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.

SUSPENSION SYSTEM SPRINGS

The vehicle body or frame supports the weight of the engine, the power train, and the passengers. The body and frame is supported by the springs on each wheel. The weight of the frame, body, and attached components applies an initial compression to the springs. The springs compress further as the wheels of the vehicle hit bumps or expand such as when the wheels drop into a hole in the road. The springs cannot do the complete job of absorbing road shocks. The tires absorb some of the irregularities in the road. The springs in the seats of the vehicle also help absorb shock. However, the passengers feel little shock from road bumps and holes.

The ideal spring for an automotive suspension should absorb road shock rapidly and then return to its normal position slowly; however, this action is difficult to attain. An extremely flexible, or soft, spring allows too much movement. A stiff, or hard, spring gives too rough a ride. To attain the action to produce satisfactory riding qualities, use a fairly soft spring with a shock absorber.

Spring Terminology

There are three basic types of automotive springs - coil, leaf, and torsion bar. Before discussing these types of springs, you must understand three basic terms - spring rate, sprung weight, and unsprung weight.

SPRING RATE refers to the stiffness or tension of a spring. The rate of a spring is the weight required to deflect it 1 inch. The rate of most automotive springs is almost constant through their operating range, or deflection, in the vehicle. Hooke's law, as applied to coil springs states: that a spring will compress in direct proportion to the weight applied. Therefore, if 600 pounds will compress a spring 3 inches, then 1,200 pounds will compress the spring twice as far, or 6 inches.

SPRUNG WEIGHT refers to the weight of the parts that are supported by the springs and suspension system. Sprung weight should be kept HIGH in proportion to unsprung weight.

UNSPRUNG WEIGHT refers to the weight of the components that are NOT supported by the springs. The tires, wheels, wheel bearings, steering knuckles, or axle housing is considered unsprung weight. Unsprung weight should be kept LOW to improve ride smoothness. Movement of high unsprung weight (heavy wheel and suspension components) will tend to transfer movement into the passenger compartment.

The coil spring is made of round spring steel wound into a coil (fig. 8-10). Because of their simplicity, they are less costly to manufacture and also have the widest application. This spring is more flexible than the leaf spring, allowing a smoother reaction when passing over irregularities in the road. Coil springs are frictionless and require the use of a shock absorber to dampen vibrations. Their cylindrical