154 



ROLLING FRICTION 



CH. X 



For example : if P = 10,000 pounds, r= 2.5 feet, 

 and A = 0.05, which is the constant derived from a 

 number of experiments made on a road of the same 

 character as that for which this resistance is now to 

 be computed, R will equal 0.05^^, or 200 pounds, 

 g-V of the load. If P = 10,000 and the radius of 

 the wheel is increased to three feet, the rolling fric- 

 tion will be reduced to 166.6 pounds, eV of the load. 

 If the quality of the road changes for the worse, 

 the value of A will increase. On a road covered 

 with loose gravel two inches deep, Morin, p .144, 

 found the value of A to be 0.165, and with this co- 

 efficient the resistance will become o.iG^X 1 -^^ 

 = 660 pounds, or T V of the load. 



As to the influence of the width of the tire in 

 diminishing the draught, Morin's experiments give 

 the following results (p. 132) : — 



In thick mud, on a road, a 7 in. tire 



a 4)4 in. 

 On a dry earth road 



On a hard road 

 On a pavement 



a 7 in. 

 a 4)4 in. 

 a zy% in. 

 a 7 in. 

 a 4 y 2 in. 

 a 7 in. 

 a 4% in. 

 a 2V% in. 



In English units. 



gave A = 0.097 

 A = 0.084 

 A = 0.073 

 A = 0.068 

 A = 0.084 

 A = 0.052 

 A = 0.045 

 A = 0.035 

 A = 0.030 

 A = 0.030 



On roads, therefore, while there does not seem to 

 be much difference in the resistance of wide and 



a ratio which is the same whether feet or metres are used, and there- 

 fore the French/ and the English /have the same value. 



