REIATION BETWEEN HORSE AND MASTER 223 



the wheel must travel, while the depression of a soft road surface 

 by the wheel creates a grade up which tlie wheel is continually 

 being drawn, in effect, the degree of the grade being proportion- 

 ate to the depth to which the wheel cuts. Experiments have 

 sliowTi that tlie traction of a given load over a connnon 

 road is from three to five times as great as in the case of a well- 

 paved surface. 



3. Friction of the box on the axle. The degree of friction 

 depends, primarily, upon the weight of the load, but may be 

 modified by the style and condition of the axle and bearings. 



4. Width of the tire. On the ordinary road the wide tire 

 reduces the necessary draft by distributing the weight borne by 

 the wheel over a greater area of road surface, thereby reducing 

 the tendency of the wheel to cut into the roadway and giving 

 tlie effect of a hard road, with its advantages. In an unusually 

 soft or muddy road the wide tire may be a disadvantage. The 

 saving in draft of as much as 120 per cent has been effected by 

 the use of a six-inch tire instead of a one and one-half inch tire 

 on ordinary going. 



5. Size of wheel. It has already been shown that the de- 

 pression of the road by the wheel results in that wheel being con- 

 tinually pulled up a grade, the steepness of which is in inverse 

 proportion to the diameter of the wdieel. Furthermore, the lia- 

 bility of tlie wheel to cut is in direct proportion to its diameter. 

 The larger the wheel the greater the base of support for the load 

 and the less the tendency to depress. Therefore, the smaller 

 the wheel the greater the draft. Also, the gi^eater the diameter 

 of the whe^l the more easily it is lifted over the obstacles which 

 tJie rough surfaces of some roads present (Fig. 125). 



6. The distribution of the load on the wagon. When only 

 part of a load is carried it is customary witli teamsters to place 

 it well forward on the wagon for obvious reasons, but when a 

 full load is put on it should be so distributed as to balance and 

 divide the weight equally on all four comers, if the wheels are of 

 equal size, or if the hind wheels are somewhat larger, as is 

 usually the case, more weight should be allowed on the rear 

 axle. Lightening tlie load for^vard has the advantage of per- 

 mitting a certain amount of play in the front axle over rough 



