Squeaky brakes

[C M Smith <cmsmith@domain.elided>]

The most common reason for squeaky disc brakes is harmonic amplification of 
slip/stick induced vibration of the pad on the rotor. The sharp corner of 
the pad becomes the focal point for this vibration which is then amplified 
through the rotor into an audible squeal.

Remedies include lubricating the back of the pad so that vibration of the 
pad cannot get started, or if started the amplification has trouble getting 
going harmonically speaking, and chamfering the edges of the pads before 
installation to round off the sharp corner that is the source of the noise 
in the first place. The mystery of the chamfering is that the pads regain 
that sharp corner pretty soon afterwards due to brake pad wear but the 
squeal rarely returns.

Proper bedding in of brakes also helps prevent squeal from starting. It is 
important to season the rotor faces with new pad material both for good 
braking efficiency and long life, without squeal. Most people baby their 
new brakes too much, some go to the opposite extreme and cook their new 
brakes. Several moderately hard stops are desirable to bed in new brakes, 
not for the reason often assumed, which is to key the pads to the rotor 
faces in a shape sense, but to grind pad material into the porous surface 
of those cast iron discs (they may not look very porous but they are). If 
the brakes are applied moderately hard several times from new, then 
material from the new pads is ground into the faces of the discs, which 
then develop more braking friction as well as smoother contact with the pads.

My suspicion is that pad squeal is most common when putting new rotors in, 
or when changing brands of pads with old rotors. I recommend new rotors if 
you change brand of pad, or at least a good honing of the old rotors if 
they can take it. The pad and rotor work best when the rotor has pad 
material ground into the surface by firm operation of the brakes. If you 
keep using the same brand with old rotors you rarely get squeal until you 
put new rotors in and forget to bed in the new brakes.

For that matter, it is good practice to apply the brakes moderately hard 
from time to time if most of your driving is of the non aggressive variety. 
Brakes tend to develop squeal if they are not exercised periodically.

Cheers


Michael Smith
White 1991 164L
Original owner
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