Quote:
Originally Posted by Angel67
I would also note that so far I quite like my setup which uses 3 Group N bushings - one each in the toe and trailing arms, plus one on the inboard side of the lower camber arm (the rest being balljoints except for the normal bushings on the outboard side of the M3 upper arms). Since the Group N bushings are very stiff, they provide not only additional spring on compression but also additional rebound, below normal or static height. Above static height, they work against the metal spring, pulling the suspension back to static height. I have a 521 lb spring and say about 180 lb equivalent spring rate from all the bushings. So on compression, below static height, together they add up to about 700 lb, but above static height the effective spring becomes around 340 lb. This helps the sway bar do its job and the car levels very quickly...
So there is a reason besides rules to use Group N...
a
Stay tuned
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The rules of springs in parallel say that you add spring rates together, so the total spring rate is the steel spring rate plus the sum of all the rubber spring rates. That is true at all compression or extension positions. The fact that the rubber is at zero force at static ride height and the steel spring is compressed does cause some confusion, but doesn't change that rule. It just changes the ride height at which the total spring force would be zero - a position that generally will be beyond the maximum droop permitted by the shock.
Stiff, short rear springs could potentially come unseated under full droop as set by the shock. Potentially rubber bushings could keep the springs seated by preventing the suspension from getting to this mechanical limit.