Traction control (also called ASR - Acceleration Slip Regulation) is a system built into some cars that is intended to minimise wheelspin. There are several ways in which this system operates, but they all work to actively slow down wheels that are spinning significantly faster than the others. Virtually all traction control systems "piggyback" the car's Anti-Lock Braking System since it has a component infrastructure that serves the ends of both. Before continuing a description of traction control, a brief description of ABS is probably appropriate:

Anti-Lock Brakes are intended to give the driver more control under heavy braking situations. If one were to be more specific, ABS systems are meant to enable the driver to steer the vehicle while braking heavily. Before ABS was introduced, if a driver were to brake heavily the wheels of the car would often lock up (they would stop rotating altogether, even though the car was still moving) and it would slide. Control is virtually impossible in such situations and given that heavy braking is almost always used to avoid some hazard, a way of retaining some control would be most useful.

The best way of "unlocking" locked-up wheels is to stop braking. Any reader who drives a non-ABS-equipped car may be familiar with the method of pumping the brake pedal, progressively depressing and releasing the brake pedal to slow down without locking the wheels up. This is the best compromise between braking hard and not locking the wheels. ABS systems merely automate this and evolve the idea slightly. A system of sensors (one for each wheel) monitor the speed each wheel rotates at, and the ABS control unit compares these speeds. If any excessive difference in wheel speed is detected (i.e. one or more of the wheels stops rotating but the others don't), the system will activate. Assuming a four-wheeled vehicle, Speed differences are usually measured for between:

  • The two drive wheels (assuming two-wheel drive)
  • Either of the two drive wheels and the two at the other end, which are often treated as a single unit.

Now, how does ABS stop wheels from locking up under braking? Without going into detail (although I really want to), the system detects when any wheel is rotating at a significantly slower speed than the other three. In such a situation it very quickly releases and re-applies the brakes over and over until the driver stops braking, or the car stops. This modulation is very fast: I'm talking the brakes being released and re-applied up to 15 times per second (more modern systems are even faster than this). Users with ABS-equipped cars - i.e. virtually all cars made since about 1995 - might have experienced a pulsing, vibrating or grinding feedback from the brake pedal while braking heavily, especially in slippery conditions. This is the ABS working. It is meant to do that.

So, if your car has ABS and you have to stop suddenly, do not pump the brake pedal. If it is an emergency, just brake as hard as you can and let the ABS work. Concentrate on avoiding any hazards, which ABS is intended to make easier. Bear in mind though that the cost of better control in a heavy braking situation is increased stopping distance. A moving car with locked wheels might be virtually uncontrollable but it will probably stop quicker than a car with ABS. Even if it is in a ditch afterwards.

Now, traction control. One might be able to understand how this system could work with the ABS. It can purloin the ABS system's wheel speed readings and use them itself, but to monitor acceleration instead of deceleration. Some cars have ABS and TC as a single system, while with others the two systems are separate but work together.

There are several different ways in which traction control can work but they all serve the same end: to stop wheelspin. In a typical 'TC situation', wheelspin will occur, the TC system will 'sense' this with the readings from the wheel sensors. In less than one second the system will kick in to stop wheelspin, ensuring the tyres maintain the maximum contact possible with the road surface. The real substance is the methods in which wheelspin is stopped.

Automatic Braking

This is where the connection between the ABS and TC systems pays off. If wheelspin is detected, the ABS system can apply the brake on the spinning wheel until it has regained its grip on the road surface. However it is not ideal because it is effectively braking not just the wheels, but the engine as well. Not the best for the engine, drive train or the brakes.

Limiting Power

This is the most common method for traction control in modern vehicles, though again there is more than one way in which power can be limited.

Retarding Spark Frequency

This was an early method of traction control, whereby the frequency of spark plug firings would be reduced until the wheels stopped spinning. This required a connection to the engine control unit. However it was hardly ideal because fuel and air still entered the cylinders, meaning that disproportionate detonations would take place when the spark plugs did fire. Think flooding the engine. Think backfiring.

Retarding Throttle

This is a common method of slowing those spinning wheels down, and there's a couple of ways this can work. In older cars, the system may use a butterfly valve mounted in front of the throttle valve (the part that is controlled by the accelerator pedal). The effect of such placement is that the valve takes precedence over the driver's accelerator control. So if this valve is closed any accelerator control by the driver will have no effect. Some cars are 'drive-by-wire', meaning there is no physical connection (i.e. a throttle cable) between the accelerator pedal and the throttle butterfly. Acceleration control by the driver is converted into signals which control the throttle. In such cars the TC system will take over control of the throttle butterfly from the driver when needed and no second valve is necessary.

If wheelspin is detected, the appropriate valve will be partially or fully closed by an electric motor, effectively 'choking' the engine until the wheels regain stable contact with the road surface. The exact position of this valve (see the diagram in throttle body: the valve is similar to this) can be controlled very precisely by the TC ECU, regulating how much air reaches the cylinders (bear in mind the idle air control valve bypasses the throttle body completely, so the engine will not cut out if the traction control valve is closed completely).

Retarding Fuelling

This requires a link between the TC, ABS and engine control units (which usually is present anyway). In virtually all vehicles since about 1994, the ignition timing in motor vehicles is electronically governed so the 'co-operation' of the ECU is needed to effect this method. When wheelspin is detected, the engine ECU reduces the amount of fuel entering the cylinders, which effectively 'starves' the engine until the wheels stop spinning. Some TC systems use this method for all situations, while others use it only for preventing wheelspin at high speed, using other methods for 'launch control' (low-speed traction control).

Distributing Power

This is a more recent possibility, though it's probably more at home in a stability control node. In it, power from the engine is selectively distributed between the wheels so that the wheel(s) with the most traction get the most power from the engine and those that are slipping get less. This is achieved with a limited slip differential or with a viscous coupling. The latter is most frequently used to distribute power between the front and rear wheels in four wheel drive systems.

There are some situations that TC isn't suited to and should be switched off (which it always can be) for. When rocking the car to get it out of deep snow or mud for example, with the TC system on you wouldn't get anywhere because the wheels would just stop spinning. You want them to spin.

Although TC is useful, most car user manuals are careful to stipulate that it and the ABS systems are not a substitute for driving with some sense. TC won't enable you to pull out in front of someone 20ft away from you in the wet and get away with it. Like ABS increases stopping distance, the TC system has its price, which is acceleration. If it operates, the car will almost certainly accelerate more slowly than it would have done if you'd been sensible with the accelerator.

How does traction control feel in operation? To one unaccustomed to driving with it, TC can be a peculiar experience. There will still be a small amount of wheelspin (it must be detected before it can be stopped!) but the car will react differently depending on the system. Power generally slumps and throttle response drops to zero until the wheels stop spinning - power is usually gradually increased back up to the amount the driver is applying with the accelerator. More advanced systems can more accurately reduce or distribute power so the effect on acceleration is lessened but other, earlier systems tend to overcompensate. Built in 1994, my car has the latter; my personal experience of traction control is that while effective at stopping wheelspin, it makes the car feel like it's driving through treacle.

  • Memmer, Scott; "Traction Control"; <>
  • Johansen, Ray; "Traction Control";
  • Personal experience.

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