----------------------+-----------------------+-----------+---------- | | Tractive | Tractive | |Resistance |Resistance Type of Surface | Condition of Surface | in lbs. | in lbs. | | per ton | per ton | | 10 miles |12.4 miles | | per hr. | per hr. ----------------------+-----------------------+-----------+---------- Asphalt | Good | 20.4 | Asphalt | Poor | 22.6 | 25.5 Wood block | Good | 24.2 | 25.3 Brick block | Good | 24.6 | 26.6 Granite block | Good | 40.3 | 45.75 Brick block | Slightly worn | 25.1 | 28.0 Granite block with | | | cement joints | Good | 25.5 | 30.2 Macadam, water bonded | Dry and hard | 23.3 | 25.8 Macadam, water bonded | Fair, heavily oiled | 35.9 | 38.7 Macadam, water bonded | Poor, damp, some holes| 36.3 | 41.6 Tar macadam | Good | 25.7 | 28.0 Tar macadam | Very soft | 36.8 | 38.7 Tar macadam | Many holes, soft, | | | extremely poor | 52.4 | 60.6 Cinder | Fair, hard | 27.5 | 30.6 Gravel | Fair, dusty | 30.4 | 33.0 ----------------------+-----------------------+-----------+---------- [Illustration: Fig. 8] =Effect of Grades.=--Grades increase or decrease the resistance to translation due to the fact that there is a component of the weight of the vehicles parallel to the road surface and opposite in direction to the motion when the load is ascending the hill and in the same direction when the vehicle is descending. In Fig. 8 _W_ represents the weight of the vehicle, acting vertically downward, _w_ is the component of the weight perpendicular to the road surface and _W_{2}_ is the component parallel to the road surface. _W_{2}_ = _W_ tan _THETA_. tan _THETA_ = 0.01 x per cent of grade. _W_{2}_ = 0.01 _W_ x per cent grade. _W_{2}_ = 0.01 x 2000 x per cent of grade, for each ton of weight of vehicle. Hence _W_{2}_ = 20 lbs. per ton of load for each one per cent of grade. The gravity force acting upon a vehicle parallel to the surface on a grade is therefore 20 lbs. per ton for each one per cent of grade and this force tends either to retard or to accelerate the movement of the vehicle. Let _F_ = the sum of all forces opposing the translation of a vehicle.