Brake (motor vehicle)
During the usual braking process, the kinetic energy ( kinetic energy ) is converted into thermal energy and dissipated ( energy loss) through friction or adequate processes , which is why the choice of materials and the cooling of the brake are of greater importance. Newer technologies allow the conversion of kinetic energy into electrical energy and storage for later drive or electrical consumers through recuperation with a regenerative brake . In addition to the service brake, all vehicles are legally required to have an additional braking device that must function independently. This is usually designed as a mechanical parking brake (" handbrake ") and secures the vehicle against rolling away when it is stopped.
The first passenger cars were only decelerated by the rear wheels. There were often block brakes on the wheels themselves , and an external band brake on the transmission output shaft that acted on the rear wheels via the cardan shaft. Later, the drum brake prevailed. The Zwickau car manufacturer Horch introduced the Horch 10, the first car with four-wheel brakes in 1924 . The first standard equipment with a disc brake was found in the German Tiger tank from World War II, the first car with four disc brakes was the American Tucker '48 (1948). The first production car in Europe with four disc brakes was the Jaguar C-Type racing car (1952), followed in 1955 by the Citroën DS with two disc brakes.
The braking process
The effect of a braking system is defined by the braking deceleration , which is defined as the decrease in speed over time. The braking deceleration is usually given as positive and is to be understood as the negative acceleration of the vehicle. In sports cars, too, the performance of the braking system significantly exceeds the engine output. In practice, the maximum achievable braking deceleration is not limited by the brake system but by the static friction of the tires and is therefore dependent on the weather and the road surface. If the brakes are applied too forcefully and no anti-lock braking system (ABS) regulates the braking force, then the static friction limit is exceeded, the wheels lock and the vehicle begins to slip, with about 15% worse braking under sliding friction , the vehicle can no longer be steered and tends to break out.
The static friction limit means that the braking force must not be greater than the static friction . This includes the vehicle mass, the braking deceleration, the acceleration due to gravity and the coefficient of static friction . For rubber on dry asphalt it is just below 1, so you can brake a vehicle with up to about one (the values are slightly lower for commercial vehicles). On a wet road it drops to a value of about 0.5, on ice even to 0.1, which leads to a doubling or tenfold increase in the braking distance compared to dry asphalt .
Because the braking force of a car is applied to the roadway, but the inertia is somewhat higher in the center of gravity of the vehicle, a moment acts around the transverse axis (pitch axis) of the vehicle, whereby the front axle is additionally loaded and the rear axle is relieved ("brake nod") . Thus, in the event of full braking, the locking limit on the rear axle is already reached with a much lower braking force than on the front axle. Blocking the rear axle when the front axle is not blocked would result in the vehicle becoming unstable and swerving. Therefore, a higher braking force must be applied to the front axle than to the rear axle. Vehicle manufacturers and others take this into account. a. with larger brake discs on the front wheels.
The inspection of the brake system is an important part of the general inspection according to § 29 StVZO (colloquial: TÜV) and the safety test (SP). A visual, functional and effectiveness test takes place here. The effectiveness of most vehicles is tested on a brake test bench. By irregularities in the surface condition of the brake disc or imbalance arises brake judder . The driver notices this through pulsing the brake pedal, torsional steering wheel oscillations and vibrations. If the brakes of a vehicle fail, the last option is " sheet metal braking ": Steer the vehicle to the edge of the lane and brake via contact with the lane boundary. Usually, however, such a situation rarely occurs because the service brake has at least two circuits ( two-circuit brake system - regulation in Germany since 1967) and the parking brake can still be used.
The brakes of a car must convert energies of several megajoules , in the event of an emergency braking with an output of more than 200 kW.
In order for the brakes to function properly, there must be sufficient brake fluid in the brake lines on the one hand and there must not be too much air in the brake system on the other. In the event of repairs or changes to individual brake components, what is known as venting is then necessary. If the proportion of air is too high, the pressure of the brake pedal does not provide the desired braking of the vehicle, which has a direct effect on the operational suitability of the vehicle and can thus lead to a hazard in traffic.
Brake pedal and lever
The brake pedal (including foot brake lever) is the actuating device of the service brake system in almost all motor vehicles . With two-wheelers there are no pedals for braking, but levers. On motorcycles, a hand lever on the right handle for the front wheel and a foot brake lever, also on the right, for the rear wheel.
Here, the driver's foot force is increased for the first time in order to generate sufficient brake pressure with the available pedal travel. The reinforcement is generated by the lever system of the foot brake lever. With the appropriate dimensions, a five-fold increase in foot strength can be achieved. In today's vehicles, however, this is not sufficient to brake a vehicle or bring it to a standstill, so that a further boost must be provided by a brake booster .
In the past, attempts were made to combine the accelerator and brake pedals into one control lever in order to relieve the driver and to cope with the increasing density of traffic. One of the inventors was G. Peiseler from Leipzig with the Peiseler pedal . Such a retrofitted Opel Kadett was accepted by the KTA of the GDR and was in operation in Berlin . The biggest argument in favor of this construction method is the significantly shorter stopping distance to this day, as the foot does not need to be moved and a shocking experience can be converted directly into stretching the leg. For reasons that have not been documented, the gas-brake pedal has not been able to establish itself until today.
According to current regulations, motor vehicles and most trailers must be equipped with brakes. The EC Directive 71/320 / EEC of 1971 differentiates between three brake systems:
- Service brake system (BBA)
- Auxiliary braking system (HBA)
- Parking brake system (FBA)
In addition, so-called decelerators (e.g. retarders ) can also be used, which are wear-free and support the normal braking system.
Service brake system (BBA)
The service brake system is used to slow down the vehicle during normal operation and bring it to a standstill. It must be gradable and affect all wheels of the vehicle.
In almost all multi-lane motor vehicles, the driver operates the service brake using a pedal that can be operated with the right foot . On motorcycles, the front wheel service brake is operated using a hand lever on the right-hand side of the handlebars and the rear wheel service brake is usually operated using a pedal operated with the right foot or (more rarely, e.g. on scooters) using a hand lever on the left-hand side of the handlebar.
With the service brake, a deceleration of 5.0 m / s 2 must be achieved in most motor vehicles ; up to 2000, 2.5 m / s 2 was sufficient.
The transmission of the force from the actuation device to the actual wheel brakes is nowadays mostly done hydraulically (fluid brake) or pneumatically (compressed air brakes in trucks and in railway technology). Up until the 1960s, mechanical transmission devices were also common in cars. Today these are almost only found in bicycles or mopeds.
Electrical transmission devices are already widespread today (as of 2009) in the compressed air brakes of heavy trucks (Wabco / Knorr EBS ). However, these systems still have a pneumatic transmission device as a fall-back level.
Purely electrical transmission systems are in the test phase, but are still a long way from being introduced into series production (as of 2009).
Common types of service brakes are:
- Drum brakes with inner jaws in simplex and duplex design including their subspecies. Drum brakes are used e.g. B. on mopeds or as rear brakes in low-powered cars. They are widely used in commercial vehicles (e.g. trailers, tractors, construction vehicles) because of their low operating force and insensitivity to dirt.
- Disc brakes as floating caliper or fixed caliper brakes. Disc brakes are characterized by a high, even braking performance with low weight. They are used, for example, on motorcycles, cars, commercial vehicles and racing vehicles.
With the use of dual or multi-circuit brake systems, redundant systems are established as fail- safe systems within the service brake . This ensures safe stopping even if one of the systems fails. Today the driver is supported by a large number of auxiliary systems such as brake boosters, ABS and brake assistants.
The principle of braking with energy recovery, also known as regenerative braking, is becoming increasingly important in hybrid vehicles in particular . Here, braking takes place by operating a generator, which in turn feeds the energy gained into the battery for the electric drive.
In large machines such. B. wheel loaders, the hydraulically operated multi-disc brake is mainly used. The further classification of the braking systems is often made according to the type of operation:
Hydraulically operated brake
The first hydraulic brake (for wagons) was designed by Hugo Mayer from Rudolstadt in 1895 without this having any noteworthy consequences. In the early 1920s, the American Malcolm Loughead (co-founder of the Lockheed Aircraft Company) received a patent for a hydraulically operated braking system. Hydraulic brakes were first used in motorsport by Duesenberg - with great success - on the occasion of the French Grand Prix in 1921. The first production car with hydraulic brakes was the Chrysler B-70 from 1924, the first in Europe the Triumph 13/35 . Alfred Teves acquired the rights to the Lockheed brake for Germany in 1926 . The first German passenger car with hydraulic brake was the Adler Standard 6 from 1926. In the same year Krupp introduced it in commercial vehicles , followed in 1927 by Büssing and in 1931 by Mercedes-Benz (Lo-2000), MAN , Henschel and Saurer from Switzerland.
Until the beginning of the 1960s, cars were still manufactured with mechanically operated cable brakes ( VW Beetle - standard model until March 1962). The brake was maintenance-intensive because the braking effect was uneven when the pads were worn. In addition, high braking forces can only be generated mechanically using large levers. As a rule, hydraulic brakes are therefore used for the service brake in passenger cars , as they always ensure uniform brake pressure on all wheels. In addition, the different diameters of the master cylinder ( master cylinder ) and slave cylinder (wheel brake cylinder) provide an additional hydraulic transmission and thus an increase in force. By pressing the brake pedal, a hydraulic piston with a small diameter is moved in the master brake cylinder, which converts the force into a pressure of the brake fluid in the hydraulic line . This pressure then acts on the pistons in the wheel brake cylinders with a larger diameter, which then exert higher forces on the brake pads (disc brake) or the brake shoes (drum brake) and move them.
Since the 1920s, cars have been required by law to have at least two independent brake systems. These are available as service brakes and parking brakes ("hand brakes") in every vehicle. The hydraulically operated service brake can be designed as a single-circuit or dual-circuit brake system . With the two-circuit system, the two circles can be divided between the front and rear axles or diagonally (right front wheel / left rear wheel and left front wheel / right rear wheel). A special feature is the three-wheel brake system (LL system) from Volvo , which was used in the Volvo 140 from 1966 . Each of the two brake circuits brakes both front wheels (via separate brake pistons) and one rear wheel each. Even if a circuit fails completely, three wheels are still braked and 80% of the total braking power is still available. The electro- hydraulic brake is a further development of the hydraulic brake .
Components of the hydraulic vehicle brake:
- Brake pedal with mechanical connection to
- Master cylinder
- Brake booster
- Brake fluid reservoir
- Brake pressure distributor (depending on incline)
- Pipe system made of metal, flanged with T-pieces and connection pieces
- Brake hoses
- Wheel brake cylinder
- Vent valves
Trucks and buses have a pneumatic brake. In the case of pure air brakes, as used in vehicles with a total weight of around 7.5 t or more , the wheel brakes are applied by compressed air and not by brake fluid as in cars. This means that the braking force achieved is no longer directly dependent on the force exerted on the brake pedal by the driver's foot; the driver only controls this using the pedal travel. Membrane cylinders are mostly used as clamping elements. In newer vehicles, the cylinder pressures are controlled electronically by a so-called EBS (Electronic Brake System), which includes all modern functions of active safety, such as B. ABS (anti-lock braking system), ASR (traction control) and sometimes ESP (electronic stability program). The commercial vehicle pioneered these brake-by-wire systems, long before the car. In addition, the ALB valve regulates the brake pressure depending on the load; this is controlled via a linkage, for example.
Trucks with a total weight of up to around 7.5 t often have mixed systems (fluid brake operated by compressed air).
Truck trailers are equipped with compressed air brakes with a continuous braking system, which in principle work in the same way as on the truck. The required compressed air is obtained from the towing vehicle via connecting hoses. Two hose connections are necessary: a supply line marked in red, which is used to continuously supply air to the trailer, and a brake line marked in yellow, which is responsible for controlling the braking process on the trailer. For newer trailers that are equipped with EBS, there is an electronic control line as a supplement to the pneumatic control line.
Until the 1980s, there were single-line braking systems for trailers. Today these are only approved for agricultural trailers up to 25 km / h. Here one and the same hose connection marked black is used to supply pressure and control the trailer brake. When the brake is released, there is a pressure of 5.3 bar, which is reduced when braking. The trailer brake valve then allows an inversely proportional pressure build-up from the reservoir in the trailer's brake cylinders. The disadvantage of this system is that braking is no longer possible if there is insufficient air. With the two-line system, on the other hand, a drop in pressure in the "red" connection ( supply line ) of normally 7.3 bar triggers an automatic braking of the trailer until sufficient air is fed in again from the motor vehicle. If the brake line is defective (yellow), emergency braking is initiated as soon as the first brake signal from the tractor unit is given.
The compressed air is generated by a motor-driven compressor (air compressor) that always runs. The pressure regulator switches between "fill" and "blow off" depending on the supply pressure in the brake system. The air delivered by the compressor is either pumped into the braking system or blown into the environment without pressure. The air supply to the individual compressed air circuits takes place via a multi-circuit protection valve , which has usually been designed as a four-circuit protection valve since the early 1980s . The circles are then divided as follows:
- Circuit 1: service brake circuit 1
- Circuit 2: service brake circuit 2
- Circuit 3: Spring-loaded parking brake and, if necessary, trailer supply
- District 4: Secondary consumers (air suspension, door control, horn)
The service brake circuits control the front axle and rear axle (s) separately so that the braking effect is still available if one circuit fails. Axle-by-axle control keeps the line lengths short, whereas in cars with hydraulic brakes, diagonal control of the wheels is common in order not to influence the driving behavior too much if one circuit fails. The service brake works with pressure build-up, i. H. Air only flows to the brake cylinders when the pedal is depressed. As a result, the braking effect can be better metered than with a brake that works with a pressure drop.
The spring-loaded parking brake works according to the "Knorr" system, i. H. when the system is fully pressurized, the brake is open: if there is a loss of pressure, the brake linings are applied; a pressureless system locks the vehicle. The system was invented because in the early days of the railway, individual wagons often decoupled from the train due to coupling defects and then sped down to the valley without braking. After the introduction of the patented Knorr-Bremse, the pressure line tore when uncoupling and the cars were forced to brake. This system was later adopted for the air brakes of road vehicles. Due to the poor controllability, this principle is only used for the parking brake. At the same time, this ensures that the parking brake works without a motor. A pressure loss can occur in the system after long periods of inactivity. For this reason, air pressure must first be built up in the brake system by operating the engine when the vehicle is stationary until the operating pressure is reached and the parking brake can be released. Up until the mid-1950s, when the brake system was depressurized, the driver saw a vertical red rod in his field of vision on the dashboard, which slowly lowered into the rest position when the system was filled. Only then could you drive off. Today the warning function is taken over by indicator lights. If a modern compressed air system is already depressurized after 24 hours of inactivity, the tightness of the system must be checked and repaired. Important: Vehicles that only have one storage container lose air even when additional functions are operated. If the bus breaks down, the doors should therefore be opened and closed manually to reduce air consumption.
Continuous brakes work with engine drag torque, an engine dynamic pressure brake (e.g. exhaust flap), a hydrodynamic retarder or an eddy current brake . They protect the service brake from overloading ( fading ) and reduce wear and tear, for example on long downhill journeys. The legal requirement for the permanent brake is that it should keep the fully loaded vehicle on a slope of 7% over a distance of 6 km at a speed of 30 km / h. The main features of a permanent brake are the wear-free design and the design for continuous operation. Since the air supply can be exhausted with pneumatic brakes by frequent actuation, a constant or permanent brake is required in addition to the service brake on buses and heavy trucks.
Parking brake system (FBA)
Parking brakes keep two-lane vehicles safely at a standstill, even in the absence of the driver. They are rarely used on two-wheelers. In cars, the cable brake is usually used as a parking brake. It is operated mechanically or hydraulically, electrically or electrohydraulically with a lever or a pedal. Disc brakes as well as drum brakes with inner or outer jaws can be used as parking brakes. When using drum brakes, parts of the service brake are often also used as parking brakes. The device, also known as the handbrake , usually acts on the service brake of the rear axle. The braking force is transmitted from the operating lever to the two wheels via a cable, where the brake shoes are pressed apart or the brake pads are pressed together. As an alternative to the handbrake, it can also be operated with the foot on a pedal ( foot parking brake ), as in most models from Mercedes-Benz and some models from Audi, BMW, KIA and VW. Electromechanical parking brakes are used in some of the new models in the upper middle and upper class. With these, the brake shoes are brought up to the brake disc via servomotors on the rear brake calipers at the push of a button.
The spring brake is used as a parking brake on buses, trucks or trailers. When the brake is actuated, the wheel brake is applied by spring force. To release the brakes, the spring force must be overcome with compressed air, thereby releasing the brake.
With the parking brake, a deceleration of 1.5 m / s 2 must be achieved in most motor vehicles .
Brakes for trailers
The towing vehicle decelerates and the trailer "pushes" on the trailer coupling of the vehicle, the brakes of which must also decelerate the trailer. This system has two disadvantages:
- The braking distance is considerably longer, since the maximum possible braking forces depend solely on the weight of the towing vehicle, while the pushing effect of the combination depends on the weight of the towing vehicle and the trailer.
- The trailer pushes very hard when braking. This reduces the directional stability of the combination, as it tends to buckle when braking. If the towing vehicle turns in during heavy braking, the trailer increases this effect.
Since January 1, 1991 (date of first registration), unbraked trailers behind cars or trucks have been allowed to have a maximum permissible gross weight of 750 kg in Europe (EC Directive 71/320 / EEC). Older trailers without brakes and trailers behind tractor units without brakes can still have a higher permissible total mass today.
Not continuous brakes
In the case of a discontinuous braking system, the brakes of the towing vehicle and trailer have two different energy sources and two different actuation devices. The most common case is the overrun brake. Energy source towing vehicle: muscle power, energy source trailer: overrun force. Control device towing vehicle: brake pedal, control device trailer: overrun device.
Overrun brakes are used on car trailers, lighter truck trailers and caravans . When the towing vehicle is braked, the trailer runs into the towing vehicle. This force is transmitted from the trailer coupling to the brakes of the trailer via mechanical levers. The braking force depends on the one hand on how much the towing vehicle decelerates and on the other hand on the actual weight of the trailer. The trailer also presses against the trailer coupling when its brakes are acting, which is why the trailer also worsens the stability of the combination. Compared to the unbraked trailer, however, the pushing effect is minimal and is automatically controlled by the braking delay.
However, a problem arises when reversing. Since in this case the towing vehicle pushes the trailer, i.e. the trailer coupling is under compressive stress, the overrun brake would also be activated without further measures. This is prevented by a so-called reverse lock. In the past, manual reverse locks were used, in which a bolt, a pin or a pawl had to be inserted in order to prevent the brake from being applied when reversing. Since January 1, 1991 (date of first registration), this reversing block must be automatic (automatic reversing).
The permitted total mass of trailers with overrun brakes are also restricted by law, but are higher than for unbraked trailers. In Europe, overrun brakes are generally permitted on trailers behind cars and trucks up to a maximum permissible weight of 3.5 t for the trailer. (EC Directive 71/320 / EEC). With older trailers and trailers behind tractors at a maximum speed of up to 40 km / h (with all-wheel brake) or up to 25 km / h (without all-wheel brake) also up to 8 t (common on agricultural trailers).
In Switzerland, overrun brakes are allowed on agricultural trailers up to 30 km / h up to 6 t, above 30 km / h the limit is 3.5 t, as is the case with cars.
With a semi-continuous brake, the brakes of the towing vehicle and trailer have separate energy sources, but are operated by a common actuating device. Examples are e.g. B. Tractors with a compressed air system only for the trailer brake. Energy source towing vehicle: muscle power, energy source trailer: compressed air. Common control device: brake pedal in the towing vehicle.
In Switzerland, off-road vehicles with air brakes over 3.5 t are allowed to pull, provided the towing vehicle is designed for it. The BE category driver's license is sufficient, but the trailer is subject to the HVF .
Outside Europe, u. a. in the US, electric brakes are common on car trailers. Typically these are drum brakes whose linings are acted upon by electromagnets (solenoids). These are triggered by a control unit (controller) in the towing vehicle, which in turn is operated either electrically via the brake light switch or via the brake hydraulics. The latter version allows the braking force of the trailer to be regulated.
With a continuous brake, the brakes of the towing vehicle and trailer are actuated by a single energy source (foot power or compressed air) and a single actuation device (brake pedal). It is used in all heavy trucks with air brakes, but is very rare in passenger cars (ATE Hydrakup).
In Germany, the actual total mass of the trailer for trucks with a continuous braking system may not exceed 1.5 times the permitted total mass of the towing vehicle. In the case of cars, the trailer load is limited to 1.0 times the permissible total weight of the towing vehicle. In the case of off-road vehicles , 1.5 times the gross vehicle weight can be permitted as a trailer load. In both cases, a maximum of 3.5 t trailer load is permitted behind a car.
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- (4) StVZO
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- (5) StVZO
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