A shock absorber is bolted between
the axle and chassis. The mounting bolts pass through isolating bushings on each
end of the shock absorber. The isolating bushings prevent vibration and noise.
Front shock absorbers are usually
mounted between the lower control arms and the chassis. When a vehicle wheel
strikes a bump, the wheel and suspension move upward in relation to the chassis.
Upward wheel movement is referred to as jounce travel. This jounce action causes
the spring to deflect or compress. Under this condition, the spring stores
energy and springs back downward with all the energy absorbed when it deflected
upward. This downward spring and wheel action is called rebound travel. If this
spring action were not controlled, the wheel would strike the road with a strong
downward force, and the wheel jounce would occur again. Therefore, some device
must be installed to control the spring action, or the wheel would bounce up and
down many times after it hit a bump, causing passenger discomfort, directional
instability, and suspension component wear.
Shock absorbers are installed on
suspension systems to control spring action. When a wheel strikes a bump and
jounce travel occurs, the shock absorber lower tube unit is forced upward. This
action forces the piston downward in the lower tube unit. Since oil cannot leak
past the piston, the oil in the lower unit is forced through the piston valves
to the upper oil chamber. These valves provide precise oil flow control and
control the upward action of the wheel and suspension, which is referred to as a
shock absorber compression stroke.
When the spring expands downward in
rebound travel, the lower shock absorber unit is also face3d downward. When this
occurs, the piston moves upward in the lower tube unit, and hydraulic oil is
forced through the piston valves from the upper oil chamber to the lower oil
chamber. Since the valves restrict oil flow with precise control, the downward
suspension and wheel movement is controlled.
When the shock absorber's piston
moves, oil is forced through it. Since the piston's valves and orifices resist
the flow of oil, friction and heat are created. The resistance of the oil moving
through the piston must be calibrated as close as possible to the spring's
deflection rate. Suspension systems deflect as different speeds depending on the
type and size of bump and the vehicle's speed. The resistance of a shock
absorber increases with the square of its speed. If a wheel strikes a large bump
at high speed, the wheel deflection and rebound can be effectively locked by the
shock absorber. Shock absorber engineers prevent this action by designing shock
absorber valves and orifices to provide enough friction to prevent the spring
from overextending on the rebound stroke. The valves and orifices must not
create excessive friction, which would slow the wheel from returning to its
Shock absorber pistons have many
different valves and orifices. In some, small orifices control the oil flow
during slow wheel and suspension movements. Stacked steel valves control the oil
flow during medium speed wheel and suspension movements. During maximum wheel
and suspension movements, larger orifices provide oil flow control. In other
pistons, the stacked steel valves provide oil flow control. Regardless of the
design of the orifice and valve design, a shock absorber must be precisely
matched to absorb the spring's energy.
During fast upward wheel movement
on the compression stroke, excessive pressure in the lower oil chamber forces
the base valve open and allows oil to flow through the valve to the reservoir.
Nitrogen gas provides a compensating space for the oil. Since the gas exerts
pressure on the oil, cavitation - of foaming - of the oil is eliminated. When
oil bubbles are eliminated, the shock absorber can provide continuous damping
for wheel deflections as small as 0.078 in. (2.0 mm). A rebound rubber is
located on top of the piston. If the wheel drops far, the shock absorber can
become fully extended, and the rebound rubber cushions the action.