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The science behind winter tires and how they work

All-season tires are a bad compromise. On snow, ice or cold pavement, the stopping distance of a car equipped with winter tires can be 30 to 40 per cent shorter than one with all-seasons. Since the force of a crash increases as the square of impact speed, this could be the difference between life and death.

Although it's the treads that you notice, the most important part of a winter tire is actually its rubber compound, which is designed to stay soft in freezing temperatures. Like a gecko climbing a sheet of glass, a tire sticks to the road by conforming to minute imperfections. The soft rubber treads of a winter tire are able to splay and wrap themselves around minute protrusions on cold pavement, or even on what may appear to be perfectly smooth ice. Summer tires, which are designed to operate in warm temperatures, harden as the temperature falls. All-seasons, which must be designed for year-round use, cannot match winter tires in low temperatures.

Premium winter tires perform better than basic models. What you're paying for is the latest in rubber technology and tread design. What you get is traction that may be up to 15 per cent better than economy-model winter tires. If you want to see the difference between different grades of winter tires, go to an ice race. "The drivers with the premium tires are all out front," says Ontario racer and winter driving instructor Ian Law. "There's no comparison."

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It's about temperature, not snow. Winter tires should be installed when you expect temperatures to fall to 7C or below. As the temperature falls, the rubber in summer and all-season tires becomes inflexible, killing traction. Watch the thermometer and use common sense, because no one will tell exactly when to put on snow tires (unless you live in Quebec, where the law dictates that your car be equipped with winter tires between Dec. 15 and March 15.)

Winter tires should be narrower than summer models. Experts recommend that you go down one or two sizes when installing winter tires – if your car came with 215-mm-wide summer tires, for example, your winter tires should be 205 mm or 195 mm. Reducing the width of a tire increases the pressure it exerts on the surface beneath it – this helps the tire slice through snow and reduces hydroplaning.

Winter tires are designed to move water. When a tire presses down on snow or ice, it melts the top layer, creating a thin film of water (the same phenomenon that occurs as a skate glides across a rink). If the water isn't moved away from the area in front of the tire, the car will hydroplane. This is why winter tires are covered with grooves (including tiny channels known as "sipes") that move water away to the sides, allowing the tire to stay in contact with the surface.

All-wheel drive helps you accelerate, not stop. On slippery surfaces, vehicles with four driving wheels can accelerate better than those with two-wheel drive. But their cornering and braking capabilities are little different than a two-wheel-drive model. When you're trying to stop or turn, the limits are determined by the traction capabilities of your tires, not the number of driven wheels.

Black ice is not a death sentence. Good winter tires can stick to glare ice, but only if they are within their traction limits. If your car begins to slide, look straight down the road at where you need to go, and maintain a light grip on the wheel. As the car decelerates, you will gradually regain control as the rubber in your tires begins gripping surface imperfections on the ice. Slow speed and gentle control inputs will maintain traction.

The performance of winter tires has been significantly improved over the past decade by advanced rubber compounds that allow designers to make tires softer without sacrificing other critical properties, including wear and heat buildup as temperatures climb. Major manufacturers spend a lot of money on R&D. Jaap Leendertse, winter tire platform manager for Pirelli in Milan, Italy, told me the company has developed more than 300 compounds in the ongoing quest for the ideal winter tire.

In the old days, winter tires came with deep, aggressive treads designed to paddle through deep snow. This made for a noisy ride and compromised stability, since the treads deflected under acceleration, braking and cornering loads. Current winter tire technology focuses on shallower treads with closely spaced grooves that carry away the water film created when the tire presses down on ice or snow.

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Although testing makes it easy to see the performance advantages of a winter tire (you stop faster), the technology behind it is deceptively complex. Tire designers must consider a long list of factors, including tread stability and hysteresis (a process that generates heat as a tire repeatedly deforms and recovers as it rotates under the weight of a car).

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About the Author
National driving columnist

Peter Cheney launched his driving column after 25 years as an award-winning feature writer, investigative reporter and news correspondent. His writing steers clear of industry jargon to focus on human experience and the passion of driving. More

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