What is stability control?

While they're often mentioned and packaged together, traction control and stability control are not the same thing.

Traction control is an electronic system that stops or limits the driving wheels of a vehicle from spinning. The system can be applied to any drivetrain configuration; rear wheel drive, front wheel drive or even all-wheel drive.

Electronic stability control (ESC) is an advanced vehicle stability system that works to help a driver maintain steering control of the vehicle. By automatically braking individual wheels and regulating torque distribution the system influences the directional control of a vehicle in situations where grip or traction are compromised and a potential loss of control is detected.

Stability control will always include a traction control function but also incorporates a sophisticated array of sensors and computer algorithms to avoid loss of control in understeer or oversteer situations.

In essence, traction control keeps the car travelling in a straight line, stability control keeps the car cornering in accordance with the driver's needs.

Since 2013 it has been compulsory for all new passenger cars sold in Australia to be fitted with ESC. From November 2017 every light commercial vehicle sold in Australia will also be required to have an ESC system as standard fitment.

Why do we need it?

For a driver to be in effective control of a vehicle it's essential that the vehicle is stable and within the grip (or traction) limits of the tyres.

Whether it's a sudden change in road surface grip or the need for an emergency avoidance manoeuvre it's all too easy for a vehicle to end up skidding out of control.

It has been estimated that the compulsory introduction of ESC systems to all new passenger and light commercial vehicles will reduce single vehicle accidents by 30 per cent.

How does it work?

Traction control can be thought of as the opposite of anti-lock braking. Where anti-lock braking reduces braking force at a slipping wheel to recover from a skid, traction control reduces torque at a spinning wheel to regain grip.

Wheel speed sensors measure the rotational speed of each wheel. The on-board computer continuously compares the data and if a driven wheel speeds up suddenly, compared to the others, wheelspin has been detected.

Automatically, and in fractions of a second, the computer takes measures to limit the torque applied to that wheel. This can be achieved either by temporarily reducing the engine's power delivery (through closing the throttle, retarding the ignition or cutting the fuel injection), applying the brake at the particular spinning wheel or possibly even both actions combined.

Electronic stability control is a significantly more sophisticated system than traction control and uses an extensive suite of sensors and algorithms. It's the comparison of the signals from wheel speed, throttle position, steering wheel angle and vehicle yaw sensors that allow the on-board computer to decide when and by how much to intervene.

The yaw sensor is one of the most critical components. It measures the actual rate at which the vehicle is rotating around its central axis. It's a crucial piece of information that is then compared to the wheel speeds and the drivers steering input. The system can calculate the difference in how the vehicle is reacting to a particular situation compared to the driver's intentions and ultimately whether there is a need to assist the driver.

What are examples of the systems in action?

To illustrate how stability control works, consider a car entering a sharp corner at an unsuitably high speed. The driver turns the wheel but the grip of the front tyres is overcome and the vehicle starts to understeer.

At this point the system can identify that the vehicle isn't turning as the driver intended. The ESC system can then apply an increased braking force to the inside wheels to assist the rotation of the vehicle and hopefully avoid a potential crash.

Stability control can also correct oversteer, where the rear wheels lose grip and the tail of the car begins to slide.

In a car with an open differential (not a limited-slip or locking unit), one wheel will spin uselessly in mud or on ice, without traction control. That's the nature of differentials, they allow either wheel to turn at a different rate of speed for negotiating corners.

But as soon as they start turning at a different rate of speed in a straight line, traction – and therefore acceleration – is reduced. A traction control system will reduce the torque on the spinning wheel and the differential will shift more torque to the wheel with the higher grip. Acceleration is improved on low-friction surfaces.

Of course the laws of physics can't be changed and therefore traction and stability control systems can only achieve so much. However, the speed and accuracy with which these systems can react is well above the average driver's ability, and the ability to brake individual wheels independently is something the driver can't do, making these technologies invaluable for the modern day motorist.