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It’s official: Winter tires really do cut collisions

Mandatory laws in Germany, Quebec prove their effectiveness

Published November 8, 2013

Now that you can see your breath on crisp mornings, it’s time to switch to winter tires in order to ensure maximum safe grip for cornering and stopping.

As usual, there will be the doubters with their, “I’ve been driving for 40 years and never been stuck or had a crash.”

To all of you, I say: “You’re lucky.”

If in doubt about the effectiveness of winter tires, just look at the collision rates in Germany before and after winter tires were made mandatory in 2008.

In 2005, there were 12,539 personal-injury collisions. In 2008, there were just 6,033. I’ll bet those 6,506 German drivers who didn’t crash would be happy to endorse how effective winter tires can be in keeping drivers out of emergency rooms.

Of course, there were improvements in car safety during those same years, but car improvements alone did not reduce collisions by more than 50 per cent.

Even Quebec, which already had 96 per cent of drivers using winter tires before they became mandatory in 2008, showed a 5-per-cent drop in collision injuries that year.

After the law received royal assent, winter-tire use increased to 98 per cent — meaning there was a 5-per-cent drop in collisions after just a 2-per-cent increase in winter-tire use. Deaths and serious injuries declined by 3 per cent.

These improvements can be attributed to the simple fact that winter tires have more grip, make collision-avoidance easier, and stop quicker than all-season tires.

All tires develop grip by grabbing the microscopic irregularities in pavement. For this to occur, the tires’ rubber must be flexible enough to fit into those tiny hills and valleys. This relationship between pavement and tire is so brief that any hardening of the rubber starts to limit traction.

Summer tires start to harden at 7 C; all-season tires become stiff at about -10 C but winter tires remain flexible until -40 C. As temperatures drop, the molecules in a tire hold onto each other a little tighter, reducing their elasticity.

All-season tires have compounds that try to grip through a wide temperature range but, like most compromises, they do nothing really well, especially performing in winter conditions.

Winter tires use vastly different rubber compounds to stay flexible. You can test this hardening effect of polymers for yourself.

Just take a plastic pail that is normally kept indoors and leave it out in freezing temperatures. The pail that was pliable indoors will be stiff in the cold and will likely shatter if hit hard. The pail has thus reached its glass-transition temperature. What was once soft and bendable has become brittle.

Each type and brand of tire has a magic temperature at which this phenomenon occurs. Once a tire reaches this point, it loses all pretense of grip.

Winter tires also have rounder casings to cut into snow more effectively, and their tread patterns are more open, to displace water, slush and snow more quickly as the tire rotates.

Winter tread blocks have sharper edges to dig and bite into snow. Depending on the moisture content, winter tires either displace the snow by firing it out the back of the tire as it rotates or grab and hold the snow, using snow on snow for grip.

Tread blocks are covered with sipes (small slits), which open when they are at the road surface and provide more water displacement and ice grip.

Some winter tires offer even higher technology. Some brands leave small air pockets in the tire rubber, with grit embedded, to increase ice grip (Bridgestone, Yokohama). Some have little pockets to grab water at the base of the sipe (Michelin, Goodyear). Toyo tires use crushed walnut shells in the rubber to provide extra traction.

So when the temperature drops, just how quickly are you able to stop? In a recent test, stops on cold bare pavement from 50 km/h produced results of 18 metres for winter tires and 27.1 metres for all-seasons. That’s 33.6-per-cent shorter for winter tires — the difference between “crunch” and “phew!”

In a just-released survey of Canadian drivers conducted for Hankook Tires, about 66 per cent of Ontario drivers change over to dedicated winter tires. Nationally, the rate is just 50 per cent.

Another interesting statistic from that survey is that only half of Canadian drivers feel comfortable, or even know how, to change a spare tire.

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