Brake pads
Encyclopedia
Brake pads are a component of disk 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.
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 have a thin piece of soft metal that causes the brakes to squeal when the pads are too thin, while others have a soft metal tab embedded in the pad material that closes an electric circuit and lights a warning light when the brake pad gets thin. More expensive cars may use an electronic sensor.
s, 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.
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.
to organic
or semi-metallic formulations. Each of these materials has proven to have advantages and disadvantages regarding environmental friendliness, wear, noise, and stopping capability. Semi-metallic pads provide strength and conduct heat away from rotors but also generate noise and are abrasive enough to increase rotor wear.
Ceramic
compounds and copper fibers in place of the semi-metallic pad's steel fibers accommodate higher temperatures with less heat fade and generate less dust and wear on both the pads and rotors. They also provide much quieter operation due to the ceramic compound that helps dampen noise by shifting its resonant frequency beyond the human hearing range and reduced metal use (approximately 15% metal content by weight). Ceramic brake pads typically are suited for light-duty applications and not severe duty applications as encountered by medium duty trucks, etc.
There are environmental factors that govern the selection of brake pad materials. For example, recent legislation in Washington State (SSB 6557) and other states will limit the amount of copper that is allowed to be used in friction materials, to be eventually phased out to trace amounts. Other materials like antimony compounds will be monitored as well.
Asbestos was widely used in pads for its heat resistance but, due to health risks, has been replaced with alternative materials, such as mineral
fiber
s, cellulose
, aramid
, PAN
, chopped glass, steel
, and copper fibers. Depending on material properties, disc wear rates vary. The properties that determine material wear involve trade-offs between performance and longevity. Newer pads can be made of exotic materials like ceramics, aramid
fibres, and other plastics.
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 don’t lose efficiency at higher temperatures and are stable, but require higher brake pedal pressure.
.
Function
Brake pads convert the kinetic energyKinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the car to thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
by friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of 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
Disc brake
The disc brake or disk brake is a device for slowing or stopping the rotation of a wheel while it is in motion.A brake disc is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon–carbon or ceramic matrix composites. This is connected to the wheel and/or...
, 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
Suspension (vehicle)
Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...
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 have a thin piece of soft metal that causes the brakes to squeal when the pads are too thin, while others have a soft metal tab embedded in the pad material that closes an electric circuit and lights a warning light when the brake pad gets thin. More expensive cars may use an electronic sensor.
Advantages of disc brakes
These brakes offer better stopping performance than comparable drum brakeDrum brake
A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against a rotating drum-shaped part called a brake drum....
s, including resistance to "brake fade
Brake fade
Vehicle braking system fade, or brake fade, is the reduction in stopping power that can occur after repeated or sustained application of the brakes, especially in high load or high speed conditions...
" 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
Vacuum servo
A vacuum servo is a component used on motor vehicles in their braking system, to provide assistance to the driver by decreasing the braking effort. In the USA it is commonly called a brake booster.-Background:...
("Brake Booster") to lessen the driver's pedal effort.
Laser shaping
Ordinary brake pads inherit vibration characteristics since the shape of the pad and contact of the caliper piston causes vibration that distorts the contact or the transfer of force to the rotor upon contact. The new laser cutting technology of the pad decreases the contact area and focuses on more force being distributed over a smaller amount of an area for more positive contact of the pad and rotor creating more efficient quality stopping.Laser burnishing process
A precisely controlled laser beam is used to heat and condition the pad surface, eliminating the need for a traditional break-in period on the vehicle following installation.IMI-sound insulator
Spreads out and absorbs heat and energy over a greater surface area for longer brake pad life and quieter performance.Types
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 temperatureOperating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
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
Brake fade
Vehicle braking system fade, or brake fade, is the reduction in stopping power that can occur after repeated or sustained application of the brakes, especially in high load or high speed conditions...
, the problem can be minimized by installing better quality and more aggressive brake pads.
Materials
Brake pad materials range from asbestosAsbestos
Asbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...
to organic
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
or semi-metallic formulations. Each of these materials has proven to have advantages and disadvantages regarding environmental friendliness, wear, noise, and stopping capability. Semi-metallic pads provide strength and conduct heat away from rotors but also generate noise and are abrasive enough to increase rotor wear.
Ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
compounds and copper fibers in place of the semi-metallic pad's steel fibers accommodate higher temperatures with less heat fade and generate less dust and wear on both the pads and rotors. They also provide much quieter operation due to the ceramic compound that helps dampen noise by shifting its resonant frequency beyond the human hearing range and reduced metal use (approximately 15% metal content by weight). Ceramic brake pads typically are suited for light-duty applications and not severe duty applications as encountered by medium duty trucks, etc.
There are environmental factors that govern the selection of brake pad materials. For example, recent legislation in Washington State (SSB 6557) and other states will limit the amount of copper that is allowed to be used in friction materials, to be eventually phased out to trace amounts. Other materials like antimony compounds will be monitored as well.
Asbestos was widely used in pads for its heat resistance but, due to health risks, has been replaced with alternative materials, such as mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
s, cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, aramid
Aramid
Aramid fibers are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic rated body armor fabric and ballistic composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic polyamide"...
, PAN
Polyacrylonitrile
Polyacrylonitrile is a synthetic, semicrystalline organic polymer resin, with the linear formula n. Though it is thermoplastic, it does not melt under normal conditions. It degrades before melting. It melts above 300 degrees Celsius only if the heating rates are 50 degrees per minute or above...
, chopped glass, steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, and copper fibers. Depending on material properties, disc wear rates vary. The properties that determine material wear involve trade-offs between performance and longevity. Newer pads can be made of exotic materials like ceramics, aramid
Aramid
Aramid fibers are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic rated body armor fabric and ballistic composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic polyamide"...
fibres, and other plastics.
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 don’t 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 systemWVA number
WVA numbers are a reference and assignment system for brake linings, clutch facings, brake shoes and other friction materials which will especially be used in road vehicles but also in mechanical engineering. The WVA numbering system has been developed by the "VRI-Verband der Reibbelagindustrie" ,...
.