84 



THE INDIA RUBBER WORLD 



November 1, 1920 



30,000 miles on the original air. Repair molds and retreading 

 equipment are nou- in use in many parts of the country, and 

 are being placed in other localities as rapidly as possible ; so, it 

 will be no more difficult to have a pneumatic truck tire repaired 

 than any other part of the truck. 



PRACTICABIUTY OF PNEUMATIC TIRES 



The practicability of pneumatic truck tires has been questioned 

 probably more than anything else. The first thing to be dis- 

 cussed under this subject is that of delays due to changing tires. 

 In the case of detachable rims, where it is necessary to remove 

 the tire from the rim, replace it and then inflate it, it does not 

 require more than 30 minutes to perform the entire operation. 

 In the case of a demountable rim, a change can easily be made 

 in 15 minutes. The average truck driver is not required to make 

 a tire change more than once in three months. 



Most garages and service stations carry sufficient air pressure 

 to inflate tires up to the 42 by 9-inch size, and many can take 

 care of the larger sizes. By the time the largest tires are in gen- 

 eral use, there will be sufficient air pressure to keep the tires 

 properly inflated. Trucks equipped with detachable rims, or op- 

 erating in long-distance or inter-city service, are usually equipped 

 with small air compressors. These trucks experience no difficulty 

 in securing sufficient pressure. One objection, which is not men- 

 tioned so much now as when pneumatic truck tires first made 



their appearance, is the danger of the high inflation pressures. 

 Pneumatic truck tires are made to withstand three to four times 

 the pressure carried in them, so that this objection must be 

 passed to the rims. These are made to withstand many times the 

 pressure carried in the tires. If the rims are properly assembled, 

 there is small chance of accident. 



Regarding the rise in pressure caused by the heating up of 

 the tire, we have operated these large tires under the most 

 severe conditions possible and in no case have we found an in- 

 crease in pressure of more than 35 pounds per square inch. If 

 the tires are made to withstand three to four times the pressure 

 at which they are operated, it is hardly possible that this addi- 

 tional 3S-pound pressure will cause them to blow out. 



The large outside diameters of the tires are often objected to 

 because they affect the truck ability and because they raise the 

 center of gravity of the truck. In changing over a solid-tired 

 truck to pneumatic tires, there is the possibility of reducing the 

 ability of the truck. Our experience has shown that unless the 

 truck is operated over a very hilly route, its ability has not been 

 noticeably affected. Looking into the future, this question of 

 truck ability and gear ratios will be taken care of by changes 

 in design ; so, the question of change-overs is only temporary. 

 Raising the center of gravity of the truck is not as serious as 

 it might seem. 



Data on Pneumatic Tires and Rims Used on Trucks' 



By Burgess Darrow" 



THE OBJECT of this paper is to familiarize truck engineers, and 

 others interested in truck design, with facts and opinions 

 which will assist in providing correct pneumatic tire and 

 rim equipment for trucks. The sizes which have been worked 

 out during the past six years, and which are now standard, are 

 as given in Table I. 



T.^BLE I. Sizes of Pneumatic Tires 



* Original equipment on new trucks. 



* Not for original equipment: only for consumer's convenience. 



* Not yet standard witU S. A. E. practice. 



Table I also gives the rim sizes, normal tire sizes and the tires 

 which can be fitted as oversize. It shows that there is no oversize 

 possibility when 9, 10 and 12-inch sizes go out on new trucks, 

 because the oversizing plan falls down above the 9-inch size, on 

 account of the size and stiffness we are forced to build into the 

 beads as designed at present. 



LOADS AND INFLATIONS 



To the best of our knowledge tires give best average satisfac- 

 tion in the way of plenty of cushioning and not too much flexing. 

 Flexing breaks down a tire, when run under conditions which 

 produce a deflection in the tire of from 12 to IS per cent of the 

 section diameter, or the height above the rim. The deflection 

 can be controlled by regulating the load or the pressure, or both. 

 Table I also gives the standard maximum loads and the infla- 

 tion pressures. These inflation pressures are practical to main- 

 tain, the tires are built accordingly and we get satisfactory, 

 practical results in first cost and mileage delivered if they are 

 used. 



In reference to underinflation and overload, both evils result 

 in an excessive deflection of the tire. This means that an ex- 

 cessive shearing action is put on the rubber between the plies of 

 the tire, and also on the cushion built into the tire between the 

 tread and the plies, which in turn results in a separation of the 

 parts and starts the tire on the road to failure. 



Table II. Weights of Tire Equipment 



lire Size Inches 



' ^ ., Weight, Remarks 



Front Rear Pounds 



6 6 72 Weights of tires alone, without wheels 



7 7 87 cr rims, but including tubes and flap. 



8 8 119 



9 9 174 

 10 10 246 

 12 12 398 



TUBES, FLAPS AND VALVES 



Tubes for pneumatic truck tires must be designed and com- 

 pounded to retain as much of their original strength and shape 

 as possible, after being subjected in service to more or less heat 

 and to continued flexing. The tube has been one of the most 

 difficult problems in connection with large tires, but has been 

 solved partly in a mechanical way by building the tubes thick, 

 shaped like the tire, and so they are stretched very little in the 

 tire. The tube problem has been solved to a still greater extent 

 by rubber compounding. Tubes are on a par with the casings 

 hi development and render satisfactory service even in the larg- 

 est sizes. 



Flaps assume considerable importance in tires inflated to the 

 pressures we recommend for truck tires. It is important that the 

 flap should fit well, so there will be no adjustment of the flap 

 when the tire is inflated, causing a localized stretch in the tube 

 at the edge of the flap. 



The valve question had to be approached first from the stand- 

 point of holding air at pressures from 90 to 140 pounds per 

 square inch and, second, from the standpoint of ease of tire 

 change. The valve insides on all 6-inch and larger tubes is of 



• .\bstracted from Cleveland-Detroit Sections paper. The Journal of the 

 Society of Automotive Engineers, October, 1920. 



= Development department. The Goodyear Tire & Rubber Co., Akron, 

 Ohio. 



