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      12-25-2012, 10:38 AM   #18
M3SQRD's Avatar

Drives: E92 M3,R56 S,F22 240iX,F82 M4
Join Date: Jun 2009
Location: EC

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Originally Posted by italyix View Post
^^i think you can, I calculated the rear springs would need to be 600lb if using a coil over setup, so they would match the fronts of 500lb. The equivalent oem style rear spring would have to be 1200lb to match the front. At 600lb I do not thing the rear towers would give in, seeing how the gts runs a rear coil over. If you then factor in sway bars, in my case RD sport, I calculated again the front would need to be at hole 1, and the rear would have to be at hole 3, to match spring rates and effectively giving, at 2" of deflection or compression, a total of 1200lb spring. I hope this makes sense :/
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 (see below for R rate based on the correct R damper motion ratio).

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).

UPDATE: found out the rear damper motion ratio is 0.83. This means if you wanted to match the stock rear spring rate of 800 lbf/in used in the example above in a rear "coilover" setup, the "coilover" spring rate would need be ~392 lbf/in.

Last edited by M3SQRD; 12-25-2012 at 07:15 PM.