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      12-15-2020, 05:01 PM   #19
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Drives: 2009 E92 335i 6MT
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Originally Posted by Tambohamilton View Post
It's actually completely standard in passenger cars, using a long bump stop sort of as a tender spring. It allows the boffins to inexpensively create a progressive (rate increases with bump) wheel rate, which allows a better (or at least, tuned/tunable) balance between comfort and body control. But it also hugely affects 'magic number', which is the ratio of front roll rate to total roll rate... Most manufacturers will use it so that in docile driving (low speed cornering) the car has very neutral handling, but the harder you turn (and therefore the greater the body roll) the higher the magic number, which results in greater understeer. So it gives the effect of very predictable, progressive understeer.

But yeah, disintegrating bump stops will make your car feel like crap!
This is all new to me... the way I'm used to seeing this effect done is with a progressive rate spring where either the spring is wound, or two separate springs are used (usually in a coilover setup), where at ride high the "soft" section is either at or near coil bind and then the 2-3 coils of the stiff spring take over. Then at droop, the soft spring keeps the springs from unseating and ruining everything.

It appears that instead on an E9x the entire spring is "soft" and the bump stop is used instead. Granted I'm way behind on current suspension thinking, but I think for longevity in daily use I would prefer the older setup that I describe above...

Also, what this says to me is that an E9x even unlowered only has maybe 1.5" of compression travel?