First, I want to clarify what I'm talking about, since I think rake might have more aerodynamic implications... I'm talking about the difference in rear ride height versus front ride height. For example, say you have coil-overs, and you set the car exactly to factory ride height, and get the corner balances right. Then, you lower the front 2" and the rear 2", that would be zero rake, if I understand correctly. Now, say you raise the rear 1/2", things change...

Now, this is where I'd like to start the discussion. Back before NSXPO 2001, I was putting new (softer) springs on the NSX. They were also longer, so I could be not quite so ridiculously low. I was working with a chassis engineer, and we discussed where to set the ride height. He looked up the factory measurements in the helm's manual, and then ended up setting the rear lower, proportionally, than the factory specified, because I had expressed some concern about wanting to make sure the car wasn't gonna be loose on an unfamiliar track with a 2000-mile drive home. He also mentioned that the way he usually sets up a race car is to drop the front end as much as possible (travel and wheel clearance being the limiting factors), and then tune the handling with swaybars and rear ride height.

Anyway, I never did like the way the car handled with it set up that way. It plowed horribly, especially in low speed corners, and would transition pretty violently from understeer to oversteer when too much power was applied. I also got sick of looking at my near-factory ride height next to all the other lowered NSXes at the monthly lunches, so I busted out my jack and 3/8" allen wrench, and lowered it. I started by only lowering the front, by 4 turns. Suddenly the handling was totally transformed, much more front end grip (lower CG), and the car just transitioned more naturally, and was much more neutral in low-speed corners. Then I went back and did 2 or 3 more turns on the front, and 3 or 4 turns on the rear (don't remember now exactly) and things got even better, mostly just because of overall lower CG, and picking up some rear toe-in. I had never realized how much of a difference CG height could make.

So this got me thinking about FWD cars. Many many lowering spring setups out there lower the front more than the rear. I though it was just common knowledge that it was for looks (as it does look better that way). But then when I put my ground controls on my civic, with 300lb/in front and I think ~280lb/in rear (might only be 250, I really don't know, they were from my integra, and don't have the standard markings), the front springs were about an inch shorter than I wanted, so my car is about 3/4" lower in front than in the rear. I also have integra control arms front and rear, ITR swaybar in the back, and some sort of front swaybar from an integra (probably not a GS-R, and definitely not a type-r). The car is far more neutral than it seems like it should be with this setup, especially when powering through corners (as much as you can use the word power when talking about a civic DX). I haven't autocrossed it or tracked it yet, but I've driven it with some considerable anger on the streets with great results.

So now I'll try to actually form some kind of conclusion. I have a pretty good understanding of RR98ITR's dissertations on why rear-stiff is important to maximize grip on a front wheel drive car. It makes perfect sense to me, and I'm not trying to say he's wrong. However, that's really not an option for all people, especially people who only autocross recreationally, and put more importance on street behavior and ride comfort. Furthermore, for autocross, balance and transition is usually more important than maximum grip. On-the-fly adjustability is also quite useful if you know how to use it, since you're seeing new course layouts all the time, and it's simply impossible to have a single setup that's always perfect for everything.

So for autocrossers, my theory is that raising the rear ride height to loosen the back end to taste (yes, you're giving up overall grip to do this) is a good alternative solution to your regular rear-stiff setup. The other advantage of this approach is that you could easily take an extra couple minutes to change your rear ride height while you're putting on your race tires, and put it back when you're taking them off, or possibly even tweak it in grid between runs.

The biggest problem with my theory is that I don't understand roll centers at all, and I have no idea if the difference in handling characteristics I notice on my two cars is simply because of the difference in CG height, or some other change in suspension geometry.

I've got my nomex on, so flame away.

-Mike

 

I won't even begin to claim that I know how roll centers affect handling, but I do know that it can make more drastic changes in your handling than you would think.

For instance, as you change your CG and roll center, there is also an anti-roll effect that also changes. There's a reason chassis engineers are paid so much.