More on Rotor Warp

[jlandry@domain.elided (John A. Landry)]

Reapers,

This afternoon, I finally had the time to clean and repack the wheel bearings
on the passenger side hub assembly on my 76 Traveler.  You may recall that
last weekend I had to replace the hub and rotor assembly on the driver's side
due to a spun outer bearing race.

Luckily I found the grease and bearing on the passenger side in excellent
condition.  While I had the brake caliper off, I decided to check the run-out
on the rotor face using a dial indicator and magnetic mount.  The outer face
of the rotor measured .005" of run-out, which is .001" past the IH limit.
Here's the interesting part, the inside rotor face measured only .002" of
run-out.  Another thing I find interesting is that the inside rotor face is
noticeably thicker than the outer face.  The IH limit for "parallelism" or
thickness variation between the two rotor faces is a tiny .0005".  I did not
make this measurement directly (using a micrometer), but from the two run-out
measurements, it most likely exceed the parallelism spec by .0025" or more.

Time to get another hub and rotor assembly prepared to swap on the passenger
side!

One thing I've held off on mentioning to the Digest since I replaced the hub
and rotor is the fact that my grabbing braking and soft pedal problem has
almost completely vanished.  I wanted to drive the truck for a week and make
sure I wasn't imagining things.

I think it was Dan Ness who recently mentioned that every once and awhile his
brake pedal would go nearly to the floor and other times would be fine.  I've
had the same problem for a long time that seemed to occur randomly.  This had
caused me to replace my brake master cylinder... only to have the problem
persist.  Now that the driver's side rotor was replaced, the pedal works
perfectly!  My brakes are firm and smooth.

Here's my theory...  The outer bearing on my driver's side hub was failing for
some time.  This probably caused excessive heat buildup in the hub and could
have contributed to hub warpage.  This of course led to ever increasing
amounts of rotor face run-out.  Excessive run-out causes the brake pads to be
"kicked back" into the caliper as the rotor turns.  This gap means the brake
pedal must be pushed farther to take up the slack.  The result is a feel to
the driver of a long soft pedal travel.  I'm also pretty sure excessive
out-of-true parallelism is a major cause of brake "grabbing."

The moral of the story?

If you have disk brakes and the pedal or performance doesn't feel right, check
your rotor parallelism and run-out with a dial indicator (while on the truck).
You might be in for an unpleasant surprise.

Regards,

John L.
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