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TM 9-8000
the spring is being extended from the position to which it
30-16.  Purpose.  The primary function of the shock
was compressed by the vertical impact.
absorber is to regulate the suspension spring rebound so
that the primary vibrations are damped out, thereby
There is some disadvantage in this arrangement when
permitting greater vehicle speeds and mobility. These
the moving vehicle hits a hole. The spring extends so
benefits are achieved by virtue of reduced bouncing and
that the ground-contacting elements can maintain good
pitching of the body or hull and reduced variations of the
contact with the ground. The high damping force exerted
traction with the terrain. Additional benefits derived from
by the shock absorber interferes with this action,
the use of shock absorbers are: improved ride quality,
resulting in a downward acceleration of the vehicle body.
reduction of wheel dance, prevention of excessive
This is an acceptable condition, because the downward
sidesway, reduction of wheel shimmy, and general
acceleration of the sprung mass cannot exceed the
improvement of the desirable vehicle traveling qualities,
acceleration of gravity because no other downward force
collectively termed roadability.
is acting on the sprung mass.
30-17. Classification of Shock Absorbers.
30-18. Types of Shock Absorbers.
a.  Single Acting.  Shock absorbers are of two
a. Single Acting, Cam Operated. A typical single-
general classes: single acting and double acting. Those
acting, cam-operated shock absorber is shown in figure
that check only spring rebound are termed single acting.
30-21. When the sprung and unsprung masses of the
They are so designed, or attached to the suspension
suspension system move toward each other, the shock
system in such a manner, that the damping force is not
absorber arm rotates counterclockwise, moving the cam
generated  during  spring  deflection.
Their  main
to the right, thereby permitting the piston spring to move
disadvantage is that they provide damping only part of
the piston to the right. This causes the intake valve in
the time, imposing the requirement of stiffer springs in
the piston to open, allowing oil to flow from the reservoir
the system. Also, a slight preload on the suspension
into the increasing cylinder volume. Because the piston
spring is experienced due to the shock absorber return
motion and oil flow are caused by the piston spring, the
spring. This has a tendency to stiffen the suspension
shock absorber has little effect upon the spring action of
spring out of proportion for mild terrain irregularities.
the vehicle. During rebound, the cam moves to the left,
forcing the piston to the left against the oil in the cylinder.
b.  Double Acting.  Those shock absorbers that
The intake valve closes, and the oil in the cylinder is
provide damping during spring deflection as well as
forced out through the relief valve.  The restricted
during rebound are termed double acting. They permit
passage of the oil through the relief valve orifice is the
the use of softer suspension springs and allow optimum
primary factor in generating the damping force during
damping in both directions. In most cases, the damping
force developed by the double-acting shock absorber
during spring compression is much lower than is
b. Vane Type. The housing of the vane-type shock
developed during rebound. This is desirable because a
absorber, shown in figure 30-22, is divided into two
high damping force during spring compression would
working chambers by stationary partitions, each of which
have the same effect on impact isolation as would a very
contains a check valve. The central shaft is connected
stiff spring; that is, it would transmit the shock to the
to the unsprung mass through the arm and link, and has
vehicle body, causing it to displace vertically.  It is
a pair of vanes attached to it that extend into each
desirable not to interfere with the impact-isolating
working chamber.
As the suspension spring is
properties of the spring during its compression stroke,
compressed, the central shaft rotates, and the vanes
because there is no upper limit to the amount of
develop a pressure in the chamber that causes oil to flow
acceleration that the vehicle body can experience when
unrestricted through the opened check
the ground-contacting elements pass over a vertical
obstacle at high speed.  For this reason, the larger
damping force is exerted during rebound; that is, when


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