| Lbs. per Ton | μ = coefficient of Resistance | |
|---|---|---|
| Upon Steel rails | 10 | 1⁄200 |
| Sheet asphalt, good condition | 20 | 1⁄100 |
| Asphaltic macadam or concrete, good condition | 20 | 1⁄100 |
| Concrete, good condition | 20 | 1⁄100 |
| Brick, good condition | 20 | 1⁄100 |
| Broken stone water-bound macadam, good condition | 30 | 3⁄200 |
| Gravel, good condition | 30 | 3⁄200 |
| Sand clay, good condition | 60 | 3⁄100 |
| Earth, best condition | 67 | 1⁄30 |
| Earth, medium condition | 100 | 1⁄20 |
| Earth, poor condition | 300 | 3⁄20 |
Resistance Due to Grade.
—The resistance due to grade is just as marked as that due to surface. The work necessary to draw a load up an inclined plane is the same as that of drawing on a level along the base of the plane and lifting it directly up to the height of the plane. A mathematical analysis[182] based upon this fact leads to the formulas:
For a horse-drawn load,
L = t - g μ + g H. (1)
For a tractor,
L = P μ + g - T. (2)
For an automobile or truck,
L = P μ + g, (3)
| where | L | = | weight of load drawn, including weight of vehicle (subtract weight of vehicle for net load); |
| H | = | weight of horse; | |
| T | = | weight of tractor; | |
| P | = | effective tractive force exerted (available engine effort); | |
| μ | = | coefficient of road resistance; | |
| g | = | grade (gradient) = tangent of angle of incline, nearly the same for small angles as the sineof the angle of incline, that is, the height of the incline divided by its length; | |
| t | = | the direct pull of the horse divided by the weight of the horse; | |
| h | = | horse-power = work of 33,000 ft.-lb. per minute. | |
| v | = | velocity in miles per hour. |