HIGHWAY CONSTRUCTION 



(2) On solid, unyielding surfaces it is independent of the width 

 of the tire, but on compressible surfaces the resistance decreases as 

 the width of the tire increases (but there is no material advantage 

 gained in making a tire more than 4 inches wide). 



(3) It is independent of the speed. 



(4) On rough, irregular surfaces, which give rise to constant 

 concussion, it increases with the speed. 



The following table shows the relative resistance to traction of 

 various surfaces: 



TABLE 1. 

 Resistance to Traction on Different Road Surfaces. 



These coefficients refer to the power required to keep the load 

 in motion. It requires from two to six or eight times as much force 

 to start a load as it does to keep it in motion, at two or three miles 

 per hour. The extra force required to start a load is due in part 

 to the fact that during the stop the wheel may settle into the road 

 surface, in part to the fact that the axle friction at starting is greater 

 than after motion has begun, and further in part to the fact that 

 energy is consumed in accelerating the load. 



Resistance to Rolling. This resistance is caused (1) by the 

 wheel penetrating or sinking below the surface of the road, leaving 

 a track oc rut behind it. It is equal to the product of the load mul- 

 tiplied by one-third of the semi-chord of the submerged arc of the 

 wheel; and (2) by the wheel striking or colliding with loose or pro- 

 jecting stones, which give a sudden check to the horses, depending 

 upon the height of the obstacle, the momentum destroyed being 

 oftentimes considerable. 



The rolling resistance varies inverselv as some function of the 



