12.2 Suspension Systems

The suspension system works with the tires, frame or unitized body, wheels, wheel bearings, brake system, and steering system. All the components of these systems work together to provide a safe and comfortable means of transportation. The suspension system functions are as follows:

Independent Suspension

The independent suspension allows one wheel to move up and down with a minimum effect on the other wheels (Figure 12-4). Since each wheel is attached to its own suspension unit, movement of one wheel does NOT cause direct movement of the wheel on the opposite side of the vehicle. With the independent front suspension, the use of ball joints provides pivot points for each wheel. In operation, the swiveling action of the ball joints allows the wheel and spindle assemblies to be turned left and right and to move up and down with changes in road surfaces. This type of suspension is most widely used on modern vehicles.


Figure 12-4 — Independent suspension.

Suspension System Components

The basic components of a suspension system are as follows:

Spring (supports the weight of the vehicle; permits the control arm and wheel to move up and down).

Control Arms and Bushings

The control arm, as shown in Figure 12-4 (above), holds the steering knuckle, bearing support, or axle housing in position as the wheel moves up and down. The outer end of the control arm has a ball joint, and the inner end has bushings. Vehicles having control arms on the rear suspension may have bushings on both ends. The control arm bushings act as bearings, which allow the control arm to move up and down on a shaft bolted to the frame or suspension unit. These bushings may be either pressed or screwed into the openings of the control arm.

Strut Rods

The strut rod fastens to the outer end of the lower control arm to the frame (Figure 12-5). This prevents the control arm from swinging toward the rear or front of the vehicle. The front of the strut rod has rubber bushings that soften the action of the strut rod.


Figure 12-5 — Strut rods.

These bushings allow a controlled amount of lower control arm movement while allowing full suspension travel.

Ball Joints

The ball joints are connections that allow limited rotation in every direction and support the weight of the vehicle (Figure 12-6). They are used at the outer ends of the control arms where the arms attach to the steering knuckle. In operation, the swiveling action of the ball joints allows the wheel and steering knuckle to be turned left or right and to move up and down with changes in road surface. Since the ball joint must be filled with grease, a grease fitting and grease seal are normally placed on the joint. The end of the stud on the ball joint is threaded for a large nut. When the nut is tightened, it force fits the tapered stud in the steering knuckle or bearing support.


Figure 12-6 — Ball joints.

Shock Absorbers and Struts

Shock absorbers are necessary because springs do not "settle down" fast enough. After a spring has been compressed and released, it continues to shorten and lengthen for a period of time. Such spring action on a vehicle would produce a very bumpy and uncomfortable ride. It would also be dangerous because a bouncing wheel makes the vehicle difficult to control; therefore, a dampening device is needed to control the spring oscillations. This device is the shock absorber.

The most common type of shock absorber used on modern vehicles is the double- acting, direct-action type because it allows the use of more flexible springs (Figure 12-7). 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.


Figure 12-7 — Double-acting, direct-action shock absorber.

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 basic components of a typical strut assembly are as follows (Figure 12-8):


Figure 12-8 — Exploded view of a strut assembly.

In a MacPherson strut type suspension, only one control arm and a strut are 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 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 12-4, 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.


Figure 12-4 — Independent suspension.
(Repeated here for reference)

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, and its resistance to this twisting action limits body lean in corners.

Torque Arms

On vehicles with a high-performance suspension, you may encounter torque arms (Figure 12-9). These arms work with the axle to reduce axle wind up. Axle wind up occurs when the vehicle is accelerating; the torque is transferred through the axle housing and can actually spin the axle housing under the vehicle.


Figure 12-9 — Torque arm.

A torque arm provides additional resistance to help prevent axle wind up. It is attached to the axle housing and runs forward under the vehicle parallel to the drive shaft between the rear axle and transmission, cushioned through a bracket to allow some flex.

Suspension System Springs

The vehicle body or frame supports the weight of the engine, the power train, and the passengers. The body and frame are 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 four basic types of automotive springs: coil, leaf, torsion bar, and air bag. 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, and axle housing are 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.

Leaf Spring

The leaf spring acts as a flexible beam on self-propelled vehicles and transmits the driving and breaking forces to the frame from the axle assembly. Leaf springs are semi- elliptical in shape and are made of high quality alloy steel. There are two types of leaf springs: the single leaf and the multileaf. The single leaf spring, or monoleaf, is a single layer spring that is thick in the center and tapers down at each end. Single leaf springs are used in lighter suspension systems that do not carry great loads. A multileaf spring is made up of a single leaf with additional leaves. The additional leaves make the spring stiffer, allowing it to carry greater loads.


Figure 12-10 — Multileaf spring.

The most common type is the multileaf spring that consists of a single leaf with a number of additional leaves attached to it using spring clips (Figure 12-10). Spring clips, also known as rebound clips, surround the leaves at intervals along the spring to keep the leaves from separating on the rebound after the spring has been depressed. The clips allow the springs to slide, but prevent them from separating and causing the entire rebound stress to act on the master leaf. The multileaf spring uses an insulator (frictional material) between the leaves to reduce wear and eliminate any squeaks that might develop. To keep the leaves equally spaced lengthwise, use a center bolt for the multileaf spring. The center bolt rigidly holds the leaves together in the middle of the spring, preventing the leaves from moving off center. Each end of the largest leaf is rolled into an eye, which serves as a means of attaching the spring to the vehicle. Leaf springs are attached to the vehicle using a spring hanger that is rigidly mounted to the frame in the front, and the spring shackle in the rear, which allows the spring to expand and contract without binding as it moves through its arc. Bushings and pins provide the bearing or support points for the vehicle. Spring bushings may be made of bronze or rubber and are pressed into the spring eye. The pins that pass through the bushings may be plain or threaded. Threaded bushings and pins offer a greater bearing surface and are equipped with lubrication fittings. Leaf springs are used on the front and rear of heavy-duty trucks and the rear of passenger vehicles and light trucks. Trucks that carry a wide variety of loads use an auxiliary or overload spring. This auxiliary spring may be mounted on top of the rear springs and clamped together with long U-bolts, or it may be located under the axle separate from the main spring (Figure 12-11). In either case, the end of the spring has its own support brackets. When the truck is under a load, the auxiliary spring assumes part of the load when its ends contact the bearing plates or special brackets attached to the frame side rails.


Figure 12-11 — Auxiliary spring suspension.

A large portion of six-wheel drive vehicles utilize a bogie suspension which uses leaf springs (Figure 12-12). This suspension is a unit consisting of two axles joined by torque rods. A trunnion axle acts as a pivot for the drive axles and is supported by bearings that are part of the spring seat. The ends of each spring rest in the guide brackets bolted to the axle housings. Mounting the springs on a central pivot enables them to distribute half of the rear load onto each axle. As a result, this type of suspension allows the vehicle to carry a much heavier load than a single axle without losing its ability to move over unimproved terrain.


Figure 12-12 — Bogie suspension.

When one wheel of a bogie suspension is moved up or down because of an irregularity in the road, the spring pivots on the trunnion shaft and both ends of the spring deflect to absorb the road shock. This causes the load to be placed on the center of the spring resulting in equal distribution of the load to both axles. The torque rods ensure proper spacing and alignment of the axles and transmit the driving and braking forces to the frame.

Torsion Bar

The torsion bar consists of a steel rod made of spring steel and treated with heat or pressure to make it elastic so it will retain its original shape after being twisted. Torsion bars, like coil springs, are frictionless and require the use of shock absorbers. The torsion bar is serrated on each end and attached to the torsion bar anchor at one end and the suspension system at the other end (Figure 12-13). Torsion bars are marked to indicate proper installation by an arrow stamped into the metal. It is essential that they be installed properly because they are designed to take the stress in one direction only. The up-and-down movement of the suspension system twists the steel bar. The torque resistance will return the suspension to its normal position in the same manner as a spring arrangement.


Figure 12-13 — Torsion bar.

Coil Spring

The coil spring is made of round spring steel wound into a coil (Figure 12-14). 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 shape requires less space to operate in. Pads are sometimes used between the spring and the chassis to eliminate transferring vibrations to the body. Because of its design, the coil spring cannot be used for torque reaction or absorbing side thrust. Therefore, control arms and stabilizers are required to maintain the proper geometry between the body and suspension system. This is the most common type of spring found on modern suspension systems.


Figure 12-14 — Coil spring.

Coil spring mountings are quite simple in construction. The hanger and spring seat are shaped to fit the coil ends and hold the spring in place. Cups that fit snugly on each coil end are often used for mounting. The upper cup can be formed within the frame, in the control arms, or as part of a support bracket rigidly fixed to the cross member or frame rail. The lower cup is fastened to a control arm hinged to a cross member or frame rail. Rubber bumpers are included on the lower spring support to prevent metal-to-metal contact between the frame and control arm as the limits of compression are reached.

Air Bags

The air bag is a rubber air chamber that is replacing either of the aforementioned springs, usually in your higher end luxury vehicles (Figure 12-15). The unit is closed at the bottom by a piston fitted into a control arm or strut shock absorber. The top usually provides a means for inflating and monitoring the pressure within the bag. This bag replaces the metal spring that is usually installed to provide suspension in most vehicles.


Figure 12-15 — Air bag suspension.

An onboard air compressor will charge the air bag to a set pressure. Usually air bags are installed to provide a level ride for the vehicle. Sensors are in place to check if the vehicle needs more or less air in each bag.

Suspension System Service

A suspension system undergoes tremendous abuse during normal vehicle operation. Bumps and potholes in the road surface cause constant movement, fatigue, and wear of the shock absorbers, or struts, ball joints, bushings, springs, and other components.

Suspension system problems usually show up as abnormal noises (pops, squeaks, and clunks), tire wear, steering wheel pull, or front end shimmy (side-to-side vibration).

Suspension system wear can upset the operation of the steering system and change wheel alignment angles. Proper service and maintenance of these components greatly increase reliability and vehicle life.

Suspension Bushing Service

Rubber bushings are commonly used in the inner ends of front control arms and rear control arms. These bushings are prone to wear and should be inspected periodically.

Worn control arm bushings can let the control arms move sideways. This action causes tire wear and steering problems. To check for control arm bushing wear, try to move the control arm against normal movement. For example, pry the control arm back and forth while watching the bushings. If the control arm moves in relation to its shaft, the bushings are worn and must be replaced.

Generally, to replace the bushings in a front suspension requires the removal of the control arm. This usually requires the separation of the ball joints and compression of the coil spring. The stabilizer bar and strut rod are also unbolted from the control arm. The bolts passing through the bushings are then removed, which allows for the control arm to be removed from the vehicle. With the control arm placed in a vise, either press or screw out the old bushings and install new ones.

With new bushings installed, replace the control arm in reverse order. Torque all bolts to the manufacturer’s specifications. Install the ball joint’s cotter pin. Check the manufacturer’s service manual for information concerning preloading control arm bushings.

 

Note

Always refer to the manufacturer’s service manual for exact directions and specifications. This will assure a safe, quality ride.

Ball Joint Service

Worn ball joints cause the steering knuckle and wheel assembly to be loose on the control arm. A worn ball joint may make a clunking or popping sound when turning or driving over a bump. Ball joint wear is usually the result of improper lubrication or prolonged use. The load-carrying ball joints support the weight of the vehicle while swiveling into various angles. If the joints are improperly lubricated (dry), the swiveling action will cause them to wear out quickly.

Grease fittings are provided for ball joint lubrication. If the ball joint has a lube plug, it must be removed and replaced with a grease fitting. Using a hand-powered grease gun, inject only enough grease to fill the boot of the ball joint. Do not overfill the boot, because too much grease will rupture it. A ruptured boot will allow dirt to enter the joint, which causes the joint to wear out quicker.

Ball joints can be checked for wear while the wheel is supported, as shown in Figures 12-16 and 12-17. Axial play or tolerance, also called vertical movement, is checked by moving the wheel straight up and down. The actual amount of play in a ball joint is measured with a dial indicator.


Figure 12-16 — Checking ball joints in front suspension with coil spring.

 


Figure 12-17 — Checking ball joints in front suspension with a torsion bar.

Figure 12-18 shows the dial indicator clamped to the lower control arm. The dial indicator tip rests against the leg of the steering knuckle. With a pry bar, try to raise and lower the steering knuckle. If you use too much force, the ball joint may give you a false reading. You want to measure the movement of the wheel and ball joint as the joint is moved up to the load position. Note the movement as indicated on the dial indicator.


Figure 12-18 — A dial indicator mounted to measure the amount of end play in a ball joint.

Rocking the wheel in and out at the top and bottom checks radial play or tolerance. This action also is known as horizontal movement. Grasp the tire at the top and bottom, and try to wobble it. However, now we are assuming that the wheel bearings have been checked and either adjusted or properly tightened. Therefore, we are now checking the horizontal movement of the ball joints. Some manufacturers do not accept horizontal movement as an indicator of ball joint wear.

The actual specifications for allowable wear limits of the ball joints are listed in the manufacturer’s service manual. Refer to the specifications for the vehicle you are checking. Any ball joint should be replaced if there is excessive play.

Ball joint replacement can usually be done without removing the control arm. Generally, place the vehicle on jack stands. Remove the shock absorber and install a spring compressor on the coil spring. Unbolt the steering knuckle and separate the steering knuckle and ball joint. The ball joint may be pressed, riveted, bolted, or screwed into the control arm. If the ball joint is riveted to the control arm, replace the rivets with bolts.

 

Note

For exact ball joint removal and installation procedures, consult the manufacturer’s service manual.

Strut Service

The most common trouble with a strut type suspension is worn shock absorbers. Just like conventional shock absorbers, the pistons and cylinders inside the struts can begin to leak. This reduces the dampening action and the vehicle rides poorly. When a strut shock absorber leaks, it must be replaced, and ALWAYS as a pair.

Basically, strut removal involves unbolting the steering knuckle (front suspension) or bearing support (rear suspension), any brake lines, and the upper strut assembly-to- body fasteners. Remove the strut assembly (coil spring and shock) as a single unit.

 

CAUTION

Do NOT remove the nut on the end of the shock rod or the unit can fly apart.

A strut spring compressor is required to remove the coil spring from the strut. After the coil spring is compressed, remove the upper damper assembly. With the upper damper assembly removed, release the tension on the coil spring and lift the spring off the strut. Inspect all parts closely for damage.

WARNING

When compressing any suspension system spring, be extremely careful to position the spring compressor properly. If the spring were to pop out of the compressor, serious injuries or death could result.

With the coil spring and upper damper unit removed, you can now remove the shock cartridge. A new shock cartridge can be installed in the strut outer housing to restore the strut to perfect condition. Some manufacturers recommend that the strut shock

absorber be rebuilt once the strut shock absorber is repaired or replaced. The strut can be reassembled and installed in reverse order of disassembly.

 

Note

For exact procedures for the removal, repairs, and installation of a strut assembly, refer to the manufacturer’s service manual.

Spring Service

Springs require very little periodic service. Leaf spring service usually involves bushing replacement. Torsion bars require adjustment, and coil springs require no periodic service.

Spring service requirements can be found in the service manual of the vehicle you are working on.

Spring fatigue (weakening) can occur after prolonged service. The fatigue lowers the height of the vehicle, allowing the body to settle toward the axles. This settling or sagging changes the position of the control arms, resulting in misalignment of the wheels. This condition also affects the ride and appearance of the vehicle.

To check spring condition or torsion bar adjustment, measure curb height (distance from a point on the vehicle to the ground). Place the vehicle on a level surface. Then measure from a service manual specified point on the frame, body, or suspension down to the shop floor. Compare the measurement to the specifications in the service manual. If the curb height is too low (measurement too small), replace the fatigued springs or adjust the torsion bar.

 

Note

For instructions on the removal and installation of springs, refer to the manufacturer’s service manual.

The vehicle should also be at curb weight when checking spring condition and curb height. Curb weight is generally the total weight of the vehicle with a full tank of fuel and

no passengers or cargo. Also, make sure nothing is in the passenger compartment that could possibly increase curb weight. Curb weight is given in pounds or kilograms.

Test your Knowledge

2. In a vehicle equipped with MacPherson struts, what components are required to support the front-wheel assembly?

A. Strut assembly and lower control arm
B. Steering knuckle and upper damper unit
C. Strut assembly and upper damper unit
D. Steering knuckle and lower control arm