Solid Rubber Tires for Commercial Vehicles.
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By " Homoc."
The scope of an article on the above subject would be far from large if the advice, which experience has proved over and over again to be right, had been adopted by all users, namely, the fitting of the best tire of the largest possible section compatible with the size of the motor. It happens only too frequently that users take many years trying all sorts of devices before they gain the necessary experience, and it therefore behoves all those closely interested in the industry to bring before the user's notice, as often as possible, the saving effected by a large section of tire, in order to disabuse their minds from a pennywise pound-foolish policy. The chassis-manufacturer's selling or business man has been, until quite lately, a difficult man to handle; he is getting better, but there still is much room for imurovement. His object is to sell his firm's chassis—that is the sum total of his existence : such etceteras as tires matter little to him. If the customer has bought small and cheap tires, he will be able, the salesman reflects, to change them when they are worn out, and to have others fitted. A few words of guidance might have saved his customer a lot of money, and have resulted in not only helping his firm to future orders for chassis, but preventing the statements one hears, such as " Costs of tires are prohibitive." The cost of tires should he quite reasonable —anything from id. per mile on a one-ton van to 2-d. per mile on a five-ton petrol lorry.
Unfortunately for the user who has bought a small and perhaps cheap section of tire, there may be attached to them a so-called guarantee of, say, 10,000 miles: that is the mileage which the tiremaker assumes the tires will do. This guarantee is really only of value if the tiremaker wants to retain the business, and I will try and explain how it operates. The user must keep a record of mileage run, and, assuming after 3,000 miles there is one tire failure out of the six tires, he writes to the tiremaker pointing out the failure, and asks for a new tire to replace the one worn out. If the user's figures are accepted (mileage recorders are not infallible, and returns may be queried), then the user gets a new tire. for which he is charged in full, whilst a credit of 7,000 miles is allowed. Of course, there has been a delay; the wheel has to be taken off the vehicle, sent up to the tire manufacturer, and returned. In about another three weeks, another tire probably fails, and the process is repeated de novo. The user then begins to find that he is losing a lot of working time with his vehicle, for which he gets no value; he is busy checking invoices and "worrying around " ; and finally after, say, six months ot incessant annoyance, he decides he will have a decent make and large section of tire. He then finds that he has to bear the loss of the mileage between that accomplished by each tire and the elusive guarantee of 10,000 miles, which thus proves of no value. This may mean twothirds of the invoiced cost. Then, he has to bear the expense of wheel alterations, fitting new steels and tires, at a considerable capital cost, whereas all this trouble might have been avoided if good advice had been tendered in the first instance.
I do not wish to attack any one type of tire, but it is very necessary to point out that, however good a tire may be, it is practically useless if its fixing is other than positive, and that is the case with all clinched or gripsection tires. In the case of light vans, I know of in stances where such tires have worn splendidly, and, with a low first cost, the mileage costs have naturally been extraordinarily low ; but, fortunately, there is no cer tainty about them, and a sharp application of the brakes may set a tire creeping, and in a very short while that tire wants renewing. I know of a case where a set of
grip-section tires lasted—on a two-ton van—over 12,000 miles, and the firm got a splendid testimonial, and yet the
next set could not stand up for 14 days. This irregularity and lack of uniform service goes to prove that any tire other than of the band section is a " fluky " tire, even for light vehicles, and quite out of the question for heavy Maus and high speeds.
It is no use anybody's imagining that they have the maximum duty consideration when they say the threeton motorvan goes only 12 miles an hour : they may average 16, but on occasions they do over 20, and it is at high speeds that tires are chiefly tried. For high-powered vehicles, a larger tire is essential, though the imposed vehicle-load be not increased. A four-cylinder vehicle carrying the same load as a two-cylinder vehicle requires stronger tires. It is true that the vibration and individual impulse from a two-cylinder engine is greater than
from a four-cylinder one, but the extra molecular stress in the tire, which is generated by the quicker acceleration of the latter, both starting and in traffic necessitates a larger section. I purpose, at the end of this article, to give a tablesuggesting a certain-sized tire for a given gross weight, but, before doing so, it is important to point out that a larger-section tire is not necessarily a greater consumer of petrol, due to its broader tread; otherwise, some engineers may consider that the saving in tire cost per mile will be exceeded by the added petrol consumption. Experience has shown that, to all intents and purposes, there is practically no difference in petrol consumption if a slightly-larger section of tire is used, and experiments made with a section 15 per cent. larger, under the same weights and over similar country, during a period of some months, have specifically proved this. The solid-tired vehicle, if carelessly driven on greasy roads, is a bad one for skidding, and many devices, such as grooves in the treads, have been adopted by manufac turers to minimise this difficulty. They are effective, to a certain extent, but a good driver is the best non-skid, and cheapest in the long run. I hear of one or two devices, which appear to be effective, but they all have some fault. If any maker of a device of the kind wish success, his must be so made that it is entirely independent of the driver's control, and is always in operation.
Some four ways of buying tires are offered by tiremakers to-day, but of these many modes there is only one really satisfactory one, namely, go to the best band section makers, buy the biggest-section tire possible, and buy it for cash without a guarantee as to mileage. The maker will always guarantee workmanship for his own reputation's sake; that is to say, supposing after a few 100 miles of running a tire failed utterly, proving faulty vulcanising, the tiremaker would replace with a new tire at once. Failures of such a nature are, however, rare. The next best mode is to buy tires with a 10,000-mile guarantee. This sort of purchase naturally costs more than a clean sale, and the users pay for the risk : it is, in a way, an insurance, and the users pay a heavy premium for the assurance. The premium charged would not be so heavy, were it not for the difficulty to which the tire maker is put in proving any short mileage. It is so easy for a user to say a tire has run only 7,500 miles, tho roughly believing what he states to be true; but it is very likely that the tire has run well over its guaranteed minimum of 10,000 miles! A run from London to Ascot and back is, say, 64 miles; but, when delivering goods at places en route, and at the terminal village, the distance run may be nearer 85 miles, and this excess, multiplied by 100 days, makes all the difference. The third manner of buying tires is to pay a given sum per mile run. This is practical, where vehicles run over a surveyed route, as do buses, or where a firm owns a large fleet of vehicles, and it is worth the tiremaker's while to accept the business. Fourthly, some firms enter into a yearly contract, the tiremaker running all risks as to mileage run. It need not be pointed out that the user really pays more in the end on this and all alternative schemes, than if he buys clean and without any conditions which handicap the vendor.
Let us take the 10,000 miles guaranteed tires: the premium added generally equals, or is greater than the cost of buying the next-sized larger-section tires, which would give him a greatly-increased mileage over 10,000. Again, here, I can quote from practical experience. A 41--ton van, carrying full load daily, with 41 in tires all round, ran 20,300 miles with two sets of such tires, i.e., each set averaged over 10,000 miles. The price for a set was about £97. A set of 51 in. tires was fitted, costing approximately £120, and the life, over the whole set, was 17,200 miles ; hence, the cost with 41 in. tires equalled approximately 2.33d. per mile, whilst the cost with 5.1i in. tires equalled approximately 1.08d. per mile.
The mileage-basis purchase, unless the contract be a very large one, is hardly worth the manufacturer's while, so he quotes a very high—and even then, perhaps, not profitable—figure. The booking-up of the returns, the collection of payment, and the capital outlay makes this necessary. Finally, with the yearly contract, or " go-asyou-please " payment, it is a toss up whether the manufacturer makes or loses, and it is only if he wants orders badly, or for some special reason, that he is induced to quote a figure which will lead to business, and then, should it be found unprofitable, it proves to be only a temporary arrangement, since no tire maker, or any firm, remains in business long if he effect losses.
In the end, what pays the user best is the business arrangement whereby a tire maker can afford to sell his tires at a small but dean profit, and the sale outright— no mileage guarantee—will prove the best to all parties in the long run. On that point there is no doubt.
The accompanying table will be of use to all parties who buy and sell tires, and if rigidly observed will advance the interests of all concerned. The list of sizes, i.e., widths of tires, refer to a band-section tire in which the rubber is vulcanised to a steel band, and in which the life of a tire is dependent on absolute wear of the tire surface presented to the road, and does not take into consideration a make of tire which becomes often detached at its base, as is the case with a grip or clinched section. I would advise the buying of tires without a mileage-guarantee arrangement, but with a specific guarantee against faulty vulcanising; the latter is easily proved by the tire's coming away from the base after a few hundred miles of running, in which case a new tire should be supplied free of charge. In the hand-section type of tire, this is, as previously stated, only an occasional occurrence, and manufacturers thereof would not charge any premium for such a guarantee, whilst it would safeguard the user on the one point where failure would be excessively expensive and unfair to him.
The width mentioned is the measurement at the base of the rubber tire. The height of tires should be approximately three-quarters of the base or width measurement; hence, a 4 in. width should have a 3 in. height of rubber —a much greater height induces lateral strains, and may damage the base when in work, whilst a lesser height of rubber means so many miles less life.
To obtain the best results, the gross weight of loaded vehicle set down must never be exceeded. Wheeldiameters are not given, as each maker has his own ideas on the subject, but nothing less than a 30 in. diameter on the front wheels in small vehicles should be employed, and 34 in, in larger-sized vehicles; the back wheels should be from 32 in. up to 42 in. for large and heavy loads. In cases where the load is well forward, and where the engine is situated under the driving seat, an extra halfinch on the front tires can be safely added. It is of the utmost importance again to point out that any largersized tire pays for the extra capital outlay in an increased life.
The last-sized tire (160 mm.) is suitable for 12-ton (gross) steamers; petrol vehicles have not such heavy gross weights. Trailers shod with rubber tires, and towed by rubber-tired vehicles, have not yet been taken up by ninny firms, since the law does not permit of a greater speed than 5 m.p.h., and it rests with the
powers that be ” to get this speed limit altered at the earliest possible date; I am convinced that in this direction lies one of the most profitable forms of mechanical transport. Tire makers have little experience in such work, but seem afraid of the proposition. Personally, I do not believe the towing of trailers will much affect rubber tires, provided they are of large-enough section.