Opinions and Queries
Page 66
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Are Modern Tyres Much Better?
'TOUR correspondent, J. E. G. Brewster, writing in your
issue dated April 17, doubts whether tyre design and tyre textiles have developed at the same rate as, the vehicles to which the tyres are fitted, and feels that tyre loads for a given sectional dimension should have increased by 25 per cent. in the course of 29 years. Undoubtedly, it would be possible, using current design methods and materials, to build a cover of the same nominal 8-in. section which would carry 50 cwt. under the conditions of speed and probable overload prevailing 30 years ago.
In fact, tyre development has taken a different course. In the first place, inflation pressures have been reduced progressively during the period under review, from 105 psi. to 85 p.s.i. for a cover of 40-cwt. carrying capacity. Secondly, the current tyre has been designed to operate satisfactorily under conditions of speed and overload which the product of 30 years ago would not have withstood.
For the size of tyre referred to, particularly, this overloading arises from the operation of vehicles designed and equipped to conform with the old 12-ton maximum laden weight regulation, at the present maximum of 14 tons without a change of tyre equipment. The increase in operating speed is the consequence not only of the change in legal limit from 20 m.p.h. to 30 m.p.h., but also of improvements in vehicles and loads which allow the legal speed to be maintained or exceeded for much of the running time.
Thus, although a cover of approximately the same cross-section is scheduled for the same maximum load as it was 30 years ago, the current product will provide a greatly increased mileage with greater comfort and reliability, at higher speeds and with a higher overload mazgin than its predecessor.
Nevertheless, the development of tyre textiles has been such that, whereas the old 8-in. section tyre had 12 or 14 plies of cotton cord, the current 9.00-20 cover of 12-ply rating commonly contains 10 plies of high-tenacity rayon cord, so that the actual weight of textile used has been reduced by at least 30 per cent. This development has contributed materially to the improvement in tyre performance by reducing running temperatures and providing a greater margin of safety under severe conditions. At the same time these textile improvements have been an important factor in holding down the price of tyres in the face of rising costs of materials and labour, and have helped to provide tyre mileage at an economic cost.
London, WA. DONALD GRANT, Marketing Division, Courtaulds, Ltd.