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The Claims of The Extra-low-pressure Tyre

1st April 1938, Page 38
1st April 1938
Page 38
Page 39
Page 38, 1st April 1938 — The Claims of The Extra-low-pressure Tyre
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An Authority Puts For ward Some Sound Arguments on Why This Class of Tyre is Likely to Displace the Popular High-pressure

Type in the Future

TYRES for commercial vehicles, from the point of view of running pressures, fall into three main classes. First, there is the highpressnre type, which, numerically, is by far the strongest class at present. It is standardized on most vehicles from 2-tonners upwards. Then there is the low-pressure type, which has a larger air space and a more flexible

carcase. .

It is of great value on passenger vehicles, by reason of the greater comfort that is afforded, but it is also used on goods vehicles to quite a 'large and increasing extent. Lastly, there is the extra-low-pressure tyre. This is to the low pressure as the low pressure is to the high-pressure type.

I should say that this tyre will oust the other two in the course of the next few years. That is a sweeping statement, perhaps, but actually this kind of thing has been going on ever since tyres were invented. Gradually there has been a tendency for them to grow bigger in section, smaller in rim diameter, and lower in pressure.

On tyres for private cars the process of evolution has already reached the extra-low-pressure stage and it will not be long before commercial vehicles follow suit, although, at the moment, the extra-low-pressure type is the smallest of the three classes.

• They Can Take It •

It is hardly'reasonable to deny that low-pressure tyres can stand up to hard work. The strongest highpressure tyre, the 44-in. by 10-in., is capable of carrying 55 cwt., yet we have a low-pressure tyre, the 15-in. by 20-in., which will carry 94 cwt. True, this is not an extra-lowpressure; the latter is, comparatively, so new that its capacity for dealing with great weights has not yet been fully exploited, and, at the moment, it is made in only a few sizes.

Nevertheless, the above example gives convincing proof that a highpressure tyre is not absolutely necessary for the transport of great weights.

So the merits of the extra-lowpressure tyre lie not only in the comfort which it gives; other advantages, however, are so numerous that it is almost impossible to make a comprehensive survey ol them in an article 3f this length.

First, let us deal with its ability to B24 do the work. The extra-low-pressure tyre runs at approximately half the air pressure of an ordinary tyre, but will, nevertheless, carry the same weight. For example, a 32-in. by 6-in. H.D. tyre will carry 22i cwt. at a pressure of 90 lb., whilst a 190-in. by 20-in, extra-low-pressure tyre will carry 22-24 cwt. at a pressure of 50-55 lb. (The alternative figures for the latter are due to the variations in different makes.) • Flexibility a Feature • This type of tyre is much more flexible than the high pressure, as there are fewer plies of cord used in its construction. This also reduces the amount of heat generation, thus not only prolonging the life of the tyre from the point of view of wear . and tear, but also rendering it less liable to bursts.

This desirable state of affairs is brought about by the fact that there is less movement or "working" between the adjoining plies of an extra-low-pressure tyre. Also, there is a greater area for the dispersal of heat. The running temperature is approximately 25 per cent, lower than that of a high-pressure tyre.

It is often argued that a tyre which is so comparatively light in construction is more liable to cuts and concussion breaks; but, actually, the reverse is the case. The tightly stretched cords of the high-pressure tyre are much more liable to cut or snap by reason of their tautness.. If this point were more generally realized, one of the most popular prejudices against extra-low-pressure tyres would be overcome.

Another objection often raised is that the extra-low-pressure equipment is liable to increase fuel consumption owing to the "road drag." Theoretically this is true, but in practice it seldom happens, because the loss from " drag :' is often coUnterbalanced by new economies in power transmission.

In extreme cases, an increase in fuel consumption of 4 per cent. may be encountered, but in other cases it is possible that a saving may be effected. In any event,. the-average increase in fuel cost, say, 2 per cent., will be saved many times over in other directions.

The outstanding feature of the extra-low-pressure tyre is its capacity for shock absorption. This is the primary duty of any tyre, but the extra-low-pressure type performs it more effectively than any other. What do we gain by this? In the first place, the shock is absorbed at its source of _origin—the point of contact of the tyre with an irregularity in the road.

• Shock Not Transmitted •

This, without question, is far more effective than the use of mechanical shock absorbers, for the simple reason that the generation of the shock is prevented in the first place, and is, therefore, not transmitted through the unsprung parts to be counteracted ultimately by springs and mechanical devices.

The shock is absorbed, or its generation prevented, by the deflection of the tyre. The deflection of the extra-low-pressure tyre is many times greater than that of the highpressure type.

Let us assume that a vehicle on high-pressure tyres is driven over a brick which is 3 ins. high. Owing to the tyre having a very small capacity for deflection, the whole vehicle, from tyres to driver, is lifted suddenly almost 3 ins, from the ground, and when it has passed over the brick it drops to the ground again with considerable force. In the case of the extra-low-pressure tyre, however, its higher power of deflection, due to its reduced pressure arid its flexible carcase, enables it to absorb most of the height of that brick.

To put the matter concisely, the extra-low-pressure tyre will reduce shocks to a quarter of the amount of those sustained on the high-pressure class. In other words, the upward and downward movements caused by road irregularities will, on a vehicle equipped with high-pressure tyres, be four times greater than those sustained by the same vehicle on extralow-pressure equipment.

Think of the difference this makes to the comfort of the driver and to the wear and tear on body and chassis. Think, also, of the saving of damage to breakable or delicate loads.

• Examples Prove Advantages •

Perhaps two actual illustrations of the advantages of extra-low-pressure tyres would not be out of place. A firm of wholesale fruit merchants were regularly transporting delicate fruits from Covent Garden to their headquarters, 50 miles away. Their vehicle was equipped with highpressure tyres and it was noticed that the fruit frequently arrived in a badly damaged condition. It was ultimately decided to give extra-lowpressure tyres a trial. The conversion solved this problem with highly satisfartory results and now the percentage of damaged fruit is negligible.

In another case an operator was having a great deal of trouble with the radiators of certain vehicles_ Owing to vibration these were almost constantly springing leaks, and the cost of repairs and replacements, to say nothing of the loss of use, was considerable. This trouble went on for a long time, but was eventually cured, permanently, by fitting extralow-pressure tyres to the front wheels.

High-pressure tyres frequently leave. the road as a result of the upward blow of road shocks. Although this is seldom visible, it occurs every few seconds if the vehicle be travelling at moderate or high speeds along a slightly bumpy road.

The greater the speed and the greater the road irregulaiity, the more pronounced will be this upward lift, and the evil effect is two-fold. In the first place, it represents a loss of power through wheelspin, which, in turn, means that fuel is being wasted. Secondly, when the tyre reaches the ground again it is spinning slightly faster than it would relative to the speed of the vehicle, and so at the moment of contact there is severe abrasion between the tyre and the road. This greatly accelerates tyre wear.

The extra-low-pressure tyre, however, with its capacity for absorbing shocks, closely hugs the contour of the road and will leave the ground only in exceptional circumstances. The loss of tread rubber due to wheelspin Ls, therefore, practically eliminated and more mileage results.

Another outstanding feattre of extra-low-pressure tyres is the extra area of road contact. This is the result not only of the greater size of the tyre, but also of the greater amount of deflection. The area of contact between a tyre and the road is elliptical in shape. AU tractive, braking and weight-carrying stresses are borne by this ellipse; the rest of the tyre is" lying idle" until the moment when it is brought into play by wheel rotation.

• Improved Braking a Feature • On an extra-low-pressure tyre this area is some two-thirds greater than on the corresponding size of the highpressure type. The stresses of propulsion, braking and load are, therefore, distributed over a greater area. This makes for greater efficiency in propulsion and braking owing to the superior grip of the tyre. The added grip makes for economy in power and a reduction in propulsion stresses per square inch.

In the same way, when the brake is applied, the tyre offers a far greater co-efficient of friction with the road, and skids are reduced to a minimum. But as the area of road conta,ct is so much greater, the frictional wear caused by braking is proportionately less.

Vinally, there is the question of weight distribution over the elliptical area of road contact, on which, as previously mentioned, the whole weight of the vehicle and load is borne. Supposing that on a sixwheeled lorry the gross weight is 6 tons, i.e.., 1 ton per tyre. If, with high-pressure tyres the area of road contact of each tyre be, say, 30 sq. ins:, we have the following result per tyre :—

2240 • --se 74f lb. load per sq. in. of tyre. 30 But if extra-low-pressure tyres be fitted and the road-contact area be increased by two-thirds (to 50 sq. ins.), we get the following: 2240_ = 441 lb. load per sq. in. of tyre. 50 The load-carrying stresses per sq. in. are, therefore, greatly reduced by increasing the area of road contact. This results in far less stress on each and every part of the tyre.

• Ordinary Low-pressure Tyre • It may be noticed that throughout the foregoing notes the case for the extra-low-pressure tyre has been compared with that of the high-pressure type, and little has been said of the intermediate type, the ordinary lowpressure equipment. This is because the high-pressure unit is the tyre of the moment. and the extra-lowpressure type is, in my opinion, the tyre of the future.

The ordinary low-pressure tyre may be just a stepping stone by which the extra-low-pressure class reaches its full popularity.

The high-pressure tyre will die hard. It is extremely popular both with the operator and with the vehicle manufacturer, and, at the moment, the latter is, to some extent, impeding the pregress of the extralow-pressure equipment because, in some makes of lorry, there are difficulties in the application of these tyres"to present-day types of chassis, which are primarily built to accommodate high-pressure tyres. So soon as vehicle builders realize the advantages of extra-low-pressure tyres they will, I feel, adopt them to a far greater extent than at the present

time. L.V.B.

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