Braking distance

Braking distance refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. It is primarily affected by the original speed of the vehicle and the coefficient of friction between the tires and the road surface, and negligibly by the tires' rolling resistance and vehicle's air drag. The type of brake system in use only affects trucks and large mass vehicles, which cannot supply enough force to match the static frictional force. The braking distance is one of two principal components of the total stopping distance. The other component is the reaction distance, which is the product of the speed and the perception-reaction time of the driver/rider. A perception-reaction time of 1.5 seconds, and a coefficient of kinetic friction of 0.7 are standard for the purpose of determining a bare baseline for accident reconstruction and judicial notice; most people can stop slightly sooner under ideal conditions. Braking distance is not to be confused with stopping sight distance. The latter is a road alignment visibility standard that provides motorists driving at or below the design speed an assured clear distance ahead (ACDA) which exceeds a safety factor distance that would be required by a slightly or nearly negligent driver to stop under a worst likely case scenario: typically slippery conditions (deceleration 0.35g) and a slow responding driver (2.5 seconds). Because the stopping sight distance far exceeds the actual stopping distance under most conditions, an otherwise capable driver who uses the full stopping sight distance, which results in injury, may be negligent for not stopping sooner. Derivation Energy equation The theoretical braking distance can be found by determining the work required to dissipate the vehicle's kinetic energy. The kinetic energy E is given by the formula: , where m is the vehicle's mass and v is the speed at the start of braking. The work W done by braking is given by: , where μ is the coefficient of friction between the road surface and the tires, g is the gravity of Earth, and d is the distance travelled. The braking distance (which is commonly measured as the skid length) given an initial driving speed v is then found by putting W = E, from which it follows that . The maximum speed given an available braking distance d is given by: . Newton's Law and Equation of Motion From Newton's second law: For a level surface, the frictional force resulting from coefficient of friction {displaystyle mu } is: Equating the two yields the deceleration: The  form of the formulas for constant acceleration is: Setting  and then substituting  into the equation yields the braking distance: Total stopping distance This section needs expansionwith: This is a very simplistic and conceptual theoretical model for how to calculate braking distance. It assumes that the only parameters for the friction force are μ (coefficient of friction) and g(gravity of Earth), which is not the case in real world...

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Emergency brake assist

Emergency brake assist (EBA) or brake assist (BA or BAS) is a generic term for an automobile braking technology that increases braking pressure in an emergency. The first application was developed jointly by Daimler-Benz and TRW/LucasVarity. Research conducted in 1992 at the Mercedes-Benz driving simulator in Berlin revealed that more than 90% of drivers fail to brake with enough force when faced with an emergency. By interpreting the speed and force with which the brake pedal is pushed, the system detects if the driver is trying to execute an emergency stop, and if the brake pedal is not fully applied, the system overrides and fully applies the brakes until the anti-lock braking system (ABS) takes over to stop the wheels locking up. This is a lower level of automation than a collision avoidance system, which may initiate braking on its own if the onboard computer detects an imminent collision. Overview Many drivers are not prepared for the relatively high efforts required for maximum braking, nor are they prepared for the "buzzing" feedback through the brake pedal during ABS operation. If an emergency develops, a slow reaction and less than maximum braking input could result in insufficient time or distance to stop before an accident occurs. EBA is designed to detect such "panic stops" and apply maximum braking effort within milliseconds. It interprets braking behaviour by assessing the rate that the brake pedal is activated. If the system identifies an emergency, it automatically initiates full braking more quickly than any driver can move his or her foot. Emergency stopping distances can be shortened, reducing the likelihood of accidents – especially the common "nose-to-tail" incident. An electronic system designed to recognise emergency braking operation and automatically enhance braking effort improves vehicle and occupant safety, and can reduce stopping distances by up to 70 ft (21 m) at 125 mph (201 km/h) Brake Assist detects circumstances in which emergency braking is required by measuring the speed with which the brake pedal is depressed. Some systems additionally take into account the rapidity of which the accelerator pedal is released, pre-tensioning the brakes when a "panic release" of the accelerator pedal is noted. When panic braking is detected, the Brake Assist system automatically develops maximum brake boost in order to mitigate a driver's tendency to brake without enough force. In doing so, Brake Assist has been shown to reduce stopping distance by a significant margin (up to 20% in some studies). Systems Mercedes-Benz In December 1996 BAS premiered to the world on the Mercedes-Benz S-Class and SL-Class. In 1998 Mercedes-Benz became the first company to make Brake Assist...

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Engine braking

Engine braking occurs when the retarding forces within an engine are used to slow a vehicle down, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes. The term is often confused with several other types of braking, most notably compression-release braking or "jake braking" which uses a different mechanism. Traffic regulations in a large number of countries require trucks to always drive with an engaged gear, which in turn provides a certain amount of engine braking (viscous losses to the engine oil and air pumped through the engine and friction losses to the cylinder walls and bearings) when no accelerator pedal is applied. Type Gasoline engines The term "engine braking" refers to the braking effect that occurs in gasoline engines when the accelerator pedal is released. This results in the throttle valve that controls intake airflow closing and the air flow through the intake becoming greatly restricted (but not cut off completely). This causes a high manifold vacuum which the cylinders have to work against—sapping energy and producing the majority of the engine braking force. While some of the braking force is produced due to friction in the drive train, this is negligible compared to the effect from the manifold vacuum caused by the air-flow restriction. Diesel engines Diesel engines do not have engine braking in the above sense. Unlike gasoline engines, diesel engines vary fuel flow to control power, rather than throttling air intake and maintaining a constant fuel ratio as gasoline engines do. Since they do not maintain a throttle vacuum, they are not subjected to the same engine braking effects. This is partly why non-turbo diesel-engined vehicles can coast in-gear for longer than an equivalent gasoline engine. The higher compression ratio in diesels means they are harder to start, but once they are running the energy expended in compressing air is regained during the expansion stroke when the compressed air is allowed to "spring" back, so the higher compression ratio causes negligible engine braking via energy being lost as friction and heat of compressed air to engine block. Compression release brake A compression release brake (also known as a Jacobs brake or "jake brake"), is the type of brake most commonly confused with real engine braking; it is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, so the large amount of energy stored in that compressed air is not returned to the crankshaft, but is released into the atmosphere. Normally, during the compression stroke, energy is...

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Transmission brake

A transmission brake or driveline parking brake is an inboard vehicle brake that is applied to the drivetrain rather than to the wheels. Historically, some early cars used transmission brakes as the normal driving brake and often had wheel brakes on only one axle, if that. In current vehicles, these brakes are now rare. They are found in some makes, notably Land Rover, usually for light off-road vehicles. Simple transmission brakes could be found in large vehicles too, such as the 16 inch single disc parking brake used in the M19 Tank Transporter of World War II. The transmission brake is provided solely as a parking brake or handbrake. Normal wheel brakes are still provided for use when driving, drum brakes originally, now almost always disc brakes. Driver's manuals usually caution against using the transmission brake when driving, as it is neither powerful enough nor robust enough and so will not work effectively and may even be damaged by trying to stop a moving vehicle. Land Rover 90 rolling chassis, with drivetrain painted yellow. The transmission brake is the yellow drum, to the right rear of the transfer box. Transmission brakes use drum brakes, rather than disc brakes, as they are intended as a static parking brake, rather than a high performance dynamic brake. Drum brakes allow simpler adjustment with cable-actuated hand lever mechanisms. The brake is mounted to the rear output shaft of the transfer box. As the transmission brake is mounted inboard of the final drive and its reduction gearing, the brake rotates relatively faster, but with less torque, than a wheel brake. The apparently undersized transmission brake thus has more holding ability than its small size might suggest, but is less suitable for driving loads. The braking forces would also be passed through the final drive and axle drive shafts, with possible risk of overloading them. One advantage of a transmission brake is that it locks the entire drivetrain, including all four wheels of a four wheel drive vehicle. However any differential action, either within an axle or front-to-back on an all wheel drive (permanent 4×4) vehicle can still allow movement. For this reason a transmission brake is convenient as a parking brake, but should not be relied upon if a mechanic is to be working beneath the vehicle and wheel chocks should be used instead. A second advantage is that they remove the need to provide cable connections to the wheel brakes, on off-road vehicles where such may be prone to damage. Automatic transmissions Pawl transmission brake, inside an automatic transmission A form of transmission brake is commonly fitted to automatic transmissions. These brakes...

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Electric friction brake

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. Mechanical principle This describes the electrically controlled drum brake principles.   The brake is built with the brake shield (1) as a base that contains the mechanism. The brake shield is mounted on an axle/spindle using the holes in the centre. The brake shoes (3) are the items performing the braking by pressing outwards at the drum that covers all the innards. The brake shoes are held in place by reactor springs (2) and an adjuster (7) spring. There are also some minor clips not pictured to keep the brake shoes in place. Braking starts with applying a current proportional to the desired brake force to the electromagnet (5) which is pulled axially towards the drum. If the wheel is rotating the drum will then pull the actuating arm (4) either to the left or to the right depending on the rotation of the wheel. The actuating arm is pivoted on the black round pin that is anchored to the brake shield. This in turn applies pressure on one of the brake shoes which comes into contact with the brake drum. The first brake shoe then tries to follow the rotation while asserting friction and thereby propagate the movement onto the second brake shoe through the adjuster which also pushes against the drum. The friction force is then caught by the stopper (Black trapezoid) mounted on the brake shield. The braking force asserted is caused by the friction between the electromagnet and the face of the drum which depends on the current through the electromagnet (as stated before). The force applied on the brake shoes is counteracted by one of the reactor springs (which one depends on the direction of the rotation) so that when the current through the electromagnet is withdrawn the spring ensures that the actuating arm is returned to its resting position and the brake shoes are retracted from the drum. The electric current controlling the brake force is supplied by a trailer brake controller. Electrics The electric current controlling the brake through the electromagnet is provided from a brake controller which provides the control current from the towing vehicle. There are different types of brake controllers on the market, each with their own advantages and disadvantages. The current controlling the brakes from the towing vehicle is originating in the battery/alternator of the towing vehicle via the brake controller and then...

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Brake

A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. Background 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 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. Types Rendering of a drum brake Single pivot side-pull bicycle caliper brake Brakes may be broadly described...

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Brake wear indicator

A Brake wear indicator is used to warn the user and/or owner of a vehicle that the brake pad is in need of replacement. The main area of use for this is on motor vehicles with more than three wheels. However brake wear indicators are also useful for brake pads in industrial applications, including wind turbines and cranes. This article refers to disc brakes as an example, but the principle is the same for other types of friction brakes. Types of indicators There are different types of wear indicators for brake pads: Ocular inspection: A cut is made in the pad material to the depth where it shall be replaced. Requires manual inspection of the pads. Mechanical: A metal plate is designed to scratch the brake disk causing a noise when the pad has worn down to the desired level. Electrical: A metal body is embedded in the pad material that comes in contact with the rotor when the desired wear level is reached. This will light an indicator in the instrument cluster. Position sensor: A sensor that measures the position of the brake mechanics and indicates to the driver when the desired position has been achieved. Of the alternatives above the first three are simple and cheap since their lifetime coincides with the service life of the brake pad. The last one is more expensive since the sensors needs to be designed to survive the designed service life of the vehicle. This means that the last alternative is usually only seen on heavy duty vehicles. The idea is that in all cases alert the driver and/or owner of the vehicle that it is time to replace the brake pads to ensure that the traffic safety is preserved for the vehicle. Detailed description Pads B are mounted on carriers G. These are pushed against the rotor A by the piston D which is pushed by the brake fluid E. This induces wear on the brake pads. The rotor A also experiences some wear, but to a lesser extent than the brake pads. The modules C are joined to the cylinder that houses the piston D and acts as counter-force to the piston D. Sufficient wear to validate a change of brake pads is considered when one of the following cases are applicable: The vXbox gap 1 is no longer visible or soon to be no longer visible. The embedded sensor in the brake pad 2 contacts the rotor and creates a connection to ground of the sensor. The metal plate 3 contacts the rotor and creates a noise. The distance between the cylinder for piston D and the carrier G becomes too large, causing the sensor F to send a signal outside the permitted range through the sensor wire 4, or ground the sensor wire 4 if F is a contact. F can either...

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