Page 115 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

ROCK, SOIL, AND MUD 3.9

Sometimes a well-worn tire will give as good or better traction than the new one ordered to
replace it.

FLOTATION

Flotation means the weight-supporting ability of a tire, crawler track, or platform on soft ground.
This ability, or lack of it, is the result of a relationship between the weight, the area of contact,
and the load-bearing ability of the ground.
The weight divided by the contact area in square inches gives the downward pressure in
pounds per square inch (psi). If this pressure is greater than the load-carrying ability of the
ground, the machine will sink until it finds enough contact area to support it. Sinking increases
rolling resistance. If it is severe, it may prevent the machine from moving under its own power.
Tire Pressure. When a tire carries enough weight that it tends to sink into the ground, its behavior
depends largely on the relationship between its inflation pressure and the load-bearing ability of
the ground.
Ground with a bearing strength of 50 pounds per square inch (psi) will allow a loaded tire with
75 pounds of air pressure to sink into it until the same area of tire is in contact with the ground as
there would be if the tire had 50 pounds of pressure and flexed to spread out on the surface.
It follows that if a larger tire were used, that could carry the load at 50 pounds of pressure, it
would not sink in at all. It does not always work out just this way, but there is no doubt that bigger
and softer tires greatly reduce problems with vehicles sinking in soft ground.
The formula for finding the area in square inches over which a tire will contact the ground is
.9 tire load
Contact area
inflation pressure
On a truck with a gross weight of 60,000 pounds, of which 70 percent is on four drive wheels
equipped with 14.00 24 tires, 20-ply, with 75 pounds of pressure, we would have a load on each
1
tire of ⁄ 4 of 60,000 .7, or 10,500 pounds. Then, by our formula,
.9 10,500 9,450
Contact area 126
75 75
If the tires were 16.00 24, 16-ply, 45 pounds of pressure, we would have
.9 10,500 9,450
Contact area 210
45 45
Since these loads are within the rated capacity of the tires, they will show only normal flexing,
and any additional bearing surface needed will be obtained by sinking into the ground.
To find out whether the ground will support the load, we use the formula
.9 tire load
Bearing area
ground bearing capacity
If the ground has a bearing capacity of 50 pounds per square inch,
.9 10,500 9,450
Bearing area 189
50 50
We find that the necessary ground area to bear this load is 189 square inches. The harder tire has only
126 inches of ground contact, so it will sink until an additional 63 square inches of its surface are
in contact. On the other hand, the tire with 45 pounds of pressure spreads its load over 210 square
inches, and should not sink at all.
Figure 3.7 shows the bearing capacities that may be expected of various types of soils. The fig-
ures should not be trusted very far, as there are many unknowns in this field.