Disc brakes can be used on all four wheels of a vehicle, or a combination of brake types can be used, with disc brakes on the front wheels and drum brakes on the rear. When the brake pedal is depressed, a pushrod transfers the force through a brake booster to a hydraulic master cylinder. The master cylinder converts the pedal force into hydraulic pressure, which is then transmitted via brake lines and hoses to one or more pistons at each brake caliper FIGURE 32-4. The pistons operate on friction pads to provide a clamping force on a rotor that is attached to the wheel hub. This clamping action is designed to stop the rotation of the rotor and the wheel.
The master cylinder converts the pedal force into hydraulic pressure, which is then transmitted via brake lines and hoses to one or more pistons at each brake caliper.
The hub and hubless rotors.
Caliper mounting methods.
The rotors are free to rotate with the wheels due to wheel bearings and the hubs that contain them. The hub can be part of the brake rotor or a separate assembly that the rotor slips over and is bolted to by the lug nuts FIGURE 32-5. On rear-wheel drive vehicles, the rotor is mounted onto the driving axle or hub and may be held in place by the wheel. On front-wheel drive vehicles, it can be mounted on the front hub and wheel bearing assembly.
The brake caliper assembly is normally bolted to the vehicle axle housing or suspension FIGURE 32-6. In most cases, the brake is positioned as close as possible to the wheel, but there are exceptions. Some high-performance cars with independent rear suspension (IRS) use inboard disc brakes on the rear wheels. The calipers are mounted on or next to the differential, which is directly mounted to the vehicle body. Manufacturers claim improved vehicle handling for this design because it reduces the vehicle’s unsprung weight by taking the differential assembly and brakes from the suspension and mounting them to the body. Because the wheels and axles are now lighter, the vehicle’s springs can do a better job of keeping the wheels on the ground, especially on uneven road surfaces.
Disc brake pads require much higher application pressures to operate than drum brake shoes because they are not self-energizing. This additional clamping pressure is created by increasing the diameter of the caliper pistons. Unfortunately, this means the brake pedal travel is lengthened to move the additional fluid being displaced by the larger caliper pistons. Building in more pushrod travel would require more room under the dash. Manufacturers have overcome this problem by equipping most disc brake systems with a power booster. Because of the high forces needed to apply a disc brake, using it as a parking brake is more challenging. Some manufacturers have chosen to design more complicated calipers, while others have built an auxiliary drum brake assembly into the center of the rear disc brake rotors to provide for parking brake operation. This is referred to as a top hat design.