PERFORMANCE OF STEAM TRACTORS 65 



A standard basis for tractive rating, however, would be of 

 great assistance to purchasers, since the differences in design 

 will enable one tractor to deliver a greater percentage of the 

 brake horsepower at the drawbar than another. This is a 

 matter which has been the subject of considerable discussion by 

 agricultural engineers, but as yet no solution has been reached. 



Unfortunately, no scientific tests of tractive efficiency have 

 been conducted in this country, at least, not for the benefit 

 of the public. The pulling power of the tractor depends on a 

 great many different factors, such as the total weight, the 

 weight in relation to the area of the supporting wheels, the type 

 of transmission, and the distribution of weight upon the front 

 and rear wheels. The tractive efficiency has usually been 

 estimated by comparing the brake horsepower developed in 

 one test with the tractive horsepower developed in another. 

 However, this gives no check on the comparative amounts of 

 power developed at the crankshaft, and is apt to be very 

 misleading. After holding down the power of any tractor on 

 a brake test it can be made to show a high proportion of this 

 power at the drawbar simply by increasing the brake horse- 

 power developed during the tractive test. This automatically 

 increases the fuel consumption in a given length of time, hence 

 the increased drawbar horsepower will be at the expense of 

 fuel consumption. A more accurate but still imperfect 

 method has been to base the tractive efficiency on the relative 

 fuel consumption in the brake and tractive tests. 



In July, 1909, during tests at Winnipeg, four steam engines 

 had a total weight of 585 pounds per inch in width of drivers, 

 and 456 pounds for each brake horsepower developed on 

 economy tests. Over a firm plowing course they were able 

 to show 56.6 per cent, as much power at the drawbar as at the 

 belt in the economy tests. Over a hauling course, which 

 presented nearly every possible condition of ground from pave- 

 ment to sand patch, the limitation of the load by the bad spots 

 reduced the drawbar horsepower to 29.3 per cent, of the economy 



