By mounting the mustache bar solidly you are moving the majority of the loads into the mustache bar mounts and the rear diff cover. Depending on power and tire size/grip you can start cracking in those areas.
The ZX and 510 semi trailing arm suspension simply makes the long bar that the control arms and diff nose attach to, into the equivalent of the S30 mustache bar, it just happens to be at the front of the diff. Its mounted to the chassis the same way our M-bar is, with bushings on a body pin. Same same, not odd, just kinda backward. ZX ties the suspension hanger and M-bar together, rather elegant I thought.
The point of these things, either kind, is to use a long bar to transmit the twisting diff torque to the body. The other end, nose on s30, and tail on zx/510 just need to be held steady from lifting/dropping, so need a simpler lighter mount. The 510 actually has a little m-bar on the back of the diff, total overkill, but that's why they work so well... Oh right, Z forum, back on track, sorry...
The bushings at the ends of the Mbar are for noise isolation and vibration damping and to remove the stress related solid mount failures that John alludes to.
Edited by z240, 21 May 2015 - 12:47 PM.