Stability control and traction control - do you need them?

27 July 2015
, CarsGuide

The ability to detect that potentially life-changing moment when your car starts slip-sliding sideways, and then being able to control and correct that slide without panicking and spinning backwards into a ditch full of broken glass, is the kind of thing you’d expect to form part of every road user’s driver training.

You’ve probably noticed that it’s not, however, and in the not so distant past this was all too often a problem. Fortunately, car companies long ago realised that most humans were far too flawed, flakey and uncoordinated to be trusted with this kind of responsibility, so they set out to develop software to do it for us.

Since the advent of early, basic and, frankly, slightly annoying traction-control systems - which used to cut the power on you at the slightest suggestion of wheel spin - cars have become cunningly adept at keeping us on the road. So much so that grateful governments have made it illegal to sell new cars without stability-control systems fitted as standard.

Australia made ESP (Electronic Stability Program) compulsory for all passenger cars in 2013, and on November 1 this year the law will also apply to all new-model light commercial vehicles.

So, what is stability control?

Stability control is a full suite of technologies that includes traction control, but also a series of complex programs designed to keep the car both stable and under control - regardless of road conditions or driver ham-fistedness.

Of course, there are a few caveats with stability control; while the systems are a feat of engineering that use more computing power than was required to send men to the moon, they still can’t stop you from parking in a tree if you decide to overstep the immutable laws of physics.

Stability control operates under the assumption that having traction is best, always, and that the car should be going where the driver intends. They seem like simple enough parameters, but the way in which stability-control programs work is pretty extraordinary.

Everything starts with an Electronic Control Unit (ECU), plus a series of sensors dotted around the car that measure how quickly the wheels are spinning, how fast you’re going and how far the car is rotating around its central axis, plus a raft of other variables that can include how far you’ve turned the steering wheel and even how quickly you’ve leapt off the throttle to stamp on the brake pedal.

Should the sensors indicate an imminent excursion into the scenery, the ECU swings into action to make sure that your car stays where it’s supposed to be and not on the six o’clock news.

Stability-control systems work long before you notice that you’ve lost control. If, for instance, you swing the wheel wildly to avoid an obstacle, the car may naturally plough straight ahead due to understeer.

Even if it only saves you once a year, it’s a very good bit of software

If the sensors pick up that you’re steering more than the car is moving, the system will brake the inside wheel to force the car to follow your commands.

Further complicating things, if you then react too vigorously to try and recover from swerving, it’s all too possible to overcorrect and send the car into a slide. The stability control is ready for your Tomfoolery, however, and will cut in again, braking the wheels individually to stop the car skidding out of control and keep it pointing in the right direction.

It’s impossible to track just how often ESP systems are thus saving people from loss-of-traction accidents, but it’s fair to say they probably save someone, somewhere, every day. And even if it only saves you once a year, it’s a very good bit of software.

How is that different from regular traction control?

Traction control is a comparatively simple technology compared to a full stability-control suite. It only prevents a loss of traction on driven wheels in cases where the engine’s power exceeds the level of grip from the road surface.

While traction-control systems use the same methods as stability control, their capabilities are much smaller. The only data that the traction control’s ECU gleans about the car is how fast it’s going and how quickly the wheels are spinning.

However, like stability control, traction control uses a number of systems to regain traction.

First, the wheel-speed sensors will send data to the ECU. If the ECU finds that one wheel is spinning faster than the others, it assumes that wheel has lost traction. The ECU will then activate the car’s anti-lock braking system (ABS) to carefully slow the spinning wheel down.

As the ABS system is slowing the slipping wheel down, the ECU will also instruct the engine to deliver less torque, ensuring that the traction isn’t overcome when the brakes are released.

Like stability control, all of this can happen in less than an eye blink, and often imperceptibly. The more severe the loss of traction, the more noticeable the reaction will be, with a noticeable slump in forward motion as the system sorts everything out for you.

Traction control only works on driven wheels, so if you’re driving a rear-wheel-drive car, it’ll only work on the rear wheels. In all-wheel-drive vehicles, it can, and will, brake and limit each wheel.

Should I ever turn the traction control off?

If you read car magazines, or watch Top Gear, turning off traction control is some sort of manly right of passage, like having vomit come out your nose, but in real life you might wonder why the button to do so is even included.

Most vehicles do allow you to switch off the traction control alone and in some cases it’s possible to turn the stability control off entirely. Disabling the traction control can help when off-roading in sand, snow and mud, where the braking effect of traction control will actually make it more difficult to get through the tricky bits.

If you’re one of the tiny percentage of people who take their cars to a race track, there’s a fair chance you’ll turn the traction control off the minute you arrive and leave it off until you leave, aiming for less interference and the ability to balance the car on its ragged edge for better lap times and more Clarkson-esque lairiness.

With a well-designed traction control system, though, you’ll almost always be faster leaving it on, especially with the multi-setting traction-control modes common among high-performance cars.

There’s a very good reason that traction and stability-control systems are banned from Formula 1; drivers don’t have to be as smooth with throttle, braking or steering inputs because a computer can react in a split second to ensure peak grip and stability, better and faster than even a Schumacher-level human.

That’s exactly what your traction and stability-control systems are doing every time you get behind the wheel - being better at handling tricky situations than you are - so it’s never a good idea to turn the traction control off on the road.

Even the most skilled drivers would be foolish, these days, to take away the safety net that technology has worked so hard to provide us with.