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ASE P2 Test Preparation Guide: Steering & Suspension


11/17/2008

WHAT YOU NEED TO KNOW: • Identify major suspension & steering components • Identify suspension & steering component functions • Identify related items • Provide basic use, installation and warranty information
 

MAJOR COMPONENTS & FUNCTIONS

* Springs — Support the vehicle’s weight at each wheel. Various types include coil, leaf, torsion bar and air springs. Leaf springs are typically used on the rear axle only. Air springs (rubber bladders filled with air) are used on some vehicles with electronically-controlled suspensions. Coil springs can sag with age, causing a loss of ride height and changes in wheel alignment. Coil springs should be replaced if ride height is less than specifications. Coil springs may also fatigue and break. This can sometimes happen when the protective plastic coating around the spring is damaged, allowing rust to concentrate in one spot. Leaf springs may also crack, bend or break.

Rubber air springs can develop leaks that prevent the vehicle from maintaining normal ride height. This may overwork the compressor and cause it to fail.

Various types of replacement springs are available. Heavy-duty or “overload” springs are stiffer to increase a vehicle’s load carrying capacity, but also increase ride harshness. “Variable rate” coil springs combine different coil spacing to increase load capacity without sacrificing ride comfort. Springs are usually replaced in pairs.

* Ball joints — Flexible sockets in a vehicle’s suspension that connect the control arms to the steering knuckle. Suspensions that have upper and lower control arms (called Short Long Arm or SLA) have four ball joints (two upper and two lower). Vehicles with strut suspensions usually have only two lower ball joints, though some may also have an upper wishbone control arm with one or more ball joints. Some FWD cars also have ball joints in the rear suspension.
 
A ball joint is so named because of its ball-and-socket construction. The ball stud may ride against a metal gusher bearing, or it may be highly polished to reduce friction and be enclosed in a polymer bearing. Most low-friction ball joints are sealed and do not have a grease fitting for lubrication. When ball joints wear, they becomes loose. This may cause suspension noise and misalignment. A ball joint failure will cause the suspension to collapse.

* Control arm bushings — Bushings located in the pivot points of the control arms. Usually rubber, bushings can crush and crack with age causing undesirable changes in wheel alignment.

* Sway bars —A horizontal bar that connects the lower control arms on the left and right sides of the front suspension together. The bar resists twisting motions to reduce body sway and roll when cornering. Some vehicles also have rear sway bars for added handling control.

* Shock absorbers — Dampen suspension motions to improve ride control and handling. The shock absorber is an add-on component mounted near the wheel and connected to one of the control arms. Inside is an oil-filled cylinder with a piston and valves. Movement of the suspension forces the piston to push against the oil. This creates friction and resistance to dampen the suspension.

The two basic types of shocks are twin-tube and monotube. Twin-tube shocks have an oil reservoir around the outside of the piston chamber. Oil moves back and forth from the chamber through valves in the bottom of the shock. With monotube shocks, there is no outer chamber. One end of the shock is filled with a highly pressurized gas charge (up to 360 psi), and a floating piston seal separates the gas charge from the oil. Twin-tube shocks may also be pressurized with nitrogen gas to reduce fluid foaming and shock fade.

Shocks are a wear item and eventually lose their ability to dampen the suspension because of seal wear. A leaky shock must be replaced. Symptoms of worn shocks include a rough, bouncy ride, excessive body sway, bottoming or rocking after hitting bumps, and poor handling stability. Badly worn shocks can also increase tire wear and may increase stopping distances on rough roads.

Replacement shocks with larger piston bores, increased gas pressure, adjustable dampening rates or other special features can be installed to upgrade ride control performance. Shocks are usually replaced in pairs.
 * Struts —An oversized shock absorber that is also part of the vehicle’s suspension. Front MacPherson struts replace the upper control arm and upper ball joint. MacPherson struts also have coil springs around them. Like shocks, struts may be twin-tube or monotube and gas charged to improve fade resistance and handling.

At the top of front struts is a bearing plate that allows the strut housing to pivot when the wheels are steered. Wear in the bearing plate can cause steering noise. Binding may increase steering effort and prevent the wheels from recentering following a turn. The bearing plate can be replaced separately if necessary.

*On some vehicles, a modified strut configuration is used where the spring is not around the strut but is located between the lower control arm and subframe. On “wishbone” strut suspen­sions, the strut supports the weight but an upper control arm is also used to locate the steering knuckle. Struts may need to be replaced if the shock absorbing element is worn or leaking, or if the strut housing is bent or damaged. A bent strut will upset wheel alignment and may cause uneven tire wear or a steering pull.

*Tie rod ends — Flexible sockets on the ends of the tie rods in the steering linkage. They allow the linkage to pivot and follow suspension motions. If worn, they can cause steering looseness and tire wear. Left and right side tie rod ends are usually different and may have reversed threads.

* Rack and pinion Steering — Uses a pinion gear on the end of the steering input shaft to move a horizontal bar (rack) sideways. The rack is connected to the tie rods with sockets, which are enclosed in rubber bellows. The linkage has outer tie rod ends only. On some GM applications, a “center-mount” rack is used where the tie rods bolt to the center of the rack rather than the ends. Worn inner sockets can cause steering looseness and tire wear.

* Recirculating ball steering — A type of steering gear that uses balls to reduce friction and transmit steering input. The steering box is connected to the steering linkage with a “Pitman Arm”. An “Idler Arm” supports the other side of the linkage, which includes a “Center Link”, inner and outer tie rod ends and tie rods. This type of steering is used primarily in trucks and older RWD cars. Steering wander and looseness can result if the idler arm is worn.
* Power steering pump — Provides hydraulic pressure to assist the power steering gear or rack. The pump is belt-driven by the engine. Noise and leaks are common problems that require pump replacement.

RELATED ITEMS
* Power steering fluid — Various types are required depending on application, so follow manufacturers recommendations.
* Power steering hose — High-pressure supply hose and a low-pressure return hose connect the PS pump to the steering rack or gear. Replace hoses if leaking.
* Chassis grease — For lubricating ball joints and tie rod ends.
* Steering and suspension service tools — May be required for replacing springs, struts, ball joints, tie rod ends or control arm bushings.

INSTALLING SUSPENSION & STEERING PARTS 
Replacing coils springs may require a spring compressor (usually necessary for front springs, but sometimes not needed for rear coil springs).

Replacing a MacPherson strut that has a coil spring also requires a spring compressor, unless a preassembled strut is being installed (in which case no spring compressor is needed). This is not necessary with shock absorbers.















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