THE ART OF BRAKING

Press, Volume CVIII, Issue 31801, 4 October 1968, Page 9

THE ART OF BRAKING

TJRAKING is perhaps the one aspect of driving which calls for the greatest degree of road sense. Most drivers can stop the car in a fashion, but the really difficult part of driving is to know when to start braking so that trouble can be avoided, says the chief instructor of the B.M.C. Advanced Driving School (Mr H. Shillabeer). Braking systems are vital to safety, and should never be neglected. Nevertheless, some systems do have their limitations under certain circumstances, and the first thing a driver should do Is find out just what effect braking does have on his car. This means driving out to a level, open stretch of road, and when there is no traffic in either direction, raising the speed to about 30 to 35 m.p.h. and applying the brakes, Mr Shillabeer says. Do not, however, stamp on the brakes so that the wheels are locked. Maximum braking is not possible from a locked wheel, but only from a wheel that is allowed to turn slowly. Apply braking pressure progressively and firmly, and note the reaction of the car. Take particular note if the steering is affected, the amount of free travel in the pedal, and what pressure is necessary to give varying degrees of retardation. At the first sign of the brakes being under par, have them checked by a really competent mechanic. Keeping the car stable is a

point that should be uppermost in every driver’s mind. There is no better way of inducing a skid than to brake while steering around a bend, yet one only has to follow the average driver along the open road to see him do just this time after time, Mr Shillabeer says.

As soon as a car is steered off the straight, a centrifugal force which tends to keep the car straight is produced. As this force acts upon the centre of gravity, the sum total result is that the car tends to “roll,” or lean. If the driver steers to the right the car will tend to lean to the left, and vice versa.

The amount of lean is dependent on the speed, the radius of the circle the driver is trying to take the car around, and the effeciveness of the springs and dampers. There is some degree of roll on all corners. NOT STABLE

In extreme cases, such as on racing circuits, this can result in the inner wheels lifting off the road altogether. With only two wheels really biting the road and the other two partially lifted, it can be seen that the car is anything but stable. Now, add to this the effect of braking. If one brakes hard, the front of the car dips. By doing this when the car is cornering, the driver creates the situation where only one wheel out of the four is doing any real work in keeping the car on the intended path. The golden rule is “brake only on the straight," and even this calls for a lot of road sense.

There will, of course, be times when traffic conditions dictate that a driver brake on a bend, but if he remembers the disadvantage that he is starting with and compensates for it by braking delicately, the chances of skidding will be very small.

Drivers must also remember that they cannot expect the same degree of braking

effectiveness in wet weather, and should give themselves more room to stop or slow. If you sense that the car is becoming “light” when the wheels lock, let the brakes go, and then re-apply them. A locked wheel does not have the same braking efficiency as a wheel that is allowed to turn slowly. If it is the front wheels that are locked, steering will also be lost.

If the wheels are locked, they have lost their grip with the road surface, and if they have lost their grip with the road surface, they cannot act against it to turn the car. CADENCE BRAKING A difficult situation arises when one is faced with a danger which calls for firm braking on a bad surface, and then as soon as the brakes are applied the wheels lock. The simple answer is “cadence braking,” which is no more than pumping the brakes on and off. The principle is to brake, and the moment the wheels lock to release the brakes immediately, then re-apply them.

This means that the period when the wheels are locked and steering control is lost is very small indeed. Therefore some measure of control is retained, and the stopping distance will be less than if the wheels were locked and the car was allowed to slide.

The cadence braking technique calls for some practice, as the movement of the brake pedal is quite rapid. Cadence braking is particularly valuable for a rapid stop on a shingle road. Some slight reduction in brake heating can be gained on long descents by using a slower and more gentle variation of cadence braking. A prime rule on bills, however, is to descend the hill in the same gear you would use to ascend it. This will help keep the speed firmly in check. Always remember it is not possible to stop a car “dead,”

even from 20 m.pji. The average car and driver takes up to 40ft to stop from 20 m.p.h., 20ft being “thinking distance”—the time it takes to move the foot from the accelerator to the brake—and 20ft being the actual stopping distance.

This is why approaching intersections at 30 m.p.h. Is often dangerous. Unless one can see clearly that there is nothing coming on the crossstreet from at least 75ft back, it will just not be possible to stop in time once you have seen an approaching vehicle. This is also why one must follow well behind other cars in a line of traffic. If you are travelling 20ft behind another car at 20 m.p.h. and the vehicle ahead stops suddenly, you may run into the back of it before you have even had time to move your foot from the accelerator to the brake.