HIGHWAY CONSTRUCTION 



11 



nothing to do with the condition of the road surface. The coefficient 

 of journal friction varies with the material of the journal and its 

 bearing, and with the lubricant. It is nearly independent of the 

 velocity, and seems to vary about inversely as the square root of the 

 pressure. For light carriages when loaded, the coefficient of friction 

 is about 0.020 of the weight on the axle; for the ordinary thimble- 

 skein wagon when loaded, it is about 0.012. These coefficients are 

 for good lubrication; if the lubrication is deficient, the axle friction 

 is two to six times as much as above. 



The traction power required to overcome the above axle friction 

 for carriages of the usual proportions is about 3 to 3J Ib. per ton of 

 the weight on the axle; and for truck wagons, which have medium 

 sized wheels and axles, is about 3^ to 4J Ib. per ton. 



Resistance^ of the Air. The resistance arising from the 

 force of the wind will vary with the velocity of the wind, with the 

 velocity of the vehicle, with the area of the surface acted upon, and 

 also with the angle of incidence of direction of the wind with the 

 plane of the surface. 



The following table gives the force per square foot for various 

 velocities : 



TABLE 8. 



Effect of Springs on Vehicles. Experiments have shown 

 that vehicles mounted on springs materially decrease the resistance to 

 traction, and diminish the wear of the road, especially at speeds 

 beyond a walking pace. Going at a trot, they were found not to 

 cause more wear than vehicles without springs at a walk, all other 

 conditions being similar. Vehicles with springs improperly fixed 

 cause considerable concussion, which in turn destroys the road 

 covering. 



