Chevrolet Light Duty Truck 1973, Service Manual

Page: 181 / 979

F R O N T S U S P E N S IO N 3-7
This cell, in the reservoir, acts the same as an air
chamber, expanding and contracting to compensate for
the volume of the piston rod. But since it is a gas filled
cell, there is no free air to mix with the fluid; thus,
aeration is eliminated.
A rebound or extension stroke, Figure 10, will cause the
pressure in chamber B to fall below that in chamber C.
As a result, the compression valve will unseat and allow
fluid to flow from chamber C into chamber B. Chamber
D contains air which expands to compensate for the
piston rod volume being removed. Simultaneously, fluid
in chamber A will be transferred into chamber B
through the inner piston holes and the rebound valve.
Rebound control is determined by the piston rebound
varying.
Sometimes shock absorbers are used to limit the rebound
travel of a suspension system. This type of usage is
primarily for front suspension systems and permits the
rubber rebound bump stop to be omitted. Shock
absorbers used in this manner have a different type of
internal construction which controls the rebound stroke.
This construction is called "rebound cut-off". On the
rebound stroke, all fluid passing from chamber A to
chamber B, see Figure 10, through the piston valving
parts must first flow through the piston rod. The fluid
flows through the piston rod by means of intersecting
holes, one along the axis of the piston rod and the other
perpendicular to it. As the shock absorber approaches
the end of the rebound stroke, the passage in the piston
rod enters the rod guide, gradually restricting the flow of
the fluid. As the flow is restricted, the control of the
shock absorber increases. When the passage is
completely closed off the by rod guide, see Figure 11, a
small amount of fluid is trapped in chamber A creating a
hydraulic stop which limits the full rebound travel of the
suspension system.
As the piston moves, forcing fluid through calibrated
orifices, pressure increases within the cylinder. This
pressure acting against the effective area of the piston
determines the resistance or control provided by the
shock absorber. Low piston velocities create low
pressures, whereas high piston velocities with the same
orifice result in considerably higher pressures. For
Fig. 9—Gas Filled Cell Shock Absorber
EXTEN S IO N FORCE
C YLINDER
TU BE
Fig. 10—Shock Absorber Rebound Schematic
example, body lean during a turn will result in low force
in the shock absorber due to low piston velocity; while
hitting a chuckhole at high speeds will generate high
resistance forces.
SP IR A L-G R O O V E
RESERVOIR
The system of valves and orifices in a shock absorber
which is referred to as the “valving,” is composed of
Fig. 8—Spiral-Groove Shock Absorber
three .distinct stages, which generate rebound and
compression resistance levels (control) dependent upon
LIGHT DUTY TRUCK SERVICE MANUAL