| Commonly misunderstood problem, you would need to understand there are 2 types of rotation required of tension rod bushings: 
You see 2 rotational axis (black lines). The main swivelling movement (red) is perfectly fine with urethane bushings. The problem is with the twisting movement (blue) is not really designed into the ES bushing. This movement is relatively small (vs the swivelling), and comes from changes in the angle of the lower arm as the suspension moves up and down. The urethane bushings do have some amount of give, but try it by hand and you can tell it is difficult with urethane bushings. Stock bushings are hollow filled with silicone, so they accomodate this movement much easier. Over time the lack of "twist" will stress the sheet metal of the lower arm where the tension rods bolt to and cause it to crack. How quickly this occurs depends on how much the suspension travels (racing will create this problem quicker than street), how tightly torqued the tension rod is to the lower arm, etc. Here is some discussion from a 240SX forum (they use the same tension rods/lower arm configuration as the Z32): [ http://forums.freshalloy.com/ubbthreads/showflat.php?Cat=&Board=UBB27&Number=67374621 ]
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