December 12, 2007

Keeping your car on the road in winter

By Jim Kerr

From coast to coast, winter's icy blast has left vehicles sliding off roads, into light poles and unfortunately into other vehicles. Some vehicles manage to stay in control much better than others. Part of it could be driver skill, but the vehicle you are driving also makes a big difference. Let's take a look at what can make the difference between staying between the ditches and being in one.

Tires are probably the biggest single component that will improve winter driving. According to Pirelli, seven degrees Celsius is the temperature where winter tires begin to have more grip on the road than summer tires. Rubber gets harder as it becomes colder. Compounds in winter tires remain more flexible at lower temperatures, so they conform to the shape of the road surface and have more grip.

All season tires are a compromise. They work well for three seasons but winter tires, especially those designed to work on ice, provide the best traction. Over the years I have talked many drivers into purchasing a set of winter tires for their vehicle. Once they feel the traction they have with them, they say they will never go back to all-season tires in the winter again. A co-worker recently purchased another set of winter tires for his second vehicle: his rationale was he could pay for the tires now or pay for the accident soon! The tires were the obvious choice.

Winter tires should be purchased in sets of four. This provides balanced traction at all corners of the vehicle. Some drivers install only two winter tires and then put them on the drive wheels. If you must purchase only two winter tires, always install them on the rear of the vehicle. This will help keep the vehicle under control. Good traction on the front tires combined with poor traction on the rear makes for an evil handling vehicle that will tend to spin often.

Weight balance is also important to winter handling. Front-wheel drive vehicles and pickup trucks tend to be front heavy. The light rear end of the vehicle doesn't have as much traction. A couple sandbags secured in the rear of the vehicle will help traction and handling. Make sure to remove them again in the spring to optimize fuel economy.

Electronic stability control systems have made poor drivers look good and good drivers even better. A computer on the vehicle monitors steering wheel angle, vehicle yaw rates, lateral acceleration and tire speeds to determine if the vehicle is travelling in the intended direction or it is sliding over the limits of control. If the computer determines that the vehicle is not going in the intended direction, it will reduce engine power and apply wheel brakes individually to slow the vehicle and bring it back in line.

Stability control systems are amazing technology that reacts quickly to the first indications of vehicle instability. They won't however, overcome the limits of lateral force on the tires. Go around a corner too fast and the whole vehicle may slide sideways on a slippery road. Stability control will minimize the danger but it can't overcome a lack of common sense.

Traction control is great for getting a vehicle moving on slippery roads but it won't help handling as much as stability control. Antilock brakes (ABS) however, do keep a vehicle in control when stopping on slippery surfaces. The other advantage of ABS is it allows steering control while braking.

All-wheel drive is another fantastic handling feature for both dry and slippery road surfaces. Note I didn't say four-wheel drive. Four-wheel drive systems lock the front and rear axles together, which is great for traction in a straight line but makes tires slip when cornering. A four-wheel drive vehicle will have less traction when turning a slippery corner than a two-wheel drive vehicle. Some four-wheel drive vehicles have "automatic" mode, which drives all four wheels when traction is low and only drives two wheels during cruise or deceleration. This system is better than non-automatic systems but still far inferior to full time all-wheel drive systems.

Canadian drivers shouldn't be surprised when winter arrives - after all, it seems to make an appearance every year! Some purchase tires with winter driving in mind. Those are the ones that always seem to get through when the winds start to blow and the snow begins to fall.

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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