Since this thread starts off talking about bump stops, I thought I'd share my research on this topic with the community.
While its been well documented that the E9x M3 stock suspension engages the bumpstops after just a small travel, I hadn't found any hard data on the bumpstop spring rate or what the force-compression curve looks like. For the hard core track folks who's stiff springs keep them mostly off the bumpstops this won't matter so much. But for those of us considering tweaking our street cars to still be primarily street cars, knowing the bumpstop rates helps us understand the stock suspension baseline better.
I bought a fresh new E92 front bumpstop to measure it (and not be thrown off by tired 10-year-old foam). My measurement system was a hydraulic press, a dial indicator, and a bathroom scale. The bathroom scale tops out at 350lb so that's the limit of my data but that turned out to be enough. To get the bumpstop to behave properly the center needs to be supported with a shaft as it would be on the car (otherwise the bumpstop folds over), so I fashioned a support shaft, then a top plate with a hole for the shaft to pass through and a larger collar to press down on the top plate to compress the whole thing.
I took two passes up and down in travel. I had a lot of stiction between the shaft in the center and the inner surface of the bumpstop which caused the offset in force for one of the passes. But I show all the data including this because the slopes are consistent despite the offset. Here's the data
I was actually a bit surprised to see the initial rate is so low at about 100 lb/in. Then after about an inch of travel the bumstop starts to stiffen up a lot, reaching about 700 lb/in by 1.5" of compression
From this data I'd say the stock bumpstop makes an effective progressive spring, the first half-inch or so (on my car it measures a bit over 3/4" but others including Steve Dinan reported only 1/2") of travel without engaging the stop is about 160 lb/in (stock front springs), then the next inch of travel adds 100lb/in for total 260 lb/in rate, and after that the stop starts to get rigid and becomes truly a "stop".
I also acquired an E36 bump stop to compare. The stop is slightly shorter as others have reported, but the initial rate is very similar. The transition to the stiff 700 lb/in rate occurs a couple tenths of an inch sooner, similar to the overall shorter length of the E6 vs E92 bumpstop. These characteristics make it a good option if you're looking to retain the stock bumpstop behavior but need it slightly shorter.
