re: //M3 rear suspension "thunk"

[jkerouac@xxxxxxx]

Vitaliy,
    The description you give including the difficult diagnosis fits to a
't' the thunk I was having from my E36.
    This is one time I would love to be totally wrong, since the fix is
not pretty.
What the problem is is the rear swaybar link hitting into the gap
between the two U shaped segments of the lower locating link.  This hits
when the suspension compresses a bit, like a small bump or the lip or a
driveway apron that is a 1/2 to 1 inch drop to the roadway.
    Finding the problem was easy, then I found the dealers were all
useless, wanting to demand running up diagnostic hours instead of doing
what I said would easily show what is wrong.
    To find the thunk:
jack up car
remove wheel
unbolt bottom of shock
remove the spring
remove the nut at the bottom of the opposite side sway bar link
Next:
    If on a lift, use a screwjack, if jacking up at home, use wood
placed under the tire
    either way, raise the opposite wheel back up to normal ride height,
    as it would sit if the car was on the ground.
Now wearing leather gloves and with both hands, get a good grip on the
brake rotor.
Visually stare at the lower link arm and the sway bar link above it.
See the tight clearance?
By hand, raise the wheel so the suspension goes up through its range of
movement.
you will see as you go up, the sway bar link contacting the lower drag
link, then sliding into the gap between the two U sections.
That's your "thunk"

How did it happen?
The lower drag link one way or another got bent upward.
Some ways that happens:
Tow driver used a big hook on the control arm to set the car on a
flatbed.
Jack stand was put under the lower arm.
If the trailing arm was separated at the bushing end, or the bushing cup
loosened as would be done for bolt sleeve rewelding, bushing
replacement, or by an inexperienced hacker trying to do an alignment,
then one section of the lower arm will bend if the control arm is pulled
down out of the way to make clearance, rather than separating the camber
bushing at the hub from the lower arm, as is the proper method.
    However it happened, the only way to get rid of the thunk is to
replace the bent lower arm.
Believe it or not, you have to drop the subframe to get the inner bolt
out.  Yuck.
    This is also a problem you have to fix pronto since when the swaybar
link is pressed tight against the lower arm, the suspension on that side
of the rear is now locked against any further camber deflection as the
car leans the opposite way in a turn.
    After much angst about this with BMW and their zone rep, BMW agreed
to pay half the cost of the repair.  Yet at dealer list labor and 20%
over list parts prices, half of that is little more than what I can have
the arm replaced for anywhere else.
    Good luck, once again, this is a time I hope my assessment is wrong.

"jk

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