SPEED AND SKIDS.

Press, Volume LXVI, Issue 19889, 28 March 1930, Page 9

SPEED AND SKIDS.

SOME POINTS IN DRIVING. THE ART OF CORNERING. Many motorists, either tbrougli ignorance or carelessness, do not seem to realise that it is adhesion alone which enables them to steer a safe course, and that if this adhesion bo materially reduced from some cause or other, the speed at which it is safe to proceed is proportionately altered. When proceeding along an unfamiliar main road in wet weather the driver should be alert to notice any change of surface as indicated by the colour or texture of the read material. If he is in any doubt as to the adhesion it affords he can easily make an impromptu test by braking hard when the road is clear, so as to ascertain whether or not it is safe to continue at unabated speed. If too high a speed be maintained on a treacherous surface, the negotiation of a corner or an emergency created by the sudden appearance of another vehicle may necessitate sudden steering or braking in a way which, while quite safe on a dry road, lends to disaster on one which is slippery. Before proceeding further, a few figures may help to drive this point home. Consider, for example, a fairly sharp curve of 100 ft radius requiring about half the steering lock of the average car for its negotiation; if there is no camber to complicate matters, a speed of 30 m.p.h. round such a curve would produce centrifugal force nearly sufficient to initiate an outward skid on a dry road; in other words, this is approximately the limiting speed for such conditions, when it may be assumed that adhesion amounts to about 60 per oent. of the weight of the car. Adhesion Seduced By Bain. Now, there are many surfaces on which the adhesion is reduced to about half the "dry" figure on a wet day. Assuming this to be the case the mpximum safe speed on the curve in question would be cut down to about 20 m.p.h., because the centrifugal force is proportional to the square of tlie speed. In other words, if the adhesion be halved the limiting speed is reduced by 30 per cent., and the safety, will be still further decreased if there is a pronounced camber to assist the rear wheels in sliding toward the gutter on a right-hand curve. A similar effect governs stopping distances. In an emergency, the maximum retardation is obtained on the average car when the pedal pressure is nearly (but not quite) sufficient to cause the rear wheels to lock and slide, and is therefore dependent upon the adhesion available between the tyres and the road. A car with reasonably good four-wheel brakes can be stopped in 100 ft from 40. m.p.h. on a dry road If the adhesion be halved, owing to rain, the same stopping distance cannot bo achieved unless the speed be reduced from 40 m.p.li. to 28 m.p.h

These are, of course, extreme instances, but they are by no means imaginary. The point is that a decrease in adhesion produces an enormous reduction in tne limiting speed which may be indulged in with safety, and if more drivers could be brought to realise this point the number of inadvertent skids would b6 very markedly reduced. Many people are still for too careless about the negotiation of tram lines. Cars differ considerably in their behaviour when crossing the lines, but generally speaking, those which give the most trouble nave the standard 4ft Bin track, so that their wheels fit the lines exactly. In driving such a car care should "be taken not to run with all four wheels in the lines.

On a dry day it does not matter very much because the car can be polled out on to the road surface quite easily by a quick movement of the steering wheel. When the road is greasy, however, two important factors are present, each of which tends to increase the dangers produced by the lines. In the first place the steel becomes very slippery, so that the degree of steering movement needed to pull the front wheels of the ear off the metal is considerably increased. Secondly, when the front wheels are pulled clear, their angle to the direction of motion, plus the slippery condition of the road, makes a rear-wheel skid an extremely likely occurrence. Wheel-loading. Reduced adhesion is also liable to occur through the presence of ripples or other obstructions in the road surface. These produce axle movements which, with flexible springs, may be converted into movements of the whole car, and naturally the load on each wheel will then vary continually between maximum and minimum Values. If the throttle be kept wide open a certain amount of spinning of the rear wheels may occur, and under such conditions their resistance to sideways movement is reduced practically to zero. Cars with stiff springs are less liable to ■ this trouble than those which. are flexibly sprung, and a very good tip, 'where such adjustments are possible, is to increase the damping effect of the shock absorbers for winter driving. It is equally important to maintain all the tyres at their correct inflation pressure.

Before concluding it would seem opportune to proffer some advice as to the correct procedure to adopt when a car is inadvertently allowed to skid. The most important point to grasp is that, apart from surface conditions, the resistance to transverse movement provided by a rolling, wheel is very larglly dependent upon the degree to which it is rolling freely.. By this one means that the application of tractive effort by opening the throttle, or retardation by applying the brakes, will each materially reduce resistance to skidding. From this it can be deduced that when a skid commences the safest rule is to relieve the wheels of all tortional load by releasing the brakes and, if possible, by holding the throttle just slightly open, so that the engine is neither driving nor driven, but is just "pulling its weight." If it is imperative to retard the car, the smoothest - way in which it can bo done is by using the engine as a brake, changing down, if necessary, to increase the effect. This is a particularly valuable tip for descending a slippery gradient; on a wet 'tarmac rjad, for example, it is often possible to proceed downhill quite safely in second gear under conditions such that even a mild application of the brakes would produce skidding] finally, there is the matter of correcting a skid when this has commenced. As a rule only the rea • wheels lose directional control; the icar actually swinging around the steerine heads with the front wheels continuing in their normal course. The idea behind the correction of this condition is to make the tail of the car swing in the opposite direction and this is done by steering into the skid. In other words, if the tail of the car has commenced to slide inwards toward the near-side kerb,, the steering should be given a sharp movement to the left in order to reverse the direction of sliding. Probably an instant later it will be necessary to steer in the opposite direction to check a second minor skid resulting from overcorrection.

Needless to say, this procedure cannot be learned by armchair study, and the safest course for anyone who wishes to feel complete mastery over his car in all circumstances is to proceed early one morning to a wide and slippery road where he can safely indulge in a little practice.