suspension and steering systems appendix...

NAPA FastTrack Counter Sales Training –Suspension and Steering Systems

Suspension and Steering Systems

Figure 1. Typical Late-Model Suspension

Introduction The suspension system (Figure 1) performs a very complicated function. It must keep the wheels lined up with the axis and direction of travel of the vehicle, provide steering control for the driver under all road conditions, and help the vehicle ride smoothly and comfortably. The purpose of the steering system is to turn the front wheels. The wheels constantly change direction when a driver switches lanes, rounds turns, and maneuvers roadway obstacles.

Suspension System Components Most automotive suspension systems have the same basic components and operate similarly. Their differences are found in the way the basic components (springs, shock absorbers, and struts) are arranged.


Springs The spring absorbs shock forces while maintaining the correct riding height. Various types of springs are used in suspension systems: air, coil, torsion bar, and leaf springs (Figure 2). Springs are rubber mounted to reduce road shock and noise.

Figure 2. Four Basic Types of Springs

Air springs are nothing more than thick-skinned balloons. When the air pressure in the spring is low, slight movements of the vehicle compress the spring. If the air pressure in the spring is high, higher pressures from the road are required to compress the air and allow the spring to absorb the road shock. Some electronic-ride control systems use an air spring assembled in a strut, as shown in Figure 3.


Figure 3. Air Spring Incorporated into a Strut for Electronic Ride Control

There are two basic designs of coil springs: linear rate and variable rate. Linear rate springs have a cylindrical shape and a consistent wire diameter. Variable rate springs use a combination of wire sizes and shapes. The most commonly used variable rate springs have a consistent wire diameter, are wound in a cylindrical shape, and have unequally spaced coils. This type of spring is called a progressive rate coil spring. Although leaf springs were the first type of suspension springs used on automobiles, today they are generally found only on light-duty trucks, vans, and some passenger cars. There are three basic types of leaf springs: multiple leaf, monoleaf, and fiber composite. Multiple-leaf springs consist of a series of flat steel leaves that are bundled together and held with clips or by a bolt placed slightly ahead of the center of the bundle. One leaf, called the main leaf, runs the entire length of the spring. The next leaf is a little shorter and attaches to the main leaf. The next leaf is shorter yet and attaches to the second leaf, and so on. Leaf springs are commonly mounted at right angles to the axle (Figure 4). In addition to cushioning the ride, they also serve as suspension and axle locators. A centering pin is often used to keep the axle correctly located.


Figure 4. Typical Leaf Spring Rear Suspension

The front eye of the main leaves are attached to a bracket on the frame of the vehicle with a bolt and bushing connection. The rear eye of the main leaves are attached to the frame by a shackle. The shackle allows some back and forth spring movement in response to physical forces of acceleration, deceleration, and braking. Monoleaf or single-leaf springs are made with a heavy or thick center that tapers off at both ends. This provides a variable spring rate for a smooth ride and good load-carrying ability. While most leaf springs are made of steel, fiber composite springs have increased in popularity. These springs are made of fiberglass. Torsion bars serve the same function as coil springs. In fact, they are often described as “straightened-out” coil springs. But instead of compressing like coil springs, a torsion bar twists and straightens out on the recoil. As the bar twists, it resists up-and-down movement. One end of the bar is attached to the vehicle’s frame. The other end is attached to the lower control arm. When a wheel moves up and down, the lower control arm is raised and lowered. This twists the torsion bar, which causes it to absorb road shocks. The bar’s natural resistance to twisting quickly restores it to its original position, returning the wheel to the road. Typically a torsion bar runs from front to rear. Either an A-arm or a single inner bushing control arm with a strut rod is used for the lower control arm. When the torsion bars are mounted from side to side, the bar runs across the width of the chassis back to the control arm.


Shock Absorbers Shock absorbers dampen and control suspension motion in a vehicle. They limit the movement of the springs. In spite of their name, they actually dampen spring movement instead of absorbing shock. A shock absorber works on the principle of fluid displacement on both its compression (jounce) and extension (rebound) cycles. A typical car shock has more resistance during its extension cycle than its compression cycle. Shock absorbers can be mounted either vertically or at an angle. Angle mounting of shock absorbers improves vehicle stability and dampens accelerating and braking torque. The mounting of a shock absorber determines its design and compressed and extended lengths (Figure 5). Identifying the correct replacement shock requires the proper identification of the vehicle. Shocks are replaced quite often to correct ride and handling problems.

Figure 5. How the Length of a Shock Absorber is Measured

Hydraulic shocks are available in two styles: the single-tube and double-tube shock. Most domestic shocks are double tubed. The double-tube shock has an outer tube that completely covers the inner tube. The area between the tubes is the oil reservoir. A compression valve at the bottom of the inner tube allows oil to flow between the two tubes. The piston moves up and down inside the inner tube.


In a single monoshock, there is a second floating piston near the bottom of the tube. When the fluid volume increases or decreases, the second piston moves up and down, compressing the reservoir. In addition to these hydraulic shocks, other shock designs are available. Spring-assist shock absorbers can be used to increase the load carrying capacity of the springs. They are available for the front or rear suspension. They resemble a conventional shock absorber with a coil spring fitted to it. This type of shock absorber is called a “load-stabilizing” shock absorber. Gas-charged shock absorbers operate on the same principle as hydraulic shocks – they use a piston and oil chamber. But instead of a double tube with a reserve chamber, gas-charged shocks have a dividing piston that separates the oil chamber from the gas chamber.

MacPherson Strut Suspension Components The MacPherson strut suspension (Figure 6) is dramatically different in appearance from the traditional independent front suspension, but similar components operate in the same way to meet suspension demands.

Figure 6. Main Parts of a MacPherson Strut Suspension

The MacPherson strut suspension combines the spring, upper suspension locator, and shock absorber fender panel. There is also a modified MacPherson strut system which has a spring located between the lower control arm and the frame, instead of around the strut (Figure 7).


Figure 7. Modified MacPherson Strut Front Suspension

The strut looks much like a shock absorber. In fact, the strut provides the damping function of the shock absorber, in addition to serving to locate the spring and to fix the position of the suspension. Struts fall into two broad categories: sealed and serviceable units. A sealed strut does not have a replaceable shock absorber. A serviceable strut does. These shocks are called strut cartridges. The suspension’s lower mounting is the frame. A strut still relies on a lower control arm and ball joint which serve as the lower locator for the suspension. MacPherson strut sus-pensions use sway, or stabilizer, bars. The lower ball joint is used to stabilize the steering and to retard shimmy. Coil springs are used on all strut suspensions. A mounting plate welded to the strut serves as the lower spring seat. The upper seat is bolted to the strut piston rod. A bearing or rubber bushing in the upper mount permits the spring and strut to turn with the motion of the wheel as it is steered.

Front Suspensions Front-suspension systems must keep the wheels rigidly positioned and at the same time allow them to steer right and left. Most independent front-suspension systems differ only in the way the components are arranged. A wheel spindle assembly consists of a wheel spindle and a steering knuckle. A wheel spindle is connected to the wheel through wheel bearings. The wheel spindle is the point at which the wheel hub and wheel bearings are connected. A steering knuckle is con-nected to control arms. In most cases, a steering knuckle and wheel spindle are forged to form a single piece.


Figure 8. Main Parts of a Front Suspension that uses Upper and Lower Control Arms

The upper and lower control arms (Figure 8) are basically used as suspension locators. They are attached to the frame with bushings that permit the wheel assemblies to move up and down separately in response to the road surface. The outer ends are connected to the wheel assembly with ball joints inserted through each arm into the steering knuckle. Worn or damaged control arms, ball joints, and/or control arm bushings are often replaced to correct wheel alignment problems. The ball joints allow the steering knuckle to pivot between the control arms when the car is steered. They also permit up and down movement of the control arm. In addition to springs and shock absorbers, other suspension control devices include bushings, stabilizer or sway bars, and strut rods. Rubber or polyurethane bushings are found on many suspension components such as the control arms, radius arms, and strut rods. They make good suspension system pivots. They also help to absorb road shock, allow some movement, and reduce noise entering the vehicle. A variety of devices are used with the basic suspension components to provide additional stability. One of the most common is the sway bar (Figure 9), also known as the antisway bar or stabilizer. This is a metal rod running between the opposite lower control arms. As the suspension at one wheel responds to the road surface, the sway bar transfers a similar movement to the suspension at the other wheel. The sway bar can be a one-piece, U-shaped rod fastened directly into the control arms with rubber bushings, or it can be attached to each control arm by a separate sway bar link. The arm is held to the links with nuts and rubber bushings and is also mounted to the frame in the center with rubber bushings. Sway bar bushings are often replaced to correct handling or noise problems. Entire sway bars are seldom replaced, except when the customer wants to improve handling and desires a larger diameter bar.


Figure 9. Front Suspension Equipped with a Sway Bar

Rear Suspension Systems There are three basic types of rear suspensions: live-axle, semi-independent, and independent. Live-axle suspensions are found on rear-wheel-drive (RWD) trucks, vans, and many four-wheel-drive (4WD) vehicles. Semi-independent systems are used on front-wheel-drive (FWD) vehicles. Independent suspensions can be Live-axle suspension systems (Figure 10) and consist of leaf or coil springs used with a live-axle (one in which the differential axle, wheel bearings, and brakes act as a unit). When coil springs are used, the rear axle needs forward and lateral control arms or links to hold the axle assembly in place. A semi-independent rear suspension is used on many front-wheel-drive models. On some, the suspension position is fixed by an axle beam, or cross member, running between two trailing arms. Although there is a solid connection between the two halves of the suspension because of the axle beam, the beam twists as the wheel assemblies move up and down. The twisting action not only permits semi-independent suspension movement, but it also acts as a stabilizer. A coil spring and shock absorber-strut assembly are ordinarily used with this suspension system. The bottom of the strut is mounted to the rear end of the trailing arm. The top is mounted to the reinforced inner fender panel. Independent rear suspensions can be found in large numbers on both FWD and RWD vehicles. Independent coil-spring rear suspensions can have several control arm arrangements. Coil springs are used between the control arm and the vehicle body. The control arms pivot on a cross member and are attached at the other end to a spindle. A shock absorber is attached to the spindle or control arm.


Figure 10. Live-Axle Suspension Equipped with a Coil or Leaf Spring

Some rear suspension systems have a lower control arm and open driving axles. A cross member supports the control arms, while the tops of the shock absorbers are mounted to the body. The springs are set in seats at the bottom and top of the cross member. The modified MacPherson strut rear suspension is very common for vehicles with front-wheel drive. The major parts on each side of the vehicle are the modified MacPherson shock strut, lower control arm, tie-rod, and wheel spindle. A coil spring mounts between the lower control arm and the body.

Steering Systems The steering system is composed of three major subsystems: the steering linkage, steering gear, and steering column and wheel. As the steering wheel is turned by the operator, the steering gear transfers this motion to the steering linkage. The steering linkage turns the wheels to control the vehicle’s direction. Although there are many variations to this system, these three major assemblies are in all steering systems. The term steering linkage refers to the system of pivots and connecting parts that is placed between the steering gear and the steering arms attached to the front wheels. The steering linkage transfers the motion of the steering gear output shaft to the steering arms, turning the wheels to steer the vehicle. A parallelogram (Figure 11) type of steering linkage is used with the recirculating ball steering gear and can be placed behind or ahead of the front-wheel suspension. The main parts of a parallelogram steering linkage system are the pitman arm, idler arm, links, and tie-rods.


Figure 11. Parallelogram Steering Linkage

The pitman arm connects the linkage to the steering column through a steering gear located at the base of the column. It transmits the motion it receives from the gear to the linkage, causing the linkage to move left or right to turn the wheels in the appropriate direction. The idler arm or idler arm assembly is normally attached, on the opposite side of the center link, from the pitman arm and to the car frame, supporting the center link at the correct height. A pivot built into the arm or assembly permits sideway movement of the linkage. Both pitman and idler arms tend to wear and are often replaced to correct steering or wheel alignment problems. If a customer is replacing these parts, make sure they have checked the rest of the steering linkage for wear. Links, depending on the design application, can be referred to as center, drag, or steering links. Their purpose is to control sideway linkage movement, which changes the wheel directions. Center links and drag links can be used either alone or in conjunction with each other, depending on the particular steering design. Tie-rods are the assemblies that make the final connections between the steering linkage and steering knuckles. They consist of inner tie-rod ends, which are connected to the opposite sides of the center link; outer tie-rod ends, which connect to the steering knuckles; and adjusting sleeves or bolts, which join the inner and outer tie-rod ends. Tie-rod ends are one of the most commonly replaced items in a front steering system. Rack and pinion steering (Figure 12) is lighter in weight and has fewer components than parallelogram steering. Steering input is received from a pinion gear attached to the steering column. This gear moves a toothed rack that is attached to the tie-rods. In a rack and pinion steering system, there is no pitman arm, idler arm assembly, or center link. The tie-rods are the only steering linkage parts used in a rack and pinion system. Most rack and pinion units are a tube in which the steering rack can slide. The rack is a rod with gear teeth on one end. It meshes with the teeth of a small pinion at the end of the steering column. The two inner tie-rod ends, attached to the rack, are covered by rubber bellows boots that protect the rack from contamination. The inner tie-rod connects to outer tie-rod ends, which connect to the steering arms.


Figure 12. Rack and Pinion Steering Gear and Linkage

The purpose of the steering gear is to change the motion of the steering wheel into a motion that will move the steering linkage. There are two styles currently in use. Rack and pinion steering is one type of steering gear assembly. The other common steering gear is the recirculating ball. The recirculating ball steering gear is generally found in larger cars. It relies on a sector shaft, a ball nut, and wormshaft. As the steering wheel is rotated, the wormshaft rotates, causing the ball nut to move up or down the wormshaft. The movement of the nut causes the sector shaft to rotate and swing the pitman arm. Movement of the steering wheel and shaft turns the steering gear. Most steering columns today are collapsible types. These columns reduce the chance of injury to the driver if an accident occurs.

Power Steering The power-steering unit is designed to reduce the amount of effort required to turn the steering wheel. There are several power-steering systems in use on passenger cars and light-duty trucks. The most common ones are the external-piston linkage, integral-piston, and power-assisted rack and pinion system. A power-steering gearbox is basically the same as a manual gearbox with the addition of an hydraulic assist. The external-piston linkage system is no longer common and is generally found on older cars. It consists of a power-steering pump and reservoir, control valve, power cylinder, and four hoses. The pressure hose connects the pump to the control valve and directs the pressurized fluid into the correct booster hoses to the power cylinder. The power cylinder piston provides the necessary power assistance. The integral-piston system is a very common power-steering system. It consists of a power-steering pump and reservoir, power-steering pressure and return hoses, and steering gear (Figure 13). The power cylinder and the control valve are in the same housing as the steering gear.


Figure 13. Typical Location of a Power Steering Pump

The power-assisted rack and pinion system is similar to the integral system because the power cylinder and the control valve are in the same housing. The rack housing acts as the cylinder and the power piston is part of the rack. Control valve location is in the pinion housing. Turning the steering wheel moves the valve, directing pressure to either end of the rack piston (Figure 14). The system utilizes a pressure hose from the pump to the control valve housing and a return line to the pump reservoir. The steering pump is used to develop hydraulic pressure, which provides the force needed to operate the steering gear. The pump is belt-driven from the engine crankshaft, providing pressure any time the engine is running. It is usually mounted near the front of the engine. The primary purpose of power-steering hoses is to transmit power (fluid under pressure) from the pump, to the steering gearbox, and to return the fluid ultimately to the pump reservoir. Hoses are generally a reinforced synthetic rubber material coupled to metal tubing at the connecting points. The material that these hoses are made from is specially formulated to resist breakdown or deterioration due to oil or temperature conditions. If the exact hose the customer needs is not in stock, a hose can be made up to meet their needs. These hoses have the correct fittings installed and are assembled to hold the high pressure within the system.


Figure 14. Rack and Pinion Steering Gear and Linkage

Key Terms Antisway bar – also known as the sway bar or stabilizer. This is a metal rod running between the opposite lower control arms to control body sway. Ball joint – a device that allows movement in the suspension while keeping the parts firmly attached to one another. Center link – controls sideway linkage movement, which changes the wheel directions. Control arms – suspension locators that are attached to the frame with bushings that permit the wheel assemblies to move up and down separately in response to the road surface. External-piston linkage – a power-steering system generally found on older cars. It consists of a power-steering pump and reservoir, control valve, an external power cylinder, and four hoses. 4WS – four-wheel steering. Idler arm – is normally attached on the opposite side of the center link, from the pitman arm and to the car frame, supporting the center link at the correct height. Integral-piston linkage – a common power-steering system. It consists of a power-steering pump and reservoir, power-steering pressure and return hose, and steering gear. Linear rate springs – coil springs that have a cylindrical shape and a consistent wire diameter providing a fixed tension. MacPherson strut suspension – an independent front suspension system that combines the spring, upper suspension locator, and shock absorber into one assembly.


Main leaf – the primary leaf of a multiple leaf spring arrangement, it runs the entire length of the spring. Monoshock – a single shock with a second floating piston near the bottom of the tube. Parallelogram linkage – the type of steering linkage that is used with the recirculating ball steering gear and can be placed behind or ahead of the front-wheel suspension. Pitman arm – connects the linkage to the steering column through a steering gear located at the base of the column. It transmits the motion it receives from the gear to the linkage. Rack and pinion steering – steering input is received from a pinion gear attached to the steering column. This gear moves a toothed rack that is attached to the tie-rods. Recirculating ball steering gear – generally found in larger cars, it relies on a sector shaft, a ball nut, and wormshaft to transmit the movement of the steering wheel to the steering linkage. Shock absorber – a device used to dampen or control suspension motion in a vehicle. Steering knuckle – connected to the control arms and allows the wheels to pivot in response to the steering gear. Steering linkage – the system of pivots and connecting parts that is placed between the steering gear and the steering arms attached to the front wheels. Sway bar – also known as the antisway bar or stabilizer. This is a metal rod running between the opposite lower control arms to control body sway. Tie-rods – the assemblies that make the final connections between the steering linkage and steering knuckles. Torsion bars – serve the same function as springs, but rely on the twisting of a bar. Variable rate springs – coil springs that use a combination of wire sizes and shapes and provide a different spring rates during operation. Wheel spindle – the assembly that the wheel rotates on.

suspension and steering systems appendix...

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