I've seen several people post that poly could be bad for a PHR because of bind. I don't understand.

I can see why LCA bushings pointing across the car need the ability to twist over a one wheel bump. I cannot see why the PHR bushings, pointing along the axis of the car, would, in any suspension motion.

Isn't the driveshaft splined so the rear axle doesn't have to move in an arc? Isn't the torque arm supposed to resist twisting the rear axle? Is there any good reason why the rear axle _should_ twist? Or does a little unavoidable twist in the rear axle bind the PHR bushings and prevent the PHR from moving freely? Is that important?

Can someone explain this one? Should I swap out my BMR PHR with poly? Thanks.

 

No need to swap out the BMR piece.

The motions that cylindrical PHB bushings don't particularly like are pinion angle change (as the rear suspension rises and falls, the axle centerline follows an arc as seen in side view) and axle steer (as the car rolls the axle ends move fore/aft different amounts/directions, also present in side view when you consider how both of the LCA's are moving). Although neither of these rotations can entirely be eliminated, they are relatively small. Consequently the magnitude of PHB bind is small, and I'd venture to say the consequences are largely academic. The PHB shear and bending loads that are developed as a result of axle steer would affect rear roll stiffness minutely at most. And, if anything, the PHB torsional load that occurs as the rear squats would give you a tiny bit more pinion angle control (assuming that you have less than 100% anti-squat).

Norm