HOW TYRE MILEA(
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IS WASTED No. 3—Underinflation
(External Effects)
iN last week's article 1 dealt with the . internal effects of underinflation,. Much of what was said then is applicable to a study of the external effects, but I do not propose to go over the same eound, except where brief reminders are necessary.
One statement rwould like to recall is that loss of proper working shape is the cause of all troubles arising out of underinflation.
. It was noted that the correct pressure was designed to support the tyre in a proper working shape which allowed for a safe amount of deflection to absorb road shocks. We saw, also, that a reduction of pressure caused the tyre to assume an incorrect shape, with a consequential redistribution of stresses which had serious effects on the carcase,
An impression Comparison The same applies to the tread. Just study Figs. 1 and 2 for a moment. Fig. 1 is a. tread-to-ground impression of a properly inflated tyre, and Fig. 2. is an impression taken of the same tyre after the inflation pressure had been reduced by approximately 40 per cent. Obviously, the reduction in pressure brought about an increase in deflection under load, with the result that a greater area of tread was in contact with the ground. At first glance it may appear that the greater the area of rubber in contact with the road surface the slower will he the rate of wear. This sounds all right in theory, but in practice it falls down for two reasons.
First, as the tyre is not " braced " by internal air support it exerts a soft " clinging " grip on the ground, and greater friction and heat are generated. Secondly, because it is not supported in its proper working shape, the'pressure on the ground is uneven. Owing to carcase distortion the shoulders of the tyre are forced into heavy contact with the road, and are carrying more than their due share of the load.
Meanwhile, the tread centre tends to lift upwards, away from the ground, and is carrying far less than its proper burden. That is why, in cases of underinflation, we frequently see tyres which wear much faster on the shoulders than in the centre of the tread (Fig. 3). This form of wear can be regarded as conclusive proof of underinflation, as it is unlikely to, occur from any other cause.
It must not be thought, however, that all cases of underinflation result in this form of wear. Often it takes the irregular form shown in Fig. 4, .which is due to a cause that is seldom appreciated. If a properly inflated tyre is viewed from the side it is quite circular, except for a slight flattening in the area where it touches the road. The amount of flattening is controlled by the factor of inflation pressure in relation to load.
If an underinfiated tyre be studied from the same viewpoint it will be seen that the flattening has extended fore and aft, in addition to an increased bulge 'in the wall area. In other words, the tyre is no longer operating in its true working shape.
The increase in fore and aft flattening, added to the general flabbiness due to underinflation, causes a tendency for the tread to pile up slightly ahead of the point of road contact. This does not, of course, apply when the vehicle is at a standstill, and in any case it cannot be detected by the eye.
The loosely supported tyre, as it rotates, forms a tread ruckle, just above the point where it touches the ground. This causes a scuffing and wiping action to take place, and the irregular movements of the tyre give rise to uneven wear of the tread.
In the case of twin tyres it is a common practice, after a puncture in one of the twins has been sustained, to continue to run on the sound tyre rather than to change a wheel. It is a popular belief that the sound tyre will carry the load and protect the deflated tyre from harm. Nothing could be more erroneous, In the first place, the stress on the sound tyre is considerable, because it is carrying double its normal load. This, in itself, is sufficient to cause failure. But even if the sound tyre survives, the deflated tyre will suffer greatly. The fact that it is carrying no load does not hell:cit. The absence of air pressure means that the tyre is not securely held on the rim. The centrifugal force generated by wheel rotation causes it to thrash about in a flail-like manner, doing considerable (if not fatal) damage to carcase and tread. The abrasion between tread and road is terrific.
I have seen tyres suffer more wear in a few hours of this treatment than they would suffer in 20,000 miles of normal running. Fig. 5 is a specimen. Note the scoopy wear, which is typical in these circumstances. This example illustrates a case of total deflation over a prolonged period.
There is an intermediate stage— partial deflation—where the damage is similar but not so pronounced. This is shown in Fig. 6. It is the typical saw-tooth "heel and toe " wear which arises from tread-block distortion.
Under conditions similar to those mentioned above there is a tendency for the tyre to creep around the rim.
This results in a scoring and whitening of the bead, due to friction against the flanges (Fig. 7). It is not a serious matter, but is mentioned because it is one of those effects of underinflation which the operator must learn to recognize if he intends to analyse his tyre troubles.
Another harmless warning is the black ring which appears midway around the tyre wall when an underinflated tyre has been rubbing against its twin. It is, perhaps, necessary to add that the same effect can be produced by overloading, or by using wheels with insufficient offset. More will be said about this in the article on twin tyres, to be published later in the series.
Visible Evidence Lacking
So far, we have dealt with the reasonably drastic forms of tread wear which result from a fairly considerable degree of underinflation. All these are easily recognized and are an indication that something is wrong. Slight degrees of underinflation will take their toll of tyre life without leaving such plain indications. The only fact of which the operator is aware is that the tread has worn down rather rapidly, but, because there is no particular visible evidence, he is at a loss to trace the cause. In the aggregate, more miles are wasted in this way than in the more serious cases of deflation.
Look at the matter this way. If a tyre be run at a pressure 20 per cent. below the normal, it may not
develop any noticeable symptoms, but it will lose 17 per cent, of normal mileage. This is not guesswork. It is a fact.
Maximum mileage is impossible without correct pressure, and the loss is in strict proportion to the degree
of underinflation. If the pressure be reduced to 35 per cent, below normal, the loss in mileage is 45 per cent, Tyres run at 50 per cent. of normal pressure (and this is by no means uncommon) will give only about 25 per ct.nt. of the mileage a which they are capable under proper conditions of use.
fhese losses are due to two main causes. (1) Increased friction, owing to the irregular movements (previously described) of the tyre on the road. (2) Heat caused by 1 and by the increased area of tyreto-road contact.
The causes, in turn, are due to the tyre's loss of working shape. If proper pressures be maintained, the trouble will be entirely eliminated Another danger cannot be disregarded. In addition to mileage 13sses, underinflation renders a tyre more susceptible to other forms of damage. A properly inflated tyre will stand a far greater degree of shock than one which is inadequately inflated. Concussion bursts, ' due to sharp impacts, are far more common in cases where pressures are neglected.
Then, again, there is the question of carcase fatigue. In these days, when every worn tyre should be surrendered in a remouldable condition, it is most important to guard against the carcase fatigue which inevitably follows continued underinflation. The carcase may not be visibly damaged, but the prolonged stress will have so undermined it that it is incapable of surviving a second tread.
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Seek the Driver's Aid
One of the chief problems is to gain the intelligent co-operation of the driver. All too often he regards a wrecked tyre as something for which he must try to evade responsibility. How often have we heard— when the tyre is found to be totally ruined : "I pulled up immediately and changed the wheel.' The fact that the tyre is now useless is definite proof that he did not pull up immediately.
True, he may have pulled up so soon. as he detected the puncture, but by that time the damage was already done. Probably the tyre had been running for miles in an under inflated condition. On the other hand, he may be the type of driver who deliberately runs on a flat tyre, taking a chance on whether it surviyes or not.
Under normal loads, no tyre will suffer any injury if the wheel be changed immediately the puncture
occurs. It is safe to .say, therefore, that all cases of tyre wrecking from this cause are due to carelessness, either deliberate or inadvertent, on the part of the driver. That is an unassailable fact.
The driver must be made to realize these two vital points :—(1) That the tyres must be inflated to correct pressure at regular intervals, and (2) that correct and frequent inflation does not necessarily eliminate all underinflation troubles.
Many tyre users, because they inflate their tyres conscientiously every week, think that this gives them immunity from all troubles arising from underinflation. It helps, of course, but there is still the danger that between one week and the next a very slow puncture may develop and remain unnoticed. Consequently, the tyre is running in an underinflated condition for five or six days.
Even a daily check is not infallible, but at least it will provide immunity so far as is practicable. The longer the period between checks, the greater the risk, not because of normal air losses, but rather because of slow punctures, valve leaks, etc.
The first job, then, is to ascertain the correct pressure for the tyre for a given Wad. The accompanying table gives the correct pressure for maximum loads of all the popular giant sizes, together with their carrying capacity.
For general purposes,_ the check should be made at least twice weekly, but all long-distance vehicles should be checked daily. In addition, it is the driver's ditty to keep a constant watch so as to see that nothing is wrong. In no circumstances should there be any delay in changing a tyre if paitial deflation be suspected.
It is no good pumping up a tyre which has lost a lot of air, unless the causehas been ascertained. This is the lazy method, and, like most lazy methods, it is expensive. .It is often used as a get-me-home measure, but the tyre usually "goes home," too.
A cover may .lose up to 15 per cent. of air pressure weekly without there being any detectalge cause. Provided that inflation be carried out twice a week, this will have no ill-effect. But any losS over 15 per cent, per week is usually traceable to either a puncture or a• valve leak, in which case the tyre should be removed and
the trouble cured. •
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Summary of Failures To sum up, the following are the chief effects of underinflation: (1) Carcase failure, which may take place at almost any stage in the tyre's life, according to the circumstances.
(2) Rapid and irregular tread wear, involving losses up to 80 per cent. or 90 per cent., according to the degree of underinflation.
(3) Susceptibility to other damage, such as concussion bursts.
(4) Carcase fatigue, due to prolonged stress on the cords, renders the tyre unfit for remoulding, even if no damage be visible.
In concluding this article, I would like to remind the operator of what I said in the first of this series, i.e., that every adverse influence to which tyres are subject will manifest itself in some characteristic form of damage
to the tread or carcase. It is the operator's duty to learn all these typical forms of tread wear, and carcase failure, in order that he may analyse his tyre troubles and prevent
recurrences. A close study of the accompanying illustrations will give some idea of the peculiar forms of tread wear which are associated with
underinflation. L.V.B.