How Tyre Mileage is Wasted
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No. 13.—Weather Influences and Working Heat
How the Differing Tempera lure Conditions of Summer and Winter Have a Direct Bearing on the Life of Tyres
EVERY mile, travelled by a road vehicle on a hot summer day wears away three times as. much rubber as a mile covered on a wet day in winter. Consequently, if your tyres could be run perpetually under winter conditions they would last three times as long. Furthermore, the. working heat generated in summer is often high enough to lead to bursts, creating a double danger.
It rimy seem out of place to talk about sum mer conditions in the middle of winter, but if this series of articles is to be of any real value it is
necessary to cover all those influences which
cause mileage loss. In any case, I do not wish to discuss summer condi tions only. Rather, I want to talk about weather influences gener ally and, in particular, the ' effect of working temperatures. '
Heat is responsible for . Thearly all tyre failures, and for most cases of rapid wear, too. I am not contradicting myself. I know that I said in previong articles that most tyre failures were due to underinflation and overloading, but really it is the heat generated by these two causes which brings about the tyre collapse.
At a rough guess I should say that heat, due to various causes, can be blamed for at least 80 per cent, of casing failures. Surety it is worth a
. few minutes' study.
Heat is generated in tyres in two ways—internal and external. External heat is caused by (1) high atmospheric temperatures, (2) contact • with hot road surfaces, (3) friction between tread and road. • _ The effect of these eauses is further aggravated by (1) • high speeds, which result in greater friction between tread and road ; (2) harsh braking—for the same reason ; (3) underinflation, because the area. of tread to ground contact is increased by excessive deflection and the tractive resistance of the tyre is increased ; (4) overloading, for the reason given in (3) and because the downward pressure of the excessive load leads to increases in friction.
Atmospheric heat is natural and cannot be controlled, but the mechanical heat generated by the four aggravating factors mentioned above is well within the driver's control. , The effects of external heat are twofold. For one . thing, rubber wears much more rapidly when hot, but it also deteriorates. The deterioraf ion which it suffers while hot will cause it to wear more rapidly even when cool, because it has lost some of its tensile strength. External heat mainly causes external effects.
How External Heat is Dissipated
The external heat which a tyre generates is, to some extent, dispersed by the currents of air in which it runs, and if conditions be normal this cooling process can be depended upon to offset its evil influence. Under abnormal conditions, however —if the tyre be subject to any of the aggravating factors—natural dispersal is insufficient, and rapid wear will result.
In any case, it must be remembered that the higher the atmospheric temperature the less effective the dispersal becomes. Heat is More readily absorbed by cool air than by air which is hot.
Despite the fact that most external heat is dissipated, it is inevitable that some of it „reaches the inside of the tyre. It allies itself with the internal heat—and then the trouble begins. Internal beat is mainly caused by theadjoining cord plies of the casing " working " against each other, and the thicker the tyre (i.e., the greater the number of, plies of cord in its casing) the greater will be the amount of " working." Thinner tyres have a greater capacity for 'deflection without excessive ply movement. As the tyre revolves, each ply is called upon to deflect
several times per second.
This leads to movement and friction, which, in turn, leads to heat.
Increased deflection causes increased "working." so you see how, once again, our old enemies, overloading and underinflation, come into the picture. You just cannot get away from them. 'Both of them cause excessive deflection. Excessive deflection causes excessive "working." Excessive " working " causes excessive heat. And excessive heat ca uses—BURSTS.
The trouble with internal heat is that it is "bottled up." Both rubber and cotton are materials of very low thermal conductivity. They cannot conduct the heat away, ' so it is trapped within the tyre. Gradually the high temperatures begin to take effect on both rubber and cotton. Cords begin to separate and break and; eventually, the heat finds a weak spot. One bang—and another precious tyre is lost.
Some idea of what takes place in an under-inflated tyre can be obtained from the fact that there are many cases on record of underinflated tyres which, as a result of their condition, have built up so much internal heat that the resultant expansion of air has actually caused considerable over-inflation. Thus a vicious circle is completed. First the tyre is under-inflated, but generates so Much heat that it becomes overinflated, Over-inflation reduces the amount of deflection and the tyre starts to cool again. But so soon as it cools the pressure drops, the deflection increases, the heat bands up—and off we go again.
This reminds me of an interesting query which was recently addressed to me at the "CM." Tyre Advisory Bureau. An observant operator mentioned that after checking his tyres before starting .a journey he had noted, after travelling some distance, that the pressures had increased. Iii both tests, the same gauge was used. The increase, as he suggested, was due to working heat.
T h e problem w a s whether-he should let the tyres down to the proper pressure? The answer, of course, is .." No." This increase in pressure due to heat is taken into account in ,the maker's recommendations, a n d should be allowed to remain .
Arising out of this is another important point. Pressures should always be checked when the tyres are cool, and adjustments made with air at normal atmospheric. temperature. Any increase which the pressure subsequently develops can be disregarded, provided that it be the result of normal conditions. If not, the correct procedure is to adjust the conditions, not the pressure. Pressure balance will be restored automatically.
In the _article on "Speed and Braking " (No, 7 in this series), I showed that speed ha S a very evil influence on tyres, but its effects pale into insignificance when compared with heat. take a look at the ,figures below. For the purpose of comparison, 20 m.p.h. has been taken as a, standard speed, and 40 degrees as a standard atmospheric temperature. These conditions have been assunied to give a rate of wear represented by the figure of 100 per
• cent. The other figures show the ielative rates of wear at various speeds and temperatures. Atmospheric Tempera20 30 40
Lure in:p.h. m.p.h. m. p: h.
40° F. 100% 108% 150% 60° F. 191% 217% 275%
80° F. 317% 367% 450% 1000 F. 491% 558% 717% NOTE.—The figures given in this table were arrived at a few years ago after much careful research. It is possible that subsequent developments in tyre con'strudtion may, to -some extent. have 'offset the effects of heat and speed, • but, so far as I am aware, no later data are available. The figures can to-day be regarded as reasonably accurate.
Now, this table is most • interesting. For a given
• temperature of 40 degrees F. we notice that an increase of speed of 50 • per cent. (i.e., to 30
• m.p.h.) causes an
increase in wear of 8 per cent. Doubling the speed to 40 m.p.h. increases the rate of wear by 50 per cent.
But look at the effect of temperature. Keep to the speed of 20 m.p.h. but 'increase the temperature by, 50 per cent (i.e., to 60 degrees F.) and we get an .increase in wear of .91 per cent.—almost double. If we d o u ble the temperature to 80' degrees F. the rate of wear is up by 217 per cent.—over three times as fast. An increase in temperature of 100 per .cent. causes wear four times as tapid as would be caused by an equivalent increase in speed.
Conditions Created by' Weather Influences
Let us see how all this can be applied to a • study of weather Influences.
In summer, we have atmospheric heat and hot, dry roads. Speeds are greater, because road surfaces are safer. Brakes are more frequently used, because of these higher speeds, whilst there is usually more traffic. Tyres in summer have a very .bad time.
Summer heat finds out the . weak spots. Patches on inner tubes will
lift, unless they have been very carefully fixed. This causes slow leaks which give rise to gradual TJeflation. The trouble is that these patches often settle down again before the tyre is .ranoyed to trace the cause of deflation, so the •mYstery remains unsolved. All doubtful patches should be ruthlessly removed and *the tube can then be properly repaired by vulcanizing.,
Repairs in covers may suffer, "too, No matter how efficient a repair
. may be, it is always a weak spot, and is liable to give trouble when used . under abnormal summer conditions. By 'abnormal, I mean, of course, conditions orunderinflation, or overloading, which aggravate the already high working temperature of the tyres.
Another sign of heat is the " blister," which somdtimes forms at the shoulder of the tyre (Fig. 1). The • thickest rubber on a tyre is that in the area of the shoulder. Consequently, heat built up in this regimi is stored, and cannot escape. The heat eventually causes a slight separation 'of the rubber from the casing and this leads to movement and fric-timi in this area.
• Constant rubbing of the rubber against the cord framework gradually wears away tiny. " filings " of rubber which forth into an almost solid mass and cause further separa tion. If a blister of this type be opened up it will be found to be full of tightly compressed rubber "crumbs."
How can the: adverse influence of high atmospheric temperature be off. set? Well, you have seen the figures above, and it is obvious that some saving can be effected by an adjustment of speed. But the most . important point is to fortify the tyre against' heat by inflating it to the proper pressure. if, after doing this, you spoil all the good work by overloading, well—you are just letting trouble in at the back door after kicking it out at the'front.
Before proceeding to disr:oss winter conditions, there is one 'source ofheat which I would like ' to recall from a previous
article. As it has been Mentioned before.
deal with it briefly. You will remember that large brake drums — those
which almost entirely fill the " dish" of the wheel—are responsible for a considerable amount of heat being transmitted to the tyre bead. Venti" lation is bad because the air space around the drum is small; consequently, the heat generated in braking is not properly dispersed. It is conducted to the tyre and often leads to bead failures.
The greatest danger is in prolonged braking periods, when enormous temperatures.are built up. Mainly, these occur when descending steep hills. I suggest, therefore, that on all vehicles having brake drums of this type, the descent of steep or long hills should • be made in low gear, to avoid the use of the brakes.
In winter, as I said at the beginning, the rate of lyre wear is only one-third of that in summer. Working temperatures are reduced because both the atmosphere and the roads are cooler. Often th'e roads are wet, and this watery film acts as a lubricant between the tread and the road. Speeds are lower because the danger of skidding is increased. Con. sequently the use of the brakes is considerably reduced, whilst a lessening Of traffic also tends to reduce the need for their use.
All these things are to the good, and the tyre enjoys a much:needed respite after its arduous task of the summer months. Incidentally, one of the leading tyre manufacturers announced some time ago that investigations it had made proved that tyres fitted during the winter months lasted longer, than those which commenced their working life during the summer.
It was assumed that a debut under winter conditions was beneficial in that it " acclimatized " the tyre for its task. Tyres fitted in summer, which did not enjoy this " breakingin " process, and which were flung straight into the "front line" of hotweather conditions, gave noticeably less mileage.
But the winter is not without its dangers. In wet weather tyres cut much more easily than in dry weather. Sharp points, glass, etc., will cut a tyre much more readily when the roads are wet. Once the tread is cut, water and grit will enter the incision and do further harm. If they reach the cord foundation they will gradually rot it away and the damage will extend.
Watch your tyres in winter, just as carefully as you would in summer, but look for cuts rather than for the effects of heat. If you find them get them repaired at once, before the damage extends to a degree which is
beyond repair. L.V.B.