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Brake pads are a component of disc brakes used in automotive and other applications. Brake pads are steel backing plates with friction material bound to the surface that faces the disc brake rotor.
Brake pads convert the kinetic energy of the car to thermal energy by friction. Two brake pads are contained in the brake caliper with their friction surfaces facing the rotor. When the brakes are hydraulically applied, the caliper clamps or squeezes the two pads together into the spinning rotor to slow/stop the vehicle. When a brake pad is heated by contact with a rotor, it transfers small amounts of friction material to the disc, turning it dull gray. The brake pad and disc (both now with friction material), then "stick" to each other, providing the friction that stops the vehicle.
In disc brake applications, there are usually two brake pads per disc rotor, held in place and actuated by a caliper affixed to a wheel hub or suspension upright. Although almost all road-going vehicles have only two brake pads per caliper, racing calipers utilize up to six pads, with varying frictional properties in a staggered pattern for optimum performance. Depending on the properties of the material, disc wear rates may vary. The brake pads must usually be replaced regularly (depending on pad material), and most are equipped with a method of alerting the driver when this needs to take place. Some are manufactured with a small central groove whose eventual disappearance through wear indicates that the pad is nearing the end of its service life. Others are made with a thin strip of soft metal in a similar position that when exposed through wear causes the brakes to squeal audibly. Still others have a soft metal tab embedded in the pad material that closes an electric circuit and lights a dashboard warning light when the brake pad gets thin.
Disc brake advantages
Disc brakes offer better stopping performance than comparable drum brakes, including resistance to "brake fade" caused by the overheating of brake components, and are able to recover quickly from immersion (wet brakes are less effective). Unlike a drum brake, the disc brake has no self-servo effect—the braking force is always proportional to the pressure placed on the braking pedal or lever—but many disc brake systems have servo assistance ("Brake Booster") to lessen the driver's pedal effort.
There are numerous types of brake pads, depending on the intended use of the vehicle, from very soft and aggressive (such as racing applications) and harder, more durable and less aggressive compounds. Most vehicle manufacturers recommend a specific kind of brake pad for their vehicle, but compounds can be changed (by either buying a different make of pad or upgrading to a performance pad in a manufacturer's range) according to personal tastes and driving styles. Care must always be taken when fitting non-standard brake pads, as operating temperature ranges may vary, such as performance pads not braking efficiently when cold or standard pads fading under hard driving. In cars that suffer from excessive brake fade, the problem can be minimized by installing better quality and more aggressive brake pads.
The five most important characteristics that are considered when selecting a brake pad material are as follows:
· The material's ability to resist brake fade at increased temperatures
· The effects of water on brake fade (all brakes are designed to withstand at least temporary exposure to water)
· The ability to recover quickly from either increased temperature or moisture
· Service life as traded off vs. wear to the rotor
· The ability of the material to provide smooth, even contact with the rotor or drum (rather than a material that breaks off in chunks or causes pits or dents).
For many years straightforward asbestos was viewed as having an optimal performance in all five categories. However, as the serious health-related hazards of asbestos became apparent, other materials had to be found. Today, brake pad materials are classified as belonging to one of four principal categories, as follows:
- Non-metallic materials - these are made from a combination of various synthetic substances bonded into a composite, principally in the form of cellulose, aramid, PAN, and sintered glass. They are gentle on rotors, but produce a fair amount of dust and have a short service life.
- Semi-metallic materials - synthetics mixed with some proportion of flaked metals. These are harder than non-metallic pads, and are more fade-resistant and longer lasting, but at the cost of increased wear to the rotor/ drum which then must be replaced sooner. They also require more force than non-metallic pads in order to generate braking torque.
- Fully metallic materials - these pads are used only in racing vehicles, and are composed of sintered steel without any synthetic additives. They are very long-lasting, but require even more force to slow a vehicle and are extremely wearing on rotors. They also tend to be very loud.
- Ceramic materials - Composed of clay and porcelain bonded to copper flakes and filaments, these are a good compromise between the durability of the metal pads and the grip and fade resistance of the synthetic variety. Their principal drawback, however, is that unlike the previous three types and despite the presence of the copper (which has a high thermal conductivity), ceramic pads generally do not dissipate heat well, which can eventually cause the pads or other components of the braking system to warp However, because the ceramic materials causes the braking sound to be elevated beyond that of human hearing, they are exceptionally quiet.
There are environmental factors that govern the selection of brake pad materials. For example, the bill SSB 6557 adopted in Washington State in 2010 will limit the amount of copper that is allowed to be used in friction materials, to be eventually phased out to trace amounts, because of the negative impact of high copper levels on the aquatic life. For its substitution, different material combinations have been developed, though no direct replacement is yet available. Other materials, such as compounds made with antimony, are being studied.
Vehicles have different braking requirements. Friction materials offer application-specific formulas and designs. Brake pads with a higher coefficient of friction provide good braking with less brake pedal pressure requirement, but tend to lose efficiency at higher temperatures, increasing stopping distance. Brake pads with a smaller and constant coefficient of friction do not lose efficiency at higher temperatures and are stable, but require higher brake pedal pressure.
There are different systems for the cataloguing of brake pads. The most frequently used system in Europe is the WVA numbering system.
The cataloguing system used in North America, and recognized around the world, is the standardized part numbering system for brakes and clutch facings issued by the Friction Materials Standards Institute (FMSI). FMSI's mission is to, "Maintain and enhance this standardized part numbering system for all on highway vehicles in use in North America."