Listers,
I sent the email below to Dave and Alan, and also copied the list. But it
didn't show up yet on the list, so I am sending it again...
Chris
Message:
Dave & Alan,
The urethane bushings have different mechanics compared to the original
bonded rubber bushings. The latter have a rubber "outer bush" bonded to an
inner "metal bush" through which the bolt passes. When you tighten the
bolts, the flanges on the brackets press against the inner metal bush, thus
preventing rotation of the inner metal bush. Because the rubber bush is a
press fit, the outer rubber bush is held fast against the trailing arm at
its outer boundary. When the suspension moves, the bush accommodates the
movement by the rubber outer bush twisting against the inner metal bush to
which it is bonded. This is why the trailing arm brackets must be tightened
only when the approximate weight of two people is in the car. This sets the
bushing at a "non-deflected" state with the car loaded, and allows for
twisting of the bush for both up and down suspension movement.
The urethane bushes work in a different way. The urethane bush is not
bonded to an inner metal bush -- the metal tube is simply inserted into it,
and the urethane bush is free to rotate around it. As with the rubber
bonded bush, the inner metal tube is crushed and held firm when the brackets
are tightened. The outer boundary of the urethane bush is also tight
against the trailing arm. To accommodate suspension movement, the urethane
does not "twist" like the rubber bush. Instead, it rotates around the inner
metal tube. Some urethane bushings don't even have the inner metal tube --
the urethane bush rotates around the bolt itself (this is often found to be
the case for front wishbone bushings).
Because the urethane is rotating against the inner metal tube, there can be
"stiction" if there is not sufficient lube between the urethane and metal.
This can cause the infamous "squeaking" characteristic of urethane bushings.
So, the metal sleeve need to be lubed on the outside, or the urethane bush
needs to be lubed on the inside, or both. There are some urethane bushing
kits out there that have drilled securing bolts, with a small hole then
drilled into the side. There is a corresponding hole in the inner metal
bush tube, and the bolt is fitted with a zerk. This allows greasing of the
urethane/metal bush interface, which is otherwise impossible.
So, in addition to using anti-seize on the bolt (to allow the whole thing to
be disassembled later), make sure you put plenty of lube into the center of
the urethane and also on the outside of the inner metal tube.
Chris
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