HIGHWAY CONSTRUCTION 



The mechanical advantage of the wheel in surmounting an 

 obstacle may be computed from the principle of the lever. 



Let the wheel, Fig. 2, touch the horizontal line of traction in 

 the point A and meet a protuberance B D. Suppose the line of 

 draught C P to be parallel to A B. Join C D and draw the perpen- 

 diculars D E and D F. We 

 may suppose the power to be 

 applied at E and the weight at 

 F, and the action is then the 

 same as the bent lever E D F 

 turning round the fulcrum at 

 D. HenceP:W::FD : D E. 

 But FD :DE ::tanFCD:l, 

 ~~A B~~ and tan F C D = tan 2 



Fio- 9 



(DAB); therefore P = W 



tan 2 (D AB). Now it is obvious that the angle DAB increases 

 as the radius of the circle diminishes; and therefore, the weight W 

 being constant, the power required to overcome an obstacle of given 

 height is diminished when the diameter is increased. Large wheels 

 are therefore the best adapted for surmounting inequalities of the 

 road. 



There are, however, circumstances which provide limits to the 

 height of the wheels of vehicles. If the radius A C exceeds the 

 height of that part of the horse to which the traces are attached, 

 the line of traction C P will be inclined to the horse, and part of the 

 power will be exerted in pressing the wheel against the ground. The 

 best average size of wheels is considered to be about 6 feet in diameter. 



Wheels of large diameter do less damage to a road than small 

 ones, and cause less draught for the horses. 



With the same load, a two-wheeled cart does far more damage 

 than one with four wheels, and this because of their sudden and 

 irregular twisting motion in the trackway. 



Grade Resistance is due to the action of gravity, and is the 

 same on good and bad roads. On level roads its effect is immaterial, 

 as it acts in a direction perpendicular to the plane of the horizon, and 

 neither accelerates noj retards motion. On inclined roads it offers 

 considerable resistance, proportional to the steepness of the incline. 



