Front Brake Trouble

I have a technical front brake question. My '88 NX650 front brake
seems to be grabbing and releasing with every revolution. I've
replaced the disc and shimmed to try and get any wobble out of it but
it still seems to be having trouble.

Unfortunately I thought it was the rotor and purchased a new one about
a year ago. With the new rotor installed the problem is still there.
So the problem is the hub? And the problem is not a pulsating brake
lever, instead it is a diving up and down front end as I apply the
front brake gradually. The use of shims has helped minimize the
diving up and down but not eliminated it. There are 2 pins or
actually two bolts going through the caliper and bolting to the front
fork. Ideally the caliper has some self-centering freedom on those
bolts. So I was thinking how could the cast hub be bent without
breaking. If some manufacturing variance is expected in the cast hub
(maybe 1/1000 or 2/1000 of an inch), then maybe that variance is
expected to be compensated by the front brake caliper being allowed to
float. If my 1988 caliper bolts have worn and now have a groove in
them where the caliper normally rests, then that groove would be
limiting the movement of the caliper and limiting the forgiveness for
the imperfect hub/disc.
Does any of this make sense or sound probable?

Has anyone else experienced/fixed this?

Wayne

 

Check the rotor for any variation in thickness. Anything over .0002"
out of parallel can be felt when you apply the brake. The brake will
pulse at low speed and feel like a jack hammer at racing speeds.
Waviness in the disc is more tolerable but it will also cause the
symptoms you describe. You'll need a micrometer that reads in tenths
and a dial indicator to check disc runout. Any machine shop can do
this for you if you don't have the tools Take them the wheel with the
disc mounted and the axle. A good grinding shop can fix the disk. Tell
them you want the sides parallel within .0002" and flat within .002"
without, of course, grinding it under minimum thickness.

From the sound of it, you just replaced the disc with another one with
the same problem. Replace the worn bolts.

Bill Smith

 

I fixed a mild case of this. In my case, the whole rotor was slightly
cocked. I shimmed between the carrier for the rotor and the wheel
hub using a couple thicknesses of foil. A layer or two of foil between
the hub and carrier will translate into a couple thousandths of
an inch at the outer edge of the rotor.

Start by identifying the high and low spots on the rotor. Put the
bike on its center stand, spin the wheel slowly and mark the spots
where it's binding. If there are multiple high and low spots, then
the rotor's warped. If there's a single high and low spot, then
you've got a wobble that you may be able to shim away. The rotor
will rub against the pad on one side and have excessive clearance
on the other. Rotate the wheel 180 degrees and the rotor will be
rubbing on the other pad with excessive clearance on the opposite side.

 

I'll check, but I think it's more of a wobble. So I'll try some more
finite shimming and new caliper bolts. Then, I'll work my way up to
the the more invasive.

Thanks to everyone for the help,
Wayne