Introduction to Friction Measurement
The friction between a rubber tire and the surface is a very complex process,
involving several hundred parameters, including, on the tire side e.g. the rubber composite,
the air pressure in the tire, the weight on the tire (number of passengers in the car),
the tire design. On the surface side, some of the parameters are the roughness of the surface,
the contaminants, e.g. water, ice, pollen, or sand. In addition, the travelling speed
of the vehicle is very important: The higher the speed, the lower the friction.
The so-called slip speed of the tire is defined as the difference between the vehicle
travelling speed and the tire rolling speed, or said another way, how much the tire
is sliding on the surface. As an example, when the tire is freely rolling, its rolling
speed is the same as the car speed, and the slip speed is zero. If the tire is locked,
and not rolling at all, the slip speed is the same as the car speed. In between these two points,
the tire will be rolling some, and slipping some.
It turns out that the friction is highly dependent on the slip speed, and if you can keep
the tire at exactly the right slip speed during the braking process, you will achieve
significantly better steering, and shorter stopping distance. The figure on this page
of a "Friction Curve" shows a typical relationship between the friction and the slip speed:
You can see that at the correct slip speed, the friction is almost twice as high as
with locked wheels at the end of the curve.