RACQ Car Safety Fact Sheet

Anti-Lock Brakes

These days, ABS, or ALB (the two common acronyms for anti-lock brakes) is offered as standard equipment on all new cars and many light and heavy commercials.

How it works

  • When braking under adverse conditions, such as slippery roads, the level of grip between the tyre and road surface may be exceeded, allowing wheels to lock and skid. When this occurs, braking efficiency is very much reduced, the car may spin out of control and the driver is likely to lose the ability to steer accurately. 
  • An ABS equipped vehicle has a conventional hydraulic brake system, with some additional components, including wheel speed sensors, an electronic control unit (computer) and a hydraulic modulator unit. 
  • The wheel speed sensors constantly send information to the computer, enabling it to detect impending lock-up of any or all wheels during braking. When lock-up is imminent, the computer instructs the hydraulic modulator to apply and release the brakes on the wheel/wheels that are locking up, up to 15 times per second. 
  • In this way the car’s braking and directional stability is maintained and the driver retains the ability to steer the car to avoid a potential collision. 
  • At all other times the brakes work like a normal braking system, only entering ABS mode when necessary, with vehicle speeds above about 6kph.

ABS Anti-lock brakes schematic diagram

Brake pedal kick back

  • During ABS brake operation the brake pedal may pulse or “kicking back” under the foot, and thumping noises from the pedal area, and vibration in the vehicle body and steering may be evident. These are all normal characteristics, though many drivers may find these sensations a little alarming at first. 
  • Under ABS braking conditions it is very important to maintain pressure on the brake pedal, to allow the system to take control. The driver must concentrate on steering to avoid hazards.

Brake Assist and other enhancements

  • ABS systems often incorporate other functions. These include:
  • Brake Assist, which detects a rapid or emergency brake application and applies maximum brake force, regardless of the actual pedal pressure applied by the driver. 
  • Hill Hold prevents the vehicle rolling backwards during hill starts
  • Traction Control prevents the drive wheels from spinning on low traction surfaces.
  • Dynamic Stability Control reduces the risk of loss of control during manoeuvres.
  • Hill Descent Control is used to maintain traction and control vehicle speed during off road hill descents

Stopping distances

In general, ABS equipped cars will have similar braking distance to non-ABS cars. 
  • However, this distance will vary according to several factors, including the type of road surface. 
  • On some roads, especially those covered in snow or loose gravel, stopping distances may be longer than with a non-ABS car. In these conditions, the wedge of loose material pushed up in front of the locked wheels also helps slow the car. 
  • The primary function of ABS is to ensure the car’s directional stability under adverse braking conditions. 
  • ABS can’t prevent an accident caused by following too closely.

When it stops working

The system incorporates a dash warning light to advise the driver of a system fault. 

  • If the lamp illuminates while driving, the ABS system will be disabled, the system will return to standard non-ABS brake operation and other related functions may cease to operate. 
  • With the standard brakes operating, the car can still be driven, though it should be taken promptly to a repairer for attention to the ABS system (Consult the owner’s handbook for further detail).

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