Figure 8-13.Auxiliary spring.
from the main spring. In either case, the ends of the
spring has it own support brackets. When the truck is
under a load, the auxiliary spring assumes part of the
load when its ends contact the bearing plates or special
brackets attached to the frame side rails.
A large portion of six-wheel drive vehicles utilize
a bogie suspension (fig. 8-14) which uses leaf springs.
This suspension is a unit consisting of two axles joined
by torque rods. A trunnion axle acts as a pivot for the
drive axles and is supported by bearings that are part of
the spring seat. The ends of each spring rest in the
guide brackets bolted to the axle housings. Mounting
the springs on central pivots enable them to distribute
half of the rear load onto each axle. As a result, this
type of suspension allows the vehicle to carry a much
heavier load than a single axle without losing its ability
to move over unimproved terrain.
When one wheel of a bogie suspension is moved
up or down because of an irregularity in the road, the
spring pivots on the trunnion shaft and both ends of the
spring deflect to absorb the road shock. This causes the
load to be placed on the center of the spring resulting in
equal distribution of the load to both axles. The torque
rods ensure proper spacing and alignment of the axles
and transmit the driving and braking forces to the
The torsion bar consists of a steel rod made of
spring steel and treated with heat or pressure to make it
elastic, so it will retain its original shape after being
twisted. Torsion bars, like coil springs, are frictionless
and require the use of shock absorbers. The torsion bar
is serrated on each end and attached to the torsion bar
anchor at one end and the suspension system at the
other end (fig. 8-15). Torsion bars are marked to
indicate proper installation by an arrow stamped into
the metal. It is essential that they be installed properly
because they are designed to take the stress in one
direction only. The up-and-down movement of the
suspension system twists the steel bar. The torque
resistance will return the suspension to its normal
position in the same manner as a spring arrangement.
Figure 8-14.Bogie suspension.