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Anti Lock Brake System Guide

An anti-lock braking system (ABS) is a safety anti-skid braking system installed in aircraft and on land vehicles like cars, motorcycles, trucks and buses. ABS functions by preventing the wheels from locking up during braking, thereby maintaining tractive contact with the road surface.

ABS is an automated system that uses the principles of threshold braking and cadence braking, techniques which were used to be once practised by skilful drivers before ABS braking systems were widespread. ABS operates at a much faster rate and more effectively than most drivers could ever manage. Although ABS usually offers improved vehicle control and decreases stopping distances on dry and some slippery surfaces, on loose gravel or snow-covered surfaces ABS may significantly increase braking distance and will still improving steering control. Since ABS was introduced in production vehicles, such systems have become increasingly sophisticated and quite effective. Modern versions may only prevent wheel lock under braking, but might also alter the front-to-rear brake bias. This latter function, depending on its specific capabilities and implementation, is called variously as electronic brake force distribution, traction control system, emergency brake assist, or electronic stability control (ESC).


The anti-lock brake controller is also called as the CAB (Controller Anti-lock Brake).

Typically ABS has a central electronic control unit (ECU), four wheel speed sensors, and at least two hydraulic valves within the brake hydraulics. The ECU continuously monitors the rotational speed of each wheel; if it detects the wheel rotating significantly slower than the speed of the vehicle, a condition indicative of impending wheel lock, it actuates the valves to lessen the hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel; the wheel then turns faster. Conversely, if the ECU detects a wheel turning significantly faster than the other ones, brake hydraulic pressure to the wheel is increased to make sure that the braking force is reapplied, slowing down the wheel. This process is repeated repeatedly and can be detected by the driver via brake pedal pulsation. Some anti-lock systems can apply or release braking pressure as much as 15 times per second. Because of this, the wheels of cars equipped with ABS are practically not possible to lock even during panic braking in extreme conditions.

The ECU is programmed to disregard differences in wheel rotation speed below a critical threshold, because when the car is turning, the two wheels to the centre of the curve turn slower than the outer two. For this particular reason, a differential is used in virtually all road going vehicles.

If a fault develops in any component of the ABS, a warning light will usually be illuminated on the vehicle instrument panel, and the ABS will be disabled until the fault isn’t taken care of.

Modern ABS applies individual brake pressure to the all four wheels with a control system of hub-mounted sensors and a dedicated micro-controller. ABS is offered or usually comes standard on most road vehicles produced today and is the foundation for electronic stability control systems, which are rapidly increasing in popularity because of the vast reduction in price of vehicle electronics over the years.

Modern electronic stability control systems are an evolution of the Anti-Breaking System concept. A minimum of two additional sensors are added in order to help the system work: these are a steering wheel angle sensor, and a gyroscopic sensor. The theory of operation is quite simple: when the gyroscopic sensor detects that the direction taken by the car does not coincide with what the steering wheel sensor reports, the ESC software will stop the necessary individual wheel(s) (up to three with the most sophisticated systems), so that the vehicle goes the way the driver intends. The steering wheel sensor also helps in the functioning of Cornering Brake Control (CBC), since this will tell the ABS that wheels on the inside of the curve should brake more than wheels which are on the outside, and by how much.

ABS equipment might also be used to implement a traction control system (TCS) on acceleration of the vehicle. If, when accelerating, the tire loses traction and the ABS controller can detect the situation and take necessary action so that traction is regained. More sophisticated versions of this can also control throttle levels and brakes simultaneously.

The speed sensors of ABS are sometimes used in indirect tire pressure monitoring system (TPMS), which can detect under-inflation of tire(s) by difference in the rotational speed of wheels.


There are four major components of ABS: wheel speed sensors, valves, a pump, and a controller.

Speed sensors

A speed sensor is used for determining the acceleration or deceleration of the wheel. These sensors use a magnet and a Hall Effect sensor, or a toothed wheel and an electromagnetic coil to make a signal. The rotation of the wheel or differential gives rise to a magnetic field around the sensor. The fluctuations of this magnetic field make a voltage in the sensor. Since the voltage gives rise to in the sensor is a result of the rotating wheel, this sensor can become inaccurate at slow speeds. The slower rotation of the wheel can lead to inaccurate fluctuations in the magnetic field and thus lead to inaccurate readings to the controller.


There is a valve in the brake line of each brake which is controlled by the ABS. On some systems, the valve has 3 positions:

  • In the first one, the valve is open; pressure from the master cylinder is passed right through to the brake.
  • In second one, the valve blocks the line, isolating that brake from the master cylinder. This helps to prevent the pressure from rising further should the driver push the brake pedal harder.
  • In third one, the valve releases some of the pressure from the brake.

The majority of problems with the valve system occur because of clogged valves. When a valve is clogged, it becomes unable to open, close, or change position. An inoperable valve will hinder the system from modulating the valves and controlling pressure supplied to the brakes.


The pump in the ABS is used for restoring the pressure to the hydraulic brakes after the valves have released it. A signal from the controller will lead to the release the valve at the detection of wheel slip. After a valve releases the pressure supplied from the user, the pump is used for restoring a desired amount of pressure to the braking system. The controller will regulate the pump’s status in order to provide the desired amount of pressure and reduce the slipping.


The controller is an ECU type unit in the car which gets the information from every individual wheel speed sensor. Whenever the wheel loses traction, the signal is sent to the controller. The controller will then limit the brake force (EBD) and activate the ABS modulator which actuates the braking valves on as well as off.


There are a lot of variations and control algorithms for use in ABS. One of the simpler systems works like the following:

1) The controller monitors the speed sensors all the time. It is searching for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will see a rapid deceleration. If it is left unchecked, the wheel would stop much more quickly than any car could. It may take a car five seconds to stop from 60 mph (96.6 km/h) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

2) The ABS controller knows that such a rapid deceleration of the car is not possible (and in actuality the rapid deceleration means the wheel is about to slip), so it helps to reduce the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this quite quickly, before the wheel can actually significantly change speed. The result is that the wheel slows down at the same rate as of the car, with the brakes keeping the wheels very near to the point at which they will start to lock up. This helps to give the system maximum braking power.

3) This replaces the need to manually pump the brakes while driving on a slippery or a low traction surface, allowing to steer even in emergency braking conditions.

4) When the ABS is functioning, the driver will feel a pulsing in the brake pedal; this comes from the repeating opening and closing of the valves. This pulsing also alerts the driver that the ABS has been triggered.

Brake types

Anti-lock braking systems use a lot of schemes depending on the type of brakes in use. They can be differentiated by the number of channels: that is, how many valves are there individually controlled—and the number of speed sensors.

1) Four-channel, four-sensor ABS

There is a speed sensor on all four wheels and a separate valve for all the four wheels. With the help of this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force.

2) Three-channel, four-sensor ABS

There is a speed sensor on all four wheels and a different valve for each of the front wheels, but only one valve for both of the rear wheels. Vintage vehicles with four-wheel ABS usually use this type.

3) Three-channel, three-sensor ABS

This scheme is commonly found on pickup trucks with four-wheel ABS and has a speed sensor and a valve for each of the front wheels, with one valve and one sensor for both rear wheels. The speed sensor for the rear wheels is found in the rear axle. This system provides independent control of the front wheels, so they both can achieve maximum braking force. The rear wheels, however, are monitored together; they both have to start to lock up before the ABS will be activated on the rear. With this system, it is possible that one of the rear wheels will lock up during a stop, thus, reducing brake effectiveness. This system is easily identified, as there are no individual speed sensors for the rear wheels.

4) Two-channel, four sensor ABS

This system, commonly found on passenger cars from the late ’80s through the mid-1990s, uses a speed sensor at each wheel, with one control valve each for the front and rear wheels as a pair. If the speed sensor detects any lock up at any one of the individual wheel, the control module pulses the valve for both wheels on that end of the car.

5) One-channel, one-sensor ABS

This system is usually found on pickup trucks, SUVs, and vans with rear-wheel ABS. It has one valve, which helps to control both rear wheels, and one speed sensor, located in the rear axle. This particular system operates the same as the rear end of a three-channel system. The rear wheels are monitored together and they both have to start to lock up before the ABS starts to kick in. In this type of system it is also possible that one of the rear wheels will lock, reducing brake effectiveness. This system is also quite easy to identify, as there are no individual speed sensors for any of the wheels.


A 2004 Australian study by Monash University Accident Research Centre has found that ABS:

– Helps to reduce the risk of multiple vehicle crashes by 18 percent,

– Increases the risk of run-off-road crashes by 35 percent.

On high-traction surfaces like bitumen, or concrete, many (though not all) ABS-equipped cars are able to attain braking distances better (i.e. shorter) as compared to those that would be possible without the benefit of ABS. In real world conditions, even an alert and experienced driver without an ABS would find it quite tough to match or improve on the performance of a typical driver with a modern ABS-equipped vehicle. ABS reduces the chance of crashing, and/or the severity of impact. The recommended technique for non-expert drivers in an ABS-equipped car, in a typical full-braking emergency, is pressing the brake pedal as firmly as possible and, where appropriate, to steer around obstructions. In such situations, ABS will significantly decrease the chances of a skid and subsequent loss of control.

In gravel, sand and deep snow, ABS increases the braking distances. On these surfaces, locked wheels dig in and stops the vehicle more quickly. ABS prevents this from happening. Some ABS calibrations decrease this problem by slowing the cycling time, thus making the wheels repeatedly briefly lock and unlock. Some vehicle manufacturers provide an “off-road” button to turn ABS function off. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain the control of car rather than go into a skid, though loss of control remains more likely on soft surfaces such as gravel or on slippery surfaces such as in snow or on ice. On a very slippery surface such as sheet ice or gravel, it is possible to lock multiple wheels at one time, and this can defeat ABS (which relies on comparing all four wheels, and detecting individual wheels skidding). Availability of ABS helps most drivers in leaving the learning of threshold braking.

A June 1999 National Highway Traffic Safety Administration (NHTSA) study had found that ABS increased stopping distances on loose gravel by an average of 27.2 percent.

According to the NHTSA,

“ABS works with your regular braking system by automatically leading to their pumping. In vehicles not equipped with ABS, the driver has to manually pump the brakes to halt wheel lockup. In vehicles equipped with ABS, your foot should remain firmly planted on the brake pedal, while ABS pumps the brakes for you so you can concentrate on steering towards the safety.”

When it’s activated, some earlier ABS systems caused the brake pedal to pulse noticeably. As most drivers rarely or do not even brake hard enough to cause brake lock-up, and drivers typically do not read the vehicle’s owners’ manual, this may not be noticeable until there is an emergency. Some manufacturers have therefore implemented a brake assist system that determines that the driver is attempting to make a “panic stop” (by detecting that the brake pedal was depressed very fast, not like a normal stop where the pedal pressure would usually be gradually increased, Some systems additionally monitor the rate at the accelerator was released) and the system on its own increases braking force where not enough pressure is applied. Hard or panic braking on bumpy surfaces, because of the bumps causing the speed of the wheel(s) to become erratic which might also trigger the ABS, sometimes causing the system to enter its ice mode, where the system severely limits the maximum available braking power. Nevertheless, ABS mostly improves safety and control for drivers in most on-road situations.

Anti-lock brakes are the subject of some experiments centred on risk compensation theory, which asserts that drivers adapt to the safety benefit of ABS by driving much more aggressively. In a Munich study, half a fleet of taxicabs was equipped with anti-lock brakes, while the other half had conventional braking systems. The crash rate was substantially the same for both types of cab, and Wilde concludes that this happened due to drivers of ABS-equipped cabs taking more risks, assuming that ABS would take care of all of them, while the non-ABS drivers drove more carefully since ABS was not there to help in case of a dangerous situation.

The Insurance Institute for Highway Safety released a study in 2010 that found that motorcycles with ABS 37% less likely to be involved in a fatal crash than models without ABS.

ABS on motorcycles

On a motorcycle, an anti-lock brake system stops the wheels of a powered two wheeler from locking during braking situations. Based on information from wheel speed sensors the ABS unit adjusts the pressure of the brake fluid in to keeping the traction while deceleration to avoid accidents. Motorcycle ABS make the rider to maintain stability during braking and to decrease the stopping distance. It also provides traction on low friction surfaces. While older ABS models are derived from cars, recent ABS are the result of research, oriented on the specifics of motorcycles in case of size, weight as well as functionality. National and international organizations evaluate Motorcycle ABS as an important factor to increase safety and reduce motorcycle accident in numbers. The European Commission passed legislation in 2012 that made the fitment with ABS for all new motorcycles above 125 cm3 to be officially mandatory from 1 January 2016. Consumer Reports said in 2016 that “ABS is commonly offered on large, expensive models, but has been spreading to different entry-level sport bikes and midsized bikes”.

Basic principle

Wheel speed sensors mounted on front and rear wheel constantly calculate the rotational speed of each wheel and deliver this information to an Electronic Control Unit (ECU). If the ECU detects the deceleration of one wheel exceeds a fixed threshold and on the other hand whether the brake slip, calculated based on information of both wheels, rises above a certain percentage and enters into an unstable zone. These are indicators leading to a high possibility of a locking wheel. To countermeasure these irregularities, the ECU signals the hydraulic unit to hold or to just release the pressure. After signals show the return to the stable zone, pressure is increased once again. Past models used a piston to the control the fluid pressure. Most recent models quite regulate the pressure by rapidly opening and closing solenoid valves. While the basic principle and architecture has been carried over from passenger car ABS, common motorcycle characteristics have to be considered during the development and application processes. One characteristic is that the change of the dynamic wheel load during braking. Compared to cars, the wheel load changes are more drastic, which can lead up to a wheel lift up and a fall over. This can be intensified using a soft suspension. Some systems are have a rear wheel lift off mitigation functionality. When the indicators of a possible rear lift off are detected, the system releases brake pressure on the front wheel to counter this type of behavior. Another difference might be that in case of the motorcycle the front wheel is much more important for stability than the rear wheel. If the front wheel locks up between 0.2-0.7s, it loses gyrostatic forces and the motorcycle starts to oscillate because the increased influence of side forces the operating on the wheel’s contact line. The motorcycle then becomes unstable and falls.

Anti-lock Braking System (ABS)

Piston Systems: The pressure release in this system is realized with the movement of a spring-tension piston. When pressure should be released, a linear motor pulls the plunger piston back and opens up more space for the fluid. The system was used as an example in the ABS I (1988) and ABS II (1993) of BMW. The ABS II differed in size and an electronically controlled friction clutch was put on the shaft instead of a plunger. Further displacement sensors record the travel distance of the piston to allow the control unit to have a more precise regulation. Honda also uses this system of pressure modulation for big sports as well as well as touring bikes.

Valve and Pump Systems: The main parts which are a part of the pressure modulation system are solenoid inlet and outlet valves, a pump, motor and also accumulators/reservoirs. The number of the valves differs from model to model because of additional functionalities and the number of brake channels. Based on the input of the ECU, coils function the inlet and outlet valves. During pressure release the brake fluid is stored in accumulators. In this open system approach the fluid is then brought back in the brake circuit through a pump operated by a motor which is felt through pulsation on the brake lever.

Combined Braking System (CBS)

Different types of cars, planes or trains, motorcycle rear and front wheels are controlled separately. If the rider only brakes with the help of one wheel, this braked wheel tends to lock up faster than if both brakes had been applied. A Combined Braking System thus distributes the brake force also to the non-braked wheel to lower the possibility of a lock up, increase deceleration and reduce suspension pitch.

With a single [rear] CBS, the brake pressure applied to the rear brake (pedal) which is simultaneously distributed to the front wheel. A delay valve cuts the hydraulic pressure to assure that only when strong braking is applied, pressure is also created on the front wheel. Honda’s first street motorcycle with a combined braking system (then known as Unified Braking) was the 1983 GL1100. This system was taken from a 1970s RCB1000 world endurance race bike.

Larger models with two front discs have a dual CBS System. The system was first installed by Moto Guzzi in the year 1975. Here, applied brake pressure at the front is also applied towards the rear wheel and vice versa. If the front lever is applied, pressure is built up at 4 of the 6 pots in the 2 calipers on the front. A secondary master cylinder at the front wheel distributes remaining pressure to the rear wheel through a proportional control valve and acts on 2 of the given 3 calipers. If strong brake force is applied at the rear, wheel force is distributed to 2 of the 6 pots of the front wheel too. More modern dual CBS have front and rear calipers (and all pots) according to a preset load ratio of front to rear. The proportioning was originally controlled by complex all-hydraulic systems interlinking the front and rear, with a fixed delay or just by sensing weight distribution changes. In the year 2001, an electrohydraulic system was introduced by BMW.


CBS helps to reduce the danger of wheel locks and fall downs but in certain situations it is possible that CBS leads to a fall down. If brake pressure is distributed from the rear wheel towards the front wheel and the friction of the surfaces changes suddenly (puddle, ice on the street) the front wheel might lock even if only the rear brake has been applied. This would give a loss of stability and a fall down. CBS is therefore combined with ABS to stop this on a motorcycle. Different approaches also are possible to realize this combination: Without active pressure Build up Single Version: A third additional channel links the rear wheel circuit with the help of a delay valve to the front brake. Strong brake pressure at the rear wheel (or both wheels) pressurises both brake circuits however this pressure is adjusted accordingly to the wheel speed and brake slip.

The dual version combines Hondas Dual CBS with a secondary master cylinder as well as a proportional control valve a modulator regulates the pressure for each. With Active Pressure Built in 2009, Honda introduced the electronic controlled combined ABS for its high performance sports bikes which utilizes brake through wire technology. The brake input of the rider is calculated by pressure sensors and the information is provided to an ECU. Together with the information of the wheel speed sensors the ECU measures the optimal distribution of pressure to prevent lockups and to provide best possible deceleration. Based on this output a motor for each wheel operates a pump which builds and regulates the brake pressure on the wheel. This system offers quite a fast reaction time because of the brake by wire functionality.

The MIB (Motorcycle integral Braking system) from Continental Teves and the eCBS (electronic CBS) in the enhanced Motorcycle ABS from Bosch are results of some other approach. These systems are based on the pump as well as the valve approach. Through additional valves, stronger pumps and a more powerful motor, the system can actively generate pressure. The input pressure of the rider is calculated with pressure sensors at the lever and pedal. The pump then builds up additional pressure adjusted towards the riding conditions. A partial integral System is designed for working in one direction only: front to rear or rear to front. A fully integral system works in all two directions.

Because these systems are electronically controlled and build up pressure actively, they offer the opportunity to adjust the motorcycle braking behavior to the rider. CBS and ABS can be switched off by experienced riders and different regulation modes with higher and lower thresholds can be chosen too just like the rain or slick mode in the BMW S1000RR.

Anti-lock Braking System (ABS) Problems

ABS problems

A vehicle’s ABS control module is designed to notify the driver with a warning light if there is any kind of malfunction in the system. Rarely is a malfunction the module or ABS by itself. It is often one or more sensors, or the wiring towards the sensors. The most common ABS problems occur when sensors become contaminated with debris or with metal shavings. 

Malfunctions also happen when sensor wiring becomes damaged, resulting in intermittent or no continuity. In more corrosive environments or serious brake system neglect, brake fluid can become quite contaminated and the hydraulic control unit fails to function.

If you have a malfunction in the ABS, first, physically check all wiring and the brake sensors. While checking the brake sensors, look for metal shavings and other debris that could cause false feedback to the electronic ABS controller. False feedback leads to the ABS to trigger when it shouldn’t, or not function when it should.

You might not have an ABS scan tool at home, but any reputable repair shop will have one, and the scan tool is invaluable if you can’t find a physical reason for your ABS problems. After the scan tool generates a fault code, you can proceed with your ABS troubleshooting. Whatever the problem might be, the repair is much easier once a fault is established.

Advantages and Disadvantages of Anti-Lock Brakes

Advantages of Anti-Lock Brakes

The main advantages of an anti-lock brake system (ABS) include.

  • Stopping on ice. An ABS stops any lock-ups and skidding, even in slippery conditions. Anti-lock brakes have been proven to save lives in some situations by helping drivers to keep the control of a vehicle.
  • Lower insurance costs. As it is a thoroughly tested safety device with a track record of effectiveness, insurers often give customers specific discounts for having an ABS system on their vehicle.
  • Higher resale value. As a feature on a car or truck, an ABS increases the market value of the vehicle. Today, where ABS technology has become standard on many vehicles, not having it could result in a lower price for resale.
  • Traction control. An ABS shares some of the infrastructure of a traction control system, where the latest technology helps ensure that each wheel has traction on the road. That makes it easy for manufacturers to install both of these features in the factory.

Disadvantages of Anti-Lock Brakes

Despite the fact that anti-lock brakes are proven to act as a safety feature in most situations, and insurers consider them to significantly lower risk for a vehicle, not all of the drivers are sold on this option for a car or truck. Here are some of the cons that drivers find in this kind of brake system.

  • Inconsistent stop times. Anti-lock brakes are made to give surer braking in slippery conditions. However, some drivers might report that they find stopping distances for regular conditions are lengthened by their ABS, either because there may be errors in the system, or due to clunking or noise of the ABS may contribute to the driver not braking at the same rate.
  • Expense. An ABS can be expensive to maintain as well. Expensive sensors on each wheel can cost a lot of money to fix if they get out of calibration or develop other problems. For some, this is a big reason to decline an ABS in your vehicle.
  • Delicate systems. It’s easy to make a problem in an ABS by messing around with the brakes.Problems have disorientation of the ABS, where a compensating brake sensor causes the vehicle to shudder, make loud noise or generally brake worse.

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