The Effects of Speed and Braking on Tyre Wear
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FEW people realize the full extent of the terrific stresses which are imposed upon tyres when the brakes are applied. Few have any definite idea of what takes place at such times. The tyre, we know, is..the medium by which all tractive and braking forces are transmitted, but what is seldom realized is that, as these forces work in opposite directions, the tyre is almost constantly being twisted between two opposing forces.
This takes place in the small area of tyre which is in contact with the road. This area, in trying to transmit the force of the brakes against the force of forward movement, exerts a force of grip on the road surface. It is this force of grip which 'causes such severe distortion of the tread rubber, and such violent abrasion between the tyre and the road.
Tread-block Distortion.
If we could closely inspect a section of the road surface, immediately alter the brakes had been forcibly applied, we should find that it was covered with tiny fragments of powdered rubber which had been ground off in the act of braking.
When a vehicle is being driven forward the tread blocks of the tyre are distorted in a forward direction which indicates their resistance to motion; but the application of the brakes causes the distortion to be reversed—the tread blocks are strained in a backwards direction, which indicates their resistance to the cessation of motion.
BO This happens every time the brakes are applied. The higher the speed, or the extent of the braking force, the greater will he the tread distortion, and the " jerk " with which the direction of this distortion changes over. It is a matter for some wonderment that a combination of rubber and cotton can withstand such terrific punishment.
A great deal of the distortion which takes place could be eliminated by having a tread pattern composed of circumferential ribs. This would certainly obviate many forms of uneven wear, and would give reasonable mileages, but the tyre would not be safe, for it is a tyre's capacity for tread distortion which gives it road grip.
Friction and distortion are necessary to safety, although these features naturally cause the tread to wear quicker. A circumferentially ribbed tyre would give a slightly greater mileage but at the expense of safety.
"Heel and toe" wear—that peculiar " scoopy " wear which everyone seems to dislike—is, in some respects, a good sign. It, at least, shows that tread distortion is taking place when the brakes are applied. Consequently, we may assume that a tyre wearing in this manner is exerting a reasonable grip.
Faulty Brakes and Tyre Wear.
So far we have seen the effect of the brakes in only their mildest light. Things are much worse if there be any mechanical faults or other adverse influences a,t work. Suppose, for instance, that there are inequalities in the braking system— a scored or oval drum, or a loose or oily facing. The most perfect system of brake compensation cannot make good these faults, nor• prevent their evil effect on the tyres.
It must be remembered that when the wheel is bolted to the hub the relative positions of the tyre and brake drum do not change so long as the wheel is not removed. So, if there be any ovality in the brake drum it always reacts on the same part of the tyre.
The effect of an oval drum is a rapidly alternating increase and decrease of the braking force at every revolution of the wheel—very much like the rise and fall of a cam. It is possible that the faulty brake, at the moment of greatest grip, is actually working far in advance of the other three brakes.
So the part of the tyre which is in contact with the ground when the braking increases is, naturally, subjected to greater road friction than the part on which the braking force fades, and, as this friction always takes place on the Same part of the tyre, rapid wear will result. That is why we often see tyres which are worn much more in some parts of their tread than in others. Sometimes the worn part shows as a small patch and at others it is quite a large area, but invariably it signifies a braking fault.
If the worn area be a small, round patch, it may look as if it had been caused by a violent skid, and many operators would probably ascribe the damage to this cause. This is quite wrong, however, for the worst of skids would not remove more than a few thousandths of an inch of tread rubber from a tyre.
A similar trouble occurs when a vehicle is subject to excessive chassis bounce. This causes a series of fiat patches running round the tyre, known as "flatting."
Link Between Speed and Braking.
The question of braking cannot be dissociated from that of speed. High speed often involves frequent and sudden use of the brakes and fierce acceleration also has its grinding effect. Always there is friction: in forward movement tread friction is designed to give traction, and to prevent wheelspin; in braking, as we know, it gives stability. High speed alone takes toll of tyres apart from the , incvitable increase in the use of the brakes. 'Tyre wear at 50 m.p.h. has been found to be twice as great as at 30 m.p.h. This is worth consideration. Every mile travelled at " 50 " costs us twice as much, for tyres, as compared with a mile covered at a modest speed of " 30.
A high brake efficiency, as we have seen, throws added stress on the tyres. We can ease this, to some extent, by observing a few simple rules. First, we must see that the brakes are really efficient—that there are no inequalities in the system. Secondly, we must do our best to ease the tyre's burden in other directions, by giving regular attention to inflation pressures, and by the restriction of overloading. Finally, having brought our brakes up to the maximum of efficiency, we must try to drive in such a manner that they are seldom called upon to prove their full worth. L. V . 13