How the Dunlop Rubber Co., Ltd., is Providing Equipment to Meet a Wide Variety of Operating Conditions
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THROUGHOUT the world, road conditions vary widely and tyres that are eminently suitable for one territory may. prove less efficient in another, where different and special conditions obtain. The Dunlop Rubber Co., Ltd., has, over a long period, given careful attention to this question of special conditions, and has developed several designs of tyre, mainly for commercial vehicles, which have proved to be of great value.
It is not so long since the solid was the only tyre available for commercial vehicles. It was suitable only for hard, smooth roads and low speeds. On soft ground it sank in, on rough surfaces its leek of cushioning, small road contact area and high contact pressure inevitably caused damage to the vehicle, resulting in high maintenance costs, damage to the road, and discomfort to the crew.
Even on good roads the damage to the chassis and the road was considerable; and, if anything more than a moderate speed was attempted, the excessive heating of the tyre introduced further difficulties.
The first improvement on the solid tyre was the introduction of the highpressure pneumatic and then the lowpressure equipment in sizes suitable for commercial vehicles and appropriately strengthened.
The low-pressure pneumatic tyre finds its general application where roads with reasonably good surfaces are to be traversed at higher speeds. The lower inflation pressure causes a reduction in running temperature and provides in
creased cushioning. It naturally follows that there is less, "hammering" of the tyre on the roads, and that the shocks to which the road surface, the tyre and chassis are exposed are much reduced. At speed, the greater deflection and the increased road contact area offered by the tyre give a constant and more certain grip, resulting in a reduction of wheel-slip. This ability to hold the road perfectly at high speeds is not the least of its advantages.
If soft ground is likely to be encountered, the Dunlop low-pressure 'pneumatic again proves its usefulness. The wider tread and greater contact area, giving better distribution of the load, prevent sinking into soft or loose surfaces. In this connection, an account of a test conducted in desert country may be of interest, as indicating the suitability of these low-pressure tyres for severe and widely varying conditions.
On a six-wheeled vehicle with a gross weight of 4i tons, 7.50-20 tyres were used, and 950 miles were covered at an average speed of 75 k.p.h., the time taken being almost 50 per cent. of that previously required when using highpressure tyres. Throughout, the tem B38 perature was 80 degrees F., and, whilst chains had previously been essential with high-pressure tyres, at no period of the test were they found necessary. The following table indicates the gvary lug surfaces encountered and the pressure employed for each Rolling resistance was found to be greatly reduced and when the tyres. were carefully examined at the end of each journey, no undue heating was observed. Tyres that had covered 2,088 miles over these and similar surfaces proved to be in excellent condition.
This, of course, is hardly an example of a special tyre, being rather an instance of a standard tyre in use under conditions other than those for which it was primarily intended. Turning now to tyres of special design, a popular pattern is the Dunlop Trakgrip. Introduced six years ago, this tyre has achieved success in many export territories where the conditions obtain for which it was built.
On particularly soft or muddy surfaces, on snow, either soft or frozen, the co-efficient of friction between an ordinary tread and the road is too low to permit of the power of the engine being transmitted, and spinning or slipping results. To prevent this circumstance, the Dunlop Trakgrip has a bold and open tread. The load, instead ef being distributed, is concentrated to give a greater intensity of pressure between the tyre and the surface.
On soft ground or .snow, therefore, the tread bites into the surface Much as one gearwheel meshes with another, and spinning is avoided. At the same time, the shape of the tread bars gives a powerful grip sideways, as well as fore and aft.
Yet another advantage is a Consequence of the wide spacing of the tread
bars. Where an ordinary tread, on soft ground or snow, might readily become clogged, with a further loss of traction, the Trakgrip tyre tends to squeeze snow or mud from between the bars, having a self-cleaning action. On frozen or slippery roads, again, the concentration of the load on the sharp edges of the tread bars tends to prevent wheel-slip.
This statement is borne out by the experience of a motor coach company operating on snow-covered roads in mountainous territory. 'Using Trakgrip tyres, the coae.hes were enabled to operate
throughout t h e winter without employing chains. In the spring, when the roads were covered with melting snow and halffrozen slush, lico difficulty was ex
perienced, n o r, again, 'were chains employed. The drivers reported that over every surface they had to encounter, the Trakgrip tyres gave excellent adhesion, and even on frozen roads they were able to brake with no fear of skidding or slipping being caused.
By way of contrast to the conditions just described, in tropical and subtropical territories extensive tracts of fine, loose sand may often be en countered. Ordinary tyres are not effective under such conditions, for they sink deeply into the sand and wheel spin develops. If the sand be very fine, even the standard low-pressure tyre, with its greater contact area, may not be effective. Therefore, to meet
(Above) The special Sand Tyre made by the Dunlop Rubber Co., Ltd.
these special conditions, a special tyre, the Dunlop Extra Low Pressure, has been designed, and we are informed that it is meeting with success in counties overseas. An accompanying illustra
tion -well exemplifies typical desert conditions.
As its name implies, this tyre has extremely low inflation pressure. Its large section and flexible casing provide a big contact area and, the load being distributed in this way, the tyre has no tendency to dig into the ground. This type of tyre is an excellent compromise where conditions are severe and widely varied. For commercial vehicles, and where fine sand is frequently met with, the Dunlop Sand Tyre is useful. Like the Extra Low Pressure, it has a flexible casing, but the tread is particularly wide. A comparison of the contact pressure between tread and road surface -is interesting. For the purpose of the table given below a load of 24/f cwt. is used in each instance :— It may frequently happen that no information is available regarding the condition of the road ahead during a long journey, and it is not impossible that, whilst the greater part of the route may consist of hard read, sections may be encountered where heavy rain as made the road a marsh. Dunlop Skid Straps were developed for just such emergencies.
The Skid Strap is easily fixed to spoked wheels. In the case of disc wheels, the strap is passed through I-in. service holes around the dist.. Being made of fabric and rubber, and provided with a patent fitting device. damage to the cover is avoided.
The fitting device, which is instantaneous in operation tightens the strap on the cover, eliminating snatch. In addition, the Skid Straps can quickly be fitted, even when a vehicle is already bogged. They are made in a number of sizes, and the chain links of the fitting device allow of adjustment for wear or for slight variations in size. A picture published on this page shows the straps fitted to a Dunlop lowpressure giant pneumatic tyre A mention of Dunlop Land Tyres may be of interest as indicating the work being done by the manufacturer in developing tyres for special purposes. For farm carts, for instance, tyres of 8-in. section and 35-in. overall diameter, complete with wheels and axles, are available. On soft ground the superiority of these over the ordinary :3-in, iron tyre is most marked.
Their greatest advantage is that. with these tyres, horses and oxen can draw at least 50 per cent, more than the usual load and with less effort. These tyres: are being employed on sugar plantations, where transport conditions are indeed severe.
For work on the road, horsed-vehicle tyres of smaller section are available. Even the humble wheelbarrow has not been neglected. The Dunlop concern has produced a pneumatic tyre, available complete with wheel and axle, which enables heavier loads to he carried and prevents the marking of lawns.
For farm tractors, too, Dunlop equipment is proving that the whole range of ground cultivation can be covered to an extent not possible with steel-wheel equipment, and the picture on the opposite page is of interest in this connection.
The illustration of a vehicle and trailer operating in an undeveloped area gives an example of the manner in which the Standard pneumatic tyre will work under adverse conditions. The tyres shown have covered 5,000 miles over the roughest of ground, and they are, we understand, still in good condition.