LOW-PRESSURE TYRES
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for use under
ABNORMAL CONDITIONS
Remarkable Results Achieved With Michelin Special Tyre Equipment. Yielding Terrain and Sand Traversed Without Difficulty
AN article concerning the Michelin special low-pressure pneumatic tyres and their advantages was published in our issue dated September 1st. We are now able to give further particulars of the application of these tyres to special conditions of working.
The operation of motor vehicles away from the beaten track presents many problems, not the least of which is the negotiation of yielding ground in which ordinary tyres, and even specially equipped wheels, have difficulty in obtaining sufficient grip for traction or actually sink below the surface and cause the vehicle to be bogged.
The new Michelin tyres are specially designed to utilize much lower Inflation pressure than have hitherto B30 been the practice with pneumatic tyres employed on commercial vehicles. These have already proved most satisfactory on ordinary roads, especially for passenger transport, as they are highly efficient in the absorption of road shocks and give almost vibrationless riding, but now they are shown also to be strikingly efficient in soft soil, whilst in tropical countries they have proved a solution of the difficulty of running transport over loose sand.
Factors Which Prevent Bogging.
The ability of a tyre to traverse yielding surfaces without becoming bogged depends upon two factors— (1) the area of contact of the tyre with the ground; (2) the ability of the tyre to flex. Of these the latter is probably the more important, and it is claimed for these new tyres that they give a greater contact area and a greater deflection than any other type. For example, a 38-in. by 9-in. tyre loaded to 43 cwt., and inflated to a pressure of 115 lb. per sq. in., has an area of contact with the ground of 42 sq. ins., and the load imposed is thus, approximately, 115 lb. per sq. in. The equivalent special low-pressure tyre carrying the same load is the Michelin 300/20, which is inflated to 60 lb. per sq. in. Its area of contact with the ground is 77 sq. ins., giving a loading of 63 lb. per sq. in., or a little over half that of the normal tyre.
Thus, considering only the factor of area in contact, this special form has the advantage of 83 per cent. over the high-pressure.
Importance of Tyre Flexion.
Let us now consider the factor of tyre flexion. This quality applies in two directions—flexion of the tyre Walls and of the tyre tread. A high-pressure type, with its rigid carcase, simply digs its way into yielding surfaces until it finds either a firm foundation or else compresses the yielding material of the surface to a degree sufficiently to allow the tyre to roll over it.
In the case of wheels to which driving power is applied the general consequence, in either mud or loose sand, is that the tyre sinks and the motive unit is completely immobilized, and after having been dug out the vehicle must be run on planks, faggots or other materials to give a firmer surface.
In the case of wheels which do not take any drive the behaviour of a high-pressure tyre is slightly different. Now let us consider a trailer on high-pressure tyres which is being hauled by a tractor either direct or through the medium of a winch or cable. Here the tyres on the trailer merely sink into the surface, and any effort to move the trailer simply pushes the surface material in front of the tyres, and these do not roll, but merely slide, which means slow, heavy haulage and constant stops to clear the tyres of the accumulation of surface material.
With the special low-pressure tyres, trailers can be hauled out directly behind the tractor unit or can be easily winched out, as the combination of the big area of contact and the low pressure prevent the tyres from digging into the surface and allow them to roll over obstructions.
Test-vehicle Tyre Equipnient.
The various photographs which we reproduce were taken during tests carried out in collaboration with the Overseas Mechanical Transport Advisory Committee and Latil Industrial Vehicles, Ltd. The Ford vehicle used for the tests in sand was equipped with Michelin 250/20 tyres on the rear wheels and Michelin 230/20 at the front. All these were run at a pressure of 20 lb., and the rear-axle load was 2 tons 15 cwt.
The high-pressure tyres used on the same machine were the standard 32-in, by 6-in., deflated to 45 lb., the normal working pressure being 85 lb. to 90 lb., but, despite this under-inflation, it will be noted that the tyres. became bogged.
The timber trailer used for haulage through deep mud was equipped with 350/20 tyres, inflated to only 70 lb., compared with the 120-lb. pressure of 40-in. by 8-in, twin highpressure equipment.
A Leading Article Recalled.
In this connection it is rather interesting to refer to a leader which we published on August 12th, 1930. In this article we stated that
"some of our best-informed military authorities believe that the question of conquering the difficulties of crosscountry work, such as represented by the Sahara, Egypt, Syria, etc., is almost entirely a matter of tyres. Given sufficient torque at the driving wheels, these terrains can be negotiated fairly easily if tyres designed to operate at sufficiently low pressure be available. Tyre deflection seems to be the most important factor--:-more so, in fact, than pressure per sq. in."
One of the most remarkable exploits achieved by. commercial vehicles was the expedition commanded by M. Bernard le Pontois, which recently made a journey of more than 10,000 miles, of which 6200, were in the Sahara Desert. The three Laffiy lorries utilized carried 3 tons, their total weight, in each instance, being 51 tons. They were equipped with Michelin special low-pressure tyres. The journey was finished without accident, and the tyres were so little worn that those who Saw them could hardly believe that they were really the same that had rendered possible the accomplishment of such a difficult trip. They stood up to running over vast stretches bristling with sharp rocks, dried salt lakes, burning sand and over hard thorns, which aloae would present great difficulties to normal-tyre equipment.
The solution, sometimes adopted when traversing yielding terrains, of lowering the pressure in ordinary. tyres, cannot be considered as in any way satisfactory. The walls and treads still remain rigid and difficult to flex, whilst if forced to do so they cannot give a long life and are apt to generate extreme heat.
We hope later to publish a further article on this question of heat generation and the performance of tyres under tropical temperatures.