The lower half of a
shock absorber is a twin tube steel unit filled with hydraulic oil and nitrogen
gas. In some shock absorbers, the nitrogen gas is omitted. A relief valve is
located in the bottom of the unit, and a circular lower mounting is attached to
the lower tube. This mounting contains a rubber isolating bushing, or grommets.
A piston and rod assembly is connected to the upper half of the shock absorber.
The upper portion has a dust shield that surrounds the lower tube unit. The
piston is precision fit in the inner cylinder of the lower unit. A piston rod
guide and seal are located in the top of the lower unit. A circular upper
mounting with a rubber bushing is attached to the top of the shock absorber. A
nitrogen gas charge is located in the oil reservoir to prevent oil cavitation
and foaming, which provides more positive action but does not provide for spring
Some heavy-duty shock
absorbers have a dividing piston in the lower oil chamber. The area below this
piston is pressurized with nitrogen gas to 360 psi or 2482 kPa. Hydraulic oil is
contained in the oil chamber above the dividing piston. The operation of the
heavy-duty shock absorber is similar to that of the conventional type. The other
main features of a heavy-duty shock absorber include a higher quality seal for
longer life, single tube design to prevent excessive heat buildup, and a rising
rate valve to provide precise spring control under all conditions.
spring-assisted shock absorbers are also available. These are conventional shock
absorbers with a small coil spring mounted over them. The coil springs on the
shock absorbers help the springs support the vehicle's weight.
struts or shock absorbers, often called air shocks, are used with an electronic
height control system. An onboard air compressor pumps air into the rear shocks
to raise the rear of the vehicle, and an electric solenoid releases air from the
shocks to lower the rear chassis. An electromagnetic height sensor may be
contained in the shock absorber, or an external sensor may be used. This sensor
sends a signal to an electronic control module in relation to the rear
suspension height. The module controls the air compressor and the exhaust
solenoid to control air pressure in the shock absorbers. This action maintains a
specific rear suspension trim height regardless of the load on the rear
suspension. If a heavy package is placed in the trunk, the vehicle chassis is
forced downward. However, the load-leveling shock absorbers extend to restore
the original rear suspension height.
Aftermarket air shock
absorbers are available. These contain an air valve connection ,and a shop air
hose is used to supply the desired pressure in these air shock absorbers.
Most automotive shock
absorbers are a double acting and control spring action during jounce and
rebound. The piston and valves in many shock absorbers are designed to provide
more extension control than compression control. An average shock absorber may
have 70 percent of the total control on the extension cycle, and thus 30 percent
of the total control is on the compression cycle. Shock absorbers are so
designed because they must control the heavier sprung body weight on the
extension cycle. The lighter unsprung axle, wheel, and tire weight are
controlled by the shock absorber on the compression cycle. A shock absorber with
this type of design is referred to as a 70/30 type. Shock absorber ratios vary
from 50/50 to 80/20.