Cadence braking
From Wikipedia, the free encyclopedia
Cadence braking is an advanced driving technique used to allow a car to both steer and brake on a slippery surface. In the past it would 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. Cadence braking is supposed to maximise the time for the driver to steer around the obstacle ahead, as it allows him to steer while slowing. It needs to be learned and practiced. For most drivers it has been entirely superseded by ABS.
Maximum braking force is obtained when there is approximately 11% slippage between the braked wheel's rotational speed and the road surface. Beyond this point, rolling grip diminishes rapidly and sliding friction provides a greater proportion of the force that slows the vehicle. Due to local heating and melting of the tires, the sliding friction can be very low. When braking at, or beyond, the peak braking force the steering is ineffective since the grip of the tire is entirely consumed in braking the vehicle.
Cadence braking involves pumping the brake pedal fairly rapidly but deliberately, to make the wheels lock and unlock. This is done primarily to maintain steering control, at least in part. While cadence braking is effective on most surfaces, it is less effective at slowing the vehicle, than keeping the tires continually at the optimum braking point which is called threshold braking. The latter is is an expert driving technique that is even more difficult to learn than cadence braking, and again has been largely superseded by ABS. When ABS is present the best emergency stop will usually be obtained by simply pressing very hard on the brakes, forcing the ABS to perform, and steering to avoid the obstacles ahead.
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, causing a wedge of loose material to 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.
Threshold braking, or a good ABS, generally results in the shortest stopping distance in a straight line. ABS, cadence and interference braking are intended to preserve steering control while braking.
