SKIDDING PREVENTION. |
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The New York Times
December 31, 1922
Motor car skidding is due to the action of the brakes operating through the rear wheels, explains a rubber company official. While the rear wheels grip the ground and rotate, they control the direction of the car. However, as soon as the rear wheels are locked by the brakes they begin to slide and, as they slide sidewise practically as easily as forward, they lose the power to direct the car.
It takes comparatively little brake friction to lock and slide rear wheels on slippery roads, because there is little friction between the tire and the road surface. When once the driving wheels begin to slip, the car is kept in motion by the force of inertia acting at the car's centre of gravity or mass. This forward motion is opposed by the resistance of all four wheels on the ground, a resistance that is greater in the rotating front wheels than in the sliding rear wheels. Unless the combined ground resistance of all wheels, tending to stop the car, acts on the same point as the force of inertia tending to carry the car forward, there results what is called a "force couple" which causes the car to skid or spin around.
After the wheels once start to slip, a very small "force couple" will produce skidding, and the skid distance is dependent upon the magnitude of the "force couple," which in turn depends on car speed and weight. Turning the front wheels in the direction of the rear wheel skid helps to reduce skidding. Also periodically releasing and reapplying the brake so the rear wheels can get a grip, offsets forward slip and consequent side skid.
Two means employed to prevent skidding are in the use of non-skid or rough-tread tires and the other by anti-skid chains. Non-skid tires are designed to give forward traction and to prevent side slip. As long as there is traction the rear wheels will turn, and as long as the rear wheels turn there will be no serous side slip, unless the front wheels are blocked.
Non-skid treads on tires were first used on rear wheels. Later non-skid tires were placed on front wheels, because the treads prevented front wheel skidding, as well as permitting the rotation or exchange of the front and rear tires.
Because they make available in road friction the total weight of the car instead of only half the car weight, four wheel brakes are coming into use, especially in Europe. Not only can cars be stopped more quickly but they can be driven safely on wet days and in ice and snow without chains. In stopping a car equipped with four wheel brakes all four wheels may slide and the car as a whole slip forward. However, there appears to be little or no side skidding. This is especially true if the brakes are connected diagonally in pairs, that is, left-hand front wheel and right-hand rear wheel. With this arrangement, opposite wheels are able to roll freely, retaining their directing qualities.