I'm getting ready to install shocks, springs, and M3 rear control arms on my 328i. When I'm done, I'll have M3 spring rates, M3 shock valving, plus sway bars and lower control arms from the M3. My car is 10% lighter than the M3, so it won't be exactly the same. Plus, my spring rates are a little lower (3-5% lower).
It might look like I have some kind of fetish for the M3 -- and that may be true! -- but the thing is, the M3's suspension setup is a convenient target for a complete system of parts that were all designed to work together. I'm especially thinking of springs, sway bars, and shocks.
I have several goals, which are: A) Eliminate the unsettled behavior over large bumps, especially on the freeway. B) Eliminate oscillation after a bump. C) Achieve a neutral steer car. D) Improve steering feel. E) Make the car enjoyable to drive!
In order to do all this, I'm raising the car up a little, on the theory that part of the bounciness (A & B) is caused by compressing the bump stops too much. I'm increasing travel by 1) raising the car, 2) shortening bump stops, and 3) using shorter mounts.
I'm selecting spring rates that are more or less the same as the M3. Since the weight is off by 10%, and the springs are off by 5%, the damping coefficient is off by 7.5%. That's for damping body oscillations.
The other thing about the spring rates I wanted were to keep the "flat ride" characteristic, and make a "balanced" neutral steer car. My spring rates are an estimated 156 in front, and 525 in rear. My targets were the M3 rates, which are 160 and 550, respectively. (Those M3 numbers are for the coupe.)
To make the rear a little easier I decided to convert to the M3 camber link. This lets me use an original M3 shock, or an OEM replacement. I couldn't get a used M3 rear shock cheap, so I just got a Bilstein HD shock. To round out what I'm doing in the rear: Hyperco 525# racing spring plus a height adjuster made by AST.
I'll say more soon, but in the meantime, here's a photograph of the rear to hold your attention for a while.
