PNEUMATIC TYRES FOR HEAVY-DUTY VEHICLES.
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By Henry Sturmey.
THERE ARE very few of the motorists of to-day who can remember when touring cars were run Upon solid tyres; yet, for the firstaseven or eight years of motoring, that was our experience.
The pneumatic tyre had been a commercial article and the accepted wear on bicycles for more man a decade when the early motorcars made their appearance, and the thought of the pioneers—who were, almost without exception, enthusiastic cyclists —naturally turned to the air tyre as a most desirable thing, provided it could be made to stand •up to the work. There never was any question about its use and desirability, if materials and manufacturing methods proved equal to the task, and many were the adventures of the early motorists who essayed their use.
But, for a number of years, manufacturers failed to produce anything practical. The life of a tyre could be measured-by days, and even hours, and our English makers virtually gave up the task until, more prominently in touch with the motorcar in France than we were here, Michelin succeeded in producing a reasonably practical tyre. Once practicability was proved,4adoption -followed as a rapid sequence, and, by the time air tyres were sufficiently perfected to -be used on high-powered cars, 'attempts were made to apply them to commercial cars, but all to no purpose. They failed utterly to stand up under dead
loads of more than 10 OT 12 cwt. .
. When the Royal Automobile Club organized its first van trials, about 1906, if my recollection serves me rightly, the Palmer Tyre Co., having evolved the cord system of pneumatic tyre construction, and finding it gave superior results, boldly essayed the task, and entered a van in the 1 ton class. But, alas for their hopes, the car was knocked out by tyre troubles in the first 1,000 miles, and this public demonstration of their fallibility led the trade and public to accept it as an axiom that pneumatics would not stand up in commercial work under more than 15 cwt. loads. And so we went on ouriway for the best part of another decade, and, by that time, we were in the war with both feet, and had little time to worry about such things.
America, however, as will be remembered, kept out
of the war for three years, during which time her manufacturers were free to pursue the national development of their industry, and the merits of the .cord tyre having made .themselves clear, not only was its manufacture taken up for touring car use by several of the leading rubber houses, but the possibilities in connection with commercial vehicle use were seriously examined. At about the time we were commencing hostilities with the Germans the United States Tyre Co. was experimenting with an 8 ins, cord pneumatic tyre on Commercial vehicles, apd was achieving passable success. As soon as this became known two of the other big tyre houses entered the field, the Goodyear and Firestone companies,. and, ever since then, all three of these firms —the Goodyear in particular—have been giving very serious attention to it, with the result that the cult of the big cord tyre has "caught on" so forcibly in the States that it is estimated that about .10 per' cent-. of America's production of commercial vehicles up to 31U.S. tons capacity, or say 35,00-0 in alt, will this year be fitted with giant pneumatics-of 6 ins., 7 ins., 8 ins., 9 ins., 10 ins., and even 12 ins. section, in accordance with the loads to be carried, the principle appearing to be accepted that the sectional diameter, coincidently with the circumference of the wheel, must increase proportionately to the load to
be carried. •
During the past few months a few giant pneumatics have been imported hare, and vans and lorries have been running on them, and, .at the last Olympia ShowAhe Dunlop Co. staged a sample, showing that English firms are now .giving the Matter -c.serious attention. So far only a very few have been seen on our roads, chiefly through the initiative of the Goodyear Tyre Co. ; but, as we mentioned in The Commercial Motor recently, they appear, so far, to have come through the ordeal successfully.
In the United States, as mentioned above, they are now recognized as at least practical, and sufficiently reliable for adoption, and out of the experience which has been obtained with them there many new facts have developed, whilst it is claimed and admitted that we are only, as yet, on the eve of 'a new era, the industry is already assurrling shape, and developmenr is proceeding along clearly defined lines, so that both English manufacturers and English users should keep an eye on it in order to be able to " keep up with the procession."
Generally speaking, it is claimed for the new development that it will revolutionize the design of commercial motor vehicles, which, not having to withstand the heavy read shocks experienced with solid tyres, will be, not only built lighter, but geared higher, and furnished with more power, so that heavy road transport will be faster. Americans recognize that if speed in light transport is an advantage, it is equally so with heavy traffic, and that if the speed of transport. is quickened up, the cost of transporter ten will be reduced, whilst it. is also claimed that the use of these big pneumatics, whilst adding very substantially -to first cost, will effect considerable saving in upkeep expenditure..
New Developments Arising from the Use of Large Pneumatics.
With the experience already .gained many new facts have been learned, and new developments are taking place in many directioes, same of which it will be useful to record. Thus, standardization has already been adopted in one. particular, and that is in wheel—as apart from tyre—diameter, a uniform wheel diameter of 24 ins, being adapted, whilst the tyres themselves are standardized in 1 in. grada
tions. Thus, the tyre. sections are either 6 Ms., 7 ins., 8 in., 9 ins., or 10 ins. and 11 iris. and 12 ins. sections are under experimental trial. This results, as all tyres have the same internal ring size; in full wheel, diameter automatically increasing with the section, each addition of 1 in. to tyre section meaning an increase of 2 ins, in wheel diameter, from which it will be seen that a 6 ins. tyre will have an effective wheel diameter of 36 ins., a 7 -me. tyre 38 ins., and so on up to the 10 ins. tyre, which makes a 44 ins, wheel.
Thentyre pressures with these big tyres and big weights are greater, a 36 ins. by 6 ins. tyre requiring an inflation pressure of 90 lb., whilst a. 44 ins. 'by 10 ins: will call for 130 lb., and this fact," in turn, calls for fresh devtlopment in two further directions. First, it is foundthat tyre valve apertureis too small, and larger valves are required on. account of the long time necessary, otherwise, to inflate to required pressures, and the call for these. high pressures rules the hand-pump out, as beingealtogether too slow and laborious.
Inflation by Means of Motor Power.
Hence mechanical pumps, engine operated, become anecessity. Such pumps have long formed a part of the standard equipment of many American cars, but here again, further development has been necessary, as the sizes already in use" proved in sufficiently powerful, as well as too slovi for the work to be done, and this has resulted in the development of powerful duplex pumps, which will inflate a 44 iris. by 10 ins. tyre to 130 lb. ip.10 minutes.
Furthermore, in this direction also, standardization has already taken place, as pump manufacturers have, for the past 12 -months, worked to a common standard as regards mounting them on the gearbox and driving them 'from one of the gearwheels, whilst makers of commercial vehicle gearboxes in the States are now providing a standard size opening in the side of the case.
Then again, with a definable tyre, the need for a jack is more apparent than when solid tyres are employed, and, when this is looked into, it is found that the jack which will do for a solid-tyred heavy truck will not do for one with giant pneumatics, and so a series of new types of " jacks has been developed, which have a wider range of action, experience showing that, whilst the jack should be able to go under a 9i ins, axle when the tyre is deflated, it should be able to lift up to 22 ins., in other words, it should have a lifting range of beyond 12 ins.
So far as the rims go, it need scarcely be said that a beaded edge on a giant pneumatic would be an impossibility, and it has been the adoption, so universally in America, of the straight-side tyre with detachable rim edge, quite as much as the development of the cord principle of tyre construction, which has made the big air tyre possible. With this type oftyre, even with a heavy stiff cover, as these large covers must.necessarily be, it is not difficult to change a tube, or a cover, and in the event of a burst: or puncture, the drop of the truck
not so great as might be imagined.
As above said, the introduction of the big tyre for heavy trucks is resulting:in a lot of attention being given in American factories towards redesigning chassis to meet the new conditions, and, in eene direction, there:appears to be'siemovement towards the multiplication of the available gear ratios, i.e., _ the number of gears, as many as seven or eight speeds beingadvocated.. Both larger engines, and either faster engines, or, by preference, a higher ratio otgearing as between engine and wheels, are also being experimented with, it being recognized that the increased speed of the pneumatic-tyred commercial vehield. should be obtained, not so much .by increased speed, per se, but by more rapid acceleration and the elimination of much of the slow time hest new in getting up speed from a start.
The Prospeas for Vehicles with Over Three Ton Loads • s -Up to the present the giant pneumatic has not been fully developed for vehicles beyond 3 tong capacity, but the Goodyear Co., which has, perhaps, given more serious and assiduous attention to this development than any other firm, has been steadily experimenting with a view to its application under still larger loads, and the accompanying illustration will show an entirely new line of 'thought, which is being followed to attain this, end. This experimental vehicle, which has been built by the Goodyear Tyre Co. has been, for many months, running under 6 U.S. ton—over 5 ton English—loads, and the tyres have stood up successfully. It will be seen that it is furnished with six wheels instead of four, so that. the load is distributed over six tyres instead of four. This design opens. up an entirely new line of -thought and study, for it is quite different in inception from the six-wheeled semi-trailer, like the Scammell. The two rear axles are mounted se as to support the rear end of the vehicle, more after the style adopted for the bogey'wheels of a long railway coach, and consideration of this, idea leads to the conclusion that it may not. be a 'weight increaser, as would at first appear, but the reverse, it being .contended that two 3 ton axles will' weigh less than one axle heavy enough to carry 6 tone, and that it will be the same with the wheels, tyres, and springs. Undoubtedly the stresses set up.on the frame by ,rason of road shock must be less if transmitted through. four pneumatic tyres instead of two. So far as brakes are concerned, experience will appear to show that 'they are more effective with pneumatic tyres than when solid tyres are fitted, on account of the greater area of road surface contact of the,tyres, :but the limit of wheel diameter. to 24 ins, necessitates that' the diameter of the brake drums must not exceed this size,. so ,that the course is being adopted of making the brakes the :same size as the wheels, and, where necessary on the larger sizes, increasing the width of the drum and band. From all of this it would appear that we may see
dome very, startling changes in commercial vehicle design during the next few years, and may look for the development of a type of machine which will show a much more radical departure from the models of past years than has been made hitherto in the history of commercial motor vehicle construction.