342 



SCIENCE 



[N. S. Vol. XLV. No. 1162 



internally frictionless, in overcoming the road, 

 tire and still-air resistances on a level surface. 

 It may be expressed either in pounds weight 

 per short ton of total moving mass, or in kilo- 

 grams per metric ton, or in per cent, equiv- 

 alent grade. Thus a 1 per cent, equivalent 

 grade tractive resistance means that a car 

 without axle friction or other internal mechan- 

 ism losses would require to be pulled on a 

 level road at a constant stated speed a force 

 of 20 pounds per short ton of 2,000 pounds, in 

 order to overcome the resistance of the tires, 

 road-bed and still-air displacement. This 

 force would obviously serve to propel the 

 same vehicle up a 1 per cent, grade in the 

 absence of tire, road-bed, still-air and internal 

 frictions. At a given speed, therefore, this 

 tractive resistance depends upon the wheel, 

 the tire, and the road, and also on the air-dis- 

 placement resistance of the truck. 



The truck tested was a 1,000-lb. (450 kg.) 

 electrically-propelled delivery wagon equipped 

 with single solid rubber tires, one on each of 

 the four wheels — the tire rating being 36 in. 

 by 2J in. (915 mm. by 63.5 mm.). The tests 

 were made by running the car at, as nearly as 

 possible, constant measured speed, in alter- 

 nate directions, over selected lengths of stand- 

 ard roads in and near Boston. From the ob- 

 served storage-battery outputs during these 

 runs, the corresponding tractive resistances 

 were evaluated after correcting for all losses 

 internal to the truck mechanism, wind losses, 

 grade and incidental accelerations. The in- 

 ternal losses of the truck mechanism from 

 battery terminals to wheel spokes were deter- 

 mined from laboratory tests with the car 

 raised from the ground on jacks, and by driv- 

 ing tested dynamos from the rear wheels. 



The following is a summary of the results 

 obtained as applying to urban roads with this 

 truck between the speed limits of from 13 to 

 25 km. per hour (8 to 15.5 miles per hour). 



1. The over-all efficiency of the test-truck 

 mechanism between battery terminals and 

 rear-wheel treads reached a maximum value 

 of about 78 per cent., under the most favor- 

 able conditions. 



2. The mechanical efficiency of transmis- 



sion from motor shaft to rear-wheel treads, for 

 the truck tested, shaft-driven through a 

 siiigle-reduction worm gear, was found as 

 high as 90 per cent. 



3. Under the conditions of these tests, the 

 tractive resistance on level roads, in the ab- 

 sence of wind, is composed of (a) displace- 

 ment resistance, (&) impact resistance and 

 (c) air resistance. 



By " displacement resistance " is meant 

 that portion of the tractive resistance which 

 depends on the lack of resilience of a smooth 

 road surface and of the wheel-tire material; 

 i. e., on the energy losses due to inelastic dis- 

 placement of tire and road-surface materials. 



By " impact resistance " is meant that por- 

 tion of the tractive resistance which depends 

 on the lack of smoothness of the road sm-face, 

 and which is due to the impacts given to the 

 moving vehicle by the irregularities of the 

 road. 



By " air resistance " is meant that portion 

 of the tractive resistance due to air pressure 

 on the moving vehicle necessary to displace 

 the air in the absence of wind. 



4. 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 very soft tar-macadam road, 

 and was practically constant, for all speeds 

 considered, on any given road. 



5. The impact resistance increases with the 

 velocity, with the total weight of vehicle, and 

 with increasing road-surface roughness. In 

 these tests, the impact resistance of good 

 asphalt or bithulithic or other smooth pave- 

 ment, was practically negligible, and reached 

 its highest values (about 1.5 per cent, equiv- 

 alent grade at a speed of 20 km. per hr. (12.4 

 miles per hr.) on granite-block roads with sand 

 filled joints, and on badly worn macadam 

 pavements. The rate of increase of impact 

 resistance with speed was most marked on the 

 roughest roads. 



6. At ,the vehicle speed of 20 km. (12.4 

 miles) per hour, the air resistance for the 

 vehicle tested, assumed to be dependent only 

 on the speed, was roughly 0.11 per cent, equiv- 

 alent grade; i. e., from 4 per cent, of the high- 



