Because of poor design and extensive abuse, stock BMW rear shock mounts mounts often fail with rubber deteriorating which then allows the inner metal sleeve to start moving (or eventually separate from the rubber). Usually, this manifests itself as squeaking, rattling, clunking and deflection of the rear shock mount, leading to poor suspension performance and a great deal of annoyance. Replacing the RSM's with OEM units provides temporary relief, but invariably they fail again, usually in 20K miles or so, but often MUCH less when aftermarket high performance shocks are used.
What about those "heavy doodie" factory style mounts?
There has been the introduction of a factory style mount touted as "heavy duty", "taper-core", "ultimate", "stage 1" and many more marketing names. This revised version of a factory mount addresses a single issue (of many) problems with factory style mounts.
A rear shock typically exerts a load (F) on the rear shock mount. The claim being made by these "ultimate doodie" mounts is that the redesigned center steel core with "wings" (indicated by the blue arrows) prevent the center core from tearing, separating and falling through the bottom of the rubber mount. The problem which still exists, when the rubber deteriorates and fails. When cracking/tearing rubber begins at the top (highlighted in yellow), then entire rubber assembly (with very secure steel core!) comes down with it all.
Like factory mounts, these revised versions are not serviceable and only will only eventually take up room in the local garbage can when they fail. It's black-painted, stamped steel housing cannot resist corrosion the same way aluminum and 303 stainless steel.
Another shortcoming of these mounts is that they are not made for 12mm diameter rear shocks (which is the size of many aftermarket, performance shocks). Then again, why would anyone want to put this on a performance shock? These mounts are really for those looking for a cheap, inexpensive fix for a car they no longer plan on driving in a performance capacity (preparing to sell, trading into a dealer, etc.).
Rogue Engineering Rear Shock Mount
Our rear shock machined from 6061-T6 aluminum, we are able to attain a more stable and stiffer platform which to mount the top of the rear shock. However, rather than use race-only spherical bearings or even overly stiff urethane bushings, we specify Shore A 60 hardness rubber bushings (approximately 30% stiffer than the factory bushings) to allow for street comfort and compliance. Our findings show that overly stiff urethane bushings do not allow the rear shock to move, making the attachment of the lower part of the shock under strain.