TRACTIVE RESISTANCE. The resistance to traction of a vehicle on aroad consists of three independent elements: axle friction, rolling resistance, and grade resistance.
The tractive power required to overcome the above axle friction for American carriages of the usual proportions is about 3 to 3i lb. per ton of the weight on the axle; and for truck wagons, which have medium-sized wheels and axles. is about 3i to 4/4 lb. per ton.
Table 3 shows the results obtained by Mr. T. I. Mairs at the Missouri Agricultural Experiment Station,* with three different sized wheels. The 50-inch represents 44-inch front wheels and 56-inch hind wheels; the 38-inch represents 36-inch front and 40-inch hind wheels; and the 26-inch represents 24-inch front and 28-inch hind wheels. The tires were 6 inches wide. The load was practically If tons in each case.
Morin concluded that the resistance varies inversely as the first power of the diameter of the wheel; Dupuit. that it varies as the square root; and Clarke claims that it varies as the cube According to some experiments made in England in 1874,t the tractive resistance varied more rapidly than the first power of the diameter of the wheels. The mean results in Table 3 vary nearly inversely as the square root of the mean diameter—certainly more nearly than as either the first power or the cube root. For obvious reasons, the experiments can not be very exact; and apparently the tractive resistance varies differently for different surfaces. The exact determination of the law of variation is of no great importance.
Width of Tire. If the wheel cuts into the road surface, the tractive resistance is thereby increased; but with surfaces for which there is little or no indentation, the traction is practically inde pendent of the width of tire.
Table 4, page 24, shows the results of an elaborate series of experiments by the Missouri Agricultural Experiment Station.* The load in each case was 1 ton. These results show that on poor macadam, poor gravel, and compressible earth roads, and also on agricultural land, the broad tire gives less resistance except as follows: (1) when the earth road is sloppy, muddy, or sticky on top and firm underneath; (2) when the surface is covered with a very loose deep dust and is hard underneath; (3) when the mud is very deep and so sticky that it adheres to the wheel; or (4) when the road has been rutted with the narrow tire. The last conclusion was established by a large number of experiments not included in Table 4, page 24.