Issued February 12, 1917. 
United States Department of Agriculture, 
OFFICE OF THE SECRETARY—Circular No. 72. 
WIDTH OF WAGON TIRES RECOMMENDED FOR LOADS 
OF VARYING MAGNITUDES ON EARTH AND GRAVEL 
ROADS. 
repared by E. B. McCormick, Chief, Division of Rural Engineering, Office of 
Public Roads and Rural Engineering. 
“The recommendations in this circular relating to widths of wagon 
suitable for use on country roads of earth and gravel, as well as 
pon those of a more improved type, are based upon two factors: 
1) The unit weight for width of tire commonly used for road rollers, 
nd (2) the results secured from a large series of traction tests con- 
acted by the Office of Public Roads and Rural Engineering, extend- 
g over several years and made in widely scattered localities ae 
it the United States. 
An examination of several of the road rollers in general use shows 
at they are designed for a weight per inch of width of tire varying 
om 450 pounds to 650 pounds, more of them ranging from 500 to 
0 pounds than within any other 50-pound division. It is apparent 
at, as this is the maximum weight applied to the foundation and 
rface of the road during construction, it is not advisable to exceed 
is weight except in cases of emergency, and then only occasionally. 
Curves shown in figures 1 to 5, inclusive, are plotted from the 
sults of traction tests recently conducted on a dry earth road which 
is constructed by the use of a 10-ton road roller having a unit 
ight on the wheels of 450 pounds per linear inch of width of rim, 
d with a constant gross load on the wagon of 5,000 pounds. The 
rve on figure 5 is the average unit draft, by which is meant the 
nds pull per ton of gross load, for the full 40 trips; the vertical 
lance in each case is the measurement of the draft in pounds per 
_ The left-hand point on the diagram represents the draft of the 
ach tire, the next point that of the 2-inch tire, and so on through 
d-inch, 4-inch, 5-inch, and 6-inch. This same curve is drawn on 
of the other Redicer: It will be noted that it approximates very 
j_ to a curve representing the average of the points recorded. 
8974°—17 
