DEVELOPMENT OF TYRES.

Taranaki Daily News, 8 March 1924, Page 14

DEVELOPMENT OF TYRES.

TESTS AND RESULTS. SYSTEMS OF CONSTRUCTION. One of the most important of the many factors which make motoring today extraordinarily cheap is the longevity of the modern pneumatic tyre. Whereas, before the war, 5000 miles was about a 6 long a Life as one could reasonably expect from the canvas-constructed cover, nowadays one may say that 15,000 miles is quite a normal figure for a welltreated cord cover, and extraordinary results have been attained in certain instances.

In order to understand the reason for the superiority of the cord construction as regards the strength of the walls of the tyre, it is necessary to devote a little attention to the strains which the cover is expected to withstand. To begin with, the portion of the tyre in contact with the road is compressed by the load, and, when the car is running, all parts of the tyre are alternately compressed in this way and then extended by the air pressure as the wheel goes round. While this process is going on, the walls of the cover are forced inwards and outwards, and relative motion is bound to occur between the layers of which the cover is composed. This motion is resisted by friction, and the heat set up is highly destructive. That this is no theoretical contention is proved by the fact that in solid rubber tyres the heat generated is sometimes sufficient to liquify the rubber core. The grea-t advantages of the cord system of construction, compared with the older canvas types, is that the internal friction is considerably reduced, the heating effect minimised, and the strength of the walls increased. Consequently, on e may say that, short of occasional manufacturing defects, it is seldom that a modern pneumatic gives way in the walls. Another effect which places considerable strain on the walls is the outward load or centrifugal ■'force produced when rapid cornering is indulged in. The car actually holds tlie road by virtue of the friction between the treads of the tyres and the road surface. The combined effect of the centrifugal force and the friction, acting in opposition, is a tendency to wrench the tyre sideways off the rim. ♦ As had already been mentioned, in the modern tyre the walls of the cover are now adequately strong, and it remains to increase the life of the tread. The tread is, of course, gradually worn away through abrasion when running over rough surfaces, and misusage, such as wheelspin, due to excessive acceleration, or skidding due to violent braking. In this condition it is very interesting to J note that some manufacturers are reverting to the practice of using a combination tread embodying steel studs. STRAJGHTSIDE TYRES. Straight-sided tyres may also be mentioned here, although so far they have not been successful in ousting the beaded edge variety on English ears. They have, however, attained considerable popularity in America, and the greater majority of American cars—especially in the price class —running in New Zealand are equipped with the straight-side rim. Ease in removal and refitting, and the avoidance of rusty rim troubles are the points claimed in favour of straightside tyres. Perhaps the most interesting development to be recorded is that of the large, low pressure or balloon tyres. Briefly, the aim of these new designs is to provide a large section holding a considerable quantity of air at a comparatively ' low pressure. Such a type will absorb minor inequalities in the road surface without transmitting shock to the rest ; of the car. BALLOON TYRE TESTS. I One would imagine that the petrol , consumption is bound to be higher with I a balloon tyre, but it is probable that ihe difference is only slight. Similarly, the maximum speed of which the ear is capable on a good road may be somewhat reduced by fitting larger tyres; but on the other hand, the average speed over rough surfaces is greatly increased, owing to the efficient absorption of shocks. It is steted that there is no noticeable effect in the steering qualities of the ear, and it will be interesting to see the results of comprehensive tests which the English Society of Motor Manufacturers and Traders have requested the R.A.C. to carry out early this year on the relative merits of the balloon and more orthodox tyres. At present the great difficulty standing in the way of the low-pressure tyre by the man who already owns a car, is that a new set of smaller wheels is required. In order to equip a car of medium size with four new wheels and low-pressure tyres, the expenditure will amount to £4O or £5O, which is too much to expect the average owner to pay. This difficulty does not, of course, arise in the case of a new car for which the manufacturer cares to standardise smaller wheels and tyres of large section. *