Epilog: Rear Trailing Arm Bushing Replacement

[jkerouac@xxxxxxxxxxxxx]

Thanks for the tips and comments I received on Rear Trailing Arm Bushing 
replacement.

     New OEM bushings don't flex sideway into the mounting cup but they 
aren't designed to resist a side load.  The side swagger sufficient for 
the rubber to hit the insides of the mounting cup started by 20k miles 
or less.  What kills them is the side flex fatigues the inner 'folded' 
section around the metal sleeve, and then the tearing begins.  The 
Ground Control shims prevent the side flexing from happening.  The 
difference in rear suspension response from old bushings to new is 
similar to replacing worn out rear subframe bushings on an old 2002 or 
E21/E30 with new ones, like jogging on the beach versus running on the 
sidewalk.  I don't think the shims help handling so much as prevent the 
degradation of OEM bushings.
     One idea I thought while doing the job yesterday is a possible 
scenario that leads to the bolt sleeves cracking out of the floor where 
they are welded.  This happens when bad bushings are not replaced.  It 
happened on my E36 when one of the original bushings suddenly broke at 
28k miles.  BMW replaced the bushings and repaired the damaged sleeve 
welds under warranty.  If I had had nonstock springs or bars, however, 
they said they would not have.
     The welded sleeve assembly may be strong enough for any load 
cornering the car could reasonably generate.  But if the bushing is 
flexing so drastically, the front of the trailing arm may make a nearly 
undamped impact sideways into the cup.  The iimpact would place more 
sudden load on the sleeve welds than any cornering force that could be 
transmitted by a nondamaged bushing which would be dampening the side  
movement.
     BMW has never come out and said why these sleeve welds sometimes 
fail, so this scenario could be correct.
     Btw, my torque wrench broke putting the wheels back on.  The real 
problem with my torque wrench is it brand.  Craftsman give a warranty 
and it never lost calibration, but if I was a Snap On, it would not have 
broken.
     One guess what my new torque wrnch will be?
     BBtw I took the car for a ride late last night to check out the new 
bushings, found no one in sight on a smooth stretch of the 101 north 
from San Jose so got the car up to 151 (indicated).  By how it was still 
pulling, tops would be at least 155 but I'm not going to try again 
anytime soon.  The ellipsoid headlights with 100 watt H1 bulbs gave 
plenty of visibility even going that fast, but there was traffic ahead 
so I had to slow down.  Slowing 150 to eighty on medium hard braking was 
stable, no side wiggle.  With the old bushings, the rear was noticeably 
less surefooted on braking much over 100 mph.
Onward,

'jk

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