What is the Future of The Low -loading Tyre?
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A Review of Practical Requirements Considered in Conjunction with Legislative Influences Points Towards its More
General Adoption
ALTHOUGH the design of commercial vehicles has never been at a standstill, probably more changes have come about in the past few years than over any previous period, whilst it is almost certain that other important changes will be revealed in the near future. It is the object of this article to discuss these trends from the viewpoint of their effect on tyres and wheels.
It can be mentioned at the outset that many of the changes have been hastened by recent road regulations, including, of course, a general order encouraging the use of pneumatics on vehicles where, a few years ago, such tyre equipment was considered undesirable or impracticable.
The floor height of the modern vehicle is usually. required to be as low as possible, whilst another trend is to build the body around rather than over the wheels. This is partly responsible for the change towards longer vehicles, although another cause of this development is the present common practice of employing three axles instead of two.
• Taxation a Big Factor.
The system of taxation by unladen weight is an important factor, and a result is a greater demand for single tyres than for twins. There is also a desire further to reduce weight by using a smaller tyre, the reduction in loading height being possibly incidental.
Small refuse collectors and milk vehicles have in the past largely been equipped with solid tyres to obtain the necessary low height, but now legislation demands a change to pneumatics, and the nature of these vehicles compels the use of a small tyre. Another need for the low-loading tyre is found in the trailer.
The factors mentioned above deal, mainly, with the cause of low-loading tyre development, but the operator will be more interested in their advantages over normal-sized tyres. To show these advantages more clearly it will be best to give actual figures.
The 27-in. by 6-in. tyre is popular for low loadersand has a carrying capacity of 184 cwt. With twin rears and single fronts, this size will give a gross carrying capacity of 5i tons. On vehicles equipped with 27-in. by 6-in. tyres, the axle height is 14 ins, and the outside diameter is 29 ins.—figures which show clearly the usefulness of this size.
As important, is the fact that this tyre will clear a 14-in, brake drum, and can thus be used on practically any chassis in the 30-cwt. or 2-ton classes, giving a substantial increase in carrying capacity.
The 13.50-in./16-in. tyre is also becoming popular on the larger vehicles, and has an axle height of approximately 20 ins., with an overall diameter of 44 ins. It will clear a 15-in.
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brake drum, and will carry a gross weight of nearly 7 tons per axle.
It is interesting to compare the 13.50-in./16-in. tyre with the 40-in. by 8-in, heavy-duty type, which would otherwise be fitted in twin form on a similar vehicle. There is a considerable reduction in the overall Width of the new tyre equipment, for the 40-in. by 8-in, twins would measure approximately 19i ins, across, whilst the 13.50-in./16-in. tyre measures only 134 ins, This means a reduction of 12 ins, in the width of the vehicle. Equally important is the fact that twin 40-in. by 8-in. tyres would weigh approximately 1,000 lb. for four, on the rear axle, against 750 lb. for two of 13.50 ins./16 ins. These weights include wheels and tyres, and show a gross saving of 250 lb. on the rear axle. The axle height of the 13.50-in./16-in. tyre is approximately the same as of that measuring 40 ins. by 8 ins., whilst it is only 2 ins. greater in overall diameter. The advantages of this new tyre to the commercial-vehicle operator will readily be seen.
It can be mentioned here that other popular low-loading tyres have the following measurements: 23 ins. by 5 ins. for 1-ton and 30-cwt. vehicles, and 8,25 ins./10 ins. and 10.50 ins./13 ins, for tractive units and their trailers, respectively.
Some Difficulties.
The present standard wheels are of the disc type, with rims riveted and welded on, and bellied or dished sufficiently to take twin tyres. Owing to the fact that low-loading tyres have such a small rim diameter, it is not practicable to mount them on wheels of this type, particularly as low-loader tyres are rarely fitted as twins. Moreover, the belly on the wheel is a disadvantage, due to the need for having a flat face on which to drill the stud holes.
As a result of these factors, the prevailing low-loading wheel is made in halves, bolted together in the middle. Such pressings are at once cheap and easy to make, and are becoming increasingly popular.
In addition, the side flanges on wheels designed for these small-diameter tyres are usually made slightly smaller in height than on standard wheels. If this were not the case, there would be a tendency for the side flange to press unduly into the side wall of the tyre and cause a break down in the carcase. Difficulties cf this nature caused the low-loading tyre to be viewed doubtfully by many users, but such breakdowns rarely occur to-day.
On refuse collectors and other vehicles demanding a low-loading height, the small tyre is already generally fitted, but it is spreading to other types of vehicle. The most popular dimensions at the moment are 18 ins. by 7 ins. This tyre carries about 12 cwt. On these low vehicles the load is usually evenly distributed, and the need for twin tyres is negligible. As a result the 18-in, by 7-in, size gives a carrying capacity of 24 tons, whilst the axle height is some 8 ins, and the overall diameter of the tyre about 164 ins.
Increased Flexing.
It is noteworthy that the small diameter of the new tyre means that the walls must flex a great deal more than hitherto, because the wheel makes many more revolutions per mile. On this subject designers have carried out a great deal of research, and although all the difficulties have not yet been overcome there is every prospect that the remaining problems will eventually be solved.
The obvious benefit of the low-loading tyre is, as its name implies, the provision of a vehicle of satisfactory carrying capacity with facilities for low loading, but some authorities are convinced that the small tyre will eventually be standard where low height is not a first consideration. It is a point of much importance that small tyres are being constructed with one eye on transport regulations, and there is little doubt that, provided the tyre maker can give his assurance that small wheels and tyres are capable of standing up to reasonable stresses, this adoption will become more general. From the point of view of wear it would be reasonable, at the moment, to expect a rather shorter life than normal.