CH. X 



DRAUGHT I 6 I 



per ton (of 2000 pounds), as the total resistance 

 to motion in a coach on a good hard road (see p. 

 163), that resistance for a weight of 3400 pounds, 

 will amount to 119 pounds, of which 9 pounds, or 

 7.5 per cent., will be axle friction, and 110 pounds, 

 or 92.5 per cent., rolling friction. 



The total resistance to forward motion on a level 

 surface is therefore represented by the rolling 

 friction and the axle friction added together, or 



by the formula P = A ^ +/- — ^7^ in which: — 



P is the power required to draw the vehicle. 



A is the constant for rolling friction for a given 

 road surface (see p. 153) ; for a hard road 0.0615. 



W, the total weight of the coach and load. 



w, the weight of the wheels. 



R, the radius of the wheel (or the mean of the 

 radii of the front wheels and hind wheels). 



r, the radius of the axle-arm. 



f, the coefficient of friction, 0.065. 



Example of computation for a coach : — 



I [ 



