Current ABS braking systems are great, but they do have their disadvantages, says Jake Venter. This is especially true when you find yourself travelling on dirt
THREE MODERN SYSTEMS
ABS brakes employs wheel-speed sensors at each wheel and an electronic control unit (ECU) to sense when one or more wheels are slowing down faster than the other wheels, during braking. When the wheels are close to stopping the ECU will reduce the brake pressure at those wheels to avoid a lock-up. The relevant wheels will then speed-up, but if they speed up too much, the system will re-apply brake pressure while the driver continues to brake. This pressure cycling usually happens many times per second. These brakes often reduce stopping distances, but the major advantage is that it improves vehicle stability by preventing wheel lock-up that makes a car unstable. Most ABS systems stop working at speeds below about 12 km/h because a car is not likely to become unstable at low speed.
In 1986, a traction control system (TCS), became available. When one or more of the wheels start to rotate faster than the others the ABS wheel sensors will inform an ECU and this unit will either brake the relevant wheels or reduce engine power by drastically retarding spark timing.
A further development occurred in 1995 when ESP was introduced. This is an electronic stability package that employs ABS and TCS technology augmented by a steering wheel angle sensor plus a yaw sensor (Yaw = rotation about a vertical line) to determine when a driver is on the point of losing control during cornering. When this happens the car is either on the point of rotating clockwise or anticlockwise. The system will brake one of the wheels to counter this movement.
WHAT THE SYSTEMS HAVE TO COPE WITH
All three systems have reduced the chances of having an accident, but they’re not foolproof. The control systems have to take the following disturbances into account:
- Changes in road surface adhesion, and especially differences between the surface underneath left hand wheels compared to the surface underneath the right hand wheels, such as one often finds under off-road conditions.
- Changes in wheel loading, such as occurs while cornering.
- Irregularities in the road surface, such as corrugations that may cause the wheels to vibrate.
- Out-of-round tyres or wheels.
- Differences in wheel circumference, brought on by buying fitting only two new tyres at a time, or using a new spare wheel and three worn tyres.
- Variations in brake pressure being applied by the driver. This has the potential to confuse the ABS control unit, but many new cars are fitted with a sensor that measures the brake pedal speed. If the driver moves the pedal fast, this usually implies panic braking and the system will respond by applying maximum braking power.
DISADVANTAGES OF ELECTRONIC SYSTEMS
I recently came across this statement on the site of an American road safety organisation:
There is no technology such as ABS, TCS, ESP, 4×4, diff-locks or brand new tyres that will allow safe travel on an icy road at a speed greater than 70 km/h. These features will improve a driver’s ability to control the vehicle, but do not eliminate the danger of losing control on a road with a low and varying traction coefficient.
This statement is also to large extent true of dirt roads. One can argue about the limiting speed, which could well be lower or higher than 70 km/h, depending on whether its sand or loose gravel, but the remarks are valid because dirt roads often manage to confuse these modern electronic systems. On many dirt roads, the soil can move under the wheels during braking, and this may confuse the ABS system. The brake/release cycle that works so well on a concrete or tar surface may be completely wrong, so that the brakes release for too long, or by too much. On a corrugated surface, the ABS control may be so flustered that the brakes don’t work at all. I experienced this some years ago on a double-cab bakkie.
One often finds that on some loose gravel surfaces a non-ABS system will result in shorter stopping distances, even if the wheels lock. The soil or gravel builds-up a wedge in front of the tyre and this helps to stop the vehicle, but you may lose steering control because the vehicle will follow the camber of the road. An experienced driver may be able to stop sooner by using a rally driver’s technique called cadence braking. The brakes are cycled on and off in a manual equivalent of what ABS does, but a lot slower and under full driver control.
HOW DOES THE INDUSTRY COPE WITH THE PROBLEM?
- Some of the earlier off-road vehicles were equipped with on/off switches for ABS, but this has been outlawed in some countries.
- Most ABS systems are automatically deactivated when in 4×4 mode, or when a diff-lock is applied.
- Some vehicles, such as the Mercedes-Benz M-Class, have special software that can cope with most dirt road conditions, but usually only at low speeds.
- The VW Amarok has button labelled “Offroad” that enables hill-descent control and also recalibrate the ABS system to cope with dirt roads.
- Some drivers deactivate ABS for dirt roads by removing the relevant fuse.