A.D. Tupper & Associates Limited

As an object rounds a curve, the outwards force, which is called centrifugal force, is equal and opposite to the centripetal force, which goes towards the centre of a curve.

If a vehicle is going around a turn, the centrifugal force tends to makes it move to the outside of the curve, but that tendency is offset by the friction between the tires and the road. For every curve, however, there is a "critical speed" at which the centrifugal force equals the available frictional force generated between the tires and the roadway. If traveling above the critical speed, the vehicle, meaning the rear wheels, will tend to slide to the outside of the turn.

Canadian drivers have all experienced this phenomenon. If the road is icy, you tend to slide off the side of the curve at speeds which would be safe under other conditions.

This principle of balance between the vehicle’s centrifugal force and the available frictional force is useful in determining the speed of a vehicle as it traverses a curve. If a car spins out, for example, and leaves characteristic curved marks which display bars or striations indicating that the wheel was rotating while it was sliding, its speed was exceeding the critical speed.

By placing scaled drawings of the vehicle on scaled plan drawings of the marks, we can get the radius of the path of the car’s CG. If the frictional characteristics of the road surface are known, it is possible to relate that radius and the friction and calculate the critical speed of the path of the car, which path will probably be different from the regular travel path.

The vehicle will have been traveling at a minimum of the calculated speed when it lost control.