274 



MECHANICS 



depends on the materials of the roller and supporting sur- 

 face, and is proportional to the normal pressure exercised by 

 the roller on the rolling surface. It depends also on the 

 diameter of the roller, being less for large rollers than for 

 small ones. On highways with soft compressible surfaces, the 

 resistance is also affected by the width of the wheel tires, being 

 greater for narrow tires than for wide ones. 

 ROLLING FRICTION FOR DIFFERENT ROADWAY 

 SURFACES 



The accompanying table gives the maximum, minimum, and 

 mean values of the coefficient of rolling friction for different 

 roadway surfaces. They are expressed in pounds required to 

 overcome the resistance on a level road of a gross ton (2,240 

 lb.). The mean value is also expressed as a ratio between 

 the frictional resistance and the load. 



