June 1, 1921 



THE INDIA RUBBER WORLD 



655 



Truck Tires and Road Damage 



THE relative effect of the variuiis kinds ui automobile truck 

 tires used on highways under various conditions of stress is 

 comprehensively shown in a recent report on motor truck 

 impact tests.' The research work has covered a period of two 

 years and has concerned itself primarily with better road design 

 to meet the hea\'y traffic which the rapid extension of truckporta- 

 tion has occasioned since the war. Four factors have been dealt 

 with by the Federal engineers, namely : impact, pressure or weight 

 of the passing load, horizontal shear and tractive forces, and 

 subgrade and soil conditions, the report on impact being the lirst 

 available. 



In dealing with impact, or the force with which a truck wheel 

 strikes the road surface, it is explained that the injury to the 

 road, and incidentally to the tire depends much upon the type, 

 construction and condition of the tire. Impact will be less when 

 the tire is new, full, and of good quality than when worn, flattened, 

 or almost lacking in cushion. The true, so-called "cushion" 

 tires have a decided advantage in reducing impact, and to such 

 as have less than 40 to 50 per cent of the deflection of a pneu- 

 matic tire the writer would deny the name "cushion." Tire de- 

 flection is far more important in considering impact than width 

 of tire or load per inch of tire width. Contrary to popular 

 belief, a very wide but thin solid tire will give much higher im- 

 pact forces than a narrow thick one. 



Tire dimensions and deformations are shown in Fig. 1. For 

 each tire a curve is drawn which gives the deflection or de- 

 formation under different loads. 



Fig. 2 shows a comparison of the effect of tire equipment upon 

 the value of impact. A 2-ton truck carrying a 2-ton load was 

 used. Four different tires were each tested using in each case 

 the speed as the independent variable. The comparison is shown 

 with both the drop and the obstruction test. In the obstruction 

 test the impact value (the ordinate) shows a considerable change 

 with only a slight change in the tire deflection. The impact 

 value, with all conditions the same except the tire, is the greatest 

 for the solid rubber tires and the smallest for the pneumatic tire, 

 the cushion tire giving an intermediate value. At low speed 



three limes, and the solids 4.3 to S.l times. In this particular 

 case, the cushion tire gives an impact value of 63 per cent of 



SOLID 



No 3 • W- !(>■ 

 No IclC'tO - ■ New 

 N0I6-*0«10' • -WTl-H-lt 

 DUAL. SOLID 



Deformation of Rubber Tire 

 Static Load 



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NoZi-4o'ie"-pNtuMM.c co-o r — »,» — v~- 

 T.nt ..i%fcAT.o« i;o" / \ 



•*o &■ J6'.7*-l . . ., , ., 



NOlS-40110* • OlAMOWOTltCAD-VVT'» 



SOLID- BADLY WORN AND WCA1HERE0 



, 4-M"'S'- • • O.0I.V ■wieJ'-M'.'i'ft-i' 



8j-H'2'-6£r DUSL SOLiw 



Ne IJ-«0''6'-Hc«- W-l?J-H-I*-6-4l 



Nal4 W'ifc'- ■ Dmuo^o T"tyiO-W'ltl'-H'i'-B'4' 



No;4-40'.6*-5i..6«Tir Wobn » Wt«THeKtO-M-lJ" B-3i 



No 6 • M • 4- 



NotO- W 

 No 3- 36 



No re-«'«8"-v.-o n-i 



(*o;t-40Vio-W'icf-M- 1 J" 



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V 



DUAL CUSHION 

 tJo8-36'.b' Ntw-W.|0(-H-3-B-4 



Public Roads 



Fig. 1. St.\tic Load vs. Deformation of Rubber Tires 



the solid tire average, and the pneumatic only 36 per cent. The 

 impact value for the pneumatic tire increases only slightly with 

 the increase of speed. 



2 4 e B 10 12 J4 i6 



Public Roads 



Fig. 2. Truck, 2-Ton A— Load, 2 Tons Fig. 3. Truck, 3^-Ton P— Load, 4}^ Tons 



Fig. 4. Solid and Pneumatic on Two 

 Similar Trucks 



the difference is not so great. At a speed of 17}4 miles an hour 



the pneumatic tire gives an impact value of only 1.75 times the 



rear wheel pressure on the road surface, the cushion tire over 



' "The Motor Truck Impact Tests of the Bureau of Public Koads," 

 by Earl V,. Smith.^Hrreau of Public Roads, in Public Roads, Vol. 3. No. 35, 

 United States Department of Agriculture. 



Fig. 3 shows a comparison of the effect of tire equipment. 

 The truck was of 3j4-ton capacity, loaded with 4;/^ tons, the un- 

 sprung weight being equal to 1,300 pounds. The total load at 

 one rear wheel was 7,000 pounds. Here, it should be noticed 

 tliat with pneumatic tire equipment the impact force was only 



