What I'm wondering is in terms of spring rates for front and rear. Lets say you have a choice 10kg/mm front and 6kg/mm rear or 16kg/mm front and 12kg/mm rear (this is purely for example sake by the way). Now I know there is a 4kg/mm difference between the two so I would expect the same balance, but besides being stiffer is the car actually going to behave differently even though the front and rear ratios are the same?

 

Yes, I believe that Spring Stiffness alone will make a difference in the way a car handles, but the question I would also like to know is whether your chosen rates are really the same ratio(F/R)... if you do the math. Not to take away from your question though.

 

it will change the handling for sure, but after all anything is ok, if it feels good for you. some people like stiff stiff springs, while some like very soft springs. its all up to the driver i believe

 

in general, smoother drivers prefer stiffer setups and 'jerky' drivers prefer softer setups. This goes for chassis rigidity also...

 

But the tendency of the car to over/understeering will remain the same as long as the ratio is equal correct?

 

Welp, I am by no means an expert on this, but I might be able to help you ask a better question. In the examples you provide above, you're comparing springs rates (F/R) of 10k & 6k, and 16k & 12k, both of which have the front rates 4k higher than the back. However, the ratio (or bias) of these two scenarios is different. With the 10k/6k scenario, the front rates are about 67% stiffer than the rears, and with the 16k/12k scenario, the front rates are about 33% stiffer than the rears. The end result between these two setups would likely yield two very different handling characteristics for the same car.

The question I thought you were asking is if you kept the same bias or ratio between front and rear spring rates (for the sake of example, let's go with the NA proclivity for stiffer rear springs) and you were to change from (F/R, in #s) 300/400 to 600/800 (or 450/600 to 675/900), would the balance stay the same? To me, that's a great question, but one where my very limited understanding physics and engineering leaves me flat on my face. With any luck, we'll get Scott (RR98ITR) to weigh in on this one.

 

Without going too far into it, and because it's all front high rate, I'll just say that the characteristics won't differ enough to be greatly concerned. 900lbs on the front of a street car doesn't seem like the best idea to me, but then I like keeping the tires on the ground most of the time.

In the general case with a rear high rate bias a change in bias (at the same total roll stiffness) will affect the transients based on the relative speed with which the inside rear unloads.

Most of the literature makes reference to the specific front/rear roll stiffness distribution and it's preservation (if optimal) when changing rates for other reasons. Other considerations (mostly for street cars comfort) are maintaining a difference in natural frequency (F/R), and conjugacy (don't ask).

Roll stiffness total/bias changes effects on the terminal cambers on each corner can have a huge effect on balance. Remember, this is a System - there really is no such thing as changing one thing at a time.

Scott, who suggests that there's no short answer....if you want the answer give yourself a BS in Race Car Engineering or hire a real race shop to set up your car....


[Modified by RR98ITR, 9:50 AM 10/4/2002]