THE FRENCH SUBVENTION TRIALS.
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Some Comparisons Between the Performances of the Four-wheel-driven Vehicles and the Chain-track Machines on Forest Routes.
THE question of wheeled versus track machines has been the subject of much discussion in the past, but we have bad very -little opportunity for comparing the virtues of a wheel versus a caterpillar track in any form of competition. Iii trials such as those recently held at Shrawardine, near Shrewsbury, it is true,that there were-examples of track vehicles, but, the conditions under which these machines pulled ploughs in .;different fields at considerable distances apart made it difficult to make a comparison between their performances and those of tractors which were carried on wheels, and,%further, thereiwas a considerable variation in the relative sizes of the vehicles, The French Subsidy Trials, near Versailles, offered an almost -unique competition; in which the agricultural tractor machines all weighed 3 to 34. tons, 'were carried on wheels or on tracks, and were driven by engines of approximately similar horse-power. Each machine had the same standard load, i.e., 4 tons, and amongst the whole eight vehicles the track type and the wheeled typo were equally represented, there being four of
each; also all the vehicles travelled over the same route on the.same day, and almost within sight of each other.
The worst part of the Trials was conducted over a forest track, which was practically unmetalled, and as a matter of fact, was so badly damaged by the efforts of the various vehicles that its further use was prohibited by the Forestry Department. This track :included some short, severe gradients, both straight-and curved, and it was at these curved gradients that the greatest difficulties arose. It was here that the wheeled vehicles showed. their superior manoeuvring power, and, as a whole, showed better adhesion and did less damage to the ground than the chaintrack vehicles.
As an example of the difficulties experienced, one point consisted of a circular hill, a gradient of about 1 in 6, which was covered with a greasy clay surface. Two track machines had serious
difficulty in descending this hill. In one case the trailer took charge and slewed the tractor across the road. Considerable time was occupied in getting the vehicle free. In the other case the mathine only just got down the hill without accident, all the time showing signs of slipping owing to the thrust exerted by its trailer. All the wheeled vehicles Came down this hill without any difficulty or showing signs of control being lost.
Much more difficulty was experienced, particularly by the chain-track vehicles, at the sharp bend in the road, which at 'the same time was up a short hill at a
i slope of about 1 n 10. Trees and stumps 'prevented the vehicles moving much off the. track, -which was only about 7 ft. to 8 ft. wide.
The first two-wheeled vehicles ,got round the corner without any difficulty.
They were followedaby the four chaintrack vehicles, all of which stopped and
got into difficulties. The first chain.. track . vehicle attempted the corner without fitting non-skid irons. It climbed up a pert of 'the hill, and then the tractor and its load slipped -backwards, and a large-amount of the subsequent troubles were due to the difficulty of palling its 4 ton trailer .out of the side.of the road, where it bad become bogged. Eventually another chain-track
machine helped it with a tow-rope, and then some further delay was experienced owing to steering difficulties—again due to the tracks slipping.
The other pair of chain track vehicles with a tow-rope were unable to pall the load of one reund the corner in a reasonable time, and so the remainder of the wheeled vehicles, which were waiting to go through, i.e., the Pavesi tractors, when permitted to do so, took the corner without any serious difficulty. The two Pavesi machines had been waiting whilst the four track machines made the surface like a ploughed field.
The Pavesi machines went through and round the corner, one after the other, without any real serious difficulty, although each machine made one or two short halts. After the Pavesi had gone through, the remaining track machines, after extensive manceuvring, managed finally to get round ,the corner.
This particular portion of the trial was very interesting, and the superior results achieved by each of the wheeled vehicles were doe largely to their better adhesion to the ground surface and their superior steering abilities. The performance of the Pavesi tractors was most creditable, in view of the extremely broken state of the ground through which they had to pull their loads, and it must be remembered that each vehicle_ pilled its load without any external assistance from another tractor, as had been necessary in the case of the chain-track vehicles.
During the progress of these tests in general, it was shown that the wheeled vehicle had superior steering, and the opinion of the spectators as a whole was that the performance of the vehicles on wheels was superior to that of the track machines.
It is probable that the track machines were handicapped by the want of, or the employment of unsuitable, nonskid devices. It was noted, however,' that the non-skid chains employed on one of the wheeled machines were not so satisfactory as the angle-iron grousers fitted to the Pavesi.
On whole, it is thought that the question of non-skids requires investigation, so that in future trials the most suitable type of non-skid may be employed It was observed that the angleiron extensions fitted to the Pavesi appeared, on the whole, to absorb less engine power and to give a better grip of the ground than the triangular-shaped spuds which were fitted to the track machines. It is suggested that if a spade-shaped spud is employed, then it should be designed so that it may enter the ground and be withdrawn from the ground surface with the minimum disturbance of that ground. It is obvious that if the spud acts as a spade and disturbs the ground it must absorb more engine power than the short penetration which the angle-iron spud gives. The angle-iron type of grouser proved more satisfactory also in the drawbar effort test. This particular test. was carried out on strong grassland, and while the Renault tractor, fitted with
spade-type non-skids, registered a drawbar pull considerably greater than the results shown for a similar make and type of tractor in the report on the trials at Shrawardine, near Shrewsbury, 1921, yet the Pavesi tractor, fitted with the,angle-iron type of grouser, appeared to get a much better grip of the ground, and registered the best result for the ratio of drawbar effort to weight of tractor.
In considering the general features of the wheeled vehicle, the advantages of speed and durability offered by the rubber-tyred wheeled vehicle when employed on hard roads will not be disputed, and, while there is no reason to suppose that any of the wheeled machines would show any signs of appre-. ciable wear at the end of the trials, it was observed that in the case of the track machines the track parts were all more or less worn, and it is probable that about one-quarter or one-third of their useful life had been taken off by the trials.
It must further be remembered that in these particular trials the chain-track vehicles were used as tractors; that is to say, they were required to pull their load. It is probable that very different results would have been shown by the chain-track vehicles if the loads had. been carried on the machines by both the wheeled and the track vehicles respectively, and the conclusions drawn suggest that the chain-track vehicle is eta disadvantage if employed to tow a load as compared with the wheeled vehicle.