the field artillery journal - Fort Sill - U.S. Army

THE FIELD ARTILLERY JOURNAL
The differential is of a type which does not use judgment in
controlling the relative movement of the rear wheels, and
should one get into soft mud it will spin and nicely bury itself
while the other one stands still.
The arrangement of the other class, covered by the general
name of four-wheel drive is shown at the top. Power is
transmitted to all wheels and the increase in grip on the ground
over the first class is the same as that of the light railway
switch engine with all its weight on the drivers, which can pull
as long a string of cars as a heavier road engine with part of the
weight on a front truck. When extraordinary power is required
the normal turns of the engine at each speed to one turn of the
wheels may be doubled by throwing in mesh extra gearing in a
subtransmission and the power at the wheels becomes so great
that it is practically impossible to stall the engine although the
truck may fail by slipping of the wheels on the ground. All the
weight is useful in increasing friction on the ground and the
differentials are so constructed that while they will permit
relative motion of the wheels on corners they will not permit
any wheel to spin while the others stand still.
At the rear of the truck there is a drum or winch such as
shown in Fig. 3 with a long length of wire rope. The rope can
be wound up by power to pull a tow or another machine out of
difficulties, or it can be attached to a tree or other anchor to
help the truck out of trouble. All four wheels are steering
wheels giving about half the usual turning radius. Brakes are
provided on all four wheels and in addition a transmission
brake, the power of which is multiplied by the transmission
gearing through which it acts on the wheels.
Fig. 4 shows graphically the relative merits of the two
classes for military purposes. The truck at the left is an
ordinary commercial 3½-ton truck, loaded to capacity, and the
other is a two-ton truck of the four wheel drive type with
subtransmission, loaded to capacity. The two-ton truck without
use of the subtransmission is having no difficulty in pulling the
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