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      12-28-2012, 04:53 PM   #6
exdos
Second Lieutenant
England
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Drives: Z3 M Coupe(S54) and Z4 M Coupe
Join Date: Apr 2008
Location: UK

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The Z4MC's suspension is too soft at the front which allows pitching on braking and lifting on acceleration, which makes the steering floaty and imprecise, and at the rear it's too stiff, which makes the ride very uncomfortable. The 3 centre coils of the rear springs are 19mm thick whereas the front springs are just 11mm thick: obviously there's a huge mismatch between front and rear spring rates, yet BMW cars supposedly have a weigh balance F:R of 50:50.

I've fitted AC Schnitzer Racing suspension (basically Bilstein PSS9 made to ACS's specification), which has rear springs of 16mm and front springs of 14mm, which redresses the mismatch of F:R spring rates. Instantly, this helps to overcome the inherent ride and handling problems of the Z4MC with OEM suspension. The adjustable dampers then permit the fine tuning of the spring oscillation rate so that for street use there is sufficient damping to make the car sufficiently compliant on uneven and bumpy surfaces, yet by simply adjusting the rebound/compression on the dampers, the oscillation rate of the springs can be increased so that the suspension is far more reactive for track use.

The fact the spring rates of the F&R suspension of the ACS suspension is a much closer match than OEM, means that the load-shifting caused by braking and acceleration is reduced, irrespective of damper adjustment, which in turn, translates into better handling. In the case of the Z4MC, the softer rear springs give a great ride with no loss to handling.

I also have a Z3MC to which I fitted KW V3 (with genuine coilovers at the front) and with OEM suspension the Z3MC is way too soft with excessive bodyroll, and KW V3 and H&R ARBs have transformed the handling of that car, and with damper adjustment the car can ride like a sedan or be suitably stiffened to give awesome track handling without a rock hard ride.

Yes, you can have a suspension that can give you the best of all worlds, BUT, you've got to make the damper adjustments to make the car perform optimally on the street and then make further adjustments to make the car handle optimally on the track. IMO, it's no big deal to make the changes when required and no different than getting dressed in different clothes to do specific jobs.

With regard to KW V3, there is separate adjustment for rebound (at the top of the dampers) and bump/compression (at the bottom of the dampers). This adjustment is easy to do on the front dampers, but on the rear dampers, this is much more tricky, especially for bump/compression. I like being able to make separate damper adjustments but if you don't intend going on track much, then the single adjustment of rebound and bump/compression, as used on the ACS Racing suspension works well too.

In the case of the Z4MC, the ACS suspension lowers the rear end by a fixed 10mm and I've lowered the front by 20mm to increase the rake angle. I wouldn't want to go any lower.

Hope this helps.
Appreciate 0