Pneumatic• Tyres to meet modern conditions By
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C. D. Law, B.Sc.,
Technical Expert, Dunlop Rubber Co., Ltd.
The Concluding Part of an Article Comprising an Informative Analysis of the Tyre Situation, the First Part Having Appeared in our Issue for Last Week
Tyre Problems on Doubledeck Buses: MOST of the double-deck buses are in use on town services. Average speeds are not high, but braking conditions are particularly seVere, owing to the frequent stopping. The efficient modern braking systems of these vehicles have provided the tyre manufacturer with another problem, and two new types of peculiar wear can be definitely attributed to this cause. These are irregular or " spotty " wear, and "flatting," which results In the development of tw6 patches of severe wear at diametrically opposite points.
In cases of " flatting " it will usually be found that, on jacking-up the wheel, two high-spots on the brake drum are in existence, correspondiug to the Eats on the tyre. A brake-testing machine (of the type where the vehicle wheels rest on rotating drums) is useful for detecting troubles of this sort.
Low-pressure tyres are perhaps slightly more susceptible to these types of wear than high-pressure equipment, owing to the greater flexibility of the casing and the tendency of the tread to " shuffle " on the road surface.
On the other hand, passengers' demands for increased comfort will eventually result in the standardization of the low-pressure tyre for both front and rear wheels of double-deck buses.
Severe Operating Conditions.
Many double-deck vehicles nave to operate over streets paved with cobblestones or granite setts; this is particularly the case in Northern areas, and the conditions impose particularly severe strains on highpressure tyres, due to the constant hammering effect. Loading conditions are usually more severe on front axles than on rear, and there is, in addition the added load which is thrown on to the front tyres every time thei brakes are applied. Under these circumstances the low-pressure tyre has been found to have a great advantage. In cases where difficulties associated with hammering have been experienced on 8-in. high-pressure tyres, they have been • almost eliminated by the fitting of 9.75-in. or 10.50-in. low-pressure sizes.
Another feature of double-deck bus operation affected the tyre at the bead, due partly to the transmission of heat from brake drums and partly to the intense side strains
c30 Imposed on the tyres by the swaying of the body. The general tendency to reduction in rim diameters in order to bring down the loading level was a contributory cause of the former, and so soon as the facts of the case were presented to the vehicle manufacturers, precautions were taken 'to ensure adequate clearance between brake drums and rims.
Improvements in tyre design have been introduced to cope with the latter typesorfallure.
Single-deck Coaches.
It is with the single-deck type of vehicle that the most serious effects of speed are felt. In general Wailing conditions are not severe, but on long non-stop runs high average speeds are maintained which compare favourably with the averages made by a good many car drivers.
Comfort is an all-important factor and for this reason alone the lowpressure tyre is almost indispensable, and is rapidly becoming standard on vehicles of this type.
The absence of severe overloading reduces the liability to structural failure of tyres and failures of this type are almost non-existent on these services, so that the life of a tyre is governed almost entirely by the wearing qualities of its tread.
This presents another problem which is not quite as straight-for
ward cas it ,appears. It is not possible to increase tread' life indefinitely by merely adding. greater and greater depth of rubber. Every additional millimetre in tread thickness results in an increase of a few degrees in running temperature, and eventually this may become so high that separation between the tread and casing occurs and the additional potential life built into the tyre is lost.
Tread pattern also has some bearing on the type and rate of wear that takes place, and in producing a pattern the tyre designer has to consider, in addition to problems of wear, the problems of appearance, resistance to skidding and noise.
It is principally in connection with fast passenger coaches that the low-pressure principle has been taken a stage farther, and still lighter tyres of larger section are being produced. For instance, normal 8.25-in. section tyres of 10ply casing construction may be replaced by 8-ply tyres of 9.00-in. section, operating at a considerably lower pressure, and with a consequent improvement in comfort.
Special Tyres for Special Purposes.
It is not intended to deal witl-tr fie subject of special tyres for/special purposes, at any great 'length, but reference may be D -1de to tyres having a particelsirly bold tread pattern designed i;s1. work on soft ground or unrr,dde roads. An example _of this type is the Dunlop " Trak which consists o bars. This tyre h fully used over sur the ordinary tyre w bogged, even if fitted with c Another recent development is t low-loading tyre for rims of 10-in., 13-in. and 15-in. diameters. These have been introduced for -use on trailers with a low loading level and on tractor vehicles of the mechanical-horse type. Speeds are generally low and problems of tread wear are therefore less acute than they otherwise would be.
ip," the tread of series of helical a been successces in which Id become ins.
Future Developments.
It has already been pointed out that, hitherto, tyre development has steadily proceeded on orthodox lines and that no great changes in fundamental principles have taken place. It does not necessarily follow, however, that this will continue to be the case. The two features which impose the greatest restriction on the tyre designer at the moment are vehicle speeds and the demand for increased comfort. Any major changes in these items will necessarily have their effect upon the design and development of the pneumatic tyre. There is now a full realization of the fact that the pneumatic tyre is not to be regarded as a separate and distinct part of the vehicle, but that it is an essential factor in the springing system, inseparably related to the design, construction and performance.of the vehicle as a whole,