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Cold, hard facts on all-seasons

My team is gathering information on the benefits and drawbacks of all-season, winter and ice tires.

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Q: I’m a paramedic in a region where heavy snow squalls off Georgian Bay are common, and road conditions can change rapidly. Regardless, ambulances still go out on calls.

My team is gathering information on the benefits and drawbacks of all-season, winter and ice tires.

Our ambulances, with 6.2 L diesel engines and dual wheel in the rear, are equipped with Michelin LTX all-terrain tires. Would ice-rated or snow tires outperform our A/Ts?

A: Will a knobby, all-season, all-terrain tire work as well in snow as a dedicated winter tire? The short answer is a resounding no. Here’s why:

All traction relies on the flexibility of the rubber compound. When you examine a contact patch at the microscopic level, you find that the peaks and valleys of the rubber surface grab the peaks and valleys of the road surface. As the tire rotates, flexibility is needed to get good interaction. Otherwise the two surfaces don’t interlock and traction is reduced.

All-season rubber starts to lose flexibility as temperatures drop, which means grip loss.

Tire company tests have shown that a crossover zone of grip among summer, all-season and winter tires occurs at about 5 to 7C. In that range, all three have equal grip.

But as the mercury drops, the winter tire gains grip and the other two lose it. By -14C, the all-season tire is finished, its compound is no longer working at all, so you give up as much as 60 per cent of grip.

But the winter boot, depending on brand, still maintains significant grip right down to -25 to -30C.

Snow traction also depends on a tire’s ability to flex and on how many leading edges it has. So your A/T tire has edges but no flex when it gets cold.

Ice traction depends on flexibility of the rubber and sipes (tiny cuts in the tread blocks). The more sipes, the more traction.


Email tire questions to John Mahler at

thetireguy_1@hotmail.com.

Please include vehicle’s make, model and year, tire brand and size and your name, address and phone number. Letter volume prevents personal replies.

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