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AUTO TECH: Put the brakes on spinning your wheels

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A traffic light sits on the ground at the corner of Harrow Street and Taylor Avenue after being struck by a car.

If you live in Canada, you know what it's like to drive on slippery roads. Regardless of the type of vehicle or tire you have installed on them, you will sometimes find road conditions that just don't provide enough traction for your tires to grip.

Many of us have become used to the modern technologies such as traction and stability control, and they are there to assist us, but we shouldn't become reliant on them. Operating a vehicle on slippery roads requires driving techniques that can be applied to any type of vehicle.

When you pull away from a stop at a slippery intersection, the engine torque tries to accelerate the tires. The friction of the rubber on the slippery road combined with the biting edges of the small sipes or cuts in the tread provide the traction to get the vehicle moving.

When the grip isn't there, the tires spin and, on today's vehicles, several things happen in the blink of an eye. Traction control systems attempt to slow the spinning tire. The most common way to do this is to slightly apply the brake on that wheel. As the spinning tire slows, the torque from the engine is then transferred through the vehicle's differential to the other side, hopefully where that tire will have traction and move the vehicle.

In all-wheel-drive vehicles, the torque can also be transferred between the front and back axles. For example, if both front wheels are spinning, more torque is transferred to the rear by applying the front brakes. Transfer case design and computer programming determines what percentage of the torque is transferred between axles, with some only transferring about 50 per cent while a few can transfer nearly up to 100 per cent.

Applying the brake on a spinning wheel is an effective way to transfer torque to other wheels, but it has a couple drawbacks. First, it takes a little time for this to happen. A second of time may not seem like much, but in that time-frame the tire has accelerated to a high rate of speed, so other methods are used to help reduce wheel spin even quicker.

One method is to retard engine timing. This reduces power to the wheels and can happen as fast as the engine computer can operate. Unfortunately, this also increases engine emissions so it's only used for an instant. Then, other methods are used, such as closing the throttle -- an easy task with computer-controlled throttles on modern drive-by-wire engines. There are a few vehicles where fuel is cut off to some cylinders to reduce power as well.

All of this helps reduce wheel spin much faster than applying the brakes and, when the brakes are applied, the reduced engine power now doesn't overheat the brake pads and rotors.

Traction control systems are designed to let the tires spin a little faster than the vehicle is moving. This lets the tire "dig" through ice and snow to a point where it can get better traction. Spinning tires not only don't provide good traction but have poor lateral stability. So, under low-traction conditions, stability systems on vehicles are integrated with the traction control to slow spinning tires selectively and help maintain vehicle stability.

How did we drive before these electronic systems were there to help? Smoothly and carefully! By applying light throttle when accelerating from a stop, braking gradually when slowing and using smooth and small steering inputs, drivers were able to maintain traction between the tires and slippery roads.

It's important to practise these skills even on modern vehicles. There could be times when the traction control, stability control and ABS systems are not operating. All of these systems require speed inputs from sensors on the vehicle. If a sensor becomes damaged, perhaps by ice or mud hanging on the sensor wiring, broken by stones hitting them or corroded by salt water on winter roads, the systems will stop working. Warning lights on the dash will indicate this, and then it's up to the driver.

Many drivers have become accustomed to just stepping on the gas and letting the systems control wheel spin when pulling away at an icy intersection, or stepping firmly on the brakes and allowing the ABS to stop the vehicle in a straight line. These systems work great and are a great equalizer, letting new drivers operate the vehicle under many conditions similar to a skilled driver. They help even skilled drivers, since no one can maintain that "edge" of driving awareness all the time.

But we should learn to drive without them too. That would make us all safer and better drivers.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists' Association of Canada.

kerr.jim@sasktel.net

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