[Wendall]

Originally Posted by M3SQRD

I think your calculations are not correct.

The wheel rate = spring rate*motion ratio^2
The stock motion ratios are:
Front coilover ~0.96
Rear non-coilover ~0.58

A commonly used e9x M3 dual-purpose (street-track) setup uses 500 lbf/in F and 800 lbf/in R spring rates which converts to F and R wheel rates of ~460 lbf/in and ~270 lbf/in, respectively.

Assuming the rear "coilover" motion ratio is the same as the front coilover motion ratio (0.96, assumption; the actual value is probably between 0.7-0.85?) the rear "coilover" spring rate would be...spring rate = wheel rate / motion ratio^2 - assuming you want to match the commonly used non-coilover R setup of 800 lbf/in, the R "coilover" spring rate would be ~293 lbf/in.

I'm not going to factor in effective spring rates from the F and R sway bar torsional stiffness because they are fixed (only one end link mounting hole per bar end; not everyone is using adjustable F & R sway bars) regardless of whether you run a rear non-coilover or coilover setup. However, I will say that one of the most important things that's commonly neglected when it comes to sway bars on lowered cars is the proper re-adjustment of the sway bars, using adjustable threaded end links, to prevent the sway bars from binding - very important on the F bars because the stock sway bar mounting bushings are extremely tight (i.e., no clearance).

Very good info. Thanks!