Maintaining traction is an important aspect of off-road driving. Nowadays, though, traction aids such as ABS, ASR and ESP make things much easier. Jake Venter explains how these aids work.
When I first started to go off-road I was told to drive in such a way that wheel-spin is minimised. This makes sense because a wheel normally has less traction when spinning than when it transfers power without spinning. The necessary skill is partly a matter of throttle control and partly a question of being in the right gear for the conditions.
When the first off-road vehicles appeared with electronic traction control many drivers found the system practically useless in slippery conditions. I was lucky enough to attend a Land Rover driving course in the UK and we were told to open the throttle when wheel-spin occurs in order to give the electronic system a chance to intervene and stop the spin. If this did not work one should try different gear ratios. This will not work in all situations, but then no mechanical combination four-wheel-drive or diff-lock is guaranteed to stop the spin.
ABS (antiblockier-system), ASR (Antriebsschlupfregeling) and ESP (electronic stability program) are three German systems that respectively control wheel lock during braking, wheel-spin during acceleration or when traversing a dodgy surface, and the instability that often results in a driver losing control when he corners too fast for the conditions.
ABS brakes were developed to help a driver maintain control during an emergency stop. The aid will often also result in a shorter stopping distance. A basic ABS system includes an electronic control unit, a wheel-speed sensor at each wheel and electronically-controlled valves inside the hydraulic system. The control unit uses signals from the wheel sensors to compare wheel speeds.
The ABS software is designed to take note only of the large differences in wheel speed that occur when one (or more) of the wheels is spinning, or on the point of locking-up.
When locking-up is imminent the brake pressure in the line going to that wheel is released via the electronic valves inside the ABS unit. The wheel in question will now start to rotate faster, the system will brake the offending wheel again, and in this way the brake/release cycle is applied over and over again many times per second to ensure that the wheel does its share of braking without locking up. The driver simply has to brake as hard as he can, and the electronics will take over.
An ASR (traction control) system normally shares the wheel speed sensors and the brake control system with the ABS unit. When one of the wheels starts to spin the system will brake that wheel. This action will send extra torque to the other wheels, but this will not always be enough to get the vehicle moving.
With an open diff (not locked) the torque each wheel gets is equal to one half of the torque the wheel with the least grip can transmit. This does not depend on what the engine can deliver but on the traction between the tyre and the road. To see this, grab a pencil in one hand, pretend it is a driveshaft, and send torque through it with your other hand. You’ll discover that the torque you can transmit depends entirely on how hard you grip the pencil at the other end.
This system has the disadvantage that a wheel has to start spinning before it can come into action. On slippery surfaces this is not a good idea. It will slow you down.
Some ASR systems also limit engine torque by reducing the throttle opening or retarding the ignition timing. ASR systems are not only fitted to off-road vehicles. They are common on many family cars, and are especially useful on powerful front wheel driven cars that will otherwise exhibit excessive wheel spin.
ESC utilises the wheel speed sensors as well as a steering wheel angle sensor and a yaw sensor (yaw = rotation around the vertical axis through the centre of gravity) to detect when a car is on the point of sliding out of control during cornering.
If a car is on the point of losing grip at the front while cornering to the left, this will mean that it is not rotating fast in the anti-clockwise direction. The system will brake the left rear wheel, causing an additional anti-clockwise rotation that will restore the car to the chosen path. If a car is on the point of losing grip at the front while cornering to the right, it will brake the right rear wheel.
If a car is on the point of losing grip at the rear while cornering to the left, this will mean that it is rotating too fast in the anti-clockwise direction. The system will brake the right front wheel, causing a clockwise rotation that will reduce the anti-clockwise rotation, and restore the car to the chosen path.
If a car is on the point of losing grip at the rear while cornering to the right, it will brake the left front wheel.