All About ABS

The rubber tyres on a car provide the grip that is necessary to initiate changes in direction when you turn the steering wheel. This grip is available so long as the tyres are rotating; the moment a wheel locks up (ie it stops rotating), the tyre will slide along the road and lose its grip. When this happens, a skid occurs and more dangerously, there is loss of steering control because it is the friction between the tyre and road that enables the car to make a turn. Should such a situation occur in a turn, the car will simply respond to the laws of physics and continue straight on even if you turn the steering wheel.

Many years ago, this understanding of the dynamics of wheel lock-up and their effect led driving specialists to recommend that ‘cadence braking’ be applied on wet and slippery surfaces when wheel lock-up would be most likely. Cadence braking requires rapid depression and release of the brake pedal (an action like pumping with the foot) so that the braking force applied is momentary, avoiding the likelihood of total wheel lock-up. Deceleration would occur without skidding and loss of steering control.

Unfortunately, cadence braking is something which the average driver may not be able to use as it requires practice and conditioning. For most people, the normal response when approaching a bend too quickly is to apply the brakes as hard as possible to slow down as fast as possible; not many people will be able to do cadence braking in such emergencies. There are also some people who, upon being faced with a dangerous situation, panic and do nothing…

Automobile researchers thus looked for a solution which could duplicate cadence braking without requiring the driver’s intervention. Just as disc brakes were adopted from aircraft, so too was the braking system which prevents wheel lock-up. For an aircraft landing, especially at speed on a slippery runway, loss of grip and steering control can have significant implications and so aviation engineers developed a braking system which used a form of cadence braking to avoid the wheel lock-up – even while the brake pedal is fully depressed.

Although the principles of anti-lock braking were understood as far back as the early 1930s and patented in 1936, research into anti-lock braking systems – ABS, they are commonly known now – for cars began in earnest in 1954 when a Ford engineer, Earle S. MacPherson (the same man who developed the widely used MacPherson strut) adopted the ABS of a French aircraft to a Ford model and found it to offer significant improvements to braking. However, it was only in the 1970s when such a system began to be commercialised by Bosch, starting with luxury models from BMW and Mercedes-Benz. Due to its high cost initially, it was found only in expensive cars but as costs have fallen over the decades, ABS is now available in small, lower-priced models (even 1-litre cars), pick-ups and 4WDs.

HOW ABS WORKS
As mentioned earlier, the prevention of loss of tyre grip and steering control due to wheel lock-up and skidding can be achieved by not allowing the wheels to lock up in the first place. A very good driver might be able to do this by cadence braking which requires manually pumping on the brake pedal. The rapid on-off pressure brings on some deceleration of the car but has to be done very quickly since there is usually a limited stopping distance.

ABS initiates the cadence effect automatically, using speed sensors on each wheel and assessing the relative rotating speeds of each wheel by means of an Electronic Control Unit (ECU) which is a small but powerful microprocessor. There are also mechanical systems which are not as effective and there are some other so-called ‘ABS’ units which simply rely on sensing of pressure in the system and releasing the pressure at a certain level.

Based on signals from the wheel speed sensors, the ECU will detect that, during braking, the rotational speed of the wheels drops but the amount of deceleration depends on both the vehicle speed during braking as well as the road surface conditions. In other words, the ECU constantly judges the slip condition between the wheels and the road surface from the change in the wheel’s rotational speed during braking. This is known as the ‘slip ratio’ which is the ratio between the vehicle’s speed and the speed of the wheels.

When a vehicle is being driven at a constant speed, the speed of the vehicle and that of the wheels is identical, ie the tyres are not skidding along. However, when the brakes are used, the speed of the wheels will gradually decrease and not match that of the vehicles’ actual speed. This changes the slip ratio from 0% – where the wheels are turning freely – to 100% when the wheels are fully locked up and the vehicle’s body continues to move forward while the wheels are not spinning at all. Skidding occurs and with it, there is loss of grip and steering control.

The ECU monitors the slip condition of each wheel all the time and whenever it approaches 100% on any wheel or deviates from the 10% – 30% optimum slip ratio, the ECU sends a signal to the ABS actuator to adjust the hydraulic pressure that applies the braking force on the disc brake of that affected wheel. This split-second pressure reduction is enough to adjust the slip ratio so that it is no longer close to 100% and a lock-up condition.

With the use of electronics, the ABS can apply and reduce brake pressure up to 20 times a second – a rate far quicker than a human being can achieve. In some cars, drivers can actually feel a ‘pulsing’ effect through the brake pedal as the ABS keeps adjusting the hydraulic pressure on and off rapidly. The frequency of on/off adjustments is a measure of how good the system is, with more pulses indicating a highly refined control strategy for maximum braking effectiveness.

The pulsing effect, in some early systems, was felt as a strong vibration which worried drivers who were not clear on what was happening. So over the years, the manufacturers have found ways to mask the feeling at the pedal and on some cars, it is hard to discern when the ABS comes into action.

With ABS, a driver can maintain maximum force on the brake pedal and the ECU will intervene to relieve the pressure whenever necessary. And because wheel lock-up and loss of grip are prevented, it is also possible to apply the brakes fully during cornering and still be able to steer the car round the corner. This is an important feature of ABS as a driver will be able to avoid an obstacle even while braking.

Another advantage of ABS which is especially applicable in Malaysia is the directional stability while braking on slippery and wet roads. On such surfaces, the degree of grip achieved by each tyre constantly changes (depending on the amount of water on a particular spot) and this variation can cause the car to slide out of control during braking as one or more wheels may lock up. With ABS, there is greater directional stability because no wheel will lock up during hard braking and grip will always be available.

Using ABS requires no special training but for those who are used to cadence braking, you should refrain from that style of braking. ABS works best when you apply maximum and constant force on the brake pedal. According to studies by BMW’s advanced driving school, the average driver does not apply maximum force in most braking situations to achieve really good braking. Just press hard and concentrate on avoiding the danger by steering.

But even with ABS installed in a car, it must be noted that total avoidance of an accident is not possible. For example, if the road is muddy and exceptionally slippery, and the approach speed is excessively high, the inertia of the car’s weight may cause it to deviate substantially as the grip of the tyres is exceeded. Thus, it is important to remember that even with ABS-equipped brakes on their car, the same degree of caution as with a car not equipped with ABS should be exercised. HAVING ABS DEFINITELY DOES NOT PERMIT FASTER CORNERING SPEEDS THROUGH TURNS.

Due to the softness of the rubber, it is possible for severe wear to occur when braking hard and if the wheel locks up, it means that it skids along the rough road surface and wears out only in a certain spot. This creates what is known as a ‘flat spot’ on the tyre and it is no longer round, as a tyre should be. Each time the wheel’s rotation brings the tyre to that flat spot, there is an annoying thump which becomes irritating vibration at higher speeds. There is no repair possible and you have to change the tyre. But ABS will help prevent flat-spotting since the wheels won’t lock up under any braking conditions, so the tyres will not be dragged along the road.

ABS FOR 4WDs
Because ABS is so effective for stopping a car, even on slippery surfaces, it would be assumed that it is ideal for an off-road vehicle which is being driven over loose ground. In some cases, that may be so, but in off-road conditions where the surfaces are soft and loose, it is actually useful to have the tyres lock up a bit and skid so as to cause a build-up of earth in front of the tyre. This helps to reduce the stopping distance because the earth acts like a ‘barrier’.

For this reason, ABS for off-road vehicles usually have a different program from road-going cars and it gives slightly difference operational characteristics. Furthermore, in the often bumpy conditions, wheels will momentarily lose contact with the surface and this will give a false signal to the ABS, causing it to react when it does not need to. The ABS program is also specially modified to cope with such behaviour to optimise braking effectiveness.

FAIL-SAFE OPERATION
In the three decades of service, there has not been any drawback found in ABS and the units themselves have been fairly reliable. Of course, like any other mechanical or electronic component, there can be manufacturing defects but the system itself is pretty reliable. All systems have a self-check mode which occurs with each starting of the engine and if something is not right, a warning light will come on.

No servicing is needed at all but you still need to maintain and service the brake system as recommended by the manufacturer. Brakes pads and brake fluids should be changed as necessary for optimum stopping performance. According to Allied Signal, a manufacturer of brake systems, the hydraulic side of any car’s brake system needs preventive maintenance. For example, regularly flushing out of the brake fluid and replacing it with fresh fluid goes a long way toward preventing brake problems as the vehicle ages.

It’s even more important for a car with ABS. The ABS modulator unit isn’t just complicated and expensive; it will get damaged if the fluid in it is either dirty or moisture-contaminated. Since most brake fluid naturally absorbs moisture from the atmosphere, the only way to fight this problem is to regularly flush and replace the fluid every 2 years or 40,000 kms

Should a malfunction be detected while driving, a warning light on the instrument panel will illuminate. However, failure of the ABS does not mean that all stopping power is lost as there will always be the normal braking system available. Additionally, there are dual hydraulic circuits for extra safety in the event that one circuit fails. The only thing is that the driver should be aware that the ABS is probably not working so skidding could occur.

It is advisable to go to a workshop straight away to fix the fault. Not all workshops have diagnostic equipment to handle ABS so it is best to find out which ones have the equipment and trained technicians. As ABS is a safety item, it is best to ensure that it is fixed by people who really know what they are doing. Never accept the suggestion that the warning light is ‘nothing’… some mechanics have been known to just disconnect the bulb and tell the owner they have fixed the problem!