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Name: Rafi
Status: Other
Grade: Other
Location: Outside U.S.
Country: Pakistan
Date: October 2008


Question:
Why brake pads, when pressed against a steel drum, loose friction at very high temperatures?



Replies:
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.

Richard Barrans


Hi Rafi,

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 cold.

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!

Regards,

Bob Wilson



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