE teams.

3. Next you will want to design your inboard suspension geometry. Assuming you are using rockers to activate 2 coil over shocks + an anti-roll bar here is the proceedure I would use:

3.1 Determine where you want your rockers, shocks, and anti roll bar to mount on the car. You
will want all these to operate in a plane defined by the 3 points: Outboard push/pull rod
attach, inboard push/pull rod attach and the point where you want your shock to attach to
the chassis. (Model this plane in your CAD package and when you make adjustments to the
positions of the various points it is easy to keep everything "in-plane" if you define the
points using this plane.

3.2 adjust the shock mounting points/the pivot axis location on the bell crank to try and get
a motion ratio that is ~0.7-1.

4. RCVD outlines a pretty good proceedure for sizing your ride frequencies/roll rates that you can use to size your springs and anti-roll bar. This is found in section 16.2. If you make a spreadsheet that does these calculations it makes doing the iterations pretty easy.

5. "Tune to Win" outlines a pretty good way to design your steering geometry in chapter 4 and 11. Also you probally want to read RCVD chapter 5. I think most FSAE teams use Ackerman steer, but I think depending on your tire data you might want to use anti-ackermann since some tire data shows that for some (depending on which tire you use) lightly loaded tires make more force at lower slip angles.

This is pretty general and it would be very difficult to learn all this in a short amount of time, but I'm sure if your determined you can do ithttp://fsae.com/groupee_common/emoticons/icon_smile.gif Hopefully this helps get you started.

This is kind of an iterative process since you guessed how much body roll and ride you wanted at the beginning. Using your tire data you might decide you need less body roll or you can stand to let the body roll even more. You can change how much body roll you get by varying your roll stiffnesses or if you change your A-Arm geometry you might be able to stand a little more body roll.

There is a lot of parts/geometries of the suspension to optimize that aren't in the proceedure I listed above (castor, anitdive/ anti squat etc.), but I think that you would probally want to focus on your A-Arm geometry, inboard geometry, and damping/spring rates. When you say "optimize your contact patch" I think that you probably just want to maintain good camber angles through your suspension travel.

Again I'm sure there are many different ways to design suspension. Thats just the way I do it.

[Kurt Bilinski]

Whatever you do, don't use diverging A-arms. Meaning that the upper one slopes down to the wheel, and the lower one slopes up to the wheel. I was working the enduro yesterday and saw a car with that setup. When the car leaned maybe 2 deg in a turn, this crap suspension caused about +5 deg Postive camber on the outside tire. Their suspension designer should have been trout-slapped.