The ultimate front suspension geometry guide for BMW E30/E36/E46 by MRT Engineering, page 4
Chapter 2: E36 front suspension
In this chapter we take a look at E36 chassis suspension setups, as in the previous chapter we start with stock setups, lowered to the same 150mm ground clearance from steering rack mounts bottom plane. We'd recommend to read through the E30 chapter to understand general rules that mostly apply in all similar setups.
E36 non-M front suspension geometry:
E36 non-M front suspension geometry (pdf)
First observation you make is the very gentle caster value at 4°, this is 5° reduction compared to E30 non-M value.
Caster trail and scrub radius values are neutral as we'd expect on a standard car.
Since subframes on a E30 and E36 share the same base geometry and we're using the same fixed ride height, bigger wheel diameter causes worse roll-center height because of control arms' upwards orientation.
Tie rod - LCA inner and outer height differences follow the same rules learned before and we can expect this will result in similar bump steer curves seen on a E30 chassis.
Results indeed were as expected and not bad at all considering bump steer. As a rough conversion from degrees to millimetres we can multiply by 10 and as the results are presented per wheel we need to multiply by 2 to get total toe.
Camber curve is quite flat with only 1.27° of camber gain during 100mm sweep, with short travel coilovers on a circuit use this is far from ideal.
Next we move on to M3 3.0 model suspension which is kind of a derivation of non-M parts. Common myth that these would have used the same spindle geometry is wrong, both control arm and tie rod attachment points are different. Control arm geometry is the same on both E36 non-M and E36 M3 3.0, but the latter used solid outer ends instead of rubber isolated ones and in fact it is the same exact control arm as on E30 despite a different part number.
E36 M3 3.0 front suspension geometry:
E36 M3 3.0 front suspension geometry (pdf)
As M3 3.0 used eccentric FCAB and offset top mounts caster has now increased from 4° to 7° which also increased caster trail, due to FCAB and spindle design wheel position is moved 11mm forward.
Tie rod - LCA height ratio remains approximately the same as on non-M and likely results in similar toe change.
Note that M3 3.0 has more spindle height which improved roll-center by almost 20mm.
Increased spindle height obviously results in better camber gain and small change in geometry also flattened bump steer curve a bit.
We still have one E36 stock suspension setup to be analyzed which is the improved E36 M3 3.2 model also known as EVO.
EVO had more changes on suspension than the first M3 variation which included different LCA, revised spindle, offset top mounts for more caster and less KPI, this setup also changed back to concentric FCAB since increased caster was already built in the control arms. It should be noted that control arms are not just made for more caster but they are slightly narrower to decrease KPI even further.
E36 M3 3.2 front suspension geometry:
E36 M3 3.2 front suspension geometry (pdf)
Caster has increased with changes described above to 8° which naturally alters wheel position and caster trail even further.
Surprisingly roll-center height is worse than on a M3 3.0 which is caused by less spindle height and also by a shorter control arm.
Tie rod - LCA height ratios have been extremely close to each other on all assemblies, which gives some rough baseline, but let's see what these other changes have meant for bump steer and camber curves:
Bump steer is neutral through most of the suspension travel, but short control arm in addition of bad roll-center causes sudden sag of the curve and will also result in bland camber curve.
As a conclusion all setups have their pros and cons and admittedly room for improvement. M3 geometry, either 3.0 or 3.2, is the way to go on circuit racing applications due to higher caster and better bump steer curves.
If we look at proven racing suspension concepts, such as E30 M3 Gr.A DTM front suspension, we could opt for even more caster and less caster trail, not to forget more aggressive camber curve.
In the next chapter we take a look how E36 non-M front suspension can be improved using components borrowed from M3 models and how these setups respond to added MRT Engineering geometry correction bits.