108 POWER AND THE PLOW 



plowing and 4*9.9 per cent, in hauling, when based on a 

 comparison of horsepower developed; and 53.8 per cent, 

 and 53.3 per cent., respectively, when based on a comparison 

 of fuel consumption. With high wheels, tractors are able to 

 negotiate extreme ground conditions with less loss in efficiency 

 than the low-wheel types, especially if accompanied by light 

 weight. 



In the motor contest of 1910, with an average of 70 per cent, 

 of the total weight resting upon the drivers, seven gas tractors 

 exerted in plowing an average drawbar pull of 25 per cent, 

 of their total weight or 35 per cent, of the weight on the drivers. 

 In 1909 the gas tractors at Winnipeg averaged about 17 per 

 cent, of their total weight in drawbar pull in two-hour hauling 

 tests over a very uneven course, and about 24 per cent, in 

 plowing firm level sod ground. As a mean of the two traction 

 tests they developed one drawbar-horsepower for 922 pounds 

 of total weight, or much more than could be reasonably expected 

 of horses. 



In private tests made on a good stone road, one large gas 

 tractor, drawn at its own rated speed of travel, required one 

 sixth its rated brake-horsepower to move it. At the rate 

 of two and one half miles an hour on an earth road, approx- 

 imately 25 per cent, of its maximum brake-horsepower was 

 consumed in moving. This shows clearly the loss due to mov- 

 ing the weight of the tractor with merely its traction gearing 

 in mesh and motor plant idle. Data on the loss in tractive 

 efficiency due to grades, will be found elsewhere in this 

 book. 



The gas tractor has small overload capacity, if it is run ordi- 

 narily at its most economical load. In the last motor contest, 

 eight tractors out of eleven were able to show less than 7 per 

 cent, increase in power between the economy and maximum 

 brake test. The engines were not necessarily run at the most 

 economical or the maximum points in either case, and in fact, 

 two showed a decrease in fuel consumption on developing a 



