Figure 8-7.- Upper and lower ball joint.

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 these movements.

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.

basic components of a typical strut assembly, are as follows (fig. 8-9):

STRUT SHOCK ABSORBER- piston-operated oil-filled cylinder that prevents coil spring oscillations.

DUST SHIELD- metal shroud or rubber boot that keeps road dirt off the shock absorber.

LOWER SPRING SEAT- lower mount formed around the body of the shock absorber for the coil spring.

COIL SPRING- supports the weight of the vehicle and allows for suspension action.

UPPER STRUT SEAT- holds the upper end of the coil spring and contacts the strut bearing.

STRUT BEARING- a ball bearing that allows the shock absorber and coil spring assembly to rotate for steering action.

RUBBER BUMPERS- jounce and rebound bumpers which prevent metal-to-metal contact during extreme suspension compression and extension.

RUBBER ISOLATORS- parts of the strut damper which prevents noise from being transmitted into the body structure of the vehicle.

UPPER STRUT RETAINER- mounting that secures the upper end of the strut assembly to the frame or unitized body.

In a MacPherson strut type suspension, only one control arm and a strut is used to support each wheel assembly. A conventional lower control arm attaches to the frame and to the lower ball joint. The ball joint holds the control arm to the steering knuckle or bearing support. The top of the steering knuckle or bearing support is bolted to the strut. The top of the strut is

Figure 8-9.- Exploded and cutaway views of a strut assembly.

bolted to the frame or reinforced body structure. This type of suspension is the most common type used on late model passenger vehicles. The advantages are a reduced number of parts in the suspension system, lower unsprung weight, and a smoother ride.

On some vehicles you may find a MODIFIED STRUT SUSPENSION that has the coil springs mounted on the top of the control arm, not around the strut.

Stabilizer Bar
The stabilizer bar, as shown in figure 8-6, also called the sway bar, is used to keep the body of the vehicle from leaning excessively in sharp turns. Made of spring steel, the stabilizer bar fastens to both lower control arms and to the frame. Rubber bushings fit between the stabilizer bar, the control arms, and the frame.

When the vehicle rounds a corner, centrifugal force tends to keep the vehicle moving in a straight line. Therefore, the vehicle "leans out" on the turn. This lean out is also called a body roll. With lean out, or body roll, additional weight is thrown on the outer spring. This puts additional compression on the outer spring, and the control arm pivots upward. As the control arm pivots upward, it carries its end of the stabilizer bar up with it. At the inner wheel on the turn, there is less weight on the spring. Weight has shifted to the outer spring because of centrifugal force. Therefore, the inner spring tends to expand. The expansion of the inner spring tends to pivot the lower control arm downward. As this happens, the lower control arm carries its end of the stabilizer bar downward.

The outer end of the stabilizer bar is carried upward by the outer control arm. The inner end is carried downward. This combined action twists the stabilizer bar. This action twists the stabilizer bar and its resistance to this twisting action limits body lean in corners.