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THE SECRETS OF THE PNEUMATIC TYRE.
Page 107
Page 108
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The Development and Progress of the Means for Introducing an Air Cushion Between Road and Vehicle.
ONE of the most noticeable features of heavy mechanical transport to-day is the increasing use of pneumatic tyres. This is now so well established and such excellent service is given that it has become commonplace and has ceased to excite remark. It is not too much to say, in view of the progress already made, and that which can be conceived by tyre technicians, that all other forms of tyre equipment must disappear in the natural order of evolution, save in exceptional etisf!S.
There are many reasons for this "swing over" towards the pneumatic equipment, most of which are really variations of " comfort " and "speed." Comfort is a term of rather wide application.The implied insulation from shock is not simply a matter of the passenger's feelings. It is as important to the chassis, because it has an intimate effect on every item of its upkeep cost and on its original design, present or future.
The Wear and Tear of Roads.
Another even more important aspect is that, of the road. All over the world road authorities are greatly concerned with the cost Of road maintenance and that of new road development. So serious has this become that there is a most marked tendency to inhibit in some way or other that type of vehicle which damages roads and encourage that which does not.
In fact, actually, hut not always nominally, this discriminates between vehicles with solid tyres and those with pneumatic equipment, as it is universal experience that, with the latter, the damage to the road by impact is reduced to a minimum.
It cannot he emphasized too strongly that all users of mechanical road transport and tyre manufacturers are directly interested in this, because any condition which damages roads damages the vehicle and the tyres. This eventually adds to the cost of transport, reduces the amount of transport available, adds to taxation, reduces mobility and retards progress. It becomes a sort of vicious circle which the pneumatic Lyre will prove the Most efficacious means of destroying.
At present there is no means in sight or even conceivable by which sufficient insulation from shock can be achieved in a better manner than by the application of a cushion of air to the periphery of the wheels. Such lifficulties or doubts as may have arisen are chiefly 2oncerned not with the principle, but vvith the means for applying it, bearing in mind that shock absorption at this most effective point cannot be considered without regard to the durability, safety, simplicity and cost of whatever apparatus is adopted.
The Rapidity of Giant Tyre Development.
The development of the 'pneumatic tyre from the :ycle tyre of about 1.!-In. sectional width, weighing a !ew ounces, to the veritable giant 8-in. or 9-in. section yf to-day, weighing perhaps 150 lb., covers a very short ieriod of time. The progress from size to size, each -meting heavier and heavier duty, was very steady as he motor vehicle gradually evolved, but whilst at every nage of that progress it did not seem possible to do nuch more with the few pounds of cotton and rubber which are the best materials for the tyre, it was in fact found possible.
Up to about 1920, in this country, the biggest pneuuntie tyre had a cross-section of about 135 mm. (say 'bout 5 ins.), adapted for a load having the order of L2 cwt. to 15 cwt. and giving a relatively small mileige performance in comparison with tyres of the same ;ize to-day. One of the most striking phases of tyre irogTess to those who are familiar with the industry was the comparatively sudden appearance of pneumatics which could safely and economically carry 'loads far in excess of anything which previously appeared practicable.
At the present time, tyres up to 9-in. cross-section, adapted for loads up to practically 50 cwt. per tyre, are 'so commonly used that they are now taken for granted as part of a normal programme. The mileages which these tyres are capable of delivering under proper conditions of use and maintenance are almost Incredible.
If the user to-day does not normally get about 20,000 miles per tyre under almost any conditions of use (or, more usually, abuse!) he feels that he has some sort of grievance; but many properly managed fleets average a good deal more than this.
Such relatively amazing performance has not been arrived at by accident. It is the result of very strenuous research and labour directed towards a definite goal. This research and experiment has not been confined simply to the tyre itself. It has included a very wide range of materials, every branch of science and many other industries, providing a history far too long to be dealt with completely in an article of this description, although a few of the main points may be touched upon.
• Cotton an Ideal Material f4 Casings.
Take, for instance, the tyre foundation or easing. This is made of cotton. Every known fibre or material has been investigated, but cotton possesses more of the necessary properties, and to a greater degree, than any other known at present. Contrary to popular ideas, the tyre manufacturer has, in cotton, even the commonest variety, more than sufficient strength for his requirements. '
Tyres will resist a bursting pressure more than ten times as great as that called for in use; indeed, in an inflation test to destruction, they are usually much stronger than the rims to which they are attached!
What had to he utilized or improved in cotton was the ability to bend and recover a large number of times with the minimum fatigue and resultant destruction.
A tyre 36 ins, diameter tevolves about 600 times per mile. At 20 miles per hour it revolves GOO times in 3 minutes-200 times per minute. Therefore each unit of the material composing the tyre is forcibly bent by the load and recovers itself more than three times per second. If the tyre runs 20,000 miles, each unit has been bent at least 12,000,000 (twelve million) times, at a rate which may vary between, say, 100 to 200 or more times per minute.
To state the demand on the cotton in this.simple way indicates its rather appalling nature, but it has been successfully provided for.
Rubber Will Wear Longer Than Solid Steel.
In connection with the 'tread rubber, to obtain toughness, flexibility and abrasion resistance required for economical service the' whole world has been
" combed " for the necessary materials. It is not realized that the compounded rubber used to-day on the best tyres wears longer than a corresponding amount of solid steel would do,
One of the main ingredients which assists this result is nothing more or less than a variety of soot—an impalpable amorphous carbon produced by the burning of natural gas.
Many other curious ingredients are used, especially in the mysterious process of vulcanization, but they are all added because they produce or enhance definite c41.
physical properties which are necessary for the purpose of tyres. Finality has by no means been reached in compounding ingredients, but the matter has become so specialized that it now involves the utmost skill of the research chemists and physicists, only available to firms with the largest resources. Further improvement will, of course, be gradual, because each slight advance becomes more difficult to achieve than the last.
The Many Merits of the Cord Tyre.
Apart from the matter of materials there is the no less important question of design. The chief 'factors contributing to the present results are: First, the discarding of a cotton fabric consisting of warp and weft —and the perfecting of a fabric which consists of stranded cords laid side by side, se that their flexibility is not interfered with by restraining cross-threads. This reduces internal friction, adds to the strength and flexibility of the multiple-ply tyre and permits a large measure of impregnation with rubber to insulate one cord from another. The provision of such " fabrics " and their satisfactory handling involved a complete new technique in a number of industries. In the tyre factory it offered the possibility of moulding upon inflated flexible cores, so that each thread and each ply could take up the position it would normally occupy in running.
Second, the design of the tyre itself. The so-called " straight-side " type with inextensible wired edges has, so far as British and American practice is concerned, practically eliminated the " beaded-edge " with extensible or semi-elastic edges.
It is almost certain that, without the wired type of tyre, technical and economic difficulties would still be In existence which would have greatly retarded the progress of the motor industry. This evolution of the "wired type" involved both wheel and rim manufacturers, and provided many intricate problems only solved by the closest co-operation.
It is interesting to note that the largest manufacturers of tyres in the British Empire also make their own wheels, a fact which has had a very important bearing on the whole development of the business..
Much could be said about many other factors, but probably sufficient has been stated to indicate that there are very sound and scientific causes for the present use of giant pneumatics, and also to suggest that further progress is being eagerly sought. Such progress is not likely to demand radical or fundamental change, but is much more likely to be a laborious perfecting and improvement in every detai4 The weight-carrying schedule for pneumatic tyres 6-in. cross-section and upwards is as follows :—The weights stated are the maximum laden axle weights when the tyres are used in single form, i.e., two tyres on each axle.
It follows that the maximum loaded weight capable of being handled by pneumatic tyres at present would be on a six-wheeler with twins on the rear axles and singles on the front axle, and would amount to no less than five times 95 cwt., viz., 23 tons 15 cwt.
Such a vehicle does not appear to have been constructed, although Messrs. Scammell have constructed several for work in certain overseas territories which closely approach it. These were provided with ten tyres, size 40 by 8, having a total load capacity when properly distributed of 5 by 80 cwt., equal to 20 tons. It is not expected that tyre makers will be called upon to offer much more in the way of weight-carrying capacity !
In conclusion, it should be pointed out that, whilst the giant pneumatic of to-day is able to perform the duties it is designed for with as much certainty and efficiency as any other part of the vehicle, it is often prevented from doing so by the callous indifference or ignorance of its owner or user. That, however, is "another story."