High-Tech Vehicle Safety Systems

Anti-Lock Brake Systems

Applying full brake pressure in an emergency stop situation can result in locking the vehicle's wheels. If the front wheels are locked, the driver is unable to steer, and the vehicle slides in a straight line over the roadway surface. Without the driver being able to take avoiding action by steering, the vehicle may collide with another vehicle or a fixed object in its path of travel.

In addition, braking is somewhat less effective when the wheels are completely locked than when there is some degree of rotation. Accomplished drivers, such as those involved in motor sports, use a "threshold braking" technique to bring the wheels close to the point of lockup. They are then able to develop maximum braking effort while maintaining steering control.

Anti-lock brake systems (ABS) automatically modulate the pressure in the braking system to avoid wheel lockup, providing adequate braking while allowing the vehicle driver to retain control of the steering.

Wheel slip

How do they work?

When a vehicle's wheel is rolling down a road there is very little resistance to its motion. If the brakes are applied gently, the wheel slows down, and a frictional force is developed between the tire and the roadway surface that, in turn, reduces the vehicle's speed. Engineers term this situation wheel slip.

The frictional force increases with increasing slip. A maximum is reached at approximately 20% slip, after which the braking force slowly declines to the point where the wheel is completed locked (100% slip). So-called threshold braking is achieved when wheel slip is maintained close to the peak of the braking force-wheel slip curve.

Anti-lock brake systems use speed sensors on the vehicle's wheels to determine if the wheels are about to stop rotating. Electronically controlled hydraulic valves then reduce the braking pressure on the wheels in order to allow the wheels to continue to rotate.

The wheel speed sensors generally take the form of a magnetic pickup and a toothed wheel. As the wheel rotates, the peaks and valleys on the toothed wheel move under the pickup, generating an electrical pulse that is used to monitor wheel speed.

A computer-based controller uses the information from the wheel speed sensors as inputs to a control algorithm that determines how to modify the hydraulic brake pressure. The controller opens and closes valves in the hydraulic brake lines to allow pressure from the brake master cylinder to be transmitted to the wheel cylinders, to block any additional pressure from developing, or to reduce brake pressure in order to prevent wheel lockup. In addition, the controller operates a hydraulic pump to increase pressure in the brake lines as necessary to maintain braking effort.

The control system monitors and controls brake pressure several times each second ensuring that braking effort is maintained without wheel lockup. This may result in a chattering noise, and pulsation in the brake pedal, when ABS is in operation.

Many passenger cars have four-wheel ABS in which individual hydraulic valves control each of the vehicle's wheels. Some vehicles, notably some pickup trucks and sport utility vehicles, may have two-wheel ABS where the sensing and control system operates only on the rear axle.

Are there any risks?

Drivers need to familiarize themselves with the operation of ABS in their vehicles before they encounter an emergency braking situation. (Try out the vehicle's brakes in an empty car park.)

Drivers who don't understand how ABS works may mistake the noise and pulsation for a problem with the brakes and take their foot off the pedal. Or, they may try to pump the brakes, as was often done on conventional braking systems. Neither action uses the capabilities of ABS and will likely result in much longer stopping distances. Drivers with ABS should apply firm pressure on the brake pedal and maintain this pressure while the anti-lock braking system does its job.

While ABS may shorten stopping distances under certain road conditions, the effect may only be marginal. Furthermore, because road conditions vary so widely, the anti-lock braking system will likely be unable to always develop maximum braking efficiency. Drive responsibly, being aware of the limitations of the prevailing travel conditions, and maintaining an appropriate following distance.

The use of ABS on loosely-packed roadway surfaces, such as loose gravel or deep snow, may increase the vehicle's stopping distance over what might have been achieved with full locked wheel braking. Drivers should be aware of this potential effect and drive appropriately when roadway conditions so warrant.

Dummies belted

Dummies belted

What can science tell us?

Evaluation of Anti-lock Braking Systems Effectiveness; David Burton, Amanda Delaney, Stuart Newstead, David Logan and Brian Fildes; Report No. 04/01; Royal Automobile Club of Victoria; April, 2004
A literature review of advanced technology braking systems and vehicle stability control systems was undertaken with emphasis on their likely effectiveness in preventing crashes and injuries. Some evidence suggested that vehicles equipped with Anti-lock Braking Systems (ABS) were involved in fewer crashes with opposing, adjacent or same direction vehicles compared to non-ABS fitted cars, but that they were over-involved in single vehicle run-off-the-road crashes.

Performance of ABS-Equipped Vehicles on Deformable Surfaces; Battista V; Proc. CMRSC-XII; London, Ontario; June 10-13, 2001
ABS may increase stopping distances on deformable surfaces such as snow, slush, and gravel. This paper presents the results of a test program to evaluate the stopping performance of a number of ABS-equipped late model light-duty vehicles on deformable surfaces. Preliminary testing conducted in the winter of 2000 on snow surfaces indicate that stopping distance increased between 7 per cent and 48 per cent with ABS.

Useful links

Quick Facts

  • ABS automatically prevents wheel lockup
  • You can brake hard and still steer
  • Stopping distances may be increased on loose surfaces
  • You may experience noisy operation and/or vibration in the brake pedal
  • Practice using your ABS - before you need it

Associated systems:

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