A horse can exert for a short time twice the average tractive pull which he can exert continuously throughout the day's work; hence, so long as the resistance on the incline is not more than double the resistance on the level, the horse will be able to take up the full load which he is capable of drawing.
Steep grades are thus seen to be objectionable, and particularly so when a single one occurs on an otherwise comparatively level road, in which case the load carried over the less inclined portions must be reduced to what can be hauled up the steeper portion.
The bad effects of steep grades are especially felt in winter, when ice covers the roads, for the slippery condition of the surface causes danger in descending, as well as increased labor in ascending; the water of rains also runs down the road and pulleys it out, destroy ing its surface, thus causing a constant expense for repairs. The inclined portions are subject to greater wear from horses ascending, thus requiring thicker covering than the more level portions, and hence increasing the cost of construction.
It will rarely be possible, except in a flat or comparatively level country, to combine easy grades with the best and most direct route. These two requirements will often conflict. In such a case, increase the length. The proportion of this increase will depend upon the friction of the covering adopted. But no general rule can be given to meet all cases as respects the length which may thus be added, for the comparative time occupied in making the journey forms an important element in any case which arises for settlement. Disre garding time, the horizontal length of a road may be increased to avoid a 5 per cent grade, seventy times the height.
Table 7 shows, for most practical purposes, the force required to draw loaded vehicles over inclined roads. The first column ex presses the rate of inclination; the second, the pressure on the plane in pounds per ton; the third, the tendency down the plane (or force required to overcome the effect of gravity) in pounds per ton; the fourth, the force required to haul one ton up the incline; the fifth, the length of level road which would be equivalent to a mile in length of the inclined road—that is, the length which would require the same motive power to be expended in drawing the load over it, as would be necessary to draw over a mile of the inclined road; the sixth, the maximum load which an average horse weighing 1,200 pounds can draw over such inclines, the friction of the surface being taken at of the load drawn.
Axle Friction. The resistance of the hub to turning on the axle is the same as that of a journal revolving in its bearing, and has 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 3.1-, lb. per ton of the weight on the axle; and for truck wagons, which have medium sized wheels and axles, is about 31. to 4, lb. 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: 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.