CH. X AXLE FRICTION I 57 



compared, with the results here noted : On almost 

 all roads, the wide tire diminished the traction by 

 an amount varying from 16 to 30 per cent., but 

 where the surface was sticky and there was a 

 smooth hard road below, the narrow tire cut clown 

 to the hard road, whereas the wide tire adhered to 

 the mud, showing an advantage for the narrow tire 

 of 20 or 30 per cent. In 14 experiments out of a 

 total of 21, with greatly varying conditions, there 

 was shown a decided advantage in the use of a 

 wide tire, and in all cases the road was improved 

 by the passage over it of the wide tire. 



We have so far considered only the rolling friction 

 of the wheel, but to obtain the whole resistance to 

 the forward motion of the vehicle we must add the 

 friction between the axle and its box. This is a 

 sliding friction, the laws of which, as determined by 

 large number of experiments of different investi- 

 gators, are, within the limits met with in well-de- 

 signed machinery, as follows : — 



This friction is directly proportional to the pressure 

 per unit of surface ; that is, between any two given 

 surfaces, if the pressure is one hundred pounds per 

 square inch, the friction will be twice as great as if 

 it were fifty pounds per square inch. It is therefore 

 independent of the absolute amount of surface, be- 

 cause with the same weight, if the rubbing surface 

 is increased, the pressure per square inch is dimin- 

 ished. A brick, for example, will move with just 

 the same amount of friction over the surface of a 



