How it works: Vehicle Stability Control and Traction Control

It didn't take long for automotive engineers to work out that if you could control a car's braking system to steer around objects, you could also optimise friction at the tyres for enhanced roadholding and traction.

Friction (grip) between your vehicle's tyres and the road surface is the critical force that keeps the vehicle going where the driver intends it to travel. Without tyre grip the vehicle will travel in the direction of its momentum, causing the tyres to skid across the road surface.

That of course applies in a lateral (sideways) direction as well as in a straight line.

Needless to say the condition of your vehicles tyres is crucial to how your vehicle behaves on the road. All other stability control systems (including the steering wheel) rely on this small contact patch each tyre maps out on the road surface for grip.

Vehicle stability control systems are known by a number of names, such as Vehicle Dynamic Control (VDC), Electronic Stability Program (ESP), Dynamic Stability Control (DSC)...et cetera.

ESP – a brand name – was developed by Bosch and made its mainstream debut with Mercedes Benz and BMW in the mid 1990s. Known more generically within the industry as ESC (Electronic Stability Control), the system is now mandated in Australia as standard equipment for new passenger cars.

Vehicle stability is controlled by the ABS unit in your vehicle. ABS was introduced to prevent your wheels locking-up during braking (allowing vehicle manoeuvrability under braking). Additionally, sudden changes in vehicle attitude, due to evasive steering or road conditions, are controlled by the stability control system.

This system suppresses vehicle sideslip when driving due to vehicle spin (oversteer) or drift-out (understeer) by controlling braking and engine speed (torque). As a result, the driver can calmly react and is provided leeway for the next manoeuvre, resulting in safe driving conditions.

In simple terms stability control compares the steering wheel angle (the driver's intended direction) to the lateral "G" force sensor (Vehicle body direction) and attempts to correct the body attitude to match the steering angle. This is achieved in a similar manner to the steering (skid steer) of an earth moving vehicle (everything from a Bobcat to a Caterpillar D9).

When brakes are applied to one side of the vehicle it is forced to pivot on an axis (theoretically the centre of the vehicle) and engine torque is reduced at the same time as the brakes are being applied to slow the vehicle and catch the oversteer/understeer condition.

Another function of the stability control system is traction control. Like stability control it is marketed under different acronyms, ETC, TCS or ASR, to list three. Traction control prevents the drive wheels from spinning on take off. If the drive wheels start to lose traction (start to spin) the traction control system reduces torque to the engine and brakes the drive wheels to prevent wheel spin, this happens many times per second which helps the drive wheels achieve grip. Think of it as ABS in reverse.

And the beauty of traction control is that it can 'nip' a brake on just one drive wheel to deliver what's known as 'torque vectoring' – the process of sending torque to the other drive wheel to level the vehicle's cornering attitude, whether understeer or oversteer.