Cadence braking
From Wikipedia, the free encyclopedia
Cadence braking is a technique used to stop a car or other vehicle more quickly on a slippery surface. It would normally be used to effect an emergency stop where traction is limited, though for use in an emergency requires a presence of mind that the situation itself might preclude.
Maximum braking force is obtained when there is approximately 11% slippage between the braked wheel's rotational speed and the road surface - at this point rolling resistance is maximised, and there is a small additional contribution from sliding friction - beyond this amount of slippage, rolling resistance diminishes rapidly and sliding friction alone slows the vehicle. Due to local heating and melting of the tires, the sliding friction can be very low.
Cadence braking involves pumping the brake pedal fairly rapidly but deliberately, to make the wheels lock and unlock. Because a locked wheel creates a smaller braking force than one which is just on the point of locking, this technique may cause the vehicle to stop more quickly, because the point of optimum braking is passed through multiple times. In addition, by avoiding a total lock-up, steering control can be retained, at least in part. While cadence braking is effective on most surfaces, it is less effective than keeping the wheel continually at the optimum braking point. However, this is almost impossible to do manually. Note that the principle of cadence braking is automated in anti-lock brakes (ABS), but done multiple times every second. Because of the fact the human can not modulate the brake pedal that fast, threshold braking is more effective in keeping the tires close to the optimum slippage. When ABS is present the best emergency stop will be obtained by simply pressing hard on the brakes, forcing the ABS to perform.
Cadence braking (or any other type of braking) will not help much on extremely slippery surfaces such as ice (in theory it would, but in practice the ice can be so slippery that it makes little difference -- a winter tire would make more difference). Also, on very loose surfaces, a quicker stop can be achieved by simply locking the wheels, forming a wedge of loose material will build up ahead of the wheels and create a substantial braking force. In such conditions, ABS actually increases the stopping distances. On poor surfaces, in the past, rally drivers timed the pulsing of brake application so as to take advantage of the load transfer as the vehicle pitches forwards and backwards in response to the initial braking effort. With modern overdamped, stiffly sprung, suspensions this is less likely to be effective.
