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The third post in this series looks at the challenges of the M3 rear suspension.
The rear suspension has different challenges than the front: As with the front, the static (sitting there waiting to go) compression is 4 inches (100mm) at the wheel.
The rear suspension moves the damper at 83% of the rate at the wheel (damper motion ratio is 83%). The spring motion ratio is 57% because the spring is mounted inboard of the damper, and not concentric with it (a coil-over design) as it is in other BMW vehicle series like the E39 5-series, for instance.
At this point, I haven’t done the full motion analysis of the rear suspension, but some problems with the factory design are evident. The full analysis will (probably) show that the rear suspension has more travel than the front, but that the spring design is a serious challenge for high-rate springs.
The reason I say the rear suspension is a challenge from a spring rate perspective is that the rear wheel-rate is about one-third of the actual spring rate. The stock M3 rear spring has a rate of about 600 pounds per inch, giving a wheel-rate of 200 pounds per inch. Combined with the front rate of 167, the rear rate provides a good frequency balance. The car rides very well, if a little stiff.
Now, the rear spring is mounted between the rear suspension “Camber link” and the body, and it sits just outboard of half-way between the two ends of the link. It’s a teeter-totter setup that tries to pry the rear subframe DOWN and AWAY from the body when the wheel rises and the spring compresses. What’s worse, the higher the rear spring rate, the more prying force is applied to the subframe when you go over a bump. No wonder E36 and E46 rear suspensions have had problems with body damage – every little ripple in the road is another attempt to pry the subframe off the body.
To prevent damage to the bodywork, it’s necessary to limit the rear spring rate. Later, in discussions of Type Three and Four suspensions, I’ll talk about how this is solved, but for the moment, take it as given that rear spring rates that are significantly higher than stock are a bad thing.
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