LOW-PRESSURE TYRE DEVELOPMENTS.
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The Advantages and Disadvantages of the Latest Large-section Pneumatics for Passenger-carrying Vehicles.
WHETHER very rapid developments and changes are considered to be an advantage or a disadvantage, everyone must admit that on the other side of the Atlantic engineers in charge of commercial-vehicle development are more ready to scrap existing practice in favour of new ideas than in the case in this country. Consequently, we find that whilst over here the ordinary pneumatic is only gradually displacing the solid tyre for passengercarrying vehicles, in America the pneumatic is itSelf being displaced by the low-pressure or balloon tyre of larger section. .
However, the comfort of the passenger is being more and more closely studied, and those in charge of fleets of vehicles realize also that anything that can be done to absorb road shocks at the earliest point, viz., the rim of the wheel, serves to reduce the damage done to chassis parts, particularly the axles and springs. It therefore seems opportune to enter into an impartial discussion upon the merits and demerits of the low-pressure tyre, basing this on the assumption that, in any case, a pneumatic oi! some kind is desirable on motor coaches and similar vehicles up to the 20-seater type. For larger coaches and motorbuses, it is very doubtful whether the pneumatic will come into general use during the next few years, and still less is it probable that the low-pressure tyre will be considered for such vehicles.
The basic idea of the low-pressure tyre in comparison with one of normal section is to double the volume of air present in the inner tube and, at the same time, halve the pressure employed. As this has been done more generally upon private cars than in any other type of vehicle, it will perhaps be excusable if we quote some of the figures used in this connection. Thus, on a large touring car, with each rear wheel carrying a load of 1,700 lb., the normal tyre would have a section of about 5 ins, and would be inflated to a pressure of 80 lb. per sq. in. For the same load, a low-pressure tyre oF 7.3 ins. section could be used with an inflation pressure of only 35 lb. per sq. in.
The lower pressure makes the tyre softer and, therefore, better able to absorb minor road irregularities. Furthermore, it is found that the walls of the low-pressure tyre can be made much thinner and still be able to withstand a great amount of flexing, thanks to modern methods of cord construction, so that the flexibility is still further increased. Obviously, if such points as wing clearance and effective gear ratios are to be retained at their normal figures, the low-pressure tyre must be mounted upon a rim of smaller diameter to allow for the larger section.
From personal experience we can say that the lowpressure tyre shows to greatest advantage when traversing roads the surface of which consists of a series of minor irregularities ; for example, granite setts in really bad condition can be traversed at.much higher speeds when riding on low-pressure air than ts possible with normal pneumatics, a point which should appeal to many bus and char-l-bancs nroprietorn in the Midlands and the North, where roads of this type abound. Then, again, the damage done to the roads themselves is very much reduced, because the hammering effect of the unsprung weight is largely absorbed by the flexible tyres.
The reader may now say, "this is all very well, hut what about the points directly affecting s. such as the changes in running costs involved by fitting low-pressure tyres ? " Now, apart from the decreased damage done to the chassis, due to better insulation from road shocks, tests have shown that there is a definite gain in average speed and, usually, a decrease in Petrol consumption when low-pressure tyres are fitted. . • The increase in average speed is due to the fact that the driver is not forced to slow down to such an extent when traversing bad• stretches of road, and this increase enables the vehicle to cover a greater mileage per day. The question of petrol consumption is interesting because, whilst the large tyres cause a slight increase in tractive resistance, they cut out a great deal of braking and acceleration, owing to the fact that the vehicle can be driven at a steady speed over good and bad stretches of road. In consequence, unless the route covered embraces exceptionally good roads, a change to low-Pressure tyres will almost improve petrol consumpt h e found slightly to tion figures.
The principal advantages o f lowpre ssure tyres have now been described in detail, and it remains to consider some of the
criticisms advanced against them.
First of all, we have the question of punctures, it being held by some authorities that the thinner section must necessarily make the tyres more vulnerable. it is admitted that' a sharp object can penetrate a thin section more readily than a thicker one ; on the other hand the more flexible tyre will absorb objects such as sharp flints, which would be driven right into a tyre running at a higher pressure. We next come to the question of whether a burst will be more serious with a low-pressure tyre, awing to the bigger drop of the wheel which will take place. This point has received considerable attention in America where one tyre manufacturer has even gone to the length of removing a front mudguard, placing a man on the running hoard and allowing him to fire a shot-gun at the front tyre while the car was travelling at a high speed. We are told that, even with this severe test, the tyre subsided gradually and there was no loss of steering control. The explanation lies in the fact that a large volume of air at low pressure will not rush out of the tyre nearly so rapidly as a small volume at high pressure. Consequently, the bigger drop of the wheel involved when a burst °emirs in a low-pressure tyre is compensated by the fax that it subsides slowly. As regards mileage, there does not appear to be much to choose between the two types of tyre under normal conditions, although extraordinary mileages have been obtained with low-pressure tyres in some instances. The large-section type is, however, more sensitive to wheel alignment, which must be carefully watched.
Other important questions are whether the lowpressure tyre will tend to creep round the rim, and whether, in the case of a bad skid, it could be pulled completely 'off the wheel. It seems that no trouble need be expected in these directions, always providing that the tyre is adequately held to the rim. Probably the straight-sided rim, so largely used in America, or the special Dunlop fixing, with well rim and wired-edge tyre, provide adequate safety from. this point of view. They also make it very easy to fit the tyre or remove it from the rim. There now remains to be considered the question of rolling on corners and the pitching motion sometimes set up when bad roads are being traversed. It is rather curious to find that both these undesirable features occur when certain vehicles are fitted with low-pressure tyres, whilst others seem quite immune from the trouble. Probably, this depends upon the characteristics of the spring suspension system, but nevertheless the problem of rolling and pitching with large tyres is certainly one which must be faced. In many cases it is so acute that it has been found necessary to fit shock absorbers or rebound dampers. Although the resulting combination of extremely flexible low-pressare tyres and steadying shock absorbers is in many respects ideal from the, point of view of riding comfort, the additional cost involved is certainly rather a grave disadvantage. However, it is quite possible that further research will enable the cause of the trouble to be ascertained, so that by modifying the spring suspension system the difficulty may be overcome without shock absorbers. Owing to its flexibility the area of contact of the low-pressure tyre with the road is much greater than that of the high-pressure tyre. Consequently, on many types of road surfaces it is found that the tendency of skidding is reduced and the braking capacity of the vehicle is increased. However, it must be admitted that on 'a good surface, rendered slippery by a thin coating of grease, the low-pressure tyres have a tendency to slide, owing to the fact that they cannot bite through the grease in the same way as a high-pressure tyre.
Bioactly speaking, there is very little to choose betwen the two types of tyre in respect of skidding and 'waking. The larger area of contact naturally makes steering slightly more difficult at low road speeds, such as when manceuvring the vehicle in a ct owded street.