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I can see the ride nicer part, but i dont really see the flex better. If ya think about it, rubber is alot stickier than poly, so its gonna grip that center bolt, its gonna grab the spring eye, and its gonna sorta twist.
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The issue is not during normal flat ground suspension movement. When you raise one tire higher than the other (on the same axle) something has to give.
Imagine how the axles are held in place with the springs: rubber bushing pressed into the spring, with a metal sleeve bonded to the bushing. Metal bolt passing through the metal sleeve holding the spring to the shackle. During normal suspension travel, as long as that bolt can rotate inside the sleeve, no problem. (and on trucks that are driven, I've never seen the bolt "locked" in place. Hard to remove yes, hard to rotate no) But when you raise one tire higher on one side than the other on the same axle, something has to give. Rubber will allow that spring to actually twist somewhat, which allows the bolt to continue to rotate "normally".
With poly, you are limiting the amount of spring angle that bolt can take before you put a *lot* of force on it and the corresponding sleeve. No matter if its greasable or not, that has nothing to do with the fact that if the bushing material won't deflect, you've either stopped the flex, or its transmitted to the next weakest spot. (rivets, shackles, etc) That's my suspicion as to why the early poly ones fell apart.
Even 4 link rear suspensions are this way...the angle and load is never "perfect", and ends up binding. Plenty of people running poly in the rears have noticed that the bushings "stick" in place and the ride height varies because of that.