Coefficient of Friction Change with Heat
Location: Outside U.S.
Date: October 2008
Why brake pads, when pressed against a steel drum,
loose friction at very high temperatures?
Brake fade is a problem with drum brakes, but not so much with disk brakes.
The reason is that the brake shoes are inside the drum, and press outward
against the drum to produce friction. When the drum becomes hot, it
expands. This causes the drum to move away from the brake pad, so the
brake pad must move farther to make contact. Since brake pads do not
usually move very far to contact the drum, this change can be significant.
If the brake drum gets very hot, it expands so much that the brake pad must
travel to the limit of its extension just to contact it. It is impossible,
then, for the pad to press against the drum with enough force to generate as
much friction as was possible with cooler brakes.
In your question, you have mixed up parts of a disk brake system with
parts of a drum brake system. Pads are part of the a disk brake, and
the drum is part of a drum brake. The difference is important.
A disk brake system consists of a flat disk that rotates with the
wheel, and two flat friction pads (one on each side of the disk) that
press against the rotating disk to apply the braking force. A drum
brake system is different. There, a cylindrical iron drum rotates with
the wheel and has two brake "shoes" mounted inside the cylindrical
drum. Each brake shoe is curved to exactly match the inside radius of
the drum, and each shoe occupies about half the circumference of the
drum. When the brakes are applied, the shoes are pushed outward
against the inside of the cylindrical brake drum.
In a drum brake system, the heat from braking causes the drum and the
shoes to expand somewhat. But they expand at different rates. When the
drum brake system is cold, the brake shoes fit perfectly inside the
drum, and the entire curved surface of each brake shoe contacts the
inside of the drum. This gives maximum braking force. But when the
brakes are hot, the brake shoes expand at a faster rate than the drum
(because the shoes are on the inside with no cooling). The result is
that the radius of curvature of the shoes has expanded and no longer
matches the inside radius of the drum, and now the brake shoes only
touch the drum in a few places (usually only at the two ends of the
brake shoe). This causes a big reduction in braking force that is
called "brake fade".
Disk brakes are different. The flat brake pads in a disk brake system
also expand when they get hot, but they still stay flat. So they still
make full contact with the flat brake disk no matter how hot they get,
or how much they expand, and braking force remains unchanged.
However, in a disk brake system, "brake fade" (or friction loss) is
still possible. The reason for this is that normal brake pads used in
passenger cars have a pad material that has high friction when cold,
but the material's friction coefficient is naturally reduced slightly
at extreme temperature. This is just a characteristic of the pad
material. The reduction is not much, and in fact is MUCH less than you
get in a drum brake system. This type of material is used because
passenger car brakes must work at full efficiency even if there has
been a long time since the last brake application, and the brakes are
In racing cars, a different disk brake pad material is used. The pad
material used in racing pads have a very poor friction coefficient
when cold, but the friction coefficient actually increases when they
are hot. These pads are used because they not only are capable of
withstanding the high temperatures experienced in racing, but also
because once they get applied a few times and are hot, they have an
extremely high coefficient of friction. Remember that in a race, the
brakes are constantly in use and (unlike in passenger car use) never
get a chance to cool off.
So, in fact, brakes used in race cars actually work much better once
they get hot!
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Update: June 2012