Hint #1: It has nothing to do with tire adhesion.

Ah, but it does! Although Rocketsled's description of "stiction" as it applies to mechanics is correct, the term is also used to describe tire adhesion.

The force required to slide a tire is called the

*adhesive limit of the tire*, or "stiction", which is a slang term combining "stick'' and "friction.'' This law, in mathematical form, is

*F ≤ µW* where

*F* is the force with which the tire resists sliding;

*µ* is the coefficient of static friction or coefficient of adhesion; and

*W* is the weight or vertical load on the tire contact patch. Both

*F* and

*W* have the units of force (remember that weight is the force of gravity), so

*µ* is just a number, a proportionality constant. This equation states that the sideways force a tire can withstand before sliding is less than or equal to

*µ* times

**W**. Thus,

*µW* is the maximum sideways force the tire can withstand and is equal to the stiction. We often like to speak of the sideways acceleration the car can achieve, and we can convert the stiction force into acceleration in

**G's** by dividing

*W* by, the weight of the car.

*µ* can thus be measured in

**G's**.

The coefficient of static friction is not exactly a constant. Under driving conditions, many effects come into play that reduce the stiction of a good autocross tire to somewhere around

**1.10G**. These effects are deflection of the tire, suspension movement, temperature, inflation pressure, and so on. But the proportionality law still holds reasonably true under these conditions. Now you can see that if you are cornering, braking, or accelerating at the limit, which means at the adhesive limit of the tires, any weight transfer will cause the tires unloaded by the weight transfer to pass from sticking into sliding.