Historical Notes : Tyres (Part II)



Part One of this article endeavoured to give some idea of the process by which the pneumatic tyre evolved, in an atmosphere of close secrecy, from a short-lived, very expensive but very necessary adjunct to high-speed motoring, to a standard article in 1924 that looked outwardly very like its early brothers, but which was, by that year, a cheap and long-lasting affair. No radical change had yet been made, but in 1916 or thereabouts work was proceeding behind the scenes, and in 1924 this research produced the so-called “‘balloon” tyre, which, like so many other new ideas, was at once lauded to the skies or torn to shreds by the pundits. With the new tyre came the well-base rim, which became an essential if easy tyre changing with the larger sections was to be at all possible. This was a peculiar combination of the original beaded-edge and wired-on types of Bartlett and Welch, respectively, which had previously fought for supremacy. The main complaints that were voiced against the new tyres were that they absorbed a lot more horse-power, and thus lowered the maximum speed of the motor car and caused it to use more petrol, and in addition they were alleged to make the steering heavy and encourage wheel-wobble. They were also said to look hideous and to be dangerous, due to the added “drop” they allowed when the dreaded burst occurred. As always there was a measure of truth in these allegations, but the new ideas soon spread and in a few years the controversies were forgotten and the noble art of compromise won the day. The champions of the new tyres campaigned on the basic grounds of comfort, overall comfort that is. The scantlings of the new type of tyre could be reduced, whilst still maintaining the same factors of safety, and the reduction in casing thickness greatly reduced the tendency of the cotton to break up and for the case plies to separate. “Long contact” as opposed to “wide contact” would increase resistance to traction), was a prized asset from the lateral non-skid point of view, and in this respect  the balloon tyre scored heavily.

Comparing an 815 by 105 standard tyre with a 5.25 in. by 32 in. balloon, the former inflated to 45 lb. per sq. in. and the latter to 28 lb., the new tyre showed a 15 per cent. increase in road contact for a 25 per cent. increase in diameter, the same axle weight being applied in each case. The length of contact at the same time increased by 20 per cent. As regards comfort of riding. Mr. Macbeth makes use of the term “absorbability” to describe the capacity of a tyre to “swallow” an obstacle, and for the two examples quoted the balloon tyre showed an increase of 50 per cent, in absorbability. Increased absorbability again was claimed to result in increased freedom from puncture or burst. As  regards the adverse criticism, the heaviness of the steering was acknowledged, but it applied chiefly to existing cars which were converted to the new tyres and not to those designed to use them from the drawing-board. In the matter of horse-power absorption, too, it appears to have been agreed that there was a 4 or 5 per cent. difference against the larger section, but the average speed upon the give and take roads of the early ‘twenties rose appreciably, due to the far greater absorbability.

The real trouble was, of course, that it had become essential at this stage for the tyre manufacturer and the chassis designer to work together. No longer could the tyre be thought of as a mere afterthought, it had to be viewed as an integral part of the basic design of the motor car. By 1939 this had, in fact, become fully appreciated, and words like “oversteer” and “slip angle  began to be freely heard in the land. Even immediately after 1924 the new tyre had very far-reaching effects upon the motor car as a whole. Bodywork noises at once dropped appreciably, shock-absorber design became much more important than it had been, more braking power could be dealt with and steering layouts had to he overhauled so far as ratio and alignments were concerned. Needless to add, while all this progress continued, the average motorist, as always, demanded more and more from his tyres and gave them less and less attention, and this set the tyre people further headaches. In the old days they could at least rely upon owners tending towards obeying the rules, especially as they had paid a great deal of money for their covers and tubes, but now that the price had dropped and competition was very keen, the “user neglect” problem re-presented itself. All this may seem a commonplace, but consider the problem from the tyre designer’s point of view as he approached his board in say, 1929. W.H. Paull made this point clearly before the Institute.

“. . . The navvy’s heavy boot will stand a lot of rough usage, but it could not be called a comfortable boot for long distance walking on good roads, and the lighter and more flexible boot which will give more comfort and probably last longer in walking on good roads would soon get broken up if submitted to the rough usage for which the navvy’s boot is made. No one would expect good service in navvy’s work from a light walking boot, or be surprised if a navvy’s boot proved uncomfortable and developed cracking in the stiff uppers if used for long distance walking. It is just as unreasonable to expect good service from tyres used under conditions for which they were not designed.”

From the designer’s point of view, for example, he might produce a balanced section to provide for a deflection of 20 per cent. for a given average speed and load. He might provide a tread of a type to resist wear for a given number of miles, and to offer a reasonable measure of non-skid properties. He might contrive a balance between these two, that is, he might legislate that the tyre will just outlast the tread. But of what use is that if he knows that a 30 per cent, deflection under the same conditions, due to under-inflation will rapidly fail his tyre in practice ? The casing will fail, and fail, moreover, if the under-inflation only takes place occasionally, exactly as a spring will fail if once stretched beyond its limits,

Power losses in the new and larger-sectioned tyres soon received close attention (incidentally, the term “balloon” soon disappeared) and Mr. Paull gives some very interesting figures as to what these losses amounted to in the late ‘twenties. At 25 m.p.h.. for example, the four 3 1/2-in. high-pressure tyres of a typical light car of the period might absorb some 1.8 h.p. when inflated at about 40 lb per sq. in., whereas some 2.4 h.p, would be the figure if 4 1/2-in. “balloons” at 25 lb. per sq. in were used.

If, further, the owner of the hypothetical light car in question chanced to be motoring with a 20 per cent, decrease in inflation pressure, the figures would be 2.5 and 3.0 h.p. respectively, and therein lies part of the secret as to how the larger tyre finally won through on this power absorption point. For a larger car at 50 m.p.h., 5-in. high-pressure tyres might account for 7.2 h.p., and 6.75-in. balloons for 11.9 h.p., and it is some indication of the superiority of the “cord” tyre over the older canvas that Mr. Paull considered that canvas would have increased the 7.2 to 11 h.p. in the earlier days. The practical fact, however, was that, as already indicated, most people ran their high-pressure tyres under-inflated, so that the difference between the two types was probably not quite as great as the foregoing figures would seem to indicate. Wear rates ran something as follows with the new tyres : 0.08 mm. per 100 miles at 30 m.p.h., 0.17 at 50, 0.9 at 80, 2.0 at 100 and 5.2 at 120, a convincing argument that racing still had lessons to teach those who lacked other high speed research facilities. The subsequent history of the pneumatic tyre has been a repetition of the early days, hard slogging with nothing sensational to show, but with the unseen progress going surely on. Any faults which have manifested themselves since those days have almost invariably been production process faults, in particular those due to the necessity of hurriedly mastering synthetic rubber techniques during the “artificial” war period. Not that this makes the manufacturer blameless, but there have been few design faults as such. Now we hear of tyres that require no inner tubes and no doubt in a few years they will become completely forgotten, rather than almost forgotten, components of the motor car. Speed the day, for even the most rabid vintage or veteran enthusiast cannot truthfully say that he enjoys changing wheels or tyres, and even with the present state of things the jack and brace are usually so flimsy and/or rusty that the job is almost impossible.

It would be easy to sum up by quoting strings of figures regarding the performance of individual parts of the tyre during, let us say, John Cobb’s runs, but this has been done so often that it is easy to lose the significance of it all. Suffice it to say that the modern racing motorist would be quite incapable of giving of his best unless he had complete confidence in his tyres, and it is perhaps the best indication of the state of progress to date that when there is trouble men dance about in dudgeon and utter dark threats. –  “A.B.C.”