Road Grip in Relation to Tyre Wear
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The Influence of Tread Design, Road Surface and Braking Force on Tyre Grp in Relation to Safety
pRACTICALLY every factor which adds to safety and stability, in the form of road grip, is detrimental to the longevity of the tyre. In other words, maximum mileage and maximum safety do not go together. Quite the reverse in fact.
The tyre which exerts the greatest grip on the road, all other points being equal, will yield the lowest mileage. The reason is obvious. Increased grip means increased friction, which, in turn, means increased wear and shorter life—but safety as well.
The three chief influences of tyre grip, in relation to safety, are the tread design, the road surface, and the degree of braking force. The last-named is responsible for a good deal of the friction which takes place between the other two.
A Vehicle Normally Stable.
True, there are propulsion stresses as well, which cause intense friction between the tyre and the road, but instability is comparatively rare when the vehicle is under propulsion, and, therefore, does not enter to a great extent in the present discussion.
Cornering at speed, or fierce acceleration on wet roads, may result in loss of grip, but, in most cases, grip is lost during retardation, when the brakes are applied. It is then that the relationship between the three factors, already mentioned, be
. comes important. Let us Consider each in turn.
First the tyre. For the sake of argument we will assume that all a22 tyres are equal in the quality of the rubber used, and that they differ from each other in only their tread designs. Thus, we can compare mileages and safety purely from the point of view of pattern.
Quality, then, being equal, the tyre which will yield the greatest mileage is the one with the pattern which exerts the least road friction—a ribbed tread, for instance. The long life which such a tyre offers is liable to shorten the lives of others, for it is not safe. Except under favourable conditions, a tread such as this is incapable of exerting a road grip powerful enough to counterbalance a sharp application of the brakes. In all cases, where the degree of braking force exceeds the force of tyre grip, the vehicle will skid, inevitably.
The sole object of obtaining a long mileage from a tyre is to economize, but if, as a result of the frictionless. tread, the vehicle is liable to make extensive deviations from its true course, the economy of long mileage is false. Now, take a good anti-skid pattern of tread, a pattern which is broken up into innumerable tread blocks of various designs and which offers a number of edges to effect road grip.
The tread of this tyre will distort noticeably under the stresses of pro
pulsion and braking. It will develop that heel-and-toe action which is so vital a factor to road grip. The ribbed or smooth tyre has practically no capacity for heel-and-toe distortion. That is why it wears well and grips badly.
The patterned tyre, as we have seen, develops an increased road friction which gives it a sound grip. This increase of friction will result in less mileage than would be obtained from the ribbed patterns, but will add immeasurably to the cause of road safety. This makes the loss of mileage well worth while.
Tyre Tread and Road Surface.
Anyone can make a ribbed tyre. The design is nobody's copyright. Tyre manufacturers know that it will give the longest mileage, but they also know the attendant dangers, and so they make this pattern for use only under special conditions.
The next influence to be considered is the road surface. Here again, we shall find that the surfaces which make for the greatest safety are those which result in the most rapid rate of tyre wear. A simple illustration is provided by wet and dry roads of the same type. The dry road, obviously, is far safer, yet the wet road is kinder to the tyre, because the film of water acts as a lubricant. It is safe to say that if tyres could be continually run on wet roads they would last three times as long.
But what a nightmare for the driver, to be in constant .danger of skidding. Even dry roads vary greatly in their effect on tyre wear, but always the rate of wear is a sure indication of the relative degree of safety—the faster the wear the safer the surface.
On a concrete road, for instance, the rate of tyre wear is 30 per cent. faster than on a road made of wood setts, but it is proportionately safer. Tarmacadarn is between the two; its rate of tyre wear is 10 per cent. faster than wood ,setts, but 20 per cent. slower than concrete. Thus, in order of safety, concrete leads, followed, in turn, by tarrnacadam and wood setts.
A few years ago the popular idea of road" improvement" was to transform a rough surface into one of glassy smoothness. In the days of two-wheel brakes and low road speeds, this so-called improvement did not constitute a serious -danger, although it could never be regarded as a step in the right direction. • . When braking efficiency and Wad
speeds increased, such surfaces be ' came a positive menace, and nowadays there is a tendency to revert to the rough surface. Rough surfaces, so long as they are free from irregularities, do not result in any loss of comfort; intact, a smooth-surfaced . road is sometimes bumpier than a rough one.
The final point for consideration is the braking force. When this is applied the tyre is torn between the force of the brake holding it back
• and the impetus of the vehicle draggingit on. It is a vital moment. If the tyre tread be of good pattern it will bite hard on the road surface and, under intense friction, will retain its,grip. Considerable wear will take place, naturally, but without it the tyre could not hold.
Now imagine a smooth or ribbed tyre in the same circumstances, The
brake is applied.7ith the same force as before *but as the grip, of this pattern is much less, it is incapable of resisting the braking force. Instead Of pulling up to a steady stop, the wheel locks and the vehicle simply
slides onwards. .
The present-day pattern of tyre, then, represents the result of the effOrts of the various manufacturers to obtain a compromise between safety and economy. On the one hand is the plain or ribbed tread, comparatively devoid of friction and, therefore, giving long mileages, but not without certain risks. On the other hand There is the patterned tread—more friction, shorter life, but safe. Many treads to-day are as nearly as possible the perfect compromise—safety with reasonable economy, • Extremes of economy in tread design are dangerous.
L.y.B.