October 4, 2006

Preparing for the winter tire season

By Jim Kerr

Cold weather will soon be upon us again and it is time to think of how our tires will fare this winter. If you have performance tires on your vehicle, then icy roads and snow will present difficulties. These tires are designed for handling on dry pavement and the ability to provide fair traction on wet roads. All season tires are found on many vehicles as original equipment. These tires do provide traction on ice and snow if driven carefully, but these tires would be better named "three-season" tires. They work well in spring, summer and fall, but don't provide the ultimate traction and handling that a performance tire will on dry pavement nor the traction of winter specific tires on snow.

For winter roads, a winter tire is still the best tire available. When installing winter tires on a vehicle, it is always recommended that they be installed on all four corners. Some drivers install only two winter tires on the drive wheels, thinking that will give them the traction to get out of a snow bank or up an icy hill. True - winter tires on the drive wheels will give the vehicle a better chance of getting unstuck, but a big safety problem exists when using only two winter tires: poor handling.

With winter tires on the drive axle and all-season or performance tires on the other axle, the vehicle will tend to become unstable and swap ends under some conditions. For example, if the vehicle is front-wheel drive with two winter tires on the front axle, the back end of the car may try to pass the front (spin out) during a braking manoeuvre. This may not be apparent until you have to brake quickly such as during an emergency stop. Then the vehicle spins out exactly when you need the most control.

Rear-wheel drive vehicles with winter tires on the rear axle only will tend to "push" or not turn into a corner. Even though the vehicle can accelerate, steering control is not optimum and avoiding an object or another vehicle on the road could be difficult. Winter tires will also provide better braking on slippery surfaces, so the anti-lock system doesn't need to intervene as often and the driver maintains better control. The cost of winter tires is often cheaper than the deductible on your auto insurance, and winter tires can last more than one season if used only in the winter.

When should you replace tires? It depends on tread depth and driving conditions. Tread depth is measured in millimetres from the surface of the tread to the casing of the tire. You could use a small ruler to measure it but it is easier with an inexpensive tread depth gauge. Many technicians have one in their tool box and they are available at auto supply stores usually for a couple dollars.

Tires typically have between 11 and 13 mm of tread depth when new. There are raised bands of rubber on the tire casing that sit 2 mm higher than the base tire. When these bands start to show across the face of the tread, the tire is worn to 2 mm tread depth, which is the legal minimum. Less tread than that will be very slick on wet surfaces and the vehicle can quickly loose control.

While more than 2 mm of tread will work well on good road surfaces, mud or snow traction requires a deeper tread. Some tire manufacturers are building "snow" bands into their winter tire designs. Just like the 2-mm minimum tread-depth wear-bands, these snow-bands are raised rubber across the tire face about 6 mm above the tire casing. Markings on the tire sidewall indicate where these bands are located on the tread, but you can also see them if you look at the tread. Tread depth less than this 6 mm height will not have optimum traction on snow or mud. For best winter traction, replace the winter tires at the beginning of snow season. Once they are worn beyond the 6 mm tread depth, the tires can still be used and are good for wet spring and fall roads, but replace them again before the snow flies the next winter.

Keeping two sets of tires - winter and summer or all-season - for your vehicle sounds expensive and a bother, just like replacing tires before they are completely worn out, but tires are all that connect us to the road. We hurtle down the road in two-ton vehicles with less than 2 square feet of rubber keeping us in control. If good tires prevent just one collision or one injury in the life of a vehicle, then they have paid for themselves many times over.

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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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