UK CAR Contents

ABS Anti Lock Braking

ABS was originated in airplanes. It was developed in order to shorten the distance necessary for landing. It did not appear in road cars until 1966, when Jensen FF  installed a system developed by Dunlop. That system, called Maxaret, It did not employ computers as well as wheel speed sensors. It just employed electronic sensors to avoid locking the disc brakes.

You might think that optimal braking is implemented by completely locking all the wheels. but, the laws of physics tells us that the coefficient of friction between the ground surface and a static object is always greater than a moving object. If the tyres are sliding on the road surface, the friction between road and wheel will not be maximum. Therefore, the maximum braking occurs when the wheels are braked up to the level that the wheels just do not slide.

ABS is an electronically controlled braking system that can assist the driver with the most efficient application of the brakes when entering a potential skid situation. As the name implies, the Anti-lock Brake System stops the wheels locking up under heavy braking and prevents the tyre from skidding and sliding across the road surface, making you lose the steering control of the car.

Contrary to popular belief, ABS does not make for shorter braking distances or give you better overall braking performance. It is designed to give you, the driver, steering control of the car whilst braking hard, as you would in an emergency.

To implement anti-lock braking, ABS system employs speed sensors for individual wheels. If the wheel speed detected differs from the vehicle speed, that means the wheel is sliding, thus the computer will signal the corresponding brake to loose until sliding disappear. The computer will also compare the speed of all wheels, if one or more of them run considerably faster than others, that means the car is losing control, it will apply more brake to that wheel to correct the driving path.

ABS sensor and hub

ABS components can also control Traction Control, Electronic Stability Control and Artificial LSD 

There is still the need for the driver to apply the brakes in the normal way, but if the pedal is pressed to hard the ABS will take over by rapidly releasing and re-applying the brakes, until the tyre contact patch (the bit of the tyre in touch with the road) regains grip on the road surface, allowing the car to brake and steer a little bit at a time, but not both together. The result is enhanced steering control when cornering or avoiding an obstacle as the tyres have contact with the road surface at all times.

When the system is active, it will become apparent that the pedal is vibrating quickly and there is an increase in noise when under braking. This is quite normal.

One thing that has to be remembered is that although ABS will help in bringing the car safely to rest, it will not be able to assist if the vehicle is placed in an unrecoverable situation. Some drivers tend to brake later and a lot harder relying on the ABS, but there still has to be the safe braking distance from the car in front. The practice of 'tail-gating' the car in front has been shown to result in more accidents than before ABS was introduced.

 

There are two types of ABS. Mechanical and Electronic, though nowadays the mechanical systems are rarely fitted. The electronic system consists of an Electronic Control Unit (ECU), hydraulic modulator, pump, accumulator, various valves and sensors along with the normal brake components (master cylinder, servo, wheel cylinders, pads and shoes, etc.). These parts, although usually reliable can be expensive to replace if they fail. So a healthy bank balance may be needed to maintain a car fitted with ABS.

Braking servo ABS pump and control unit

 

When the brake pedal is pressed hydraulic pressure is applied to the wheel cylinders. An electric powered hydraulic pump is used to generate the pressure and an accumulator is used to store the pressurised fluid. 

The flow of fluid to the brakes is controlled by the master cylinder and the solenoid valves (two valves per hydraulic circuit). All of these components and a fluid reservoir are all part of the hydraulic control unit that is positioned in the engine bay where you would normally expect to find the master cylinder and servo.

The rotation of the wheels is monitored by sensors that transmit the information to the ECU. If any of the wheels stop turning, the ECU will activate the system and reduce the hydraulic fluid pressure to that particular brake resulting in the wheel being able to rotate momentarily. This prevents that wheel from locking and the tyres skidding on the road surface. This action is applied to all of the wheels and the operating time can be measure in several times per second.

 

 

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