My Views about Solid Rubber (Elastic) Tires.
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By T. C. Aveling, Assoe.M.Inst.C.E.
Thomson's road steamers saw the first use of rubber for the treading of wheels of commercial motors, and, although he seemed to get encouraging results from the use of rubber 40 years ago, this design of road engine on his death practically faded from the public view. At any rate, he is the father of all rubber tires used for heavy traction, as well as of the earliest pneumatics (t844), and we owe him at least our sincere admiration for his genius in foreseeing what ultimately would appear to be a necessary adjunct to all heavy commercial motors. His son, Mr. H. Lyon Thomson, still works in the cause.
At the present time, all manufacturers of solidrubber tires have a certain section of tire for certain weights and horse-powers, and a guarantee is usually given for a certain horse-power for a certain mileage at a given weight over a period of usually 12 months, or a distance of 10,000 miles. The manufacturers of rubber tires for heavy commercial purposes have thes: certain sizes for various stresses, but what do we generally find to be the results? No two tires give the same life, and very rarely are whole sets of tires supplied of exactly the same material. The manufacturers themselves do not seem to know what will be the certain results of the tires they produce.
The mort-difficult detail in the design of the commercial motor is tnelban.ation.of the area of contact of the driven wheel with the road surface, and, although we may have the weight of the vehicle and the horse-power to guide us, the chief stress that any part of the vehicle has to undergo is at the area of contact between the driven road wheel and the road surface. This being so, the capabilities of any commercial motor are limited by this one point, and the true way of obtaining the capabilities of one should be, not the horse-power developed by the engine, but the power given off at the contacts between the driving wheels and the road surface. A machine for measuring this power is. I believe, used by the Technical Committee of the Royal Automobile Club_ Whilst for light commercial motors rubber forms the major portion of the composition of the tires, for heavier ones, up to five-ton loads, etc., various compositions are being tried. These are not so resilient as rubber, but give the necessary elastic effect, at the same time not being so costly to apply.
All the tire makers have agreed as to the necessary section of tire for certain loads with certain horsepowers, but, from present experience, no allowance seems to have been made by tire makers for the increased efficiency of the engine and transmission. This increased efficiency produces higher speeds at the road wheels, with the consequence that the maker of the most-efficient vehicle may suffer considerably from the guaranteed tires not lasting out the mileage offered. Very few users of commercial motors are willing to bear the expense of spare wheels, although the necessity for holding them becomes very apparent when, in renewing tires, tile wheel has to be sent away to the tire makers. Most of our long-mileage results are obtained in London, a city of no hills to speak of, and with a considerable percentage of asphalt and wood pavements. The results obtained in London are in no way t.) be compared with the results to be obtained in the hilly and macadamized Provinces. A great difficulty has to be met in obtaining accurate mileage results. A speedometer may be fitted, but, if the speedometer give any trouble or break down, and there be no haste on the part of the user to remedy this defect, the vehicle runs for a considerable period on estimated mileage. Nor is there any compunction on the part of the user in overloading the vehicle, and it is only by constant watching that tire _makers are able to obtain the neensary evidence of weight of loads carried. Under the circumstances, there must be e. considerable amount of give and take between the user and Hie maker of elastic tires for commercial motors.
A inileometer built into the vehicle and forming part and parcel of the wheel, making it impossible for the wheel to revolve—whether on the road or not— without giving a faithful record of the revolutions, would seem to be the only type of indicator capable of registering the requisite evidence. To obtain the full life of elastic tires, all four wheels of the vehicle should be of the same diameter and the tires of one section. in twin tires, it will be often found that one of the twins requires renewing before the other ; instead of the unsatisfactory method of renewing only one of the twins, both may be renewed and the good half-twin fitted to a front wheel when required.
The tire maker, all other qualifications being equal, who will obtain the orders will be the firm that has at its disposal tire presses at as many depots as possible, thus saving delay in changing tires, a considerable bugbear at the present time.
(a) To have all wheels of the same diameter, and all tires of the same section.
(b) No hesitation should be shown by the user in adopting the largest section it is possible to obtain, regardless of the weight to be carried, but at the same time bearing in mind the period guaranteed.
(c) Suitable tire presses should be provided by the makers at depots throughout the country. (d) A mileometer that is part and parcel of the wheel, locked, and so constructed that the wheel cannot revolve without registering.
(e) Constant supervision by the tire maker of the loads carried by the vehicle with guaranteed tires.