How Braking Efficiency Depends on Tyre Condition
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The Limiting Factor in Braking is the Adhesion Between Tyre and Road. A Contributor Considers This Fundamental Fact from the Viewpoint of the Tyre
THE most perfect set of brakes is absolutely useless if the tyres have not a sufficient capacity
for grip. No matter how firmly, smoothly, and equally their pressure may be applied, the effort is completely wasted unless the tyre exerts an even greater force in trying to grip the road surface.
A moving vehicle is a storehouse of energy, not only that emanating from the engine, but also the energy built up as a result of the momentum which the vehicle develops. This energy increases with the speed and weight of the vehicle, and can be expended and absorbed only by exerting itself against opposing forces. The opposing force which is most commonly employed to control and absorb this energy is the brake, and it does its work so sweetly and with so little apparent effort that we are apt to lose sight of the colossal task which is performed every time the pedal is depressed.
Take, for instance, a vehicle travelling at 50 1-11 . p . h . Its stored-up energy at this speed is very considerable, especially if it be heavily laden ; yet, when it is required to stop, the brake will overcome that energy and bring the vehicle to a standstill. There is nothing spectacular or impressive about that.
Evidence of Momentum
But suppose, on the other hand, that the same momentum has been absorbed by collision with a brick wall. The vehicle would be completely smashed, and the wail, no doubt, would be much the worse for the encounter. What is more, we should have visible proof of the force which was necessary to overcome its stored-up energy.
This, then, is the task which the brake has to perform. Under a given set of conditions (speed, weight, etc.), its effort in bringing the vehicle to a standstill is exactly equal to that of the brick wall, the only difference being that the expenditure of braking fOrce is spread over a longer period of time as the retardation is more gradual.
The medium through which the braking force is directed, in its effort to overcome the speed of the vehicle, is the tyre. On the one hand, we have momentum carrying the vehicle onward, and on the other we have the brake trying to hold it back, thus exerting a " twist " in the tyre which
is held between the two opposing forces.
At this moment much depends on the tyre. If it fails to retain its grip of the road surface, it ceases to be a suitable medium for the transmission of the opposing forces, with the result that each force achieves its object quite independently. The momentum of the vehicle, no longer opposed by the brake now that the tyre has ceased to grip, carries it onward. And the brake, no longer opposed by the energy of the vehicle, for the same reason, locks. The result is a skid.
It is an almost perfect example of a tug-of-war. The tyre is the rope, at one end of which is the stored-up energy of the moving vehicle, whilst at the other end is the braking force. Each side strains at the rope in its efforts to overcome the other. There are three possible results: (1) the brakes will win; (2) the momentum of the vehicle will win; (3) the rope will snap.
Now, if the rope breaks, which is equivalent to the tyre losing its grip, the connection between the two opposing teams is lost, and each achieves its object of exerting its maximum force unhindered by the other. This is the point where the vehicle, with brakes full on and wheels locked, still travels forward in a skid.
Tyre Maintains Equilibrium
The duty of the tyre is to retain a happy balance between the forces of momentum and braking. Its task is neutral and impartial, like an arbitrator who seeks to restore peace between two opposing factions. It helps both sides, but it helps them
equally. It transmits the braking power which is trying to overcome momentum, but it also transmits the force of momentum which is trying to gain supremacy over the braking power.
Whichever side wins, it must do so purely by its own superior exertion, for the tyre, though capable of transmitting all the power of both forces, is incapable of adding to this power in favour of one or the other.
But one duty it has in particular. Its personal contribution to the braking effort is stability—the job of keeping the vehicle under the driver's control, so that it does not skid. A skid represents the failure of the tyre in its task, and the complete independent success of the other two forces. For in this ease we must regard stabilityand braking separately, the one as the tyre's job and the other the duty of the brake. In a skid the maximum exertion on the brake drums, resulting in a locked wheel, contributes practically nothing to the cause of retardation of the moving vehicle and results in a complete loss of stability.
The "stability of the vehicle depends on a third force which acts tangentially to the tyre itself and is exerted in opposition to the force of braking. This force is applied on the elliptical area of tyre tread which is
in contact with the road. It is important that this force, which we will call C, shall be greater than the force of braking, otherwise skidding is inevitable.
Tyre Load and Friction
The value of C is variable according to circumstances, the main influences being the load per tyre (L) and the coefficient of friction of the tyre an the road (F). Variations in the former are entirely subject to the driver's control and are a matter on which exact figures can fairly easily be given.
Coefficients of friction are not so easily ascertained, however, for they vary with the tread pattern of the tyre, the degree of wear, and the nature of the road surface—wet or dry, rough or smooth. Taking average figures—smooth, dry road and average tyres—the value of F is about 0.6.
Force C, on which stability depends, is equal to the product of L and F. Thus, if we assume that F is constant (as it would be if we were making a journey along a smooth road on a dry day) the value of C increases as the load increases. But a more important way of looking at it is that if our load be constant the value of C is controlled by the variations in F. And variations in the coefficient of friction of the tyre on the road are, generally speaking,
much more serious, from a stability standpoint, than variations in load.
Suppose we have a vehicle equipped with six tyres (the fact that it has only four brakes does not affect the calculation) and the load on each tyre (L) is 30 cwt., i.e., a total weight of 180 cwt.
. As we have already. seen C=L X F, therefore C=30 X0.6=18 cwt.
This force of 18 cwt. is working on each of the six tyres in the opposite direction to which the vehicle is travelling. Thus the total effort of retardation is 61<18=105 cwt. But the vehicle, as we have seen, has a load of 30 cwt. on each tyre and, therefore, weighs 180 cwt, On these figures the braking efficiency would
be described as —1081<100 per cent.=
60 per cent.
Let us calculate the effect of a reduction in F.
On the same smooth road, if the surface becomes wetted by rain, the coefficient of friction (F) is reduced by two-thirds from 0.6 to 0.2. Thus,
with all other factors unaltered, C=30 x0.2=6 cwt. The maximum power of retardation becomes 6x6=.36 cwt. and the braking effi ciency is described as —36x100 per cent 20 per cent. It is evident that a fall of two-thirds in the value of F causes an equivalent fall in the braking efficiency. The stopping distance of the vehicle would be three times greater in the latter case than in the former. Incidentally, reductions in L would have a similar effect.
Why Efficiency is Lost So although the brakes themselves remain unchanged we have lost twothirds of our brake efficiency. Why? Simply because the tyre fails to maintain the force C vhen the road becomes .wet.
What can we do to control this state of affairs, and to increase the stability of our vehicle? Merely to increase the braking effort is not enough ; for unless the tangential force C is at least equal to the brake
force the wheels will lock and the vehicle will skid. To increase our power of retardation by increasing the load L is impracticable for obvious reasons. Questions of weight distribution and transference are involved which are admittedly important, but they are outside the scope Of this article and do not serve to cast light upon this discussion ; they Merely complicate the issue.
Thus we are left with F—the coefficient of friction of the tyre on the road. The road surface we cannot change. If it be smooth we cannot roughen it; if it be wet we cannot dry it. All we ,can do to raise the figure F is to ensure that our tyre treads are good and are capable of exerting 'maximum grip. Obviously. a smooth tyre has a lower coefficient of friction than one which has a deep patterned tread. So soon as the tread pattern disappears the cover should be removed and either remoulded, regrooved, or scrapped. Remember, if your tyres be no good, your brakes
are no good either. L.V.Ei.