60 POWER AND THE PLOW 



never less than 6 per cent., and may reach 15 per cent, in ordi- 

 nary footing; hence some have adopted exteme measures in 

 order to produce an efficient traction wheel. For general 

 purpose tractors, however, the problem narrows itself con- 

 siderably. There are the alternatives of high or low wheels, 

 and wide or narrow wheels, which, with the type of grouter 

 or cleat, determine the gripping power of the wheel upon 

 the soil. 



The increasing weight of tractors which has accompanied 

 the great increase in power has necessitated careful considera- 

 tion of the weight per unit of bearing surface upon the ground. 

 In standard tractors from two thirds to three fourths of the 

 total weight is thrown upon the drivers when the engine is 

 stationary, and the hitch may be so arranged as to lift con- 

 siderable weight off the front wheels and place it upon the 

 rear ones when plowing. At rest the weight per square inch 

 of bearing surface is usually from one fourth to one third 

 below the pressure exerted by horses at rest. In plowing, 

 however, the bearing surface is mostly forward of the 

 centre line of the axle, and it is probable that this weight 

 equals, or even exceeds, the figure for a horse's hoof, which 

 is about twenty to twenty- three pounds per square inch. 

 In order to increase this bearing surface and, incidentally, 

 the friction of the wheel upon the ground, wheels have 

 been increased in diameter in order to present a longer arc 

 in contact with the ground, and in width further to 

 increase the area. 



In extremely high wheels there is the difficulty of securing 

 sufficient rigidity and strength without unduly increasing 

 the weight. The low, wide wheel produces greater 

 strains upon the axle, and is at a further disadvantage 

 in comparison with the high wheel, in that as the tire 

 sinks slightly into the earth the tractor must constantly 

 propel itself and its load up a slight grade. The percentage 

 of the radius which sinks beneath the surface represents 



