Brake pad shims and guides.
Disc brakes are more prone to annoying brake squealing than are drum brakes. Brake squealing is caused by vibrations set up between the brake pad and rotor. Manufacturers have addressed this problem in a number of ways:
Using softer linings with a higher coefficient of friction, which are less prone to noise than harder linings with a lower coefficient of friction.
Adding brake pad shims and guides to the brake pads, which help cushion the brake pad and absorb some of the vibration FIGURE 32-22.
Using springs to tightly hold the pads in place to minimize vibration FIGURE 32-23.
Contouring and grooving the lining material in a way that minimizes vibration FIGURE 32-24.
Incorporating bendable tangs on the brake pad backing plate that allow technicians to crimp the tangs so they are more firmly mounted in the caliper FIGURE 32-25.
Technicians also can apply noise-reducing compounds to the brake pads. One is a type of high-temperature liquid rubber compound that is applied to the back of the brake pad. When it cures, it stays flexible and absorbs brake pad vibrations and helps reduce brake noise. Another compound is a specially designed liquid that is applied directly to the face of the lining material. This compound helps to modify the lining’s coefficient of friction slightly, making it less likely that the lining will squeal. Make sure you apply the correct compound to the correct side of the brake pad.
Example of brake pad retainers.
Brake lining grooves and contouring.
Brake pad bendable tangs.
Some manufacturers claim that refinishing their rotors removes enough material from the rotor that it can cause the brakes to squeal due to less mass, which changes the harmonic vibration qualities of the rotor. They recommend replacing the rotors any time the rotors are worn enough to need resurfacing.
AS-36: Sound: The technician can demonstrate an understanding of the role sound plays in identifying various problems in the vehicle.
Sound is a series of waves that travel through a gas, liquid, or solid and can often be heard by the human ear. Sound waves are created by vibrating objects, such as a guitar string. Moving objects that are in sliding contact with each other are highly likely to create sound. One example of this is fingernails dragging on a chalkboard. The fingernails vibrate on the surface of the chalkboard and create sound waves that are then heard by the ear.
Technicians commonly use differences in sound to assist in diagnosing disc brake problems. For example, brake pads that are worn down to the metal backing plate make a deep grinding noise when the brakes are applied. Listening to hear which wheel or wheels the noise is coming from helps identify the source of the problem. In the same way, if the disc brakes are equipped with a scratcher style of brake warning system, the brakes will make a high-pitched screeching noise. Many times this noise will happen when the vehicle is being driven when the brakes are not applied. One way to help determine which side the noise is coming from is by driving the vehicle next to a concrete traffic wall or building. If the noise is on that side, it will get much louder than when not near the wall.