490 



EXPERIMENT STATION EECOED. 



I Vol. 30 



Ma.iimvin shears and bonding moments for lypieal Irufks. 

 [Values for two wheels on one side.] 



" Bridges on improved roads should as a rule be designeil for 15-ton typical 

 trucks, but other capacities may be necessary for special conditions. ... In 

 general, impact allowance shi>uld not be less than 30 per cent of the .static 

 stress, but it should be varied to fit conditions." 



Tractive resistances to a motor delivery wagon on different roads and at 

 different speeds, A. E. Kkinnki.ly and O. R. Schurig (Proc. Amcr. Inst. Fdeei. 

 Eiujins., S5 {1916), No. 6, pp. 1011-1030, pi. 1. figs. 17; Mass. Inst. Tech., Elect. 

 Engin. Dcpt., Research Div. Bui. 10 (1916), pp. 1011-1039, pi. 1, figs. 11; ahs. in 

 West. Engin., 7 (1916), No. 11, pp. 4S4, 435, fig. i).— The results of an investiga- 

 tion conducte<l at the Massachusetts Institute of Technology on the tractive 

 resistance of asphalt, wood block, brick block, granite block, water-bound mac- 

 adam, cemented joint granite block, tar macadam, cinder, and gravel pavements 

 in conditions varying from extremely poor to good are reported. 



The vehicle used was a worm-drive single-reduction electric truck having 

 a capacity of l.CMX) lbs. The overall length of the frame was IBSJ in., the width 

 35 in., the wheel bjise 107^ in., and the wheel gage 58 in. .Solid-rubber demount- 

 able tires measuring 2i by 30 in. were used on all four wheels. The truck was 

 equipped with a direct-current .series-motor. The length of runs in the road 

 tests ranged between 400 and 2,600 ft. 



It was found that " the overall efficiency of the test-truck mechanism, be- 

 tween battery terminals and rear-wheel treads, reached a maximum value of 

 about 78 per cent, under the most favorable conditions. , Tlie mechanical cdi- 

 ciency of transmission from motor shaft to rear-wheel treads, for the truck 

 tested, shaft-driven through a .single-reduction worm gear, was found to be as 

 high as 90 per cent. 



" Tractive resistances are most conveniently expresse;! as an equivalent grade: 

 that is. a level road of definite tractive resistance may be regarded as a road of 

 zero tractive resistance, but rising uniformly x units in 100 units of road length, 

 or having an equivalent grade of x per cent. Under the conditions of the.se 

 tests, the tractive resistance on level roads, in the absence of wind, is coni- 

 lK>sed of (1) displacement resistance. (2) impact resistance, and (;{) air re- 

 sistance. 



"The displacement resistance varied from 0.85 per cent equivalent grade for 

 a hard, smooth asphalt or bituminous concrete to 1.6 per cent for a soft tar- 



