How Tyre Mileage is Wasted
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IMAKE no apology for beginning this series with the question of underinflation. It is a sore point, both for tyre manufacturer and operator. The manufacturer recognizes it as a major cause of tyre failure for which the operator is to blame. The operator regards it as the manufacturer's stock excuse for every tyre that fails.
Being neither a tyre manufacturer nor an operator I can speak impartially, and I have no hesitation in saying that the manufacturer is right and the operator is wrong. Underinflation is definitely the primary cause of tyre waste. , Let us begin at the beginning. What is a tyre for? Its job in life is: (1) To absorb road shocks; (2) To carry the load; (3) To transmit the forces of propulsion and braking.
The second and third requirements might quite well be met (although not quite so well) by solid tyres, but the first is essentially a job for the pneumatic and is the primary purpose for which such equipment is made. Shock absorption, then, is obtained by putting a flexible air container between the vehicle and the road surface.
A Combination That Gives Good Results
The only satisfactory materials so far discovered for the construction of this container are rubber and cotton. The cotton framework provides a flexible, although inextensible container, and the rubber protects this framework from abrasion, weather conditions, etc. Both materials, being vegetable products, are somewhat fragile, especially cotton. But in combination, coupled with the third vital factor—air—they become a powerful unit which is capable of withstanding any amount of rough treatment for many thousands of miles.
The manufacturer who makes the tyre knows exactly its capabilities. It is designed to do a particular job under given conditions. It is designed to carry a specific weight at a certain pressure. At this given pressure it will fulfil the three purposes which have been mentioned. (1) It will deflect just.sufficiently to absorb road shocks. (2) It will carry the weight of vehicle and load without suffering any ill effects (always assuming that the load is not in excess of that which the tyre is designed to carry). (3) It will transmit the forces of propulsion and braking to the best advantage, with the minimum of tractive resistance to propulsion, and the maximum of road adhesion when braking.
Now, consider this question of deflection. The manufacturer advises a definite pressure for a given load, so that these three duties shall be performed with the minimiim of stress on the tyre itself. This pressure is not a matter of guesswork. It is arrived at by much careful calculation.
A tyre is designed to run in a certain " working " shape, i.e., a predetermined amount of deflection at the point of road contact and a given area of tread in contact with the ground. When working in this shape the cover is not subjected to a stress which is beyond its capabilities.
Before proceeding, I must elaborate this point, as it is really most important. The tyre is the unit which holds the air, and the air, in turn, carries the load. But—and here is the point— the air, in addition to supporting the load, also supports the tyre. It supports it in the true working shape which its maker has decided is the ideal state of affairs.
Dire Effects of Loss of Working Shape,
This question of working shape is vital. If a tyre were inflated to such an extent that it did not deflect at all, it would not absorb any road shocks and would, in effect, become a solid. As shock absorption is so obviously essential the pressure is designed to allow deflection to fall within safe limits. Any material increase in load or fall in pressure will result in an excessive amount of deflection when the tyre will no longer be operating in its true working shape.
This loss of shape is the cause of all troubles arising out of underinflation.
Immediately the pressure falls (from now on I prefer to leave the question of increases in load to the article dealing with overloading) the stresses and strains falling on the tyre are no longer evenly distributed. Some parts are relieved of some of their burden and others are subjected to more than their share.
The present article deals with the effects on only the tyre carcase. The effects on tread life will be dealt with in the next, of the series. Let us see, then, what happens to the carcase.
Instead of being almost circular in
transverse section at the point of road contact, the tyre, due to lack of air support, has adopted a shape which is roughly oval, with the maximum amount of deflection at a point midway up the wall. The carcase of a giant tyre consists of anything from eight to 14 plies of cord, each succeeding ply being moulded over its predecessor on the basis of a round tyre section. • If this round section be deflected to such an extent that it becomes an oval there is, obviously, a great amount of distortion taking place. The inner plies are compressed at the point of greatest deflection (i.e., the middle of the wall area) and the outer plies are stretched tightly around them, This is a very unnatural state of affairs. The tyre was designed to ran at a pressure which would support it in an almost round shape and thus obviate any undue compression or tension of individual plies.
In fact, all plies should be carrying more or less equal stress. Moreover, it was never intended that the relative positions of the plies should change, or that there should be any movement between them.
The Reason that the Plies Disintegrate
All these things happen when pressure is reduced, for the tyre is no longer" braced "to hold up its load. Probably, the most serious of these effects is the relative " movement" or " working " of the individual plies. Every time the tyre revolves, each part of the wall is subjected to this "working " of the plies against each other.
At 30 m.p.h. a 32 by 6 HeavyDuty tyre revolves five times per second, or, in other words, 18,270 times in a 30-mile journey. Every completed revolution means that each part of the lyre wall has been subjected to ply-movement. Movement, in this case, means friction, and friction means wear and heat. Heat is most destructive, of both rubber and cotton, so you have the culmina
tion of a chain of evils all contributing to tyre failure.
Wear ,and stress take place on the individual cords of the carcase, and deterioration develops as a result of the great heat which is generated.
There are two possible results. The tyre will either collapse suddenly, due to an under-inflation burst, or else it will slowly disintegrate under the stress. Cords will begin to separate from the inner plies of the casing. Perhaps a few of them will break. Ultimately, complete cord detachment will take place and the tyre will be scrap. The form which the failure takes depends on many factors, such as the amount of deflection, the weight of the load, etc.
There are four main causes of underinflation which should be guarded against.
The first, of course, is neglect. No tyre is a perfect air seal. Small pressure losses occur even if no leak be apparent. The only way to check this loss is to test the tyres at least twice a week—more often if any trouble be suspected. On long-distance vehicles, the check should be made daily.
Secondly, valve leaks are a common cause of pressure loss. Plungers should be renewed from time to time, but more important, perhaps, is the fitting of valve caps. Even a new plunger will leak slightly under high pressure, but if an extra seal, in the form of a cap, be fitted the valve is as airtight as it is possible to make it.
A third cause is the slow puncture, brought about, maybe, by slight leakage from an imperfect repair. Slow punctures, like valve leaks, are particularly serious in their effects, because their presence is often unsuspected. If a tyre goes down with a hearty bang we know exactly where we stand, and the wheel is changed before further damage is -done. But
the slow puncture does not advertise itself; and the driver continues to run the tyre at much reduced pressures, for considerable distances, until total deflation becomes apparent. This is why a daily check is necessary on long-distance vehicles.
The fourth cause of underinflation is the use of inaccurate pressure gauges. The operator, with the best of intentions, may blow up his tyres conscientiously every day, but if his pressure gauge be inaccurate his efforts are wasted. Gauges should be tested at least once a year by the makers.
Early Recognition of Signs of Trouble We must now pass on to the discernible signs and effects of underinflation, so that the trouble may be recognized and prevented. The signs which indicate that underinflation is present are:—(1) The " flabby" tyre. This is obvious. It simply screams for attention; (2) The vehicle rolls on bends and when braking;
(3) The steering is heavy and sometimes erratic; (4) The vibration due to road shocks is somewhat reduced.
All these conditions call for immediate attention if the tyre is to be saved.
The effects of underinflation are usually to be found inside the tyre carcase in the " wall " area. They vary according to the degree of deflation and the length of time which the tyre has been subjected to it. Most of them are shown in the accompanying illustrations.
The mildest effect is a noticeable darkening of that area of the wall which has been subjected to the greatest deflection. This is caused by the removal of the chalky deposit due to friction. Fortunately, it is not serious. Often the tube will become blackened in the same area, and shows a tendency to " stick " to the walls of the cover.
A more serious result is the slight loosening of the cords, as shown in Fig. 1, the causes of which have already been discussed. Continued; running or increased inflation loss
brings about a distinct separation of the cords, some of which are broken, as depicted in Fig. 2. Finally, unless the tyre be changed, the carcase disintegrates entirely, as indicated in Fig. 3.
Sometimes, as distinct from cordloosening, the individual strands will snap, as shown in Fig. 4. The reason for this difference is mainly due to a changed principle of tyre construction. Specific makes will react in different ways. This does not necesti.rily mean that one type of tyre is better than another. None of them will stand persistent underinflation.
This, I think, pretty well covers the internal effects of underinflation. I will deal with the external effects in the next article.
There is no doubt that one of the reasons why tyre pressures are so sadly neglected is that operators do not understand why the air is necessary. They know that it is necessary, because they have been told so many times, but it is no good telling them to do something without giving good reasons. I have tried,-in this article, to show why the air is necessary, in the hope that by so doing the full importance of correct inflation may be brought home to the user.