86 



THE INDIA RUBBER WORLD 



NOV-EMBER 1, 1920 



engine speeds, rear-axle gear reduction and air brakes ; (2) 

 traction, including engine torque and transmission gear reductions ; 

 (3) shock effects, including stresses introduced and the necessary 

 factor of safety of sprung and unsprung parts; (4) emergency 

 equipment, including tire pumps and spare tires. 



2 3 4 



II 



13 



M- 



5 6 7 8 3 \0 



Load in Thousand Pounds 

 The Journal of the S. A. E. 



Fic. 1. For a Given Load the Pneumatic Tire Deflects Four 

 Times as Much as a Solid Tire 



Table I shows road speeds that we consider satisfactory, to- 

 gether with the usual rear-tire specifications for various sizes of 

 trucks. The engine speeds are figured on the basis of 1,200 feet 

 per minute piston speed, which can be considered a good aver- 

 age. Higher speeds set up considerable vibration and add dis- 

 comfort to driving. 



SHOCK EFFECT 



Fixing of allowable stress requires an investigation of the 

 cushioning effect of pneumatic as compared with solid tires. 



Fig. 1 shows the rate of deflection of pneumatic tires anc 



Fig. 2. First Application of Four Small Tires to Rear Wheels 



corresponding solid tires, together with a curve showing how the 

 solid tire depreciates in resilience with age and wear. It will 

 be seen here that for a given load the pneumatic tire deflects 

 four times as much as a solid tire. 



COST 



I believe it is possible to build a 5-ton motor-truck chassis 

 equipped with pneumatic tires for only $200 to $300 more than 



a corresponding solid-tire truck, and that the net weight reduc- 

 tion may be easily 1,000 pounds without resorting to aluminum 

 where it is not yet considered commercially practical. 



SIX-WHEEL TRUCK 



On account of the large size and weight of the 48 by 12-inch 

 pneumatic tire, we were brought to consider the application of 

 four smaller tires to the rear of the truck, instead of two of the 

 excessively large ones. Our first attempt at an arrangement foj 

 applying four small tires to the rear without using dual tires, 

 which is considered out of the question, is shown in Fig. 2. Il 



Fig. 3. Further Developments of the Tandem Axle 



Construction ^t 



consisted of a more-or-less standard rear axle with a walking 

 beam adapted to each end and the wheel mounted upon trunnion; 

 from this walking beam, the springs being mounted upon the 

 axle and attached to the frame on the inside. Chain drive was 

 made use of in this case, which is about the only feasible drive 

 with this arrangement. This construction ran successfully for 

 about 10,000 miles before serious failure occurred. We were, 

 however, inconvenienced with the chains jumping off and were 

 not able to get a brake mechanism that would work. The main 

 point against this design was its enormous weight ; however, it 

 served to show us that satisfactory tire mileage could be secured 

 from such an arrangement and that there was a good possibility 

 of adapting four small tires to the rear wheels. To further de- 

 velop this point, we built up the tandem axle construction as 



FiG. 4. Another View of the Tandem Axle Construction 



shown in Figs. 3 and 4. This construction appears to have good 

 possibilities and has at present operated some 3,300 miles, 1,000 

 to 1,200 miles over rough and uneven country roads, so rough 

 in fact that it was difficult to keep the front springs tight. Fig. 5 



