Stability control systems explained

Electronic stability control systems (SC for the purposes of this article) detect loss in traction and react to regain grip using the braking and engine management systems. Situations where the systems will come into action include understeer, oversteer, and spinning wheels.

Most new vehicles are now fitted with some kind of stability control system. There are a host of acronyms for this technology which varies according to car manufacturer…

  • Electronic Traction control (ETC/ TCS)
  • Dynamic Stability Control (DSC)
  • Electronic Stability Programme (ESP)
  • Porsche Stability Management (PSM)
  • Etc

Don’t be fooled into thinking each of these systems are unique – they all function in very similar ways (and are usually all made by the same manufacturer).

How do stability control systems work?

Sensors In order for the car to detect loss of traction it needs some sensors. These come in various different forms and determine how the car is behaving, and what the driver is trying to do. Yaw sensors, gyros, wheel-speed detectors and accelerometers are the most common sensors found in SC systems. In addition, information ranging from steering and pedal position, engine speed and gear selection is used to determine driver inputs.

How is this information used?

When the SC system determines that loss of traction is occurring, it acts using the braking and engine management controls (and in some cars even the steering system) to put the car back on track. The system reacts according to a set of preset criteria depending on the nature of the loss of traction, which can include spinning wheels or slides.

Spinning wheels

Traction control is used to reduce drive loss through spinning wheels. This can occur when driving on slippery surfaces, or when accelerating hard (usually in first gear from a stand still). Traction control reacts by applying the brakes to the spinning wheel and this forces the drive to be diverted to the wheel(s) with the best grip. Traction control usually only operates below a certain speed.

Sliding

There are two different types of slide – understeer and oversteer. SC systems react to these situations by applying the brakes to individual wheels, and reduce engine torque when appropriate to keep the car on line. During an understeer situation, torque is reduced and the resulting forwards weight transfer is usually enough to regain control, if this is not sufficient to bring the vehicle back in line, individual rear brakes will be applied. When oversteer is occurring, brake force is applied to one of the front wheels, which acts as pivot to bring the car back on line. In general, the brakes are only applied to the wheels which are likely to have the most grip.

how does the system apply the brakes?

Almost every vehicle now has ABS fitted as standard. This life-saving system allows you to continue to steer while braking by regulating the brake pressure and preventing wheel-lock. The system uses a hydraulic motor to generate brake pressure, and this same motor is used by the SC systems to apply braking force to individual wheels where possible, and valves in the ABS unit regulate the pressure.

Disadvantages of Stability Control systems

As discussed above, SC systems use both the brakes and engine management controls to reduce wheelspin or slides. Great on the road, but when you’re on a track the last thing you need is the car putting on the brakes! Most performance cars have an option to disable (or significantly reduce) the SC systems via a button on the dash. Experiment by turning off the control and see how the car behaves. If you have got into the bad habit of allowing the SC systems to sort you out round corners, you may find yourself spinning in the first bend, so be careful and build up speed gradually as your confidence improves.