Traction control
by Jim Kerr

The new model cars and trucks are now appearing at most automobile dealerships. Improvements such as more horsepower, better fuel economy, more versatile cup holders, and "hot" new colours are advertised to attract potential new owners to the dealerships. While these can be desirable features, there are other features available on some new vehicles that can make driving easier and more enjoyable. One of these improvements is called "traction control".

Traction control is not new. The concept of increasing the traction of the driving tires has been around for decades. Professional top fuel drag racers used a crude system of traction control in the 1960's and 1970's to limit tire spin and shorten their elapsed times down the quarter mile racetrack. Their approach was to make the clutch slip so horsepower to the rear wheels was limited to only the maximum the tires could handle. While this concept works, clutch repairs and maintenance are time consuming and expensive! Today, passenger cars rely on electronics to increase the vehicle's traction.

Traditionally found only in luxury vehicles such as Cadillac, Lincoln, BMW, Mercedes, Lexus, and Jaguar, traction control is quickly finding its way into lower priced family market cars. Some vehicles such as Saturn equipped with antilock brakes and automatic transmission have had it for some time.

The antilock brake system is the key to determining how much traction is available. Wheel speed sensors used for the brake system signal the difference in speed between the front and rear wheels. This difference is then signalled from the antilock brake computer to other computers on the car to control traction. Just as antilock brakes has become common on modern vehicles, so will traction control.

Maximum traction is obtained when the tires on the drive wheels just begin to slip against the road surface. Maximum traction for braking is between 14% and 20% slippage. For maximum traction during acceleration, the electronic systems on today's cars allow up to 30% slippage. The extra slippage or wheel spin helps the tires to dig through any ice or snow.

There are many different methods of controlling the traction of the vehicle: retarding ignition timing, reducing fuel delivery, controlling the transmission, controlling the throttle, or applying the brakes. These methods can be used independently or in any combination depending upon vehicle design. Some of these methods are used manually by skilled drivers when driving vehicles without traction control on slippery roads.

Each of the methods used to control traction limits power to the drive wheels, just as the slipping clutch does on a drag racing car. Retarding ignition timing is the quickest way to reduce power. The engine computer can do this in a fraction of a second. Reducing fuel delivery is the next easiest to do. The computer turns off a fuel injector for a short time and the engine has less power. The driver usually doesn't even notice the difference.

Shifting the transmission to a higher gear also reduces power to the drive wheels. The use of electronic shift automatic transmissions is common on today's vehicles so an electric signal is all that is needed from the engine computer to make the transmission shift up.

Closing the throttle can be done two ways. One is to have a throttle controlling motor connected with the throttle cable. The throttle works normally until the wheels begin to spin; then the motor pushes the throttle pedal back up and closes the throttle. This method is usually noticed by drivers.

Another method is to have a second throttle system on the engine with it's own computer-controlled motor to operate it. On this system, when the wheels spin, the computer simply closes the motor driven throttle plates and the engine slows down. Because this system is not connected to the regular throttle, the driver feels nothing.

The final method of traction control is to slow the spinning wheel by applying the brake on that wheel. This method is usually used only at slower speeds. Excessive use of this method can quickly overheat the brakes, so most manufacturers program their computers to turn off the traction control if the brakes will be damaged.

Manufacturers design the traction control systems to be always on, but pressing a switch will turn the system off until it is manually turned on again or the next time vehicle is started. You may want to turn the system off if the road is completely covered in glare ice. This will permit the tires to spin and melt their way down to more solid traction surfaces.

The next generation of traction control is now appearing on many luxury vehicles. It integrates ABS, traction control, and vehicle stability systems into a complete package to help the driver maintain control of the vehicle while cornering, as well as stopping and accelerating. These systems are amazing in operation, but still can't overcome the laws of physics. Push the car too hard and you will end up off the road!

Finally, remember your vehicle's traction is only as good as the tires will permit. A good set of tires on a vehicle without traction control will still go further than a worn set of tires on a vehicle with it!

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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