Check the Obvious!

["Steve D'Gerolamo" <steved3@xxxxxxx>]

A local BMW specialty shop installed an E39 Mov'it big brake kit on
Saturday and the customer called the shop first thing Monday morning (who
in turn contacted me) regarding a terrible shimmy problem when braking. We
went for an extensive ride this morning and the shimmy appeared to be
getting better until we did a few high speed stops from over 100mph.  The
shimmy was back with a vengeance.

The E39 cars with their bolt on hub/bearing assemblies are notorious for
causing brake shimmy problems, especially with fixed piston big brake kits.
 BMW has very loose standards for hub runout (.004" or .10mm) and as such
can point to these specs to avoid replacing the bearings under
warranty.....hub runout of .004" can extrapolate to rotor runout at the
circumference of the disk of .008" or greater.  This would easily be felt,
even with floating caliper brake systems.

The customer was forewarned of the bearing problem and had the shop put new
hubs on before installing the brakes.  However, I still had thoughts of new
defective hubs or those with a few thousands runout causing the
problem....replacing the bolt-on bearings involves moving the strut to get
at 2 of the bolts, not a simple matter.   I removed the left front wheel
and went to attach my dial caliper via its magnetic base and ran into a
problem....the strut and other major front suspension components are
aluminum.  Fortunately, I had another tool with the vice-grip attachment
which was easily clamped to the lower spring perch.  I installed 5
M12x1.5x20 bolts with washers to secure the rotor to the hub and torqued to
these to 20 ft lbs.  Runout for the new Mov'it rotor and hub assembly
measured .0024" (.06mm)...not perfect but within spec and not the kind of
measurement I had expected given the terrible shimmy.  Mounted the wheel
back up and torqued to 80 ft lbs.

Onto the passenger side wheel.  Removed the hub cap and went to break loose
the first lug bolt and it practically fell out by itself.  I had the
customer remove his remaining 4 lug bolts which were all finger tight.
Amazingly, this car with its 18" BBS factory rims was rock solid at 100mph,
even with loose lugs.  It wasn't until the caliper clamped the rotor during
braking that we detected the problem.  The rotor runout measured the same
as the left side, the wheel was reinstalled and torqued properly, and now
the problem is gone.

Moral of the story....always check the obvious first when troubleshooting
shimmy problems.  SD

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Steve D'Gerolamo - The Ultimate Garage
201-262-0412 / steved3@domain.elided / http://www.ultimategarage.com

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