American Rural Highways

by T. R. Agg

Table 6

Type of Surface	Condition of Surface	Tractive Resistance in lbs. per ton 10 miles per hr.	Tractive Resistance in lbs. per ton 12.4 miles 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

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 W₂ is the component parallel to the road surface.

W2	=	W tan θ.
tan θ	=	0.01 × per cent of grade.
W2	=	0.01 W × per cent grade.
W2	=	0.01 × 2000 × per cent of grade, for each ton of weight of vehicle.
Hence W2	=	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.

F = f_r + f_i + f_p + f_a + f_g (1)