Winter tire

Snow tires—also called winter tires—are tires designed for use on snow and ice. Snow tires have a tread design with bigger gaps than those on summer tires, increasing traction on snow and ice. Such tires that have passed a specific winter traction performance test are entitled to display a "Three-Peak Mountain Snow Flake" symbol on their sidewalls. Tires designed for winter conditions are optimized to drive at temperatures below 7 °C (45 °F). Some snow tires have metal or ceramic studs that protrude from the tire to increase traction on hard-packed snow or ice. Studs abrade dry pavement, causing dust and creating wear in the wheel path. Regulations that require the use of snow tires or permit the use of studs vary by country in Asia and Europe, and by state or province in North America.

Related to snow tires are those with an M+S rating, which denotes an "all-season" capability—quieter on clear roads, but less capable on snow or ice than a winter tire.

Snow tires operate on a variety of surfaces, including pavement (wet or dry), mud, ice, or snow. The tread design of snow tires is adapted primarily to allow penetration of the snow into the tread, where it compacts and provides resistance against slippage. The snow strength developed by compaction depends on the properties of the snow, which depend on its temperature and water content—wetter, warmer snow compacts better than dry, colder snow up to a point where the snow is so wet that it lubricates the tire-road interface. New and powder snow have densities of 0.1 to 0.3 g/cm³ (6 to 20 lb/cu ft). Compacted snow may have densities of 0.45 to 0.75 g/cm³ (28 to 47 lb/cu ft).

Snow or ice-covered roadways present lower braking and cornering friction, compared to dry conditions. The roadway friction properties of snow, in particular, are a function of temperature. At temperatures below −7 °C (20 °F), snow crystals are harder and generate more friction as a tire passes over them than at warmer conditions with snow or ice on the road surface. However, as temperatures rise above −2 °C (28 °F), the presence of free water increasingly lubricates the snow or ice and diminishes tire friction. Hydrophilic rubber compounds help create friction in the presence of water or ice.

Vehicle in cold, dry, powder snow, which offers comparatively better traction than moist snow.

Vehicles in warm, moist, granular snow encounter decreased roadway traction.

Attributes that can distinguish snow tires from "all-season" and summer tires include:

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Wikipedia