my thoughts too, its only on certain bumps like speed bumps that seem to cause it.

At least from what I can see from the design of the bush it seems more of an issue of the suspension design. As all of the lateral arms allow rotation in more than one direction, the job of constraining the rear Hub's rotation around that axis is also one by the trailing arm, and as the arm is only fixed at on place this also doesn't fully constrain it. that is in my mind why there is a flat plate on the trailing arm where it sits on the bush to stop it from 'complying' left and right as it were. this naturally over time wears down the rubber it sits on till it reaches the metal.

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