YOUR TYRES IN WINTER
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How Lower Temperatures and Wet Roads Tend Towards Increased Tyre Life. Avoiding Skids by Paying Attention to
Inflation Pressures
THE winter months are the easiest I of the year, so far as tyres are concerned. Conditions are then most favourable and their task is proportionately lighter. The most beneficial change, as winter approaches, is the gradual' fall of atmospheric temperature. Throughout the summer the tyres have been working under most arduous conditions of heat—their worst enemy— but now the temperature has fallen, the rate of wear, for every mile travelled, will also fall.
The influence of atmospheric heat on the rate of tyre wear is almost visible. During the hot. dry weather of summer, it is possible, from week to week almost, to detect the gradual decreases in the amount of tread rubber, but during the winter, except in unusual conditions, the amount of wear is negligible. A tyre is almost as good at the end of the season as it was at the beginning.
Heat, in addition to being mainly responsible for rapid tread wear, is also the cause of most carcase failures. As a result of the decrease in atmospheric temperature, bursts will be much less frequent.
Water as a Lubricant.
In winter, road surfaces are kinder to the tyres. They are cool and wet most of the time, and a film of water over the ground acts as a lubricant between the tyre and the road. The fact, too, that there is less traffic results in less frequent use of the brakes.
Drivers instinctively use their brakes as little as possible on wet roads and are, generally speaking, more cautious. Emergencies which may call for sudden stops are anticipated by gradual reductions in speed. Even if the brakes have to be used there is always, or nearly always, that wet film to ease the friction.
Winter may be an easy time for tyres so far as their actual work is concerned, but during this period they are subjected to far greater responsibilities than ever beset them during the summer months.
One of the primary duties of a tyre is to give stability and safety to the vehicle, in the form of an efficient grip of the road surface. In Summer this is comparatively easy. Newly surfaced roads offer a good frictional grip. Occasional rainy days call, perhaps, for a little extra caution, but, generally, the capacity of a tyre for road grip is quite considerable, due largely to the dry surface. Safety from skidding depends entirely on the relative values of the braking force, as compared with the force of grip between a tyre and the road surface. The latter force depends, of course, on the nature and condition of the surface, and on the value of the tread grip.
Thus, a dry road and a good antiskid (there is no non-skid) tread pattern form a, powerful combination which will, in most circumstances, be sufficient to allow the brakes to be applied with considerable force without the danger of skidding. In wet weather, however, a tyre, whilst still prepared to do its own share, loses the support of its greatest ally in the cause of safety—a dry road.
The loss of efficiency in the capacity for grip between a tyre and the road can, in theory, be offset by a proportionate decrease in the application of the braking force. Unfortunately, we are not always in a position to decide with how much, or how little, force the brakes may be applied, consistent with safety.
In an emergency we are called upon to stop and although, in many cases, we may do the correct thing instinctively, we yet have no time to work out, with mathematical nicety, the degree of force which can be applied, with safety, under the prevailing conditions.
Stopping Distances Greater.
It is a recognized fact that, excluding skids, stopping distances are three times longer on wet roads than on dry ones. This difference is effected solely by a thin layer of water between the tyres and the road. Even good tyres will lose this amount of grip ; worn tyres, of course, will lose infinitely more. On a dry road, a worn tyre has at least a little grip, but on a, wet surface it has practically none.
As the road surface can no longer be considered an ally in the cause of stability, the whole responsibility falls on the tyres, and it is the duty of every operator to see that they are equal to the task. The first essential is that they should have good tread patterns. Road grip depends largely upon the capacity for distortion of the tyre tread. The blocks of the tread pattern move to quite a considerable extent under the stresses of propulsion and braking, and in doing so they tend to develop that biting grip which is characteristic of rubber. The newer the tread pattern the greater will be its capacity for distortion and its power of grip.
Press your thumb against the tread block of a nev), tyre. It yields. Now press against the tread block of a tyre that is tweethirds worn. There is practically no movement. The nearer you get to the foundation of the tyre the smaller the movement becomes— like a pendulum. A smooth, or nearly smooth, tyre has almost no tread movement.
Importance of Correct Pressures.
The other great aid to stability is correct inflation. This I have already discussed at length in this journal. It has been explained how inadequate inflation provokes roll, which, apart from being extremely uncomfortable in itself, will often cause a fullfledged skid. Proper inflation serves a double purpose : first, it prevents roll; and, secondly, a tyre which is properly inflated exerts a better grip.
It is most important, too, that the pressures be carefully balanced throughout the vehicle, otherwise the capacity for grip of the individual tyres will vary with the pressure and will cause an inequality in the transmission of the braking force to the road surface—another possible cause of skids.
Operators seem to be divided into .two camps. On the one hand we have the man who "always has new tyres to carry him through the summer," provided, of course, that his old tyres have reached the stage where replacements will soon be necessary. Then we meet the man who "prefers new tyres to see him through the winter." There is nothing in it either way ; good tyres are essential at all times. L.V.B.