[cvc 22349a]

Originally Posted by luckyu

Hi CVC! I don't think anyone knows for sure exactly why, except for each individual vendor. However I will try to answer anyway; I hope I'm not adding to the noise...

I think vendors don't triple the rear springrate because they can't. That is, they can't raise both the front rate 3X and the rear rate 3X. Because, the way the rear spring is positioned it would pry the subframe away from the body instead of behaving like a spring.

If the rear were a coil over spring design, like the E39, then we would see vendor wheelrates that are: 1) BMW-like, with slightly higher rear, 2) the same front to rear, and 3) slightly higher in front.

It may be possible to reduce understeer on a 400#/800# car by using long bump stops in the rear. Add 150# at the rear shock to the 800# spring and you're at 360# rear wheel rate. You've "added" to the spring. I think this is what BMW is doing with the non-M cars.

Motion ratio is easy to understand intuitively, it's just leverage. Put a spring in a pair of hedge clippers, and the spring is easier to compress depending where you put it.

Motion ratio is also easy to calculate. Divide rear springrate by 3 and you have the approximate wheel rate.

I didn't mean increasing the front spring rate x3; just tripling the rear rate from the front as BMW has w/the 135, 335, e9X M3.
Examples of doubling the rear spring rate from the front instead of triplling it can include KW's street comfort c/o 286 lb F/573 lb R or HPA's c/o kits with 336 lb F/672 lb R.
In trying to make sense of what 'Orb' has written; if the rear spring rate isn't tripled from the front, will it further promote understeer and the car won't settle at the same time front to back when going over a bump? In the second example, wouldn't a 224 lb F/ 672 lb R spring set up be more congruent with the way the car was engineered?