No aspect of motor-car racing has been the subject of so much comment as the evolution of that necessary component— the tyre.
It is a truism that the strength of a chain is governed by its weakest link. That is equally true of a racing car. Only a comparatively few years ago, tyres were practically the limiting features of car racing, being uncertain as to performance under any given set of conditions and no driver could depend upon their distance and speed capabilities.
The year 1923 set a new mark in the history of speed. It was in that year that the Dunlop Company organised a Racing Department and started really to consider that phase of the tyre business.
Automobile racing occupies a more important position to-day than ever before.
It serves a very practical purpose in that, as has so often been said, the racing car of to-day is the touring car of to-morrow. This was foreseen by the Dunlop Company when they laid down their special plant and laboratories for the manufacture and testing of racing tyres, and by very exhaustive research work and tests on machine, road and track, the Dunlop racing tyre has been evolved, that so far has stood up to everything that has been asked of it.
Racing experience has definitely benefited the ordinary motorist. The whole of years of racing experience has been utilised, and built into the latest Dunlop product—the ” 90 ” tyre. From racing practice, this tyre has been designed to be cool running on the fastest of touring cars, and it retains an almost constant non-skid value throughout its life, and is singularly free from irregular wear.
Racing speeds have greatly increased these last ten or twelve years. At the end of 1923, the World’s Fastest Land Speed was 156 m.p.h., to-day it is over 281 m.p.h., recently accomplished by Sir Malcolm Campbell on his Northward run on Daytona Beach. At the beginning of 1927 when Segrave’s car was being designed in the Sunbeam works, the Dunlop Company was approached regarding tyres that would stand up to the car’s t,000 h.p., and the speed that was expected of it—for it was obviously useless to build a car unless By
N. W. H. FREEMAN, A.I.A.E.
tyres could be made for its equipment.
Nothing approaching a 200 m.p.h. had ever been made, and it was not really known whether it was a possibil:ty to make one, for it seemed incredible that articles made of a comparatively few potmds of cotton and rubber could withstand such enormous strains as would be imposed upon them.
The Dunlop Company, by research work on all materials and processes and a long series of experiments, eventually produced a tyre that emerged from its arduous test on Daytona Beach, having accomplished the two-way run at an average speed of 203.79 m.p.h. Since then tyres have been produced successfully year by year for the land speed record, which to-day stands at an average speed of 276.82 m.p.h.
It is not within the scope of this article to delve deeply into technicalities, but there are certain technical aspects which are interesting to the users of racing tyres. For example, heat is generated in all tyres to a certain extent, but the racing tyre, due to the enormous number of revolutions it makes, naturally becomes considerably hotter than a tyre used at touring speeds on the road.
To illustrate this, take a car travelling at 200 m.p.h. At this speed, the tyre—a large size—makes approximately 2,047 revolutions per minute, or 34 per second— a small diameter tyre would naturally be more. Every portion of the tyre is, therefore, hammered between the rim and the road—that is compressed and released-34 times per second, and just as a slab of iron may be made hot by continual hammering, so a racing tyre tends to get hot. The art of making a racing tyre is largely in so treating the rubber that it can withstand this repeated hammering on the ground without generating excessive heat, and so arranging its other component parts with the same object. It is this important question of heat engendered in the tyre combined with centrifugal force that makes problems of tyre construction acute with the develop
meat in speed and power of the modern supercharged racing car.
The source of heat is the power consumed by the tyre. This depends upon a number of factors, among which are speed, deflection of the tyre, and size of tyre (diameter and section).
The rate of increase in power consumed becomes greater and greater with increasing speed. For example, the effect of increasing the speed from 190 to 200 m.p.h. is approximately twelve times that of increasing the speed from i7o to 180 m.p.h., and sixteen times that of increasing the speed from 130 to 140 m.p.h.
The deflection of a tyre depends upon the load carried and the inflation pressure. An under-inflated tyre with the load constant means an increase in power consumed or, in other words, abnormal heat is generated in the tyre with naturally deleterious effects.
The size of the tyre affects its performance in two ways. The larger the diameter, the fewer the revolutions per minute, and the fewer the impacts any section of the tyre has to sustain for a given speed, consequently the lower the temperature attained. Also the greater the diameter and cross section, the smaller the deflection and the larger mass of material to be heated up. Therefore, for a given load, the lower the temperature developed.
An example may be cited. A small tyre running at the same speed as a large one, the smaller tyre revving at 1,330 r.p.m., and the larger at 1,180 r.p.m., the difference in heat after the same period of time was nearly 200 centigrade.
From the foregoing few technical points it is obvious that the racing motorist, if he expects to obtain the maximum efficiency and service from his tyres, must make certain that his car is equipped with the most suitable tyres for the particular event, and that they are inflated to the correct pressure.
The Racing Department of the Dunlop Company exists solely to give from its wide experience of racing the best advice on all matters appertaining to the tyre equipment of racing cars, and the expert staff are always at the service of the racing motorist.