What is traction control? PH Explains

A traction control system is an electronic safety aid designed to automatically prevent a car's driven wheels from spinning while accelerating. This process, besides preventing unwanted wheelspin, delivers improved vehicle control - particularly when accelerating on slippier surfaces, such as wet roads. This is why some manufacturers refer to the set-up as 'Anti-Slip Regulation'.

Modern traction control systems have become an integral part of a car's electronic stability control hardware, which grants better stability and cornering in emergency situations.

How does traction control work?

A modern traction control system uses sensors, typically shared with the anti-lock braking system, to monitor the speed of the car's wheels - as well as monitoring the driver's inputs and other key criteria. As the car accelerates, the system looks for any major difference between the speeds of the wheels. If a powered wheel suddenly starts rotating more quickly than other wheels, then wheelspin is occurring and the system will intervene.

The traction control system will then reduce the torque output of the engine until traction is restored. In modern petrol cars, this is achieved by momentarily adjusting or cutting the ignition system.

The engine's output is gradually reduced until the wheels stop spinning or the accelerator is lifted. There are other methods that can be used to alter the engine's power, including electronically closing the throttle, cutting the fuel supply or - if the engine is supercharged or turbocharged - bleeding off boost. If the vehicle has electronically controlled differentials or four driven wheels, torque can also be shunted to wheels with more traction to reduce spin.

What about using the brakes?

A traction control system may also use the car's brakes to reduce the speed of the spinning wheel, usually in conjunction with a reduction in engine output.

Applying the brake at one wheel can also be beneficial in low-traction situations. In a car with an open differential, the driven wheels receive the same torque but can rotate at different speeds. Consequently, if one wheel is on ice and the other on Tarmac, the wheel on the ice will spin when subjected to just a little torque - which, in turn, means all the available traction at the wheel on Tarmac goes to waste as it too will only receive a little torque.

To get around this, and to get the car going, the traction control system applies the brake at the spinning wheel. This generates a brake torque at that wheel, which then means the other wheel will receive torque from the differential equal to the combined sum of the braking torque and the little torque being generated by the spinning wheel's traction force.

This moves the car, helping it out of its sticky situation. If the wheels continue to spin, however, power will be cut in an effort to restore traction.

What are the benefits of traction control?

If the car has driven front wheels, traction control will improve safety and stability in corners - particularly in wet or bumpy conditions. A spinning wheel cannot generate much cornering force, so automatically reducing the power restores whatever grip is available and prevents understeer.

A wheel that's just spinning on a slick surface, or as a result of standing on the accelerator, also offers very little tractive force - slowing acceleration. A traction control system will dial back the power and make the best use of what traction is on offer, granting prompt, hassle-free acceleration and improved control.

A brief history of traction control

The earliest recorded mention of traction control dates from 1908, when an engineer proposed a method to stop accelerating trains from damaging the tracks on which they were running.

The concept was developed in the following decades and, finally, in 1970 a production automotive system arrived - in the form of Buick's 'Max-Trac'. This short-lived system worked well but, due to technical difficulties relating to newly developed emissions control systems, it was soon phased out.

As electronic systems developed throughout the 1990s, traction control began to be fitted to increasing numbers of cars. As safety demands increased it became an integral part of overarching electronic stability control systems, making it commonplace in cars today.

PH Explains homepage