Brake Device to Prevent Wheel Locking
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I T has been proved many trmes that maximum braking occurs before the wheels begin to slide. The friction between a road surface and a tyre tread is less when there is relative movement between them, that is, when the wheels are locked, than when contact is static,. Furthermore, so soon as sliding starts, control may be lost, particularly if it be the front wheels that are skidding.
With the object of preventing such sliding, or rather of checking it so soon as it begins, a device has been proposed which releases' the brake automatically when the wheel locks. The scheme is to employ a sudden stop of the wheel, that is, a high degree of angulateretardation, to close an electric circuit which momentarily diminishes the brake's action. It functions thus:
A lever, weighted at each end, is mounted diametrically on the wheel with its mid point at the hub centre. The mounting allows the lever to rotate about the wheel axis, but it is restrained •from doing so, in normal circumstances, by a spring which pulls (or pushes) it, in a direction opposite to that in which thewheel is turning, against a stop on the wheel. Provision is made for-adjusting the force exerted by the spring.
If a turning moment be imparted to the lever in the same direction as the wheel's rotation and the spring force be overcome, the lever moves relatively to the wheel and causes a .pair of electric contacts to meet.
At maximum vehicle retardation, the wheel's angular retardation is comparatively low and the momentum of the weighted lever is not enough to overcome the spring. If the wheel locks, however, its angular retardation rises rapidly, with the result that the lever continues to turn against the spring and makes the contact.
Through the medium of brush and slip-ring gear, an electric magnet is energized which operates a valve in the hydraulic brake line and reduces the pressure in the system. Experiments show that'intermittent application and release of the brakes occur, and it is of interest that, among expert drivers, similar operation of the brake is often employed under conditions when maximum retardation is, required and wheels are prone to lock.