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Disc brake on a motorcycle Disc brake.jpg
Disc brake on a motorcycle

A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. [1] It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. [2]



Most brakes commonly use friction between two surfaces pressed together to convert the kinetic energy of the moving object into heat, though other methods of energy conversion may be employed. For example, regenerative braking converts much of the energy to electrical energy, which may be stored for later use. Other methods convert kinetic energy into potential energy in such stored forms as pressurized air or pressurized oil. Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc, fin, or rail, which is converted into heat. Still other braking methods even transform kinetic energy into different forms, for example by transferring the energy to a rotating flywheel.

Brakes are generally applied to rotating axles or wheels, but may also take other forms such as the surface of a moving fluid (flaps deployed into water or air). Some vehicles use a combination of braking mechanisms, such as drag racing cars with both wheel brakes and a parachute, or airplanes with both wheel brakes and drag flaps raised into the air during landing.

Since kinetic energy increases quadratically with velocity (), an object moving at 10 m/s has 100 times as much energy as one of the same mass moving at 1 m/s, and consequently the theoretical braking distance, when braking at the traction limit, is up to 100 times as long. In practice, fast vehicles usually have significant air drag, and energy lost to air drag rises quickly with speed.

Almost all wheeled vehicles have a brake of some sort. Even baggage carts and shopping carts may have them for use on a moving ramp. Most fixed-wing aircraft are fitted with wheel brakes on the undercarriage. Some aircraft also feature air brakes designed to reduce their speed in flight. Notable examples include gliders and some World War II-era aircraft, primarily some fighter aircraft and many dive bombers of the era. These allow the aircraft to maintain a safe speed in a steep descent. The Saab B 17 dive bomber and Vought F4U Corsair fighter used the deployed undercarriage as an air brake.

Friction brakes on automobiles store braking heat in the drum brake or disc brake while braking then conduct it to the air gradually. When traveling downhill some vehicles can use their engines to brake.

When the brake pedal of a modern vehicle with hydraulic brakes is pushed against the master cylinder, ultimately a piston pushes the brake pad against the brake disc which slows the wheel down. On the brake drum it is similar as the cylinder pushes the brake shoes against the drum which also slows the wheel down.


Rendering of a drum brake Drum brake testrender.jpg
Rendering of a drum brake
Single pivot side-pull bicycle caliper brake Bicycle caliper brake highlighted.jpg
Single pivot side-pull bicycle caliper brake

Brakes may be broadly described as using friction, pumping, or electromagnetics. One brake may use several principles: for example, a pump may pass fluid through an orifice to create friction:


typical braking system for cars:
FAD: Brake disc front
FPD: Brake disc rear
FPT: Rear brake drum
CF: Brake control
SF: servo brake
PF: Brake Pump
SLF: Brake Fluid Reservoir
RF: Splitter braking
FS: Parking Brake Impianto frenante.svg
typical braking system for cars:
FAD: Brake disc front
FPD: Brake disc rear
FPT: Rear brake drum
CF: Brake control
SF: servo brake
PF: Brake Pump
SLF: Brake Fluid Reservoir
RF: Splitter braking
FS: Parking Brake

Frictional brakes are most common and can be divided broadly into "shoe" or "pad" brakes, using an explicit wear surface, and hydrodynamic brakes, such as parachutes, which use friction in a working fluid and do not explicitly wear. Typically the term "friction brake" is used to mean pad/shoe brakes and excludes hydrodynamic brakes, even though hydrodynamic brakes use friction. Friction (pad/shoe) brakes are often rotating devices with a stationary pad and a rotating wear surface. Common configurations include shoes that contract to rub on the outside of a rotating drum, such as a band brake; a rotating drum with shoes that expand to rub the inside of a drum, commonly called a "drum brake", although other drum configurations are possible; and pads that pinch a rotating disc, commonly called a "disc brake". Other brake configurations are used, but less often. For example, PCC trolley brakes include a flat shoe which is clamped to the rail with an electromagnet; the Murphy brake pinches a rotating drum, and the Ausco Lambert disc brake uses a hollow disc (two parallel discs with a structural bridge) with shoes that sit between the disc surfaces and expand laterally.

A drum brake is a vehicle brake in which the friction is caused by a set of brake shoes that press against the inner surface of a rotating drum. The drum is connected to the rotating roadwheel hub.

Drum brakes generally can be found on older car and truck models. However, because of their low production cost, drum brake setups are also installed on the rear of some low-cost newer vehicles. Compared to modern disc brakes, drum brakes wear out faster due to their tendency to overheat.

The disc brake is a device for slowing or stopping the rotation of a road wheel. A brake disc (or rotor in U.S. English), usually made of cast iron or ceramic, is connected to the wheel or the axle. To stop the wheel, friction material in the form of brake pads (mounted in a device called a brake caliper) is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop.


Pumping brakes are often used where a pump is already part of the machinery. For example, an internal-combustion piston motor can have the fuel supply stopped, and then internal pumping losses of the engine create some braking. Some engines use a valve override called a Jake brake to greatly increase pumping losses. Pumping brakes can dump energy as heat, or can be regenerative brakes that recharge a pressure reservoir called a hydraulic accumulator.


Electromagnetic brakes are likewise often used where an electric motor is already part of the machinery. For example, many hybrid gasoline/electric vehicles use the electric motor as a generator to charge electric batteries and also as a regenerative brake. Some diesel/electric railroad locomotives use the electric motors to generate electricity which is then sent to a resistor bank and dumped as heat. Some vehicles, such as some transit buses, do not already have an electric motor but use a secondary "retarder" brake that is effectively a generator with an internal short circuit. Related types of such a brake are eddy current brakes, and electro-mechanical brakes (which actually are magnetically driven friction brakes, but nowadays are often just called "electromagnetic brakes" as well).

Electromagnetic brakes slow an object through electromagnetic induction, which creates resistance and in turn either heat or electricity. Friction brakes apply pressure on two separate objects to slow the vehicle in a controlled manner.


Brakes are often described according to several characteristics including:

Foundation components

Foundation components are the brake-assembly components at the wheels of a vehicle, named for forming the basis of the rest of the brake system. These mechanical parts contained around the wheels are controlled by the air brake system.

The three types of foundation brake systems are “S” cam brakes, disc brakes and wedge brakes. [3]

Brake boost

Brake booster from a Geo Storm. 2008-05-05 1990 Geo Storm GSi vacuum servo.jpg
Brake booster from a Geo Storm.

Most modern passenger vehicles, and light vans, use a vacuum assisted brake system that greatly increases the force applied to the vehicle's brakes by its operator. [4] This additional force is supplied by the manifold vacuum generated by air flow being obstructed by the throttle on a running engine. This force is greatly reduced when the engine is running at fully open throttle, as the difference between ambient air pressure and manifold (absolute) air pressure is reduced, and therefore available vacuum is diminished. However, brakes are rarely applied at full throttle; the driver takes the right foot off the gas pedal and moves it to the brake pedal - unless left-foot braking is used.

Because of low vacuum at high RPM, reports of unintended acceleration are often accompanied by complaints of failed or weakened brakes, as the high-revving engine, having an open throttle, is unable to provide enough vacuum to power the brake booster. This problem is exacerbated in vehicles equipped with automatic transmissions as the vehicle will automatically downshift upon application of the brakes, thereby increasing the torque delivered to the driven-wheels in contact with the road surface.

Heavier road vehicles, as well as trains, usually boost brake power with compressed air, supplied by one or more compressors.


Brake lever on a horse-drawn hearse Aa horsehearsebrake 00.jpg
Brake lever on a horse-drawn hearse

Although ideally a brake would convert all the kinetic energy into heat, in practice a significant amount may be converted into acoustic energy instead, contributing to noise pollution.

For road vehicles, the noise produced varies significantly with tire construction, road surface, and the magnitude of the deceleration. [5] Noise can be caused by different things. These are signs that there may be issues with brakes wearing out over time.


Railway brake malfunctions can produce sparks and cause forest fires. [6] In some very extreme cases, disc brakes can become red hot and set on fire. This happened in the Tuscan GP, when the Mercedes car, the W11 had its front carbon disc brakes almost bursting into flames, due to low ventilation and high usage. [7] These fires can also occur on some Mercedes Sprinter vans, when the load adjusting sensor seizes up and the rear brakes have to compensate for the fronts. [8]


A significant amount of energy is always lost while braking, even with regenerative braking which is not perfectly efficient. Therefore, a good metric of efficient energy use while driving is to note how much one is braking. If the majority of deceleration is from unavoidable friction instead of braking, one is squeezing out most of the service from the vehicle. Minimizing brake use is one of the fuel economy-maximizing behaviors.

While energy is always lost during a brake event, a secondary factor that influences efficiency is "off-brake drag", or drag that occurs when the brake is not intentionally actuated. After a braking event, hydraulic pressure drops in the system, allowing the brake caliper pistons to retract. However, this retraction must accommodate all compliance in the system (under pressure) as well as thermal distortion of components like the brake disc or the brake system will drag until the contact with the disc, for example, knocks the pads and pistons back from the rubbing surface. During this time, there can be significant brake drag. This brake drag can lead to significant parasitic power loss, thus impacting fuel economy and overall vehicle performance.


Early brake system

In the 1890s, Wooden block brakes became obsolete when Michelin brothers introduced rubber tires. [9]

During the 1960s, some car manufacturers replaced drum brakes with disc brakes. [10]

Electronic brake system

In 1966, the ABS was fitted in the Jensen FF grand tourer. [11]

In 1978, Bosch and Mercedes updated their 1936 anti-lock brake system for the Mercedes S-Class. That ABS is a fully electronic, four-wheel and multi-channel system that later became standard. [12]

In 2005, ESC — which automatically applies the brakes to avoid a loss of steering control — become compulsory for carriers of dangerous goods without data recorders in the Canadian province of Quebec. [13]

Since 2017, numerous United Nations Economic Commission for Europe (UNECE) countries use Brake Assist System (BAS) a function of the braking system that deduces an emergency braking event from a characteristic of the driver's brake demand and under such conditions assist the driver to improve braking. [14]

In July 2013 [15] UNECE vehicle regulation 131 was enacted. This regulation defines Advanced Emergency Braking Systems (AEBS) for heavy vehicles to automatically detect a potential forward collision and activate the vehicle braking system.

On 23 January 2020 [16] UNECE vehicle regulation 152 was enacted, defining Advanced Emergency Braking Systems for light vehicles.

From May 2022, in the European Union, by law, new vehicles will have advanced emergency-braking system. [17]

See also

Related Research Articles

Clutch Mechanical device that connects and disconnects two rotating shafts or other moving parts

A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts. In these devices, one shaft is typically attached to an engine or other power unit, while the other shaft provides output power for work. Typically the motions involved are rotary, but linear clutches also exist.

Friction Force resisting sliding motion

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:

Disc brake Mechanism using friction to resist rotation of a circular plate

A disc brake is a type of brake that uses the calipers to squeeze pairs of pads against a disc or a "rotor" to create friction. This action slows the rotation of a shaft, such as a vehicle axle, either to reduce its rotational speed or to hold it stationary. The energy of motion is converted into waste heat which must be dispersed.

Drum brake Type of vehicle brake

A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called a brake drum.

Bicycle brake Braking device for bicycles

A bicycle brake reduces the speed of a bicycle or prevents it from moving. The three main types are: rim brakes, disc brakes, and drum brakes.

Engine braking occurs when the retarding forces within an engine are used to slow down a motor vehicle, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes.

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. Brake fade can be a factor in any vehicle that utilizes a friction braking system including automobiles, trucks, motorcycles, airplanes, and bicycles.

Master cylinder

In automotive engineering, the master cylinder is a control device that converts force into hydraulic pressure. This device controls slave cylinders located at the other end of the hydraulic brake system.

Eddy current brake

An eddy current brake, also known as an induction brake, electric brake or electric retarder, is a device used to slow or stop a moving object by dissipating its kinetic energy as heat. Unlike friction brakes, where the drag force that stops the moving object is provided by friction between two surfaces pressed together, the drag force in an eddy current brake is an electromagnetic force between a magnet and a nearby conductive object in relative motion, due to eddy currents induced in the conductor through electromagnetic induction.

Hydraulic brake

A hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing glycol ethers or diethylene glycol, to transfer pressure from the controlling mechanism to the braking mechanism.

Brake lining Consumable surfaces in brake systems

Brake linings are the consumable surfaces in brake systems, such as drum brakes and disc brakes used in transport vehicles.

Track brake Track brake

A magnetic track brake is a brake for rail vehicles. It consists of brake magnets, pole shoes, a suspension, a power transmission and, in the case of mainline railroads, a track rod. When current flows through the magnet coil, the magnet is attracted to the rail, which presses the pole shoes against the rail, thereby decelerating the vehicle.

Brake pads are a component of disc brakes used in automotive and other applications. Brake pads are composed of steel backing plates with friction material bound to the surface that faces the disc brake rotors.

Clutch control refers to the act of controlling the speed of a vehicle with a manual transmission by partially engaging the clutch plate, using the clutch pedal instead of the accelerator pedal. The purpose of a clutch is in part to allow such control; in particular, a clutch provides transfer of torque between shafts spinning at different speeds. In the extreme, clutch control is used in performance driving, such as starting from a dead stop with the engine producing maximum torque at high RPM.

An S-cam is part of a braking system used in heavy vehicles such as trucks and wheeled machinery. It consists of a shaft, usually around 4 to 25 inches long, turned at one end by means of an air-powered brake booster and lever with an 'S' shaped cam at the wheel end. Turning the shaft pushes the brake shoes against the drum, producing friction.

Brake shoe

A brake shoe is the part of a braking system which carries the brake lining in the drum brakes used on automobiles, or the brake block in train brakes and bicycle brakes. A device that is put on a track to slow down railroad cars is also called brake shoe.

The Ausco-Lambert disc brake is an unusual brake where an axially-expanding shoe assembly is sandwiched between two linked rotating discs. It may be thought of as an "inside out" disc brake: instead of pads pinching a disc, the pads expand inside a hollow disc.

Power brakes are a system of hydraulics used to slow down or stop most motor vehicles. It uses a combination of mechanical components to multiply the force applied to the brake pedal by the driver into enough force to actuate the brakes and stop a vehicle that can weigh several tons. The brake pedal is connected to the vacuum booster which is the first step of the force multiplication. The booster passes the force to the master cylinder, which applies a compressive force to a liquid and forces it through the brake lines to the brake calipers. The liquid pushes the brake calipers, which in the case of disc brakes, push against the brake rotor causing friction that slows and eventually stops the rotation of the vehicles wheels. In drum brakes, pistons push two shoes against the brake drum accomplishing the same effect.

An electric friction brake, often referred to as just electric brake or electric trailer brake, is a brake controlled by an electric current and can be seen on medium duty trailers like caravans/RVs and consumer-grade car trailers. It is related to the electromagnetic track brake used in railways which also use electric current to directly control the brake force.

Motorcycle braking systems

Motorcycle braking systems have varied throughout time, as motorcycles evolved from bicycles with an engine attached, to the 220 mph (350 km/h) prototype motorcycles seen racing in MotoGP. Most systems work by converting kinetic energy into thermal energy (heat) by friction. On motorcycles, approximately 70% of the braking effort is performed by the front brake. This however can vary for individual motorcycles; longer-wheelbase types having more weight biased rearward, such as cruisers and tourers, can have a`greater effort applied by the rear brake. In contrast, sports bikes with a shorter wheelbase and more vertical fork geometry can tolerate higher front braking loads. For these reasons, motorcycles tend to have a vastly more powerful front brake compared to the rear.


  1. Bhandari, V.B. (2010). Design of machine elements. Tata McGraw-Hill. p. 472. ISBN   9780070681798 . Retrieved 9 February 2016.
  2. "Definition of brake". The Collins English Dictionary. Retrieved 9 February 2016.
  3. "Foundation Brakes". Retrieved 2017-07-22.
  4. Nice, Karim (2000-08-22). "How Power Brakes Work". Retrieved 2011-03-12.
  5. C.Michael Hogan, Analysis of highway noise, Journal of Water, Air, & Soil Pollution, Volume 2, Number 3, Biomedical and Life Sciences and Earth and Environmental Science Issue, Pages 387-392, September, 1973, Springer Verlag, Netherlands ISSN   0049-6979
  6. David Hench (May 8, 2014). "Train-sparked fires cause explosions, destroy trailers, force evacuations". Portland Press Herald.
  7. "Mercedes explains Hamilton brake fire on Mugello F1 grid". Retrieved 2020-11-21.
  8. "Sprinter 311 Rear Brakes on fire". Mercedes-Benz Owners' Forums. Retrieved 2020-11-21.
  9. "The History of Brakes | Did You Know Cars". 28 August 2017.
  10. "The History of Brakes | Did You Know Cars". 28 August 2017.
  11. "The History of Brakes | Did You Know Cars". 28 August 2017.
  12. "The History of Brakes | Did You Know Cars". 28 August 2017.
  13. Roll Stability Control system (RSC) Archived 2011-07-16 at the Wayback Machine
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  17. "Parliament approves EU rules requiring life-saving technologies in vehicles | News | European Parliament". 2019-04-16. Retrieved 2020-08-31.