ROAD CAMBER AND TYRE WEAR.
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T" FRENCH military trials which were recently held near Versailles have shown us that three tyres on one wheel unmet adapt themselves both to cambered and flat roads as well as can two tyres. We will admit that, with regard to their conditions, these trials were somewhat unusual, but we think that they are enlightening and that a lesson
can be drawn from them which, if followed up, might lead to a very considerable improvement being made in the mounting of twin solid tyres.
To make the point clear, it is necessary to recall what actually happened at the trials. Owing to certain requirements of the military authorities, it was necessary in the case of some vehicles to fit three tyres, as shown in Fig, 1, as the loads tube carried for their purpose, combined with the weight of the chassis,
in these cases exceeded the guaranteed reliability of the. tyres. The military authorities would not allow special tyres to be made for the trials, so, to avoid overloading, there was no alternative but to fit triple tyres.
Much of the course chosen corelsted of road with a very pronounced CATnhei-. The result of this was that, in tire case 3338
of those vehicles which bad their wheels perfectly vertical, as shown in Fig. 1, the inner tyre received the bulk of the weight, which resulted in its developing considerable heat, whilst the outer tyre remained cool. Continual running under these conditions resulted in cracks developing in some of the inner tyres. The most satisfactory results seem to have been achieved by those vehicles which had their wheels set at an angle as shown in Fig. 2. One of these was the Saurer, where a live axle of very special construction is employed, which enables the wheels to run in an inclined position (when viewed horizontally) and the other was the Thewald, which has a chain drive, in which case the splaying of the wheels is an easy matter.
Admitting that the triple tyres and the long stretches of heavily cambered roads are unusual conditions, it i's sonic times by going to extremes that one can see a principle more clearly demonstrated. In all probability, had the splayed wheels been fitted with triple tyres, as shown in Fig. 3, and run on flat roads, the outer tyres would have suffered as the inner ones did as shown in Fig. 1. The lesson of these trials seems to be that the more evenly divided the weight between the tyres which carry it, the longer will be the life of the tyre. It is impossible always to confine the running of a, lorry to either flat or cambered roads; consequently a vehicle must take the roads as it finds them. If the set of the wheels is right for one class of road it, is wrong for the other. It is true that a medium angle might be struck as a compromise, but what really is wanted is an arrangement in which some yield in the wheel itself is provided, as shown in Fig. 4. This may seem like asking for the impossible. With our present knowledge perhaps it is impossible. We have, however, an inkling of such. a de. • vice, the success of which seems within the bounds of possibility or we should not have raised the point.
. It is quite evident that, in running over roads with uneven surfaces, one or other of the tyres is temporarily called upon to support the whole weight, and that for a moment one tyre is greatly overloaded. This action does not only take place when running on cambered and flat roads, but must be present whenever the wheel passes over the edge of a pot-hole, as shown in Fig. 5, The provision ofseme independent yield between the tyres may seem almost impossible,
but when it is explained that it, is, in she instance we have in mind, coupled . with a spring wheel it -will be more readily understood. Up to now we have seen. no spring wheel that can be called satisfactory hut that fact, does not necessarily mean that we never shall. We hope to be able to give further particulaats at an early date.