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WHY TYRES WEAR
Page 135
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THE tendency of heavy-vehicle users consistently to load their transport media beyond their rated capacity is one to which we have referred time and time again. The procedure is one which is strongly to be deprecated and is short-sighted, because it cannot fail to be reflected in shorter life of the vehicle and tyres, as well as in greatly increased cost of maintenance. The bright side of the matter is the way in which British-made vehicles stand up
to these heavy overloads. Moreover, the fashion in which the tyres on those vehicles support the excessive strains to which they are put is highly creditable.
Makers at a Disadvantage.
We feel, however, that the tyre makers are somewhat at a disadvantage in this matter. The vehicle manufacturers, aware of this inevitable tendency to overload, can, to some extent, take precautions. These precautions may or may not extend to the provisions which are made, in the design of the vehicle, for the tyres. If they do, well and good. If they do not, it is only fair that the user should at least appreciate the extent to which he is penalizing his tyres, particularly in the case of chassis that are equipped with twin tyres on the rear wheels.
In such cases, the excess loading is greatly exaggerated, owing to the fact that, when the machine is standing or running upon a cambered road, the burden is not evenly distributed. In those circumstances the inner tyres carry much more of their proportion of the load and are thus even more excessively stressed, whilst the outer tyres hardly bear their nominal load. It is no exaggeration to state that each inside tyre bears as much at 33j per cent. of the total load on the rear axle, one of them carrying even more than that.
In an actual example, a vehicle standing or running near the kerb upon a slightly cambered road, the following were the percentages of the total load upon the rear axle, which were carried by the individual tyres :—The near-side outer, 25 per cent.; the near-side inner, 30 per cent.; the off-side inner, 24 per cent ; the off-side outer, 21 per cent.
More Interesting Figures.
On a more steeply cambered road— not, however, so sharp as to be in any sense of the word remarkable—the following percentages of the load were carried by each tyre :—Near-side outer, 18 per cent. ; near-side inner, 38 per cent. ; off-side inner, 33 per cent. ; offside outer, 11 per cent.
The meaning of these percentages will become clearer to many of our readers if we state them in terms of tons, cwt., and qrs. On a slight camber, the following were the loads carried by individual tyres, the total load on the axle being 8 tons 11 cwt. 3 qrs. :—Near-side outer, 2 tons 3 cwt. ; near-side inner, 2 tons 11 cwt. 2 qrs. ; off-side inner, 2 tons 1 cwt. 1 qr. ; off-side outer, 1 ton 16 cwt.
On the more steeply cambered road the figures were :—Near-side outer, 1 ton 11 ewt. ; near-side inner, 3 tons 5 cwt. 2 qrs.; off-side inner, 2 tons 16 cwt. 2 qrs.; off-side outer, 18 cwt. 3 qrs.
The maximum equal load on the rear axle of any goods-carrying vehicle in this country, provided for -under the Heavy Motor Car Order, is eight tons, and the tyres on the vehicle we instanced were actually designed to carry the maximum load of two tons each. Clearly, therefore, the near-side inner tyre on any well-cambered road is carrying an overload of 1 ton 5 cwt., which is 62-i per cent.
Even, however, if the load on the vehicle had not been such as to cause the maximum on the rear axle to be more than eight tons, the innocent owner of that vehicle would, unknown to himself, have been exceeding the 'rated load on his near-side inner tyre by as much as 50 per cent., as the load thereon would have been three tons.
The Result of Overloading.
Let us consider an ordinary case of overloading, in which a vehicle designed to carry six tons, and imposing, with that load, a total of eight tons upon the rear axle, is called upon by its user to bear eight tons. With normal distribution of the load it is more than probable' that the whole of that extra two tons will fall upon the rear axle, thus increasing in proportion the load on all the tyres. In such circumstances on the more steeply cambered road the load upon the nearside inner tyre would be 4 tons 1 cwt. 2 qrs., being, therefore, more than 100 per cent. greater than that which the tyre was designed to carry.
The point we have been trying to make is that overloading of any twin-tyred commercial vehicle involves an excess burden on the tyres which is out of all proportion to the amount of overloading to which the vehicle is subject. The fact still remains, however, as has been shown, that when the chassis is carrying a full load, and when it is on a cambered road, the inner tyres hear such a high percentage of the total load that, with the best intentions in the world, it is impossible to avoid overstressing them in the manner already shown.
• Premature Wear of Inner Tyres.
This is probably the main reason why, in the ease of twin tyres, the inner ones usually fail to give the same amount of service as do the others. It is important to note that, contrary to the usual opinion, if one of the twin tyres be larger than the other (for example, a worn and new tyre running together), the larger tyre should be put on the outside of the axle, as this enables a better distribution of the load to be made between the twins.
This unequal load distribution presents a problem which is engaging the attention of important tyre manufacturers and of vehicle designers. It is rather difficult, especially in view of the limitations imposed by the overall width restrictions of 7 ft.. 6 ins., at present in force.