434: Farm Mechanics. 



DRAFT OF WAGONS ON THE LEVEL. 



There are many factors which modify the draft of a 

 wagon over a level road and some of the most important of 

 these are : 



1. Smoothness of the road-bed. 



2. Rigidity of the road-bed. 



3. Width of the tire. 



4. Diameter of the wheel. 



5. Distribution of the load on the carriage. 



6. Direction of the line of draft. 



7. Rigidity of the carriage. 



534. The Smoothness of the Road-bed. When the road- 

 bed is not smooth and has numerous ruts, stones or other 

 obstructions upon its surface, the draft of the load is in- 

 creased and the wear on the vehicle and on the road-bed 

 is also greater so that much effort and care should be ex- 

 ercised to have the road smooth. The increase in the 

 mean draft of the load is not so great, however, as the other 

 difficulties which result for the reason that when the wheel 

 enters a rut or passes down off from an obstruction there 

 is a push forward which tends always to give back a portion 

 of the energy expended in raising the load upon the ob- 

 struction or out of the rut. 



535. Rigidity of the Road-bed. A yielding road-bed is 

 perhaps the most serious defect of roads, and the one which 

 increases the draft more than any other. If a wheel is 

 steadily cutting into its road-bed it is continually tending 

 to rise over an obstruction or out of a rut, or it is doing 

 what is in effect all the time passing up a grade, as repre- 

 sented in Fig. 210, the hill being steeper in proportion as 

 the wheels are smaller. 



In Fig. 209 is represented a method of measuring the in- 

 crease in draft due to the wheel rising over an obstruction 

 whose hight is a stated per cent, of the radius of the wheel. 



