The coming of the pneumatic tyre is, of course, well documented, as such an event deserves to be. It was the very cornerstone upon which the temple of automobile progress was built. Nowadays racing folk, in particular, are apt to be somewhat “shirty” if a failure takes place, but surely the measure of their “shirtiness” is really the measure of the progress that has actually been made! Especially is this so when we recall that the materials of which tyres are made are not “engineering” materials in the true sense of the word and are, in consequence, very much more difficult to deal with.
True, those who aspire to make a commercial business out of selling such things must take the responsibility for failure as and when it does occur, but it is only fair to see the difficulties.
Recently a small stout gentleman sat outside a certain pit at Le Mans, somewhat sleepy after the recent Jaguar victory, and someone had the decency to remember the part that tyres played in that victory. “Thanks, Mac,” said the someone, and all of us who aspire to motor at all fast should likewise remember that we certainly owe our lives to the tyre folk, and especially does this apply to the racing brigade.
How many people have actually placed their hand on a tyre immediately following a fast Brooklands lap, or paused to actually count the number of tyre failures they can recall during, say, the T.T. races in the I.O.M.? “Fair’s fair,” as the saying is.
Perhaps the chief difficulty that the tyre people have had to face right from the earliest days has been the fact that they have had to legislate for an altogether disproportionate amount of “user neglect.” Other motor-car components are frequently neglected, but, with the possible exception of the battery, none to so great an extent as the pneumatic tyre. They are habitually run at other than their correct pressures, they are banged against kerbstones, seldom changed round in rotation, and even nowadays sometimes blamed for wearing out quickly on patently unsound independent front-wheel suspension systems. In the racing world, plumes of blue smoke frequently demonstrate the ignorance of the “also-ran” or the temperament of the excited Italian. It is not within the scope of this article to deal with the early stories of struggles by the wayside with pneumatic tyres, nor to repeat the stories of Michelin, Dunlop or du Cros; for all that has been done most adequately before, “Wheels of Fortune” being, perhaps, the best and most accurate work on the subject, but we are here more concerned with a few of the more technical aspects of the matter. It is equally impossible to deal, in a short article, with many of the different types of tyre that have at various times been placed on the market. There must literally have been hundreds of them, and space just does not permit.
The development of the standard tyre has not rested, by any means, with one particular firm, but has, nevertheless, followed the orthodox course of evolution, with but one major change in about 1924 with the coming of the “balloon” type on the well-base rim. From the earliest days, up to 1922, it was the story of a gradual mastery, in deep secrecy, over the details of construction, with the marketed product looking outwardly virtually unchanged during that period. The “hidden” differences, however, were simply enormous. In the beginning, tyres were dreadfully expensive (although just how much of this was due to exorbitant commercial profits is not easy to assess) and lasted a bare 3,000 miles. In fact, Mr. Filson Young tells us that ” . . . there can be no doubt that to many a man weighing in his mind the pros and cons of investing in a motor car, the tyre trouble, with its unknown quantity of expenditure, seems like the last straw that broke the camel’s back.” And that, be it noted, was in 1905. At that time they cost anything from £20 to £100 per set, and the pound was worth quite a lot in those days, and it is not therefore surprising that many hundreds of ideas were marketed in the way of spring wheels, solids, solids with holes for resilience, semi-solids, and single-tube tyres, all without lasting success. As usual, the way of progress lay not with sensational inventions, but with years of very hard labour by the so-called “back-room boys,” always along standard lines. The “cord” tyre was one of the first milestones along that path, and the replacing of the canvas in the body of the tyre by a cord of threads first dried and then impregnated with rubber was a great step forward, and these tyres were even fitted with a small band of red rubber so that they could be retreaded provided they were not worn beyond this point. These tyres (Palmer) were used with success in the Gordon Bennett Trials in 1904.
The first authoritative public utterance dealing with some of the difficulties that the orthodox tyre designer had had to face up to 1920, was that of Mr. Colin Macbeth in a paper read before the I.A.E. in that year, and the writer wishes to proclaim very firmly his indebtedness to that paper for much of his information when writing the present article. Two main types of fixing had by then been evolved, the beaded-edge type and the wired-on or straight-sided type, but the beaded-edge was by far the more popular. Standardised rims had been gradually evolved also during those years, but there were still serious variations in shape and dimensions permissible under the British system, and this, especially as large manufacturing tolerances were normally allowed, made the tyre designer’s job much more difficult as the fit of the bead into the rim was all important, and the bead shape had to be capable of dealing with all the so-called standard rims. It may, perhaps, be useful to note at this stage the standard beaded-edge rim sizes as laid down by the S.M.M.T., for these were, as has been said, by far the most common tyres of those early days. Briefly, after having purchased and examined rims of all sizes, the measurements were summarised and compared with the standards already recognised by the different makers of tyres and rims. It was proposed to make the width between the lips of the flanges some fixed proportion of the nominal size, but whilst in some cases the result approximated to the average dimensions, in others the difference was great enough to cause considerable difficulty with inner tubes and security bolts. It was therefore decided to accept the average measurements for the width. It was found that the average depth increased in steps of about 2 mm. for the smaller sections, a 65-mm. having 10 mm. depth, a 90-mm. 12 mm., and so on, while in the case of the 135-mm. the average depth was 17 mm. The other dimensions of the rims were made proportional to these depths. Five standard dimensions were agreed upon: 65 to 75 mm., 90 mm., 105 mm., 120 mm. and 135 mm., these being the nominal cross-sectional diameters for the tyre itself. For each of these sizes a number of standard nominal wheel diameters were permissible; for example, the same rim section applied in the 120-mm. size for 120 by 820, 120 by 850, 120 by 880, and 120 by 920 tyres. The attached sketch of the dimensions of one of the sections may help to make the point clear.
Safety factors of about eight were employed, reckoned on the tensile strength of the plies, the normal inflation pressure for a tyre of about 3½-in. section being about 50 lb. per sq. in. in 1920. Although the cross-section of a typical beaded-edge tyre of that date would seem to lead, at first glance, to the conclusion that the rubber used was consistent throughout, quite the reverse was, in fact, the case. The head cores themselves needed to be more or less inflexible and therefore a vulcanite or semi-vulcanite type of mix was required so that friction or movement between the cores and the adjacent parts of the tyre might not take place. Casing rubbers, on the other hand, needed to he extremely elastic and yielding, and were required to insulate one ply from another, and even one thread from its next-door neighbour. Here lay one of the secrets of long life in the tyre, as upon this casing rubber depended the resistance to water and dirt of the carcase of the tyre, and its consequent length of life as a “pressure-containing vessel.” The tread rubber introduced a further variation, requiring special properties of toughness in order to resist cuts and abrasion, without undue sacrifice of its yielding qualities. It was also absolutely essential that this rubber should be able to resist the heat of the midsummer sun and the deepest winter frost, to say nothing of oil and other injurious substances. Then there was the cushion rubber, used where the tread joined the main casing. On these points alone it will be seen how much scope there was for manufacturing errors due to lack of experience during the early years. Add to this the fact that vulcanisation, too, had to be developed to avoid cracking through excessive hardness, or excessive distortion and early breakdown due to under-vulcanisation, and that all the complicated machinery to ensure these factors being controlled in production had to be evolved, and some idea of the problem is gained.
The “proportions” of the cross-section had also to be evolved gradually, the thickness at the various points being of absolutely prime importance. Mr. Macbeth gives some examples of this. An insufficiency of rubber between the adjacent plies, causing loose plies and, in consequence, internal friction and heating. Too great a thickness at the side walls again causing the cracking of the surface, in spite of all the other features being correctly carried out in the manufacturing process. Tread separation was also often due to use of the wrong cross-section, and many of the earliest tyres were made almost circular in cross-section, a plan that experience ultimately showed not to be ideal. This, and other errors along the lines already indicated, caused most of the early failures and lack of life.
After the 1914-18 war, in spite of the fact that the tyre looked outwardly the same as its early forerunner, as has already been said, its life had greatly increased, until 10 or 15 thousand miles was not an excessive hope, given kind treatment. Prices had dropped considerably, too, and the market had become very competitive indeed, quite a number of manufacturers offering “free” insurance against all manner of mishaps, as an advertising measure. Lionel Rapson made a great effort to produce a real “unpuncturable,” wisely choosing a “Silver Ghost” as his demonstration car, as his tyre could obviously not have been as comfortable to ride upon as he would have had us believe, had we been the typical motorist of the period. As a rough guide to the rate at which tyres wore out during the vintage era, tread wear was about 0.08 mm. per 1,000 miles at 30 m.p.h. and about 0.17 mm. at 50 m.p.h., summer wear being about one-and-a-half times winter wear. The biggest difficulty with the beaded-edge tyre from the racing point of view was that it relied on the air pressure within the tube to keep it firm on the rim, and a burst usually meant that the tyre left the rim and gave the driver a horrid moment, if nothing worse. The late Sir Malcolm Campbell has left us a graphic description of the horrors of such a happening in 1923, but he goes on to add: “. . . Dunlops began a thorough investigation of the whole tyre question, because they saw far enough ahead to realise that speeds would become higher still; finally they evolved an altogether different type of tyre, which eliminated the faults even of the straight-sided type. But for what happened at Fanoe, their investigations might have been postponed for some time. Coming when it did, the work achieved by Dunlops lent an added factor of safety, not only to record attempts, but to normal motoring, because the new type of rim was eventually used on all touring cars.” Sir Malcolm was referring to the well-base rim, something of whose story the present writer hopes to deal with in Part II of this article, but it is pleasant to be able to recall this contribution to motoring progress made by the racing game, and by one of its greatest exponents. Although not strictly technical, it is fascinating, before considering the coming of the “balloon” tyre and the attendant well-base rim, to look over some of the old advertisements and re-read about some of the “gadgets” that used to be available. Gaiters, puncture-warning devices, pumps, valves, collapsible rims, Stepney wheels, steel-studded tyres, puncture-proof multi-walled tubes, and “goo” for pouring down the valve hole — verily the motorists of those days should have had no trouble. The writer even heard the delightful story, whilst preparing this article, of a puncture-proof tube salesman who proceeded to blow up his demonstration tube before a garage proprietor and then, with reckless bravado, called for the largest and sharpest nail in the place. He produced, to his own consternation, a record explosion, departing sorrowfully with no further faith in the world-shaking invention with which he had linked his post-war career. Alas! perhaps his sales manager should have told him that that tube was only supposed to be puncture-proof inside the cover. Happy days ! — “A.B.C.”