Re: [DML] brake runout specs

["Walter Coe" <Whalt@xxxx>]

I just got my car back on the road today. Yippee! Anyway about the rotor
run-out, what I was seeing when I spin the rear rotors is that they wobble
laterally by about 1mm, and the parallelism is uniform. The only way to see
this is with the rear wheels off, and you need these on to hold the rotor
evenly in place. So I bolted an aftermarket wheel spacer to the hub and
torqued it all down to 60 ft-lbs. This didn't change anything, and I tried
it on both sides. There is a chance that perhaps both 'axles' (if you can
call them that) are bent, but I don't think this is likely. When I put the
rotors on the hubs, I made sure everything was completely clean. This rules
out the possibility that an uneven layer of rust on the hub could make the
rotor seat unevenly. The next thing I'm going to do is measure the rotors
using a micrometer-caliper to measure the run-out relative to the inside of
the brake caliper. I can see how a problem with parallelism will cause
brake pulsation, but only a problem with run-out would just oscillate the
pistons back & forth. I suppose this could still be felt as a pulsation,
but on my 30 mile test drive today everything was nice and smooth.

Some observations: The brakes feel like they need more pressure than normal
to stop the car. I assume this is due to the fact that they still need to
go through a break-in period. I put PowerStop cross-drilled brake rotors on
the front. I would have done all 4 with these, but they were discontinued.
I can tell you that the cross-drilled ones are noisy. I don't consider it
objectionable, but it is there if you listen for it. I describe the noise
as kind of a cross between sand paper and the hydraulic door sound dubbed in
BTTF. It kind of gives my brakes a high-tech sound. Now that I know what
to listen for, I can even hear the noise very slightly pulsating even when
I'm not applying the brakes. The pulsation part comes from the new rotors
not being perfectly flat. I'm going to measure the run-out on those too and
keep track of them as a function of mileage. I'm going to have to convert
David T's specs to metric before I can understand them (yep, I hate the old
system! I can't even put those numbers into perspective without
converting.) The specs that Jan gave don't make much sense either. You are
only supposed to use commas every three digits, so 0,15mm makes no sense.
Do you mean 0.15mm? I think I have a lot more run-out than that. I'll get
back later with some real numbers once I get the car up on jacks again.

Walt Tampa, FL