oscillations. This device is the shock absorber. The
most common type of shock absorber (fig. 8-8) used on
modern vehicles is the double-acting, direct-action
type, because it allows the use of more flexible springs.
The direct-action shock absorber consists of an
inner cylinder filled with special hydraulic oil divided
into an upper and lower chamber by a double-acting
piston. In operation, the shock absorbers lengthen and
shorten, as the wheels meet irregularities in the road.
As they do this, the piston inside the shock absorber
moves within the cylinder filled with oil; therefore, the
fluid is put under high pressure and forced to flow
through small openings. The fluid can only pass
through the openings slowly. This action slows piston
motion and restrains spring action.
During compression and rebound, the piston is
moving. The fluid in the shock absorber is being forced
through small openings which restrains spring
movement. There are small valves in the shock
absorber that open when internal pressure becomes
excessive. When the valves are open, a slightly faster
spring movement occurs; however, restraint is still
imposed on the spring.
An outer metal cover protects the shock absorber
from damage by stones that may be kicked up by the
wheels. One end of the shock absorber connects to a
suspension component, usually a control arm. The
other end fastens to the frame. In this way, the shock
absorber piston rod is pulled in and out and resists
The strut assembly, also called a MacPherson
strut, is similar to a conventional shock absorber.
However, it is longer and has provisions (brackets and
connections) for mounting and holding the steering
knuckle (front of vehicle) or bearing support (rear of
vehicle) and spring. The strut assembly consists of a
shock absorber, coil spring (in most cases), and an
upper damper unit. The strut assembly replaces the
upper control arm. Only the lower control arm and strut
are required to support the front-wheel assembly. The
Figure 8-8.Double-acting, direct-action type shock absorber.