about brake squeal...

not just related to my '89 240, but all cars with disk brakes seem prone to
squeal. I've always wondered just what part is making the noise. The caliper
is pretty massive, as is the rotor. Since the pads are being pressed against
the rotor when the brakes are applied, I can't imagine they would be able to
vibrate much...

so, just what is it that makes the noise? I've had some luck (with this and
other makes of car) by putting some silicone sealant on the back of the
brake pad when I install them, but eventually they squeal...

New pads, old pads, makes no difference...

Just wondering....

 

I have always thought that it is the pad to disk interface that squeals.
, but perhaps something to do with the caliper piston. The pad
bracket is what takes most of the force but holding by the pad bracket
doesn't seem to help. Isolating the caliper piston seems to be the
normal cure by either anti-squeal shims or copper grease on the back of
the pads.

Personally I have never had a problem with it on 940s or 300s in the UK
in 20 years of driving, unless there was a fault. It must be a
reasonant wave like that when you rub a glass rim, changing the mass or
connection to other masses is the way to change the frequency out of the
usage range. I guess it is also temperature/humidity dependant maybe
your local climate makes a difference.

I am surprised that silicone sealant does not burn up as it is not
really rated for such high temps. Putting in something with higher temp
rating should last better, proper shims or copper grease.

Do new disks make any difference, are you using the OE spec pads? Is
there any chance the brackets or disk guards are touching the disk under
braking?

 

other than the squeal (which is not very loud, and is quite intermittant,
usually when hot), the brakes are excellent, and clearances seem ok...

I've tried with and without the shims, with and without silicone, no
combination is a guarantee! Also, it generally gets better over time, as
pads wear in, then reappears when the pads are quite worn.

I just replaced them this past weekend, and a little squeal appears at the
very end of braking to a stop. I'm hoping it resolves spontaneously, don't
feel like pulling them apart again!...

As for the silicone sealant... I agree that it's not rated for the high
temp, but I've used that trick on quite a few cars over the past 30 or so
years, and amazingly it usually works, and lasts for a year or so of regular
driving, about 10K miles, if I leave it alone... a cheap fix!...

I was just curious as to the exact source of the noise... like you, I'm
sure it's some sort of resonance, just strange it would be so high-pitched
when everything is so massive!