Brake Pads
Brake pads have friction material attached to a plate of metal. A brake pad is located on each side of the rotor. One is known as the inboard pad (nearest the piston) and the other is known as the outboard pad. They are squeezed with high force against the rotor by the caliper piston(s) when the brakes are applied.
Brake pads are relatively small sections of friction material attached to a metal backing plate. They are placed on each side of the rotor and clamped against it by the caliper when the brakes are applied. They provide very effective braking and handle heat well.
Disc Brake Hardware
Disc brakes use metal slides, shims, and anti-rattle clips to minimize brake noise and ensure proper operation during brake apply and release. Some are insulated on rubber bushings.
Because brake squeal is a byproduct of the vibration caused by brake systems, engineers generally use anti-rattle clips and springs to minimize the noise generated by disc brake systems. A shim between the brake pad and caliper or piston may also be used to reduce the potential for brake noise.
Brake Shoes
Brake shoes are C-shaped and are lined with friction material. The two inside of each drum are forced against the inside of the brake drum when the brakes are applied. They are also sometimes referred to as brake linings.
Brake shoes perform the same task as brake pads. Friction material is attached to the shoe with rivets or a bonding process. When the shoes contact the drum during braking, they increase their force by wedging into the drum.
Drum Brake Hardware
Several springs and linkages are used to keep the brake shoes in proper position and also serve to retract the brake shoes when the brake pedal is released. In addition, automatic adjusters are used to maintain proper brake pedal height as the shoe material wears. Some of the hardware is also used to apply the parking brake when the vehicle is parked.
Drum brakes use hold-down and return springs as well as automatic adjuster and parking brake components. These components are part of a hardware and adjuster kit when a brake overhaul is needed. Due to heat, these components should be replaced during servicing.
Rotors
Brake rotors are connected to the wheels and rotate as the wheels turn. When the brake pads clamp around the rotor, the rotor is slowed or stopped. Heat is generated and must be released through conduction and radiation.
Brake rotors are made of iron and generally have cooling fins when used on the front of a vehicle. Some rear rotors are solid without cooling fins. The brake pads are squeezed around the rotor surface by the caliper when the brakes are applied, which slows or stops the vehicle.
Drums
The brake drums also have the vehicle’s wheels bolted to them and they rotate as the wheels turn. When the brake shoes are pressed into the inside of the drum during brake operation, the drum is slowed or stopped.
Brake parts on older cars were made of heavy cast iron and steel. Modern cars use more aluminum and plastic. Due to reduced mass, brake noise is more common today than ever. It is imperative to do the job right using quality parts or brake noise and other problems may occur.
The brake drum covers the shoes that expand into it when the brakes are applied. Because the drum fits over the wheel studs, this action slows the vehicle.
Beyond the Basics
In addition to these basic components found in nearly every brake system, the vehicle could also be equipped with various valves and ABS components. As systems have progressed, the brake system is the basis for several other systems including the traction control system (TCS), vehicle stability control (VSC), ABS, ADAS, and stability control. These systems will continue to be key components of future vehicles, even autonomous vehicles. Chapter 9: ABS, ADAS, and Related Systems contains more information on these systems.
CHAPTER 3
DISC BRAKE SYSTEMS
Caution 1: The information in this chapter is based on general knowledge and may not be specific to your vehicle. Specific information is available in the factory service manual for your vehicle, which should be consulted for torque specifications, procedures, and safety concerns.
Caution 2: Some newer vehicles with electronic brake systems require a scan tool for servicing. See “New Technology Precautions” at the end of this chapter before servicing a vehicle with an electronic brake system.
Most modern vehicles use disc brakes on the front and disc or drum brakes on the rear. Because the front brakes do as much as 80–90 percent of the work when stopping, they tend to wear out quicker, requiring more frequent replacement than the rear brakes.
Drum brakes were used on the front and rear of cars into the 1970s. Since then, disc brakes have taken the place of drums on the front and in many cases on the rears too because they offer many advantages over drum brakes. They are much better equipped to dissipate the heat generated by modern vehicles, are easier to service, and are better suited for operation in wet conditions. They also self-adjust more reliably than drum brakes and resist brake fade (brake fade is the term used by engineers to explain a temporary loss of braking, usually due to heat). A drum can expand away from the brake shoes during heavy use, causing a lower brake pedal, but disc brake rotor expansion does not cause this condition when hot. Disc brakes also can shed gases and wear particles better than drum brakes and are less likely to cause the vehicle to pull to one side during stopping.
Today’s front-wheel-drive vehicles, SUVs, and light trucks have most of the weight up front, which is why engineers design brake systems to have more brake “bias” to the front end. If the same amount of brake force was present on the rear as the front during a hard stop, the rear brakes could lock up. This could lead to a loss of control, especially if the lockup occurs during a turn.
Disc brakes are common due to their advantages over drum brakes, but drum brakes are still found on the rear of some vehicles. Every year, more vehicles are produced with disc brakes on all four corners.
The pressure from this C-clamp is the same on both sides of the wood, even though the threaded shaft only presses on one side. One of Newton’s laws is that “For every action, there is an equal and opposite reaction.”
Operation
Imagine a large C-clamp around a 2x4 board. As you tighten the threaded shaft, both sides of the board are squeezed by the clamp the same amount, even though you are only tightening the screw against one side. Sir Isaac Newton stated: “For every action, there is an equal and opposite reaction.”
Calipers
The modern disc brake system is designed to clamp the brake pads against the spinning rotor to slow or stop the vehicle when you apply the brakes, much like the C-clamp. This device is known as a caliper.