[fmorelli]

The E46 rear end (which is the designed carried over to the E89 Z4 rear) is known for moving around at the Rear Trailing Arm Bushing. There is a fair bit of space from the sides of the bushing to the cage in which it is mounted. You can read about this exhaustively on E46 forums. The standard solution in E46-land is to use E46 M3 RTAB bushings with limiters. One could leave the factory Z4 RTABs but E46 limiters (there are no Z4 limiters) are too thick, as the E89 RTABs are 4mm wider than the E46. I don't know that the Z4 RTAB is any less dense, rubber-wise, but the change to E46 M3 RTAB is done mostly to support using the E46 limiters. This helps keep the rear trailing arms from deflecting under side load.

Hope this helps,

Filippo