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Quality Fluctuations n Synthetic Tyres A CCORDING to the United States Pl. National Bureau of Standards, methods which their scientists have evolved for testing the wearing qualities of tyres are of particular importance on account of the large variations in quality which are characteristic of
synthetic-rubber covers. It has been found that the service life of tyres of thn same size but of different makes varies as widely as between 12,000 and 41,000 miles.
The energy absorbed by tyres is an important factor with regard to both fuel consuMption and the effect of heat on tyre life, the power lost through this cause in a tractor-trailer outfit representing an outstanding example.
To measure power losses accurately in the laboratory, Government technicians have built a special machine with two dynamometers which record the power necessary to drive a tyre in contact with a steel drum and the power transmitted to' the drum shaft. The tests have shown that synthetic rubber absorbs a great deal more power than natural rubber and that composition and design influence the results obtainable.
Experiments have indicated that a tyre giving a high power loss has poor fatigue resistance to flexing, but they also show that the converse does not necessarily apply. With thfferent makes of the same size of tyre the variation in running temperature has been as much as 68° F.
The energy required to rupture a tyre is measured in a Static test with the aid of a cylindrical plunger with a rounded end, measurements being made when the cover is new and after it has been used in service to gauge deterioration of rupture resistance. It has been found that rayon and other synthetic fibres deteriorate far less quickly than cotton cords.
Cuts in Grooves Other tests include running tyres with artificially cut grooves to determine the resistance of the rubber to the spread ot cuts, this characteristic being shown by the increase in groove dimensions after a long period in contact with the revolving drum. Tread wear is measured by the weight lost during a test, and compensation ;s made for the different treatment which would be suffered by a tyre on the road according to the position it occupies on the vehicle.
Four-wheel-drive and six-wheel-drive trucks are employed for road tests of tyres on 15-in. to 20-in. rims. Weightloss measurements are made at the conclusion' of a test, the total weight of tread being estimated after the remaining rubber has been removed by buffing. From these tests it has been concluded that power loss is influenced more by the type of rubber than the type of carbon black used in the manufacture of the tyre.