What's the theory behind suspension arm bushes?

[minimoog]

Consider a standard trailing rear suspension link with a rubber or urethane bush at each end. The link is mounted on the chassis/body at one end and a live axle at the other. There is a tube or sleeve in the middle of the bush through which passes a bolt. This bolt secures the bush and therefore the link to the axle or the chassis via a U-shaped bracket.

Question: should the central tube within the bush be clamped ridgidly to the U-bracket by the nut & bolt, thereby restricting the tube from turning and imparting torsion through the bush as the arm pivots? Or should the tube (and therefore the whole link) be free to rotate around the bolt, allowing the bush only to resist twisting of the link?

Or does it 'depend'?

Enquiring minds wish to know...

[Avocet]

My understanding is that the bolt should be tight. A polyurethane bush seems to work differently to a rubber one though. The PU bushes seem to be free to slide around the inner sleeve. In fact, the ones I recently fitted to my car came with a sachet of grease for the purpose. I think the idea is that the inner sleeve presents a bigger bearing area to the inside of the bush than the bolt would have done so the wear on the PU isn't as rapid.

Rubber bushes are generally bonded to their inner sleeves and are supposed to "wind up" with suspension movement. I'm told one should always tighten them after the car's weight has been "settled" on to its suspension and is at normal ride height rather than when the job is finished and the car is still on the jack with the wheels dangling at full rebound.

Leaving the bolts loose will probably only lead to wear on the bolt and / or suspension brackets and (possibly) slightly wayward handling!

[Mr2Mike]

The steel tube in the middle of the bush should be well clamped by the securing bolt in order to prevent rotation. This applies to both metalastic and polyurethane bushes. Allowing the tube to rotate on the bolt will cause excessive wear in short order and lead to wayward handing and lots of knocking from the suspension.

Poly bushes should always be greased as well; they work ok without grease but they tend to squeak very annoyingly as the suspension moves.

[minimoog]

Thank you both.

[dickkark]

set the hubs at running height i.e. as if it was sitting on it`s wheels, before tightening or they will fail prematurely.

[Mr2Mike]

dickkark said:

set the hubs at running height i.e. as if it was sitting on it`s wheels, before tightening or they will fail prematurely.

That only applies to metalastic bushes, where tightening the bolt before lowering the car excessively stresses the rubber. With a polybush the inner is not bonded to the polyurethane and is free to rotate so can be safely tightened with the suspension as full droop.