Object of Roads. The object of a road is to provide a way for the transportation of persons and goods from one place to another with the least expenditure of power and expense. The facility with which this traffic or transportation may be conducted over any given road depends upon the resistance offered to the movement of vehicles. This resistance is composed of: (1) The resistance offered by the roadway, which consists of (a) "friction" between the surface of the road and the wheel tires; (b) resistance offered to th° rolling of the wheels, occasioned by the want of uniformity in the road surface, or lack of strength to resist the penetrating efforts of loaded wheels, thus requiring the load to be lifted over projecting- points and out of hollows and ruts, thereby diminishing the effective load the horse may draw to such as it can lift. This resistance is called "resistance to rolling" or "penetration;" (c) resistance due to gravity called "grade resistance;" (2) The resistance offered by vehicles, termed "axle friction;" (3) Resistance of the air.
The road which offers the least resistance to traffic should com bine a surface on which the friction of the wheels is reduced to the least possible amount, while offering a good foothold for horses, to enable them to exert their utmost tractive power, and should be so located as to give the most direct route with the least gradients.
Friction. The resistance of friction arises from the rubbing of the wheel tires against the surface of the- road. This resistance to traction is variable, and can be determined only by experiment. From many experiments the following deductions are drawn: (1) The resistance to traction is directly proportional to the pressure.
(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: 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 or 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 inversely as some function of the diameter of the wheel, as the larger the wheel the less force required to lift it over the obstruction or to roll it up the inclination due to the indentation of the surface.
The power required to draw a wheel over a stone or any stacle, such as S in Fig. 1, may be thus calculated. Let P represent the power sought, or that which would just balance the weight on the point of the stone, and the slightest increase of which would draw it over. This power acts in the direction C P with the leverage of B C or D E. Gravity, represented by W, resists in the direction C B with the leverage B D. The equation of equilibrium Let the radius of the wheel = C D = 26 inches, and the height of the obstacle = A B = 4 inches. Let the weight W = 500 pounds, of which 200 pounds may be the weight of the wheel and 300 pounds the load on the axle. The formula then becomes The pressure at the point D is compounded of the weight and the power, and equals and therefore acts with this great effect to destroy the road in its collision with the stone, in addition there is to be considered the effect of the blow given by the wheel in descending from it. For minute accuracy the non-horizontal direction of the draught and the thickness of the axle should be taken into account. The power required is lessened by proper springs to vehicles, by enlarged wheels, and by making the line of draught ascending.