My third German publication

has appeared in the local weekend newspaper, the ExtraTip. The question was about friction between car tires and the road. Specifically, what force causes the tires to lose traction. You can check out the article (in German, many thanks to Angela for translating) here.

Below is the original article.

We know that friction occurs whenever two surfaces in contact try to slide past one another. As our reader correctly points out, the force of friction between any two surfaces is proportional to the force with which they are pressed together (the normal force) and the coefficient of friction, a pure number that depends mostly on the surfaces and on whether the objects are at rest (static friction) or moving relative to each other (kinetic friction). When the objects are at rest, the force of friction between them increases as we push harder. The maximum force with which we can push before moving one of the objects determines the coefficient of static friction. As one might expect, the coefficient of static friction is generally larger than the coefficient of kinetic friction. Our reader asks what happens when a car brakes and loses traction. How does friction work then?

Well the principle is the same as for flat objects but there is a subtlety. Let's examine a single tire on pavement as an example. The key is that no part of the tire ever slips along the ground. (To show this, place a colored dot on the side rim of the tire and slowly roll the tire back and forth. Does the dot slide along the ground or does it only touch the pavement for a moment?) When you want to change direction or speed, you have to apply some force onto the road through the tire. The force between tire and road that effects this change is the force of static friction; the maximum rate of change is thus determined by the coefficient of static friction. So having good traction means rolling smoothly without slipping.

When we exceed the maximum force of static friction by accelerating or braking too hard or by turning the wheel too sharply, the tire begins to slip and we lose traction. If we decrease the force required either by straightening the wheel or by releasing the brakes or gas pedal, the tire will regain traction.

In my native Canada, road conditions in winter are often very slippery from ice and snow. When I first learned to drive, I was taught to pump the brakes in and out if I had to stop suddenly. This lets the car decelerate as fast as possible while maintaining control. Anti-lock brakes work this way. And if the car starts skidding, you have to turn the wheel into the direction of the skid to reestablish control then gently bring the car out of its skid. It’s scary but I can tell you it works!