An essential component of the disc brake rotor, brake pads are used in automotive and other applications. They are made of steel backing plates with compressed friction materials bound to the surface that faces the disc brake rotor.
The concept of brake pads dates as far back as early 1902 as an alternative to drum brakes which use friction caused by a set of shoes or pads that press outward against a rotating cylindrical part called a drum. Due to the high cost and inefficiency compared to drum brakes, brake pads were not implemented until after World War II. Once the disc brake technology improved, brake performance quickly surpassed that of drum brakes. The performance difference was remarkable. As late as 1963 the majority of automobiles using disc brakes were European made, with American cars adopting the technology in the late 1960s after the invention of fixed callipers that made installation cheaper and more compact.
Disc brakes offer better stopping performance compared to drum brakes. They provide better resistance to brake fade usually caused by overheating of the brake pads, and also able to recover quickly from immersion. Unlike the drum brake, the disc brake has no self-servo effect. That is to say, the braking force is always proportional to the pressure applied on the brake pedal. However, many disc brake systems have servo assistance to reduce the driver’s effort on the pedal.
Functionally, brake pads convert the kinetic energy of the vehicle to thermal energy through friction. Two brake pads are contained in the brake with their friction surfaces facing the disc brake rotor. When the brakes are hydraulically applied, the calliper clamps the two pads together onto the rotating disc brake rotor to slow and stop the vehicle. When a brake pad heats up due to contact with the disc brake rotor, it transfers small amounts of its friction material onto the disc brake rotor, leaving a dull grey coating on it. The brake pad and disc brake rotor (both now having the friction material), then “stick” to each other, providing the friction that stops the vehicle.
In disc brakes, there are usually two brake pads per disc brake rotor. These are held in place and actuated by a calliper affixed to the wheel hub or suspension upright. Depending on the pad friction material, brake pads should usually be replaced regularly to prevent brake fade. Most brake pads are equipped with a method of alerting the driver when they need to be replaced. A common technique is manufacturing a small central groove whose eventual disappearance by wear indicates the end of a pad’s service life. Other methods include placing a thin strip of soft metal in a groove, such that when exposed (due to wear) the brakes squeal audibly. A soft metal wear tab can also be embedded in the pad material that closes an electric circuit when the brake pad wears thin, lighting a dashboard warning light.