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GIANT TYRES
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Page 93
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of the
Present and Future
LOOKING at the commercial-vehicle industry as it stands to-day, it is difficult to realize that less than six years ago a controversy was raging as to whether pneumatic tyres could ever be used commercially on heavy vehicles.
In the light of our experience during the past six years, it is still more difficult to realize that there should ever have been any question on this point, for the pneumatic tyre has done all and more than its sponsors originally claimed. In the splendid progress the commercial and passenger transport industries have made the pneumatic tyre has played an outstanding part; in fact, without it that progress could not possibly have been achieved.
In the issue of The Commercial Motor dated January 6th, 1925, there appeared a report of a conversation I had with The Commercial Motor, and my views at that time are now all established facts. I have been asked to give some views on the future development of pneumatic tyres for commercial transport, and I find this somewhat more difficult than was the case six years ago.
The Advance of the Low-pressure Pneumatic.
It has all along been obvious that still further advantages could be obtained from pneumatic tyres if a lower air pressure than was originally specified could be employed. Tyre manufacturers, however, were faced with the problem of providing a carcass having side walls which could flex to a much greater extent without the danger of cracking and breaking down.
Considerable development has been made in this direction, and to-day giant balloons or low-pressure 130 tyres are in fairly common use on buses and coaches. There is, however, still a considerable divergence of opinion as to the relative merits of high-pressure and low-pressure tyres. Opinions differ almost, but perhaps not to quite the same extent, as was the case with pneumatics versus solids.
I would here venture the opinion that, just as the highpressure pneumatic has proved its superiority over solid tyres in all but very exceptional circumstances, so will a tyre employing a much lower air pressure supersede that using a high pressure. Having made that statement, I will give some reasons for my opinion.
The position to-day is almost parallel with that which obtained six or seven years ago. Then the advocates of the solid tyre said, amongst other things, that the pneumatic would not give the mileage of the solid, and its cost, both initial and final, would be too high to permit it becoming a commercial proposition.
These same arguments are being used to-day against the low-pressUre tyre, but just as they have been disproved in the one case, so will they be disproved in the other. It is true that to date the general average mileage obtained from low-pressure tyres is not quite as high as that obtained from high-pressure equipment, but there is a reason for this, quite apart from the respective merits of the tyres themselves. Owing to regulations governing the overall width and turning circle of vehicles, it has been difficult to fit a sufficiently large-section tyre to existing vehicles. The result is that a low-pressure tyre, the recommended capacity of which is very little more than that of a 6-in, highpressure one, has been generally fitted to replace a 7-in.
Comparative results, therefore, do not so far present a true picture. Sufficient .accurate data is, however, available to prove that where a size of low-pressure tyre is used equal in carrying capacity to that of the high-pressure tyre it replaces, the mileage it will give is not only as good, but better.
There is no need for me to elaborate upon the effects of overloading on tyre life, as it is well known that a tyre constantly carrying a load in excess of its recommended capacity will fail considerably quicker than one which is run within its capacity. It is also true that the initial cost of a low-pressure tyre of equivalent carrying capacity is greater than that of the highpressure unit. The difference, however, is slight and will be more than offset by lower maintenance costs of chassis and body.
It is generally known, although perhaps not fully appreciated, that heat is one of the greatest enemies of tyre life. Excessive generation of heat can be caused by a number of factors, such as over-inflation, underinflation and overloading, all of which are controllable, but probably the main cause is sustained high speed. Low-pressure tyres, because of their greater air volume and lower pressure, run much cooler than highpressure tyres. They are, therefore, more suitable equipment for vehicles which operate over long distances at high speeds. This is particularly true in tropical and semi-tropical cmuatries and where high atmospheric temperatures are constant.
For passenger-carrying vehicles the low-pressure tyre is, in my opinion, bound, in the near future, to be universally adopted, for, in addition to the advantages mentioned, there is that important factor— additional riding comfort for passengers. I do not anticipate that anyone will seriously disagree with this contention, but what of the goods-carrying vehicle?
Low-pressure Tyres for Six-wheelers.
This presents a much more difficult problem, as it is only comparatively recently that the pneumatic tyre has been accepted by the industry, and there are some types of vehicle for which it is unsuited. There is, however, one type of vehicle on which low-pressure tyres can be used with considerable advantage—i.e., the six-wheeler.
During the past year or two considerable progress has been made in the development of large-capacity six-wheeled vehicles to carry loads of 8 tons, 10 tons and 12 tons, and there appears to be a promising future for this class of vehicle not only in this country but abroad. One of the principal advantages of the six-wheeler over the standard four-wheeler is its ability successfully to operate over country where the roads are poor, or where there are no roads at all, but full advantage of this characteristic cannot be taken if high-pressure tyres be fitted.
Low-pressure tyres, 'because of their greater deflection, provide a much ,better grip on the ground and enable a vehicle to operate through mud and snow, over loose surfaces and sand, under which conditions high-pressure tyres would merely dig themselves in and stall the vehicle.
Tyre manufacturers have not lagged behind the vehicle manufacturers, as low-pressure tyres are available for these large-capacity vehicles. An accompanying illustration shows a section of a 13.50 tyre which actually measures 14.1 ins, in cross section and fits a 20-in, diameter wheel. It has a carrying capacity of 74 cwt. per tyre at 95 lb. pressure, and it is therefore possible to tyre a vehicle to take a rear-axle load of 7 tons 8 cwt. If single tyres be used and of 14 tons 16 cwt. if fitted to a six-wheeler.
This tyre is not an experiment, but has been thoroughly tested, as can be seen from the illustration of a Scammell six-wheeler which is fitted with them. This picture also illustrates the ability of these tyres to grip under the most difficult conditions.
• Aeroplane Tyres for Road Vehicles ?
The low-pressure principle, then, is the tyre manufacturer's contribution in the immediate future toward the extended use of commercial vehicles; but to what extent can this principle be developed? I am of the opinion that in the future we shall see still larger sections oftyres fitting smaller-diameter wheels. Can we, however, ever expect to see road vehicles equipped with tyres of the type developed for aeroplanes?
From the illustration of an "air wheel" it will be seen that the tyre is fitted directly to the hub ; the air pressure used is from 7* lb. to 12* lb. At the moment the mere idea seems fantastic, and I would hesitate even to suggest that it is a practical one as applied to road vehicles, but it must be remembered that all inventions seem fantastic a year or two before they become established facts.