How Tyre Mileage is Wasted
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Although Less Liable to Abuse Than the Cover, the Tube Can Be Directly Responsible for Causing a Variety of Tyre
Troubles
SO far in this series we have dealt mainly, with damage caused to . , the tyre; the tube has received little consideration. The tube, how'ever, is inseparably associated with the cover, and usually suffers consequential damage from any injury *which the latter sustains. Being a more flexible unit, however,, and being protected by the outer cover from most of the hard work, it is • rather less vulnerable under conditions of misuse. The well-being of the tube depends-, to a large extent, on the condition of the cover in which it is fitted, and on the conditions under which it is used. But, conversely, the well-being of the coverdepends largely on the tube. The tube's capacity for sustaining injury is equalled only by. its capacity for inflicting it.
Relationship of Cover and Tube
In order to see the matter in its true proportions, it is neeessary to consider the working relationship between cover and tube. First of all, we need a container which will hold gir compressed to a high pressure, carry a heavy load, and resist abrasion from the road surface, the whole to be securely anchored to therim.
On its own the tube is, obviously, useless. It would not fulfil any of these needs. True, it is airtight, but it will not hold air, unaided, at the pressure We require, The outer cover cannot be made airtight (except in certain freakish deigns), but it can fulfil all the other requirements, pre vided that it be supported by an adequate volume of compressed air.
Therefore, the , inner tube is, primarily, a rather fragile.. air contaMer; the outer cover is its protection against wear upon the road surface, its support against distension due to high pressure, and its means for anchorage to the rim. The load is': of course, carried by the air which the cover and tube contain.
Thus, we have a third factor—air —to complete this working relationship. It is the combination of the three—cover, tubb, and air—which gives us that toughness, with flexi bility„ which are the two most essential features of the pneumatic tyre.
Bearing this in mind, it is obvious that if the cover fails in any way in its task of protecting the tube, or if the tube fails in its job of holding the air, one or the other is bound. to suffer, and, maybe', both.
The tube, as I have said, is fragile.. Its welfare depends, first of all, on its being.a perfectly snug fit in the outer, cover and on the rim. If there be any irregularity, such as a small fracture in the cord casing, or a projecting flint, it will quickly penetrate the tube by reason of the pressure with which it is continually forced into contact .With, it.
Take a look at Fig. .1. It is a typical example of a small concussion fracture, which can be sustained withmkt the driver being aware of it. The impact and the violent casing distortion which results • therefrom have broken through the cord casing. But the tube, being much more flexible, may survive the immediate damage.Thus the-tyre, although injured, is still fully inflated, and from an external examination there is no indication that anything is wrong.
When the Perfect Fit is Broken
The tyre continues its work, but the perfect fit between cover and tube is now broken down. The pressure . pushes' the tube hard against the rough edges of the fracture. . Every time the tyre rOta.tes it deflects slightly as it touches the ground, and the cut opens and closes again as deflection takes place. The tube, lacking toughness, cannot withstand all this, and in a very short time it is chafed through and a puncture is caused.
If you refer to Fig. 4 in article No. 14 of this series you will see a typical chafe caused by a small concussion break in a cover. The outline of the break is faithfully reproduced in the damage to the tube.
The effects of underinflation are far less serious in the case of tubes than in covers, mainly because of their greater flexibility, which enables them to assume, without damage, a working shape which would be disastrous to the less flexible outer cover. Nevertheless, this does not mean that the tube is immune from damage.
A sign of mild, but prolonged, underinflation is a blackening of the . tube in the wall area, Usually accompanied by a definite tendency to stick to the outer cover when removal is attempted. The blaillening is the result of excessive deflection and the heat generated thereby. This is • greatest in the wall area and accounts for the tendency to stick.
A sign of more advanced underinflation is when the valve is torn out from its seating (see Fig. 2). When the pressure falls below an adequate figure the grip which a tyre exerts upon its rim is considerably reduced, and there is a tendency for it to " creep" under the stresses of Propulsion. In creeping it takes the tube with it but as the valve is more Or less anchored in the ValVe slot it cannot go with the tube. The tube and cover creep to such an extent that the valve, hanging on in its slot; is torn a wa y.
Replacing Torn-Out"'"' Valves a Mistake
Et is a common error to trY to replace -torn-out valves in thelr original seatings, adding,' sometimes, a small-rubber patch. This arrangement is foredoomed to failure. The valve seating is reinforced with a canvas insertion, and the 'act o tearing away • the . valve inevitably breaks this material. If the valve be replaced it will come away almost immediately. . The only satisfactory method of repair is to fit a new valve seating containing the. necessary canvas insertion.
-A really bad case of underinfiation will, of course, wreck the tube just as completely as the cover. Fig. 9 is an example. Prolonged running "on the rim" has torn this tube to shreds. I have actually seen tubes reduced almost to a powder from the lame cause, " Invariably this extreme damage is brought about because the vehicle is not properly equipped; either the spare tyre; or the jack, ofthe wheel brace is missing and the driver comes home " on the flat.' If fleet operators would equip their 'vehicles with the necessary tools the damage caused by running on flat tyres would be reduced enormously.
Quite a few of the troubles to which a tube is subject are centred around the valve. In the case of lorry tyres the valve usually employed is the " double bent "; this is designed to lie in a channel in the bed of the rim.
Sometimes, however, the "double bent" is used on the older types of rim, which have a simple round hole
through which the valve is passed. If the placing of the 'valve be not dead accurate there is a tendency for the rim to cut into the valve stem, as in Fig. 3, and this often extends right through to the central air passage and causes deflation. •
Valves which protrude too far may he bent slightly by contact with kerbstones. etc. • This causes the plunger to bind; instead ,of working freely in response to its spring there is a definite inclination to stick. Thus, the plunger does not seat properly, and the air escapes. All valves which are bent, no matter how slightly, should 'be replaced at once.
The valve plunger itself is by, no means a perfect air seal. It may be -quite satisfactory when new, but its effectiveness can be upset by deterioration of the rubber washer, by the advent of grit under the seating, or by other apparently unimportant causes.
Valve Cap Gives Extra Air Seal
. To offset this an extra air seal is required, and this is obtainable in the form of the valve cap. It keeps out the grit, protects the plunger, and traps any air Nyhich may otherwise escape. The Dublchek cap, as used on giant tyres, does not need to be removed for inflation. It has a valve in the head, through which the air can pass. Every valve should have its cap; ' it is only half a valve without one.
One of the most common (and least kntiwn) causes of pressure. loss is the process known as "diffusion," The air with which tyres are inflated is composed of approximately 21 per cent. oxygen and 79 per cent. nitrogen. Rubber has an affinity to oxygenand will absorb it readily. Consequently, when a tyre is inflated with normal air, there is a tendency for up to 21 per cent, of it (the oxygen) to disappear by " diffusion " into the rubber. That is why tyres lose pressure fairly/rapidly sifter being inflated with fresh air.
To carry the matter farther, the initial loss of oxygen leaVes the tube filled with almost pure nitrogen. If the tyre be topped up, then, the proportion of oxygen is far less than „formerly, i.e., instead of being 21 per cent. of the total volume it is now that percentage of only the amount of
• fresh air .which has been added. Thus, its proportions are reduced from a fifth to about a twenty-fifth.
Negligible Loss Through Diffusion Subsequent losses and inflations still further reduce the proportion of oxygen until, eventually, the tyre is filled at proper pressure with almost pure nitrogen. When this stage is reached, the loss due to diffusion is negligible. If, for any reason, the air be released from the tyre, possiply through a puncture, the filling of he tube with fresh air will start the same process all over again. The moral is—watch your tyres very closely when they have just been filled with new air.
In the course of the lifetime of a tube it usually suffers a few punctures which have to be repaired. In the article on cover repairs I pointed out that all repairs, no matter how good, are potential sources of trouble, mainly because of their susceptibility to any tyre abuse.
A sound cover, for instance, may withstand a mild degree of overloading without any evil effect, but a repaired orie may possibly succumb in the same circumstances. The same applies to the tube. All repair patches are vulnerable places. If they be properly applied they will survive all normal use, but they will not stand up to misuse.
The ordinary adhesive patch, which is much used for tube repairs, is very susceptible to heat, and heat, as we know, is brought about by atmospheric influence, by sustained high speeds, and by overloading and underinflation. Heat will soften the adhesive rubber which cements the
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patch to the tube, and will cause it to " lift " as in Fig_ 5. A loss of air results, and the cover suffers the inevitable consequences, • if the trouble be not discovered in time.
The tendency of the patch to lift is much increased if it has not been properly applied. Three, much-neglected features of successful tube repairing are: (1) the area to be repaired should be well roughed up; (2) it should be spotlessly clean and free from grease (do not use Pool petrol—it is far too oily); (3) the solution should be allowed to dry properly before the patch is applied. When I say "dry," I mean really dry. The instructions given with 'many repair outfits tell you to allow the solution. to go " tacky" before applying the patch; this is quite useless—it must be dry.
A far better method of repair is
vulcanizing. The repair material is virtually weldedinto place and there is no danger of it coining away, even under great provocation. In my opinion, all tube repairs in giant tyres should be done by this process.
Pension Off Much Patched Tubes'
Tubes which are much patched, like the one shown in Fig. 4, should be pensioned off. As the number of patches increases, so do the chances of trouble. It is easy to wreck a new tyre by fitting an old tube.
In the course of time, there is a'tendency ...for a tyre to stretch' slightly; this is known as "growth.". The tube, of course, stretches with it. The older the tube becomes the more it loses its elastic properties. Consequently, having stretched with the cover, it does not assume its normal dimensions when the air is released.
If, then, the old, stretched, tube be fitted in a new and unstretched cover it is actually too big for the job. Instead of fitting snugly, as it would if it be not stretched, it is forced into wrinkles and folds. When, the air is put in, these folds are pressed flat against the inside of the tyre by reason of the pressure; a good idea of what takes place can be seen from Fig. 8. Eventually the tube cracks along these folds and, once again, air is lost. .
The tube, then, has a great capacity for causing trouble, as well as for sustaining it. A minute leak in the tube may bring about the total collapse of the cover. The two are inseparable, and if we are to safeguard ou'r tyres we must look to our tubes. '
This series of articles, dealing with a subject of extreme national importance, will be completed by two further contributions by " L.V.B.," and at an early date it is hoped to republish them, at a reasonable price, in booklet form. He has covered all those phases of tyre use and abuse with which road.'vehicle operators are brought into contact, and the articles have received warm commendation from many important sources.