Front spring selection
I want to post the process I used to select the front spring. I have a factory style strut with no height adjustment, so I have to work from the springs that are available. Here were my goals:
1) I wanted a spring rate as close as possible to 160#.
2) I wanted the ride height to be between 14.4" and 14.8". This is a little high, but I have my reasons.
So I looked over various BMW models and estimated their spring rates. If the 335i sport spring is 145#, then with a little algebra I can estimate what the 335d sport spring rate must be (assuming it is the same length -- which it is). I get 156# for the 335d spring. In reality I think it's a little lower, but still, that's pretty close to 160#.
So what's the ride height? If I take the 335d sport front spring and put it on my car, it will deflect less, because my car weighs less. I can take the difference in corner weights (335d versus 328i), divide by 156 lbs/in, and get the difference in deflection, which is 0.9". So this spring will jack up my car by 0.9".
That's 0.9" added to the 335d sport's nominal height, which is unknown but it's probably around 14.25".
That's too high. So, to bring it down I was originally planning to use the SP lowering perch, which lowers 0.4". This gives 14.75".
Ultimately I found a better way to bring down the height, but this gives an idea how to choose a front spring.
2007 328i ZSP. M3 suspension
: custom valved Bilstein shocks, Hyperco race springs, M3 lower control arms front and rear, M3 sway bars, and M3 subframe bushings. E46 front guide supports. Euro tail lights.