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The Merits and Maintenance of

GIANT PNEUMATIC TYRES

The Height of Centre of Gravity Under Load an Important Factor in the Choice of Adequate Tyre Section for any Class of Commercial Vehicle

By Edward S. Shrapnell-Smith,

C.B.E., M.Inst.T, COMMERCIAL road transport owes more than is often admitted to improvements in pneumatic-tyre construction. One may, in fact, fairly record that during the past few years the evolution of the giant pneumatic tyre has far exceeded the most optimistic anticipations of post-war operators of either passenger or goods vehicles.

Operators have within their own control the opportunity for varying the degrees of benefit accruing to them. The economies or expenditures per mile run, or per tyre consumed, are, to a large extent—if not wholly —within their own jurisdiction, plus the hands and feet of their drivers.

Solid-tyred Buses in London.

Nowadays the public will have none of solids on buses or coaches. This discriminatory preference is practical evidence of the additional riding comfort held available. Relatively, few tyres of the old type, which in its day rendered wonderful service to the industry, survive in passenger work ; many of those which linger on the scene do so, strangely enough, in London. I know there are special reasons for these temporary retentions. It was also largely the impossibility with the solid tyre of overcoming serious overheating at required speeds on long journeys that caused it finally to give way to the giant pneumatic.

Whereas solid-rubber tyres depend for their cushioning capacity upon the distortion of a solid-rubber mass, the eomfort associated with pneumatic tyres arises from the displacement under load of. the " circle " of compressed air right around the wheel. Perhaps I might here enforce the point that the additional cushioning obtained from pneumatic tyres reduces vibration and shock to the chassis, thereby loWering both the cost of and the time required for mechanical repairs and replacements.

Cushion tyres of several designs and forms have done —and still do render—good service. I am not setting out to argue for or against them. I am satisfied, none the less, that the public demand for travelling comfort on the road could not have been met, had not the true pneumatic Lyre triumphed by reason of technical progress in manufacture. This indicated cord construction to replace canvas in the covers.

Cord Tyres Keep Cool.

Advances in the development of cord tyres above five inches in cross-section date back to al)out the year 1916. Their results are to-day's possible mileages, and the high averages that are,in fact, achieved. Cord tyres keep cool on long trips under load at speed, even

on vehicles with high centres of gravity. On this point, to which I shall return, one might well adopt the rule—" the higher the centre of gravity, the larger the tyre section needed for the same axle-weight."

There is no doubt that the increased speed afforded by the use Of giant pneumatic over. solid tyres is an asset of the greatest value, and I believe that many more of our municipal and county fire brigades will ,..79 well forthwith to convert their solid-tyrecl machines to pneumatic equipment, in order to secure this allimportant advantage. The one-time risk of failure en route through puncturing or other involuntary deflation has practically disappeared, whilst fire.station opportunities for supervision and maintenance are unequalled.

Our large stores now find that, with the pneumatic-tyred vehicle, it is possible for them to cover a,much wider area of distribution than was possible in the same time when the vehicles were equipped with solid tyres. Why do any of them retain the older class of tyrik? It dates the vehicles anyhow.

Some owners and engineers of large fleets favour outright purchase of tyres ; others pay per mile, with diminishing rates after agreed

steps of so many thousands. Those who buy outright frequently overlook the incidence in their costs sheets of serving the locked-up capital. They wrongly compare the bare running and maintenance costs per mile, whereas anything from 0.03d. to 0.054. per mile requires to be added in respect of interest on owner's capital, in order to establish a true comparison with a tyreSupplying contractor's mileage charge.

I do not seek to name all the direct and consequential advantages and economies which can, and should, arise from the use of giant pneumatics. Higher speeds and greater daily range are the outstanding ones ; bower all-in costs per mile follow. Less packing for fragile goods, and fewer chassis repairs, are outward and visible proofs of the same fundamental improvement. The carrying in stock of a reasonable proportion of spare wheels and tyres, to permit examination and repair while the service is carried on, adds to the assurance of economies from any fleet.

The Factor of Overheating.

It is, happily, a matter largely outside the necessary ken of the operator to keep tyres from overheating. Time was when it could not he avoided. I name this overheating factor again, because of the importance of its practical disappearance with cord pneumatics. At anything much over 20 m.p.h. for long runs (say, 50 miles on end, non-stop), solid tyres carrying an axleload of six or seven tons, particularly, if the vehicle has a high centre of gravity, may reach so high an internal temperature as to disintegrate. The old canvas type of pneumatic construction, too, was, under service conditions, liable to serious damage by overheating, which the cord type faithfully and regularly surmounts. The straight-side, wired tyre, held in position by a locking ring, has in this, as other respects, been proved best. I desire, in passing, to enforce the importance of height of centre of gravity in relation to the wear and over-heating of solid rubber tyres. Saddle-tank wagons, for equal loads and speeds, show much better tyre mileages on solids than do cylindrical-tank wagons. There is no other explanation than the difference of some 14 inches in e.g. height. The loss can amount to three or four thousand miles of life per tyre.

It is also interesting to recall that the overheating of solid tyres was found, on investigation, to be due to lack of opportunity for the generated heat to escape, and not to a more intense interior generation of heat. Mean results of tests appear to show that at least 12 per cent. more internal heat is, under comparable conditions, set up in some pneumatics than in some solids. The point of generation of maximum heat in a solid is, however, nearly twice as far from the surface as in a pneumatic, and rubber has a notoriously low heat conductivity. As a result, on a long run, cord pneumatics heat up to a certain non-dangerous temperature determined by the conditions, and then heat up no more with added non-stop mileage.

Users and Manufacturers Co-o‘perate.

Vehicle operators themselves have, largely contributed to the successful progress of the cord tyre. They were not all deterred by costly failures of the old

canvas-ply type, and, when even 14 or 16 canvas plies did not "do the trick," they proceeded to co-operate in trying out the more expensive cord type when it was still largely in the realm of promise. By thus working in close touch with the manufacturers, whose patient research has meant so much for road transport, the stage has been reached when results on the road have become largely dependent upon treatment in service rather than upon manufacturing efficiency. Now, bar occasional faults in production, treatment during use is the key to assured satisfaction.

Overloading and under-inflation are the arch-enemies of longevity. I have already referred to the effect of a high vehicular centre of gravity upon solid tyres, due in part, no doubt, to the greater induced rolling and swaying action. For giant pneumatics, which can, and do, accommodate themselves to the several evils which so much damaged solids, I place under-inflation as by far the more serious, although well aware of the fact that any loading to much above rating is anathema to the tyre manufacturer.

Overloading—the Recurring Evil.

Users may struggle—by good intentions and working rules—against repetitions (occasional or frequent) of overloading, but in practice it does occur.. Extra passengers do crowd on to buses, and extra loads are dumped on to lorries. The remedy for overloading is over-tyring, and that, sometimes, is but partial.

Overloading remains primarily a question of adequate tyring, with some regard for the height of the centre of gravity of the loaded vehicle. For double-deck buses the change from 34-in. by 7-in. to 36-in. by 8-in. pneumatics has proved to he necessary to meet this very factor. Overloading tends, in any event, to go a little out of control at times and, therefore, to call for margins in advance. Tyre Pressures are already scheduled to allow for their automatic rise during a long non-stop run, and it is not slight over-inflation that does harm..

The reaping of maximum revenue and traffic advantages which the use of giant pneumatics can confer is now generally admitted to depend upon regard without relaxation for a 'few important but simple supervisory and operating requirements. Yet these periodic duties are not in every case given that heed for which they call. Premature tyre failure must be the inevitable consequence.

Accept the Manufacturers' Advice.

Manufacturers of giant pneumatics have gradually built up and rendered available in convenient form complete series of warnings, precatitions and suggestions for routine duties, for the guidance of, and observance by, depot staffs and drivers. These are, as a rule, so clearly laid down, and in themselves so succinctly conveyed, that it is next door to impossible for anybody concerned honestly to ignore them. The consequences of neglect should, with increasing propriety, fall upon the user, and not upon the tyre maker. The old plea of "a bad tyre" is found to be increasingly incapable of substantiation.

The user's duty to himself is, in the first place, not to under-tyre his vehicles. He should study his class of business, not ignore the estimated centre of gravity of his loaded vehicle and then consult his tyre supplier. I offer no schedules; the makers have them. But it is the user who must make fair-weight declarations.

Is Overloading Unavoidable ?

The duty not to overload is obvious, but I contend that occasional (and in some instances regular) overloading beyond normal is unavoidable. With tyres of sizes above rimmal, the penalty of shorter life is in a measure mitigated, and I content myself by recording . that, in my experience, whilst an overload frequently In being can do a

B26 As to correct inflation, can an owner or driver justly kick anybody but himself if his pressure-tester is not put to the valves two or three times per week? In the first place, the vehicle should be Weighed under load, with its true maximum load up, and the correct inflation pressure then ascertained. For each 1 per cent. of under-inflation I estimate added wear at, perhaps, 4 per cent.

Correa alignment ranks with the two factors already named. Misalignment, of course, may do tyres to death as quickly as the knife itself ; for this there is no excuse. It can be readily checked, by simple apparatus or close observation. It is of real importance to check it after any collision.

The Removal of Sharp Flints;

I accept, as do all users, the bearing upon tyre lives of abnormal Speeds, hilly routes, frequent starting and stopping and had road surfaces. Exactly why road authorities do not use shingle from tidal sources which is rounded by Nature's action, instead of crushed or other sharp-edged stones, for "gritting" purposes, has puzzled some laymen. It is, of course, because the smooth stones will not bed themselves in the road material, but some of the sharp ones certainly do bed themselves in the tyres. The removal each night of any imbedded flints or granite chippings from the treads will definitely Prolong tyre life.

Speed creates custom by its several attractions and values. Does it send up tyre costs disproportionately? The answer is "yes" and "no." With over-loaded (under-sized) or under-inflated tyres, costs may he anything, but there is no basic reason for being frightened of speed, so far as giant pneumatics are concerned. If well chosen and well fitted they like it, they can keep it up, and they are certainly not killed by that which they alone render possible.

"Speed Does Not Kill Tyres."

All the same, it is due to the makers of these tyres to enforce the point that speed may tell on costs if driving at speed is not intelligently done. Potentialities for damage to tyres are enormously greater at 45 m.p.h. than at 25 m.p.h. The stressing effect of the sudden pull-up may be quadrupled; the likelihood Of the easy stop is greatly lessened throughout the journey, where the driver does not look well ahead and always use his head. Even where the driver's qualities are ideal he is not master of circumstances. No; speed of itself does not kill tyres, but it takes its toll.

The obtaining of best results from giant pneumatics on high-speed coaches calls for the exercise of the highest supervision and the most regular attention. They are not, I fear, always given. Covers and tubes should, for example, be periodically removed for inspection (say, once each eight or nine months), and any necessary special repairs effected, whilst it is wise Also to keep the rims clean and to paint them once a. year. No oil or grease should be allowed to get on the tyres.

Charts Issued by Makers.

I conclude by recommending still closer observation by users of the directional advice available to them for application and display in their running departments. These concern fitting and supervision. Every, good tyre supplier offers them, but they are not studied by all users to the extent which they merit. Cards and. charts for garage walls, or for superintendents' offices, embody these guiding principles and detail instructions in a manner that leaves no excuse for failtre to utilize them.

It is only by a coMbination of all.the points set forth that gialit pneumatics can be relied upon to ,cost no more perniile than did solid tyres at their best. -This result, with such care, is definitely achievable, quite anart from the, annual saving of 20 per cetb. on vehicle-licence duty.