TYRE WEAR REASONS

Taranaki Daily News, 12 September 1935, Page 12

TYRE WEAR REASONS

RESEARCH WORK BY ENGINEERS. CONSERVATIVE DRIVERS BENEFIT. The year 1934 was one of rapid evolution in automotive design. As to chassis features, there were three major develop* merits, any one of which represented more departure from the conventional than all chassis changes of the previous five years combined. These three developments are: (D Independent front springing; (2) redistribution of total weight between front and rear axles, with corresponding change in percentage of braking on front and rear wheels; and (3) the changes in the aerodynamic shape of car bodies, including the increased shrouding and shielding of tyres by fenders and side plates. In general, faster tyre wear has been experienced during this year, and it is important that the various factors which contribute to the situation be recognised. The following information, the result of research by engineers, covers the more important of these factors. Authorities agree, after many tests, that tyre wear, on the average, is increased from three to five fold for a two-fold increase of car speed. One of these tests shows that tyre wear at 50 miles an hour is more than twice as fast as 35 miles an hour. Another recent test shows that tyre wear at 70 m.p.h. is 42 per cent faster than at 60 m.p.h. With these facts in mind, it is important to survey driving habits of to-day as compared with five years ago. In those days, both because of car design and road conditions, average driving speeds were from 35 to 40 miles an hour, At the present time there are plenty of 60 and 70 miles an hour drivers. It is these drivers who complain of rapid tread wear. The conservative driver of the present time—that is, the one who drives between 35 and 45 m.p.h.—continues to get exceptionally long tyre mileage. SPEED ON CURVES. The case is even more striking when we talk about speed on curves, Modern cars not only have the engine power to take curves fast, but lower centre of gravity, nicely poised steering, anti-roll stabilisers—all of these factors permit a driver to negotiate curves at speeds which would have been impossible five years ago. Under these circumstances, tyre wear is very violently enhanced. As roads are renovated or new roads built, there is a very definite trend toward the use of surfaces and constructions of greatly increased abrasive quality for the purpose of reducing slipperiness of the road surface as a safety measure. This practice also adds greatly to the abrasive quality of the road as far as tyre wear is concerned. Carefully controlled tread wear tests have shown the present type of road surfacing to be 30 to 35 per cent, more abrasive than previous types of roads. With the increased speed and engine power of modern cars, more powerful and adequate braking has, of course, become of vital importance. Not only are the drivers of fast cars able to stop quickly, but it is common practice for them to do so. Test car experience shows that an extra replication of brakes, made only once in five miles, increases tile rate of tread wear 30 to 40 per cent. Congested traffic conditions in cities have greatly increased the necessity for quick and frequent application of brakes, and it has been found that this vigorous use of brakes not only increases the rate of tread wear, but also enhances the peculiar and irregular types of front tyre wear. Car drivers have demanded lower and lower inflation pressures for the sake of the easier ride. With any type of tyre or of tread compound, the lower the pressure the faster tread wear, because with the lower pressure there is more tread area scrubbing and grinding against the road. Numerous tests have shown that a 10 per cent, reduction in pressure causes 15 to 20 per cent, increase in rate of wear, and 25 per cent, pressure reduction increases the rate of wear 30 to 50 per cent. These percentage relationships depend on car design, steering geometry, weight distribution, and other such factors, but no matter what the circumstances one always pays for tlie softer ride with the coin of faster tread wear. STEERING EFFICIENCY. Improvements in steering efficiency, combined with the trend toward higher steering gear ratios, have practically eliminated one of the reasons why the average driver used to keep front tyre pressures up to a reasonable level. If he allowed them to get too low, the car was too hard to handle, particularly in parking; also the steering was vague and unsatisfactory at higher speeds and on curves. With modern cars, however, low front tyre pressures do not, to nearly the same Oxtent, adversely affect the handling of the «car. Hence the driver is content to leave front-end pressures low. The steady trend toward the use of smaller diameter wheels on motor-cars has been another contributing cause of rapid tread wear. The preceding discussion covers the principal factors which are responsible for the fast tread wear situation which exists. With all these factors having such very great individual effect on tyre wear, it is inevitable that a wider and wider spread between the mileages obtained by different drivers will continue to. develop. The conservative type of driver is to-day enjoying greater tyre miteages than ever before, due to the tremendous improvements that have been made in tyre construction in the past few years. On the other hand, the fast, careless type of driver is paying for his speed and carelessness with poor mileage. In the same manner, his other operating costs are proportionately higher, ror example, if he averages 18.4 mites a gallon of petrol at 30 miles an hour, he will obtain only 10.1 mites a gallon at 60 m.p.h. , „ , Considering the fact that all of the forces of driving and stopping and of lateral control are concentrated ana focussed in the small area of contact between tyre and road, it is not surprising that there is a vast difference between 25 mites an hour and 75 m.p.h. operation. This three-fold range of speed actually means a nine-fold range of force value to be handled by the tyres. Consequently it is to be expected that total tyre mileages will vary from 5000 to 40,000, depending upon the case or abuse to which the tyre equipment in each Individua! case may be subjected.