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How Tyre Mileage is Wasted

30th October 1942
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Page 23, 30th October 1942 — How Tyre Mileage is Wasted
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Which of the following most accurately describes the problem?

No. 4.—Overloading One of the Primary Causes of Failure and Waste Forms the Subject Matter of this Instalment of a Series Which is Intended to Help Operators to Obtain the Maximum Mileage from Their Tyre Equipment

THERE is 'a great similarity in theresults of

underinflation a n d overloading, because their effect on the tyre is the same. In the articles on underinflation we saw how inadequate air support caused the tyre to assume a shape contrary to the designer's intentions, and we saw, too, how the stress of working in the incorrect shape caused the collapse of the tyre.

An excessive load on a properly inflated tyre is just as bad as a correct load on an underinflated tyre. But an excessive load on an underinflated tyre is the worst possible combination. However, as we have already dealt with the question of underinflation we will assume, for the present, that the tyre is properly inflated but overloaded.

Each size of tyre has a definite maximum carrying capacity, which can be ascertained from the literature issued by the manufacturers. The tyre is designed, when run at• the proper pressure for this maximum load, to give normal mileage without trouble. It is operating in its correct "working shape." (See previous articles.)

The amount of .deflection is sufficient to absorb road shocks, but insufficient to cause carcase stress. The area of the tread in contact with the road is just right to avoid the generation of excessive heat and rapid wear. Each part of the tyre is doing its fair share of the work.

But, one day, somebody decides that it is necesSary to put on an extra couple of tons to speed up

delivery. T h e tyres are running, shall we say, 33* per cent. overloaded? What happens? The excessiveload causes an increased deflection of the wall— deflection beyond the sale margin which the tyre designer intended.

What Happens When Friction Increases The tyre -is forced into heavier contact with the road, consequently a larger area 'of tread is touching the

pc-tired. The distortion of the carcase causes a redistribution of the stresses. The shoulders of the tread are carrying more weight than the tread centre. The walls are subjected to a greater. stress than any other part of the casing.

The increased pressure on the road leads to an increase in friction. This has two effects—heat generation and rapid wear. In addition to the rapid

Furthermore, as in the case of underinflation, the stresses of overloading render the tyre more susceptible to other damage. If, for instance, it sustains a sharp blow against an -obstruction, such as the kerb, it has far less chance of surviving it than it has when ,correctly , loaded.

In the previous articles I have• so emphasized the . importance of "working shape" that it may appear that the evil effects of overloadirtk can be offset by overinflation. It may be argued that if the proportion of overload be offset by a proportional increase of overinflalion the tyre will be restored to its proper working shape and all will be well. But will it?

In the first place it must be remembered that the maximum load and the ideal pressure have already been decided after long research by the tyre manufacturer. To disregard the result of these investigations is like playing with fire. If overinflation were a satisfactory antidote for overloading you would have been told. But it isn't. You can't make a pony do the work of a carthorse.

So it is with the tyre. Its capabilities have been worked out for you with mathematical exactitude. To disregard the advice of people who know better than yourself is just plain foolishness.

There are four main consequences of overloading, as follow:—(1) Rapid wear due to excessive friction and heat generation. (2) Carcase fatigue due to increased stress. Even if the tyre does not fail this is sufficient to make it unsuitable for remoulding. (3) Complete carcase failure, in cases where overloading is regularly practised. (4) Increased susceptibility to other damage, such as concussion bursts.

Add to this list the extra wear and tear on the vehicle and it is obvious that overloading cannot• possibly pay.

The accompanying illuStrations show some of the typical effects of overloading and it is hoped that the operator will study them carefully so that he will recognize the trouble when it occurs on his own vehicles.

In Fig. 1 is shown a sign of overloading which is associated only with twin tyres. Owing to excessive deflection under load, the twin tyres have actually touched at the point of road contact and the continual chafing has worn away the rubber in the area of greatest deflection. This is an extreme case. Generally, rubbing is indicated by a black line around the tyre wall. In itself it is not serious, but it is an indication that things are not as they should be.

Failures Vary on Different Makes

The other pictures (Nos. 2, 3 and 4) show a few different types of overloading burst. It will be noticed that the failure occurs in different parts 4 the tyre. This may be due to the circumstances ruling at the time, but is more likely to be due to slight differences in tyre construction as between makes. Some may fail in the bead area, some in the wall, and others at the shoulder.

A point to bear in mind is thatan overloading burst is almost invariably beyond repair. The weakening

A2,2

of the carcase which gives rise to the burst is general rather than local. Consequently, when the failure occurs, it involves quite a large area of the casing. The sudden release of a large volume of air compressed to a high pressure naturally adds to the damage.

The operator may like to know how he can distinguish an overloading burst from, say, a concussion burst. Obviously, if we are to analyse all tyre failures with a view to preventing recurrences, there must be no doubt in our minds as to the cause. If we blame concussion for every burst tyre we may never discover that, in fact, the real cause is overloading. And, until we discover the real cause, our troubles will continue.

Externally, the two kinds of burst look much the same, but there is one difference. Concussion bursts usually (but not always) appear in the tread area, whereas overloading bursts are more frequently (but, again, not always) found somewhere between the bead and the shoulder. Conclusive proof can usually be found by internal examination.

A concussion burst is either V-shaped or star-shaped, whilst an overloading burst is more or less in a straight line. I am speaking, of course, of the main area of the burst, not of the minor dainage extending from it, which is caused by the force of the escaping air.

Before we leave the question of effects, I would like to give a few figures regarding the rate of wear on overloaded tyres, figures which prove

that, even if no burst takes place, the mileage loss is still considerable.

A 10 per cent, overload means a 20 per cent. loss of mileage, a 20 per cent. overload means a 30 per cent. loss of mileage, and a 50 per cent. overload (not so uncommon as you Might think) results in a 60 per cent. loss of mileage.

Mileage Losses for Specific Loads

If we take 5 tons as the carrying capacity of the tyres, and, say, 30,000 as a normal mileage, we arrive at the following figures. Carrying 5 tons 10 cwt., we get only 24,000 miles—a loss of 6,000. Carrying 8 tons, we get only 21,000 miles—a loss of 9,000.• Carrying 7 tons 10 cwt., we get only 12,000 miles—a loss of 18,000.

In these calculations " weight" is total weight, i.e., vehicle and load.

These figures are not baseless propaganda put over with the idea of scaring operators against overloading. They are careful calculations arrived at after years o f research b y tyre manufacturers. Mileage losses from this cause can be calculated with mathematical precision.

If, despite overloading, an operator considers that his present mileages be satisfactory, the figures serve to show how much better results could be obtained when working under normal conditions.

So much for cause and effect. Let us now deal with the cure. There arc only two courses open. (1) Either the load must be reduced to bring it within the carrying capacity of the tyres, or (2) an oversize tyre must be fitted which is adequate for the load. _ This, of course, is a question for the operator to decide. If overloads be unavoidable, he will have no alternative than to fit an oversize which, in these days, may involve a special permit from the Tyre Control if his present tyres be not in such a condition as to justify replacements. The Tyre Control, however, is prepared to give favourable consideration to such applications when it can be seen that the result will make for greater economy.

• In all cases where overloading is suspected the only real check is the regular weighing of loads. It is not sufficient to weigh the vehicle as a whole, because, in many cases, due to chassis design, it is possible for the rear wheels to be overloaded and the front ones underioaded, even when the total weight • does not exceed the stated carrying capacity of the tyres. This is due to unequal load distribution which is found on some types of vehicle.

First, the carrying capacity of the tyres must be ascertained (for this I must refer you to the table given in the article on underinflatiOn, published last week). Let us take the 32 by 8 Heavy Duty as an example. The carrying capacity of this tyre is 22/ cwt. at 90 lb. air pressure per sq. in. Thus, on a six-wheeled

vehicle, the rear axle is capable of carrying 41 tons and the front, axle 21 tons, i.e., the carrying capacity of the tyre is multiplied by the number of tyres per axle. When the loaded vehicle is driven on to the weighbridge each axle should be weighed separately. Do not forget, too, that the weight of the vehicle counts as well, not just the weight of the payload. Thus, if the weight on the back axle (vehicle and load) is in excess

of 4i tons the tyres are overloaded and a cure must be found if mileage loss is to be avoided.

Owing to the vital need for rubber conservation, the Tyre Control is taking a Jively interest in the question of overloading. It recognizes it as one of the primary causes of tyre wastage, and any operator who is persistently guilty of this form of sabotage will find himself in trouble with the authorities.


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