Disc brake pads consist of friction material bonded or riveted onto a steel backing plate (disc brakes) FIGURE 32-18. Bonded linings are more common on light-duty vehicles since they are less expensive to build and the bonding agent can fail under the very high temperatures of heavy-duty use. Riveted linings are used on heavier-duty or high performance vehicles. Metal rivets provide a mechanical connection to hold the lining to the backing plate that is less susceptible to failure under high temperatures. Since the rivets actually pinch some of the lining between the rivet head and the backing plate, the linings must be changed sooner than bonded linings. Otherwise, the rivet heads would contact the rotor and wear a groove in the face of the rotor. The backing plate has lugs that correctly position the pad in the caliper assembly and help the backing plate maintain the proper position to the rotor FIGURE 32-19. Disc brakes are usually designed so that the thickness of the pads can be checked easily once the wheel has been removed. Most disc brakes also are designed to allow the pads to be replaced with a minimum of disassembly.
FIGURE 32-18
Bonded and riveted brake pads.
FIGURE 32-19
Brake pad locating lugs.
FIGURE 32-20
Brake rotor wear.
Some pads have a groove cut into the friction surface. This groove helps ventilate gases that build up at the surface of the friction material under heavy brake application. It also can help modify the harmonic vibration quality of the friction material to reduce brake squeal. On some pads, the depth of this groove is set so that as the pad wears thinner, the remaining groove gets smaller; when it can no longer be seen, the pad should be replaced.
The composition of the friction material affects brake operation. Materials that provide good braking with low pedal pressures tend to lose efficiency when they get hot, thus increasing the stopping distance. They also tend to wear out quicker. Materials that maintain a stable friction coefficient over a wide temperature range generally require higher pedal pressures to provide efficient braking. They also tend to put added wear on the disc brake rotor, reducing its useful life FIGURE 32-20.
