Trucho-951

The main problem with variable rate springs is that they eat up valuable suspension travel. Yes, the idea of having a soft initial rate for a supple chassis to soak up a rough road surface and then transitioning to a main stiff rate that can handle the weight transfer at full “g” cornering is appealing.

If the car has enough travel to spare, then it's ok, but if not, then it will end up riding on the bump stops. Once on the bump stops, that corner, probably the outside front, will turn effectively into an inappropriately stiff spring, and that corner will wash out, i.e. the car will push like a pig.

Having said that, I'm currently working on designing a dual rate spring combo for my 951. I’m on my 2nd prototype and it seems to be working to expectations. I just have to work out a few more bugs before I call it a done deal.

wanna911

Ok back to the simple stuff, I think we all for the most part know how compression damping relates front to back. If you understeer (all other suspension being equal) then you want to soften the front and/or stiffen the back. Either way creating a higher ratio of stiffness in the back compared to the front. And vice versa for oversteer, A little stiffer in the front and softer in the back to induce a bit of understeer.


We've talked about rebound in various ways as directed straight towards handling, but how does front and rear rebound relate to each other. And in what areas will you see it? ALL ELSE BEING EQUAL ON THE SAME CAR WHILE TUNING IT.

So how does changing ratio of front to back rebound damping affect the handling on the car???

Accelerator

After finding and reading this thread, I did some measurements and calculated my static instant centers and roll centers (front view). Everything is underground on my 996 turbo that has some GT3 suspension components. I am trying to understand, from reading books, not experientially yet, why it wouldn't be better to have RC lower and control it with stiffer springs as opposed to having the RC higher with its attendant jacking issues even though the rolling moment would be lower. With my lower RC I have also lowered the car's CG.

I have made recent changes in several areas and they, as yet, are untested. I changed dampers, springs, tires and added a significant amount of front and rear adjustable downforce. I'm trying to gain more understanding with several suspension topics before a test day (excuse me, driver orientation day) I am attending at Road Atlanta next Friday. I hope to learn more about what to change when the car does whatever its going to do. This whole RC/CG rolling moment issue is one I can't find a satisfactory explanation for in any of my reading material. The Millikens and Carroll Smith explain definitions in an understandable way but no specific answer to what I posed above.

Trucho-951

As I understand it, the distance between the roll center and the car’s center of gravity is the “leverage arm” that acts on the chassis to make it roll upon turn entry.

Since the car’s center of gravity is always above ground, then having roll centers below ground level only serve to increase the length of this “leverage arm.”

Accelerator

That is the way I understand it also, hence the smaller roll moment as the RC gets closer to the CG. I'm trying to understand the tradeoffs of a lower vs. higher roll center and how it relates to the best overall handling characteristics for the car. To raise my roll centers I will have to raise the ride height of the car. This in turn raises the CG. Is it better to have the lower CG and RC's and deal with the increased roll moment via stiffer springs or raise the RC which raises CG and deal with the increased jacking forces in cornering and effects of a higher CG?

Trucho-951

I guess your question then really comes down to, “which is the worst of two evils: 1) reduction in overall traction due to “jacking effect” i.e. lifting both inside wheels, or 2) reduction in overall traction due to excessive chassis roll (i.e. too much weight transfer on a given pair of wheels).”

I’d say the jacking effect is worse, so in your case, lower CG is better.