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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]

Background

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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 a significant portion of the otherwise wasted kinetic energy of a moving vehicle into electrical energy, which can be stored in batteries for later use.[3] 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 transform the 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 utilize a combination of braking mechanisms, such as drag racing cars equipped with both wheel brakes and a parachute, or airplanes that employ both wheel brakes and 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. Consequently, the theoretical braking distance, when braking at the traction limit, is up to 100 times as long. In practice, fast vehicles typically experience significant air drag, and the energy lost to air drag increases rapidly 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. In a brake drum design, a similar action is employed, involving a cylinder that pushes the brake shoes against the drum, thereby slowing the rotation.

Types

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Rendering showing the cylinder and brake shoes inside a drum brake
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:

Frictional

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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

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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

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Main articles: Electromagnetic brake and Regenerative braking

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.

Characteristics

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Brakes are often described according to several characteristics including:

  • Peak force – The peak force is the maximum decelerating effect that can be obtained. The peak force is often greater than the traction limit of the tires, in which case the brake can cause a wheel skid.
  • Continuous power dissipation – Brakes typically get hot in use and fail when the temperature gets too high. The greatest amount of power (energy per unit time) that can be dissipated through the brake without failure is the continuous power dissipation. Continuous power dissipation often depends on e.g., the temperature and speed of ambient cooling air.
  • Fade – As a brake heats, it may become less effective, called brake fade. Some designs are inherently prone to fade, while other designs are relatively immune. Further, use considerations, such as cooling, often have a big effect on fade.
  • Smoothness – A brake that is grabby, pulses, has chatter, or otherwise exerts varying brake force may lead to skids. For example, railroad wheels have little traction, and friction brakes without an anti-skid mechanism often lead to skids, which increases maintenance costs and leads to a "thump thump" feeling for riders inside.
  • Power – Brakes are often described as "powerful" when a small human application force leads to a braking force that is higher than typical for other brakes in the same class. This notion of "powerful" does not relate to continuous power dissipation, and may be confusing in that a brake may be "powerful" and brake strongly with a gentle brake application, yet have lower (worse) peak force than a less "powerful" brake.
  • Pedal feel – Brake pedal feel encompasses subjective perception of brake power output as a function of pedal travel. Pedal travel is influenced by the fluid displacement of the brake and other factors.
  • Drag – Brakes have varied amount of drag in the off-brake condition depending on design of the system to accommodate total system compliance and deformation that exists under braking with ability to retract friction material from the rubbing surface in the off-brake condition.
  • Durability – Friction brakes have wear surfaces that must be renewed periodically. Wear surfaces include the brake shoes or pads, and also the brake disc or drum. There may be tradeoffs, for example, a wear surface that generates high peak force may also wear quickly.
  • Weight – Brakes are often "added weight" in that they serve no other function. Further, brakes are often mounted on wheels, and unsprung weight can significantly hurt traction in some circumstances. "Weight" may mean the brake itself, or may include additional support structure.
  • Noise – Brakes usually create some minor noise when applied, but often create squeal or grinding noises that are quite loud.

Foundation components

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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.[4]

Brake boost

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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.[5] 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.

Noise

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Brake lever on a horse-drawn hearse
Main article: Roadway noise

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.[6] Noise can be caused by different things. These are signs that there may be issues with brakes wearing out over time.

Fires

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Railway brake malfunctions can produce sparks and cause forest fires.[7] 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.[8] 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.[9]

Inefficiency

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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.

History

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Early brake system

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In the 1890s, Wooden block brakes became obsolete when Michelin brothers introduced rubber tires.[10]

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

Electronic brake system

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In 1966, an ABS was fitted in the Jensen FF grand tourer. There was no electronic control: wheel lock-up was detected mechanically by a Dunlop Maxaret unit fitted to the centre differential of the Ferguson four wheel drive system and brake pressure was modulated by a solenoid actuated valve.[10]

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.[10]

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.[11]

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.[12]

In July 2013[12] 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[12] 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.[13]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Brake

View on Grokipedia
from Grokipedia
A brake is a mechanical device used for retarding or stopping motion by or power means, essential for controlling the speed and direction of , machinery, and other moving systems. In automotive applications, brakes function by converting the of a moving into through between components, allowing drivers to slow down or halt safely. This process is typically initiated by pressing a brake pedal, which activates hydraulic or pneumatic systems to apply force to materials against rotating parts. The two primary types of brakes in modern passenger vehicles are disc brakes and drum brakes, with disc systems predominant due to their superior heat dissipation and performance. Disc brakes employ a caliper assembly that squeezes brake pads against a rotating disc (rotor) attached to the , generating to slow the vehicle; this design is efficient for frequent or heavy braking and is often used on all four wheels in contemporary cars. Drum brakes, conversely, feature curved brake shoes that expand outward to press against the inner surface of a rotating drum, a simpler and more cost-effective option commonly found on rear axles or in older vehicles, though they are prone to overheating during prolonged use. transmits the pedal force in most systems, ensuring even distribution of braking power across wheels. Braking technology has evolved significantly since early mechanical systems in the late , with hydraulic brakes introduced in the for automobiles and air brakes developed in 1872 for railways and heavy vehicles. Modern advancements include anti-lock braking systems (ABS), first commercialized in the late , which modulate brake pressure to prevent lockup on slippery surfaces, enhancing steering control and reducing stopping distances. and in electric vehicles further improve safety and efficiency by recovering energy during deceleration. These innovations reflect ongoing efforts to balance performance, durability, and environmental impact in braking .

Overview

Definition and Purpose

A brake is a mechanical device that inhibits motion by absorbing energy from a moving system, typically converting into heat or other forms such as or . This process applies resistance to rotating or linear components, enabling controlled deceleration, complete stops, or stationary holding of loads. The primary purposes of brakes encompass deceleration to ensure during operation, maintaining stationary positions for loads in various systems, and regulating speed in dynamic environments like machinery, vehicles, and elevators. In essence, brakes counteract the natural tendency of objects in motion to continue moving, as described by Newton's of motion, which states that an object remains in uniform motion unless acted upon by an external force. During braking, this opposing force—often generated through between brake components and the moving parts—produces deceleration in accordance with Newton's second law, where the net equals mass times acceleration (F = ma), allowing the system to slow or halt predictably. Brakes find essential applications across diverse sectors, including automotive for routine stopping and emergency maneuvers, rail systems for managing speeds on tracks, for landing and , and industrial settings for controlling heavy machinery and conveyor operations. Effective braking plays a critical role in , with advanced systems like automatic emergency braking (AEB) projected to prevent at least 360 fatalities and 24,000 injuries annually in the United States by reducing rear-end and pedestrian crashes. Studies indicate that such technologies can lower rear-end crash rates by 46-52% in passenger , contributing to overall collision reductions of up to 50% in equipped .

Historical Context

Early wheeled vehicles in ancient civilizations relied primarily on controlling draft animals to stop, with mechanical braking mechanisms emerging much later. Simple brakes, such as wooden blocks or spoons pressed against wheels, appeared in horse-drawn carriages by the . The brought significant progress, particularly in , where iron-shod wheels and basic lever systems were developed for locomotives. Early locomotives like George , introduced in 1829, lacked dedicated brakes, relying on engine reversal; manual wheel brakes were soon applied to tenders and cars in the 1830s to manage speeds on early railways. In the early , automobiles drove further innovation, with cable-operated brakes adopted by 1900 in vehicles like the , enclosing brake shoes within a drum for enhanced durability and weather resistance. Hydraulic systems followed, patented by Malcolm Loughead in 1918 for a fluid-actuated design that transmitted pressure evenly to all wheels. This hydraulic evolution addressed limitations of mechanical linkages, enabling safer and more consistent braking as vehicle speeds increased.

Types

Friction Brakes

Friction brakes are the most prevalent braking systems in automotive applications, relying on the direct contact between frictional surfaces to decelerate vehicles by converting into . These systems generate stopping force through the rubbing action of brake pads or shoes against rotating components, dissipating to slow or halt motion. The core mechanism involves applying to press materials against a rotating surface, producing a tangential that opposes wheel motion. This process follows of dry , where the maximum FF is expressed as
F=μNF = \mu N
with μ\mu as the coefficient of and NN as the perpendicular to the contact surfaces. The resulting heat from must be effectively dissipated to maintain , as inadequate cooling can lead to thermal issues. Key subtypes include disc brakes and brakes. Disc brakes feature a rotating rotor (disc) attached to the , clamped by brake pads housed in a caliper that applies hydraulic for even force distribution on both sides. This design, patented by Frederick William Lanchester in 1902, provides superior heat dissipation due to exposed surfaces. brakes, conversely, use internal expanding shoes pressed against the inner surface of a rotating , a configuration often employed on rear wheels for its self-energizing effect and integration with parking mechanisms. Friction materials have evolved significantly for and . Historically, asbestos-based composites dominated due to their high stability and , but risks from inhalation prompted a phase-out beginning in the , with most manufacturers ceasing production by the . More recently, the U.S. Environmental Protection Agency finalized a ban on asbestos in 2024, effectively eliminating its remaining use in automotive . Modern alternatives include semi-metallic pads, incorporating or fibers for enhanced heat resistance and , and ceramic composites, which use carbon or fibers for low noise, minimal dust, and operation at temperatures up to 800°C. These materials balance coefficients typically between 0.3 and 0.5 while reducing wear on mating surfaces. Friction brakes offer advantages such as high braking for rapid deceleration, mechanical simplicity in , and cost-effectiveness compared to advanced alternatives. However, wear factors like heat dissipation are critical; poor ventilation can cause , a progressive loss of effectiveness when interface temperatures exceed 500°C, reducing the by up to 50% due to degradation or vaporization. Effective cooling, often via ventilated rotors or airflow, mitigates this, ensuring sustained performance under repeated loading.

Pumping Brakes

Pumping brakes encompass hydraulic and pneumatic systems designed to amplify and transmit braking force using fluid or pressure, enabling efficient force application across vehicle wheels. These systems rely on the principle of , which states that pressure applied to an incompressible fluid in a is transmitted equally in all directions throughout the fluid. In hydraulic setups, a connected to the brake pedal converts the driver's mechanical input into hydraulic pressure within the , which then actuates slave cylinders (also known as wheel cylinders) at each wheel to apply the force. Hydraulic pumping brakes, first introduced in the late and widely adopted by automakers in the , initially featured single-circuit designs where a single fluid path served all wheels. To enhance safety through , dual-circuit systems were developed, splitting the hydraulic lines—typically front/rear or diagonally—to ensure partial braking capability if one circuit fails; these became mandatory in U.S. vehicles starting in 1967. The core equation governing generation is P=FAP = \frac{F}{A}, where PP is the hydraulic , FF is the applied , and AA is the area in the ; this allows for through larger slave cylinder areas, multiplying the force for effective braking. Pneumatic variants, commonly known as air brakes, are prevalent in heavy vehicles such as trucks and buses, where an engine-driven fills reservoirs with at pressures around 100–120 psi. Upon pedal activation, air from the reservoirs flows through valves to brake chambers, pushing diaphragms or pistons that apply the brakes; this setup suits high-load applications due to the scalability of air storage. Both hydraulic and pneumatic pumping systems offer advantages including even force distribution across multiple wheels, reducing uneven wear, and self-adjusting mechanisms that compensate for brake lining wear over time. These systems typically interface with surfaces at the wheels to generate stopping , as detailed in the friction brakes section. However, limitations include potential fluid leaks in hydraulic systems, which can cause loss and brake if not addressed, and inherent delays in pneumatic systems due to air compression and transmission, typically ranging from 0.2 to 0.5 seconds of brake lag.

Electromagnetic Brakes

Electromagnetic brakes generate stopping force through without physical contact, making them suitable for applications requiring high-speed operation or frequent engagement cycles. The core mechanism relies on , where a changing induces eddy currents in a nearby conductor according to Faraday's law. These eddy currents, in turn, create an opposing per , which resists the relative motion and produces a braking effect. Key subtypes include eddy current brakes and magnetic particle brakes. Eddy current brakes induce circulating currents in a non-magnetic conductor, such as a metal disc or rail, exposed to a moving from electromagnets, resulting in non-contact deceleration with no mechanical wear. Magnetic particle brakes employ fine ferromagnetic particles suspended in a viscous fluid; upon energizing the coil, the particles magnetize and form chain-like structures that shear under rotation, transmitting proportionally to the strength in a clutch-like manner. These brakes find prominent use in and systems, where variants provide auxiliary braking to supplement primary systems, enabling smooth deceleration at velocities exceeding 300 km/h without frictional heat. In elevators, electromagnetic brakes serve as safety mechanisms for holding loads and emergency stops, activating rapidly to grip sheaves or rails. Notable advantages encompass rapid response—typically within milliseconds due to electromagnetic activation—and reduced maintenance, as the absence of contacting surfaces eliminates wear on brake pads or linings. The braking TT for eddy current brakes follows the relation T∝B2ωr2T \propto B^2 \omega r^2 where BB is the strength, ω\omega is the , and rr is the effective radius of the conductor, derived from the power dissipated by induced currents and Lorentz forces. Despite these benefits, electromagnetic brakes require ongoing electrical power to sustain the , contributing to use, and produce internal heating from resistive losses in the induced currents or particle fluid.

Components and Mechanisms

Core Components

The core components of brake systems encompass the fundamental hardware elements that enable the conversion of into through , primarily in disc and configurations. These include rotors or , materials such as or shoes, or wheel cylinders, hydraulic lines and reservoirs, and basic sensors for monitoring key states. Designed for under high and mechanical loads, these parts ensure reliable while minimizing wear and failure risks. Rotors, also known as discs in disc brake systems, serve as the primary heat-absorbing surfaces against which friction materials are pressed. The most common material is gray cast iron due to its favorable thermal conductivity, wear resistance, and cost-effectiveness, with a specific heat capacity typically around 450-500 J/kg·K that allows effective dissipation of braking heat. For high-performance applications, carbon-ceramic composites are used, offering higher specific heat capacities of approximately 800-900 J/kg·K, lighter weight, and superior resistance to thermal fade, though at a higher cost. Ventilation designs in rotors, such as straight, curved, or pillar vanes between inner and outer plates, enhance airflow to cool the component during repeated braking, reducing the risk of overheating and warping. Brake pads (for disc systems) or shoes (for drum systems) provide the friction interface that grips the or to generate stopping force. Common compositions include organic materials, which use resins, fibers, and fillers for quiet operation and low dust; semi-metallic formulations incorporating , iron powder, and for higher heat tolerance and durability; and ceramic options with synthetic fibers for reduced and rotor wear. Typical friction coefficients range from 0.3 to 0.5 under standard conditions, balancing with minimal rotor abrasion. The bedding-in , involving controlled heating cycles through moderate to firm stops from speeds like 50-60 mph without full lockup, transfers a thin layer of pad to the rotor surface, optimizing initial friction and even wear distribution over the first 200-400 miles. Calipers in disc brakes or wheel cylinders in drum brakes house the pistons that apply force to the friction materials. Floating caliper designs feature pistons on one side only, with the caliper body sliding via guide pins to press the opposite pad against the rotor, offering simplicity and lighter weight for most passenger vehicles. Fixed calipers, with pistons on , provide more even pressure distribution and better performance under heavy loads but require more space and complexity. Sealing is achieved through O-rings and square-section elastomers around pistons, preventing leaks while allowing slight retraction for pad clearance. Hydraulic lines and reservoirs transmit from the to the or cylinders, using incompressible fluid to amplify pedal force. Steel-braided hoses, often with PTFE inner liners, enhance by resisting abrasion, expansion under , and degradation compared to rubber alternatives, making them suitable for demanding environments. Brake fluids adhere to DOT standards: DOT 3 and DOT 4 are glycol-ether based with minimum dry points of 205°C and 230°C respectively, while DOT 5.1 offers similar with improved low-temperature performance, all hygroscopic to varying degrees requiring periodic replacement. Basic sensors in pre-electronic brake systems primarily consist of mechanical position indicators, such as limit switches or cables linked to levers for engagement status, and hydraulic pressure gauges or relief valves that monitor line pressure to detect leaks or low fluid levels without electronic processing. These passive components provided essential feedback for , predating integrated electronic controls.

Brake Assistance Systems

Brake assistance systems enhance the driver's applied to the brake pedal, ensuring consistent and effective hydraulic for braking without requiring excessive physical effort. These systems primarily employ servo mechanisms to amplify pedal input, allowing for safer and more responsive control, particularly in passenger cars where driver or varying physical capabilities can impact performance. By integrating with the , assistance systems multiply the , transforming a modest pedal into substantial hydraulic transmitted to the brakes. Vacuum boosters, the most common type in gasoline-engine vehicles, utilize a diaphragm design powered by the engine's manifold to achieve . The booster housing is divided into two chambers separated by a flexible rubber diaphragm attached to a pushrod that connects to the ; when the brake pedal is depressed, a admits to one side of the diaphragm while maintaining on the other, creating a pressure differential that amplifies the force by 2 to 3 times. This design leverages the engine's partial (typically 50-70 kPa at ) to assist in generating up to 300-500 N of additional force on the piston, depending on diaphragm size (e.g., 8-11 inches in ). Hydraulic boosters, often used in diesel vehicles or heavy-duty applications lacking sufficient , integrate with the power system via master cylinders to provide assistance. These systems draw pressurized from the power , directing it through a spool mechanism in the booster to apply force to the ; the configuration features two independent pistons and reservoirs, ensuring split-circuit operation where front and rear brakes can function separately if needed. This integration allows for reliable boosting even at low speeds, with fluid pressures around 7-14 MPa amplifying pedal input similarly to systems. Electro-mechanical (electric) brake boosters represent a modern alternative, particularly in electric and hybrid vehicles that lack engine vacuum. These systems use an and gearbox to generate the boosting force, controlled by electronic signals from the brake pedal , providing precise and tunable assistance. Adopted widely since the and standard in most battery electric vehicles as of 2025, electric boosters offer advantages like integration with systems, faster response times, and reduced weight compared to vacuum setups, with market growth projected at over 9% CAGR through 2035. The core principle of power braking involves servo assistance that reduces required pedal effort from approximately 100-200 N without aid to 20-50 N with the booster active, enabling average drivers to achieve deceleration rates of 0.6-0.8 g comfortably. designs incorporate return springs in the to retract pistons upon pedal release and dual hydraulic circuits to preserve braking in at least one if assistance or a line fails, preventing total loss of function. boosters became widely adopted in passenger cars starting in the mid-1950s, following patents and initial implementations by manufacturers like Ford and , marking a shift toward safer, driver-friendly braking.

Performance and Issues

Efficiency Factors

In traditional friction brakes, the kinetic energy of a moving is entirely dissipated as through between the brake and rotors or , resulting in no recoverable and contributing to overall system inefficiency. This complete conversion to occurs because the braking generates frictional that is released into the surrounding environment, with no mechanism for storage or in conventional systems. For instance, stopping a 1600 kg from 120 km/h dissipates approximately 0.25 kWh of solely as . Repeated braking applications exacerbate inefficiencies due to thermal buildup, leading to where stopping power diminishes as components overheat. A key factor in thermal fade is the boiling of , which typically has a dry of around 230°C for standard DOT 4 fluid; once exceeded, vapor bubbles form in the hydraulic lines, reducing pressure transmission and braking effectiveness. Mass reduction techniques, such as lighter brake components or overall vehicle weight optimization, improve efficiency by lowering the that must be dissipated (KE = ½mv²), thereby reducing heat generation and fade risk. Regenerative braking addresses these limitations in hybrid and electric vehicles by converting back into for battery storage, potentially recovering up to 60% of the braking depending on system design and driving conditions. This recovery efficiency is defined as the ratio of recaptured to the total available during deceleration, offering a conceptual alternative to full dissipation. However, in traditional systems, additional inefficiencies arise from aerodynamic drag, which dissipates some as air resistance during the stopping , and from component , which amplifies the total requiring conversion to . Braking performance metrics, such as stopping distance, quantify these efficiency aspects via the formula d=v22ad = \frac{v^2}{2a}, where dd is the distance, vv is initial , and aa is deceleration rate—typically 3–5 m/s² for passenger vehicles under normal conditions. This derives from kinematic principles assuming constant deceleration, highlighting how factors like fade or directly influence aa and thus overall .

Noise and Vibration

Brake noise and vibration are significant concerns in automotive systems, arising primarily from dynamic interactions between friction components during operation. High-frequency squeal, often perceived as a shrill sound, results from stick-slip vibrations at the interface between brake pads and rotors, where alternating phases of static and kinetic friction generate self-excited oscillations in the 1-16 kHz range. This phenomenon is exacerbated by factors such as pad material composition and surface geometry, leading to unstable modal coupling within the brake assembly. In contrast, brake judder manifests as low-frequency vibrations, typically caused by rotor thickness variations or uneven wear—commonly misattributed to warping—inducing torsional oscillations that produce a pulsating feel through the pedal and steering wheel. These vibrations propagate from the brake system through the suspension components to the , amplifying perceptible harshness for occupants. Judder frequencies generally fall between 50-200 Hz, correlating with rotational speed and transmitting structural modes that can resonate with the body. Such transmission paths are analyzed using techniques like transfer path analysis to identify dominant routes, such as knuckle-to-subframe connections, where insufficient allows energy to couple into the cabin. Mitigation strategies focus on interrupting vibration sources and paths through design modifications. Chamfered leading edges on brake pads reduce initial contact instabilities, while anti-noise shims—thin layers of viscoelastic materials bonded to the pad backing—decouple vibrations from the caliper and provide damping ratios up to 20-30%. Additional damping materials, such as rubberized underlayers or filament-wound composites, further attenuate resonances. Industry standards for (NVH) evaluation, including SAE J2521 for dynamometer testing and SAE J2786 for , guide rigorous assessment to ensure compliance with annoyance limits during development. Brake pad material selection plays a crucial role in performance, with low-metallic formulations—incorporating reduced and fibers in a matrix—typically achieving 10-20 dB lower sound levels compared to semi-metallic pads due to smoother profiles and inherent . From a standpoint, brake noises exceeding 70 dB(A) are often rated as irritating, crossing thresholds even in urban driving environments where background levels hover around 60-65 dB(A). These perceptual metrics underscore the importance of subjective evaluations in NVH refinement, balancing acoustic comfort with functional durability.

Fire Risks

Brake fires in vehicles typically ignite due to excessive overheating, particularly during prolonged downhill braking where generates intense in the brake pads and rotors. In such scenarios, brake temperatures can rise significantly, with heavy trucks on steep grades reaching levels that ignite wheel bearing grease or even tires if cooling is inadequate. For instance, worn brake pads allowing metal-to-metal contact can produce sparks, while low fluid levels exacerbate overheating, directly leading to vehicle . Oil or grease on brake surfaces further lowers the ignition threshold by providing additional sources. Once ignited, fires can propagate rapidly if brake fluid leaks onto hot components. Glycol-based brake fluids used in DOT 3 and DOT 4 specifications have flash points ranging from 100°C to 150°C, enabling them to vaporize and sustain flames when exposed to . Historical incidents in the highlighted this vulnerability, with U.S. commercial fires causing nearly 140 fatalities in alone, often linked to mechanical overheating during heavy loads or descents. In modern contexts, from braking inefficiency—where converts to —can accelerate these risks if not managed. Mechanical failure or malfunction was the leading contributing factor in fires, accounting for 45% of ignitions from to 2016. Prevention strategies focus on heat dissipation and material resilience. Cooling fins integrated into rotors and increase surface area for , while thermal barriers such as specialized coatings or steel plates shield reservoirs from radiant heat. Fire-retardant alternatives like DOT 5 silicone-based fluids offer higher flash points above 260°C, reducing flammability in high-risk applications such as racing or heavy-duty vehicles. Regular maintenance, including pad inspections and fluid checks, is essential to avoid and . Regulations mandate heat resistance to minimize fire hazards. The Federal Motor Vehicle Safety Standard (FMVSS) 135 requires light vehicle brakes to undergo hot performance tests, simulating repeated stops to ensure functionality after heating without failure or ignition risk. Notable case studies illustrate these dangers. In commercial vehicles, operating with engaged parking brakes has caused wheel-end s, as documented in three SAE analyses where friction overheated components, melting tires and spreading to the undercarriage. Similarly, a 2025 General Motors recall affected over 62,000 trucks due to brake pressure sensor leaks that could short-circuit and ignite fluid. The also investigated nearly 500,000 semi-trucks in 2021 for overheating brakes prone to spontaneous fires during operation.

Developments

Historical Evolution

The development of modern brake systems accelerated in the post-World War II era with the invention of anti-lock braking systems (ABS), which Bosch patented in 1936 as a mechanism to prevent locking during braking. This innovation addressed the limitations of mechanical brakes by modulating hydraulic pressure to maintain steering control and reduce skidding. Commercialization occurred in 1978 when Bosch introduced ABS on the , marking the first widespread automotive application and significantly improving safety on slippery surfaces. In the and , advancements focused on electronic integration, with electronic brake-force distribution (EBD) emerging to dynamically allocate braking force between axles based on load and traction conditions. pioneered EBD in 1997, integrating it with ABS to optimize stopping distances without rear-wheel lockup. Concurrently, traction control systems (TCS) were introduced, starting with in 1987 on models like the S-Class, using engine and brake interventions to prevent wheel spin during acceleration. These systems laid the groundwork for more sophisticated electronic controls, building briefly on early 20th-century mechanical foundations. The 2000s saw the rise of technologies, which replaced traditional hydraulic linkages with electronic signals for faster response times, first partially adopted in Mercedes-Benz's on the 2001 SL-Class for enhanced brake assist. In hybrid vehicles, became prominent, capturing kinetic energy during deceleration to recharge batteries, as seen in the since its 2000 U.S. launch. Full systems expanded in the decade, enabling seamless integration with electric powertrains. Key milestones included the U.S. mandate for (ESC) in 2012, requiring all new passenger vehicles to feature systems that apply selective braking to prevent skids. Similarly, the introduced advanced in 2012, achieving up to 60% in urban driving. These evolutions were driven by stringent regulations, such as the Economic Commission for (UNECE) Regulation 13, which since the has set global standards for brake performance and stability. Advancing computing power also enabled , combining data from wheel speed, yaw, and acceleration sensors for real-time decision-making in systems like ESC and EBD.

Future Innovations

Advancements in systems for electric vehicles (EVs) are focusing on full to optimize during deceleration. These systems leverage independent at each wheel to distribute braking forces precisely, enabling up to 70-85% recapture of that would otherwise be lost as heat in traditional brakes. For instance, the 2023 Dual-Motor variant incorporates brake-based virtual alongside its adjustable modes, enhancing efficiency in varied driving conditions like off-road or highway descent. This evolution builds on existing electronic systems by integrating AI-driven allocation for smoother without compromising stability. Brake-by-wire technology is evolving toward fully electronic, electromechanical systems that eliminate hydraulic components entirely, relying on electric actuators at each for precise control. These "dry" systems, as developed by ZF and Nexteer, remove the need for , reducing weight and maintenance while enabling seamless integration with advanced driver assistance features like . In 2025 prototypes from Bosch and ZF, such systems support Level 4 autonomy by providing millisecond-response braking that synchronizes with autonomous navigation, allowing vehicles to handle complex urban scenarios without mechanical backups. This shift facilitates over-the-air updates for braking algorithms, further aligning with the demands of software-defined vehicles. Research into is introducing shape-memory alloys (SMAs) for self-adjusting brake components, where or stress triggers shape changes to maintain optimal pad-to-rotor contact. Nickel-titanium (NiTi) SMAs, known for their superelastic properties, are being explored in brake actuators and to dynamically compensate for wear, potentially extending component life in high-stress applications. These alloys enable adaptive surfaces that realign under operational heat, reducing uneven wear patterns observed in conventional pads and improving overall durability without manual adjustments. Sustainability efforts in brake design emphasize bio-based friction materials derived from renewable sources, such as husks, fibers, and other plant wastes, to replace and metals that contribute to environmental . These green composites maintain comparable frictional performance while lowering the of production, with studies showing effective integration in non-asbestos formulations for passenger vehicles. Recyclable components, like Brembo's 2025 aluminum calipers made from 100% recycled content, further support principles by cutting lifecycle emissions by up to 70%. The is driving these innovations through Euro 7 regulations, effective from 2026, which cap brake dust emissions at 3-11 mg/km and mandate copper-free pads to minimize non-exhaust , aligning with broader zero- goals by 2030 under the EU Action Plan. Despite these progresses, challenges persist in cybersecurity for connected brake systems, where electronic interfaces expose vulnerabilities to remote hacks that could compromise braking integrity in autonomous fleets. Potential threats include signal interception or injection into networks, necessitating robust and intrusion detection as vehicles become more interconnected. The global automotive brake system market is projected to reach approximately $30 billion by , fueled by demand for these advanced, sustainable technologies in EVs and autonomous vehicles.

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