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I though I'd try and contribute what I think is useful info. I spent a lot of time recently investigating Winter Tires for my X - and found some information that I didnt know even after buying 2 previous sets of winter tires. I know it won't apply to some of our friends south of the border - but I hope someone finds it useful.

The original definition of M+S tires is based on the geometry of the tread design. The M+S designation was first used to differentiate the knobby bias ply tires intended for use on muddy, and/or snow-covered roads from the straight rib tires used on early cars or trucks. Tires with tread designs that meet the definition may be branded with the letters "M" and "S" in several different ways (e.g., M&S, M+S, M/S, MS, etc.) at the discretion of the tire manufacturer.

When early radial ply tires were also found to deliver more snow traction than the straight rib, bias ply tires, the tire companies introduced "All-Season tires." Supported by advertising, All-Season tires have presented an unspoken promise that they, throughout their life, can provide tratcion for all seasons...through spring's rain, summer's heat, autumn's cooling and winter's snow. While this combined offering has made All-Season tires popular, many drivers have learned that a geometric definition doesn't guarantee winter snow and ice traction.

According to the Rubber Association of Canada, all-season tires tend to stiffen and lose gripping power around zero Celsius. This loss of traction on icy or snow-covered roads may become critical for safety as the temperature drops.

In 1999, The U.S. Rubber Manufacturers Association (RMA) and the Rubber Association of Canada (RAC) agreed on a performance based standard to identify passenger and light truck tires that attain a traction index equal to, or greater than 110 (compared to a reference tire which is rated 100) during the specified American Society for Testing and Materials traction tests on packed snow. The new standard helps ensure that drivers can easily identify tires that provide an higher level of snow traction.

Passenger and light truck tires designed for use in severe winter conditions will be marked on the sidewall with the M & S stamp, along with the following graphic:



According to Transport Canada, the stamp and graphic indicate the tires have met specific snow traction requirments and have been designed for severe snow conditions.

As a general rule, to maintain control and stability of your vehicle you should install identical tires on all wheels. Avoid mixing tires with different tread patterns, internal construction or size, unless specified by the vehicle manufacturer.

Although drivers in most areas will be fine with non-studded winter tires, those that live in ice-prone areas should consider studded snow tires. In reviews, studded tires brake more quickly and handle better on ice. In snow, theres little performance difference between studded and non-studded tires. Be aware that some regions restrict the use of studded snow tires, since they contribute to road damage.

According to Peter Keith – a former chief engineer in the motor vehicle safety branch of Transport Canada – studded tires are a must. "The great advantage of the stud is that the little piece of steel bites into the surface of an icy road and ... so it provides me with very good adhesion on a slippery road," he says. "In fact it almost doubles your adhesion on a slippery road. And the other thing it does, by roughing up the ice, it actually provides better traction for all other people who don't run on studded tires. So it's actually a very beneficial device.�
 
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