The Physics of Snow Tires

Today is the first of many Physics blogs about my family's Christmas vacation to Canada. YES, Physics happens outside of Hawaii, too! : ) As we went sightseeing, and out and about in Montreal, one of our primary means of transportation are our relatives' cars (the other is the METRO). Here is a picture of a tire of my aunt's car:

Friction play a very important role in ensuring our safety when driving especially when it comes to skidding out of control. As TV commercials often show, tires have different tread patterns for each road condition with each change of the season. These tread patters are very important in determining the tires' friction (against the road) or more commonly known as the skid resistance, which prevent your car from skidding out of control. During the summer or dry conditions, smooth tires are ideal because they provide a large area of contact between the tire and the road for frictional forces.

However, a smooth tire would not work as well during the rainy season because a film of water develops between the tire and the road, which reduces the friction between the tire and the road which may result in hydroplaning. Therefore, a patterned tire with grooves is ideal for rainy weather because the water on the road can collect in these grooves or channels and therefore prevents the formation of a film of water. The patterned tires also provides greater contact between the tire and the road, which is essential for friction forces to keep the car in control. The larger the area of contact between the tire and the road is, the greater the ability of the car to attain maximum traction (the friction between the tires and the road), which helps in stopping.

In terms of snow, in order to retain maximum traction, tires with wide and deep treads are ideal in order to move the snow to the sides. Snow can be a very slippery surface and slippery materials yield a low coefficient of friction for tires without deep treads (which is why smooth tires are not ideal). If F = uN, where u is the coefficient of friction, F is the force that resists motion, and N is the normal force, then u = F/N. The higher the u, the more resistance between the two obejcts sliding past each other, which in this case are the tires and the road. To increase u, tires with wide and deep treads that can dig through the soft material to provide larger traction are ideal.