Brake pad

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 disk brake rotor.

Function

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.[1] 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.

Technology

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.

Types

A set of pads for high-performance disk brakes

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.

Materials

The five most important characteristics that are considered when selecting a brake pad material are as follows:

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:

There are environmental factors that govern the selection of brake pad materials. For example, the bill SSB 6557 [3] 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. Other materials like antimony compounds will be monitored as well.

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.

Cataloguing

There are different systems for the cataloguing of brake pads. The most frequently used system in Europe is the WVA numbering system.[4]

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."[5]

Cartridge brake pad

A type of brake pad used on rim brakes.

See also

References

  1. Henderson, Bob; Haynes, John H. (1994). "Disc Brakes". The Haynes Automotive Brake Manual. Haynes North America. pp. 1–20.
  2. 1 2 Cliff Owen (21 June 2010). Today's Technician: Automotive Brake Systems Classroom and Shop Manual. Cengage Learning. pp. 27–28. ISBN 978-1-4354-8655-3.
  3. Limiting the use of certain substances in brake friction material
  4. WVA numbering system
  5. http://fmsi.org/home/
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