ABS (Anti-lock Braking System)

The ABS system prevents the wheels from locking up during emergency braking. If the wheels lock up, there is a risk of losing grip between the tire and the road and the driver may lose control of the vehicle. ABS also makes the vehicle controllable during hard braking.

The Anti-lock Braking System (ABS) is one of the basic elements of active safety. ABS prevents the wheels from locking up during hard braking or braking on slippery surfaces. The wheels of a vehicle equipped with ABS will still roll off during emergency braking. As a result, there is no loss of grip between the wheels and the road, so the vehicle remains steerable. Keep in mind that the locked wheels do not transmit any lateral force, for more information on steerability, see the article Kamm’s circle.


The principle of ABS function:

The ABS system prevents the wheels from locking up when braking by automatically regulating the braking force on each wheel so that they do not lock up. This is because if a wheel were to lock up, the grip between the tire and the road would be lost, a dangerous condition for driving. In this case, the vehicle may skid or become unsteerable! In other words, if the front wheels lost grip, the vehicle would stop responding to steering wheel movements, and if the rear wheels lost adhesion, the vehicle could have a very dangerous rear axle skid.

Let’s simplify how ABS actually works. The ABS unit is located between the brake master cylinder and the brake tubes leading to the individual brake calipers (brake rotors). If the control unit receives a signal that a wheel is locked, it will briefly reduce the pressure in its brake lines, thereby putting the wheel back into rotation. The latest generation of ABS can react in this way up to 40 times per second, ensuring near-smooth wheel rotation and therefore good vehicle handling. At the same time, braking force is maintained at the maximum possible level, i.e. at the limit of adhesion.

The ABS wiring diagram is shown in the picture below. Each wheel is equipped with a magnetic or inductive speed sensor (yellow sensor in the lower picture) and a pulse wheel (grey perforated disc). The speed sensors are used by the controller to detect the movement (rotation) of each wheel. The electronic control unit (grey box) evaluates the situation and uses a control valve (brown component in the upper picture) to reduce the pressure in the brake system if necessary to set the blocked wheel in motion again. In this way, the ABS system intervenes several times per second throughout the braking period, independently of the force applied to the pedal. When the vehicle decelerates to a minimum speed of around 4 km/h, the ABS automatically deactivates to allow the vehicle to come to a complete standstill, when the wheels are not actually turning.


Basic parts of the ABS system


History of the ABS system:

The ABS system was developed into its present form by BOSCH in 1978. However, the history goes back even further. As early as the early 20th century, there were already ideas about how to prevent the wheels from locking up under hard braking. In 1936, Bosch filed a patent for a “Device to prevent heavy braking of the wheels of a motor vehicle”. These first systems worked on mechanical principles only. It was only with the advent of electronic control that engineers could develop an anti-lock braking system that was fast and robust enough for use in motor vehicles. The ABS system first found commercial application as a special feature on the Mercedes-Benz S-Class and shortly afterwards on the BMW 7 Series.


Since the introduction of the system in 78, ABS has undergone further development and undergone many improvements. At the beginning, for example, ABS was only installed on the front wheels, which are more stressed and more important for steering when braking. The problem of integrating ABS into drum brakes was also addressed, as they are more difficult to control braking performance and have a longer reaction time. Further developments have gradually led to the miniaturization of the ABS unit and an increase in operating frequencies. In order to better distribute the braking effect to the individual wheels, the ABS system became multi-channel over time. This means that if the number of channels is the same as the number of wheels, this is the ideal situation where the control unit can control each wheel separately.

Further development gradually led to the creation of follow-up systems such as: ASR (Anti-Slip Regulation, which prevents wheelspin during starting); ESP (electronic stabilisation programme); EBV (electronic brake force distribution); etc. The wheel speed sensor has therefore become an important component for many other driving assistants that need information on the speed of the individual wheels to do their job.

Did you know that …

ABS as an optional extra

ABS, like many other systems, started its career as an optional extra. Interestingly, in 1998, if a motorist wanted to buy ABS for his Škoda Octavia 1st generation, he had to pay another CZK 26,500 on top of the basic price of CZK 350,000, with an average gross monthly salary of CZK 11,693. This was therefore an item that everyone had to consider carefully first.

ABS as a compulsory equipment of the vehicle

In 2004, the manufacturers in the ACEA association voluntarily agreed that ABS must be fitted to every newly homologated car in the EU. Since 2006, the regulation also applies to previously homologated vehicles.

Braking distance of the vehicle with and without ABS

The answer to the question of whether a vehicle with ABS has a shorter or longer braking distance is not entirely straightforward. It is a classic debate based on the thesis that on dry roads, a vehicle with ABS has a longer braking distance because ABS does not brake at full speed. A driver with driving sense doesn’t need ABS as the wheel lockup is self-monitored.

Admittedly, these claims are a bit of a beard. A lot of time has passed since the first ABS and the technology has improved a lot. As for the second part, let’s face it, most drivers’ sense of steering has diminished. Today’s driver almost mindlessly trusts the car’s electronics to work their magic to keep him on the road. That in certain situations the wheels could lock up and what that would entail is beyond the ken of most drivers today.

But everything has its limits, and these are determined by physics. The braking force cannot be greater than the grip between the tire and the road allows. However, this grip is not constant, it is influenced by a number of factors, including the so-called tire slip. Theory tells us that the tire has the most grip around a slip value of 10-30%, see graph below. Beyond this slip value is the so-called peak of friction forces. If you push the brake pedal even harder the wheel will increase its slip against the road and the friction between the tire and the road will decrease very quickly! The graph below shows that at 100% slip, i.e. a locked wheel, the friction only reaches 60 – 70% of its original potential. Logically, the braking distance increases by up to a third when the wheel is blocked!


It may seem complicated, but here lies the answer to our question. Today’s highly sophisticated braking systems can operate within the limits of ideal tire slip, i.e. between ten and thirty percent slip. When the system assesses that a peak in friction forces is approaching, which would actually mean a reduction in grip, it reduces the pressure in the brake line. In this way, the ABS system manages to make the most of the current adhesion conditions. Such a condition is difficult to achieve with a sensitive foot on the brake. Not to mention the fact that multi-channel ABS can control each wheel separately!

Yes the older versions of ABS were not so fast, some vehicles would have pedal dive during prolonged braking, or it was almost impossible to stop on uneven surfaces when the wheels were bouncing. However, this is no longer the case today.

The lower graph shows the area of action of the ABS system. It also shows the decrease in adhesion for different surfaces, from rough asphalt to unpaved surfaces to driving on snow and ice. The dashed line in the graph shows the decrease in adhesion in the transverse direction. This is the component that provides stability and the vehicle’s willingness to turn.


One more interesting thing can be seen in the above graph. The slight increase in the coefficient of friction when driving on an unpaved surface and a blocked wheel, this is caused by particles that the blocked wheel seems to be throwing in front of it. This creates a sort of wedge in front of the wheel. Volkswagen’s system called ABSplus makes very good use of this fact.

How do you know when ABS is working?

Intermittent braking can be heard when braking hard on the grip line, e.g. on wet roads. The intermittent braking is caused by the ABS releasing the pressure in the braking system. This reduces the braking force and the wheel unlocks. The pedal seems to kick during this action and can drop to the floor in older cars during prolonged braking. A less experienced driver might get stuck and release the brake pedal, but that would be a mistake! The ABS system will lock itself to prevent the wheels from locking up and the vehicle from skidding.