1 Suspensions AUTOMOTIVE COMFORT AND CONTROL In this study unit, you ll learn about common automotive suspension systems, the components that make up these systems, and how the systems and components are serviced. As you may already know, an automobile s suspension system is responsible for providing a comfortable ride. However, this suspension is also directly related to the directional control of the vehicle; that is, the suspension and steering systems are so interrelated that a suspension problem can quickly lead to steering problems. While this study unit will focus on suspension systems and their component parts, we ll begin by discussing how steering and suspension systems work together to provide a comfortable, in-control ride under a range of operating conditions. A steering system allows the driver to control the vehicle s direction. In a later study unit, you ll learn about various steering system components and the two basic types of systems: parallelogram and rack-andpinion systems. The suspension system supports the vehicle s weight while working to maintain contact between the tires and the road. Without an adequate suspension system to smooth the ride, irregularities in the road surface, such as bumps and dips, could cause tires to momentarily leave the road surface. This would interrupt traction, interfere with the ability to steer the vehicle, and potentially lead to a loss of control. You should begin to recognize how the steering and suspension systems are related. Horse-drawn wagons first incorporated crude suspension systems, often consisting of simple leaf springs. Some of these systems were the first independent suspension systems (ones that let each wheel rise and fall independent of the three other wheels). Some of today s suspension components look much like early suspension parts while others take advantage of high-tech enhancements like computercontrolled suspension response and road-surface compensation. The most advanced systems take advantage of modern computer-control capabilities to adjust the suspension to match almost any imaginable road and driving conditions. There are three major subsystems that make up the overall automotive suspension system: the frame, the front suspension, and the rear suspension. All automobiles have a front and rear suspension system, yet
2 Suspensions these systems operate independently from each other. Before discussing the front- and rear-suspension systems in more depth, let s briefly discuss the types of frames commonly found in automobiles. AUTOMOTIVE FRAMES An automobile s frame serves two primary roles: support and absorption. It relies on a strong construction to support the vehicle s overall weight. It also provides attachment points for body panels and other components. At the same time, the frame must be able to absorb (without flexing) a wide range of substantial loads that are continuously applied whenever an automobile is in motion. Loads come from vertical wheel displacements caused by irregularities in the road surface, forces resulting from braking and cornering, and the dynamic (always changing) torque delivered by the engine and other framemounted drive train components. There are three general types of frame systems to be considered: full frames, subframes, and unitized body construction. Unitized body construction combines the features of an automotive frame with those of an automobile s body. Full Frames As the term suggests, a full frame is one that supports the entire vehicle. These are used most often with heavy vehicles (trucks) or high performance-type vehicles that will encounter exceptional load on the frame. Figure 1 shows the three types of full frames. The ladder frame in Figure 1A gets its name from the cross member configuration that gives the appearance of rungs on a ladder. These cross members connect two side rails that are positioned somewhere between the center line and the outside edge of the bottom of the vehicle. When the side rails follow the outside edge of the vehicle bottom, the frame is considered a perimeter design (Figure 1B). By adding strength, this frame design improves vehicle safety in side-impact accidents. Figure 1C shows an X-frame design. Obviously, it gets its name from the shape of its intersecting side members. This type of frame is especially popular in front-wheel-drive applications where there s no need to leave space for a driveshaft. X-frames are also used to provide extra support in automobiles equipped with convertible tops.
Suspensions 3 FIGURE 1 Full frame designs can be grouped into three classifications: ladder (A), perimeter (B), or X-frame (C). (Courtesy of Chrysler Corporation)
4 Suspensions Subframes Front and rear subframes support selected systems as full frames do, but rear and front subframes aren t directly connected to each other. Rear subframes support rear-suspension components (like shocks or springs) and sometimes rear-body components. Front subframes support either the engine or front-suspension components or both. Subframes alone don t take the place of a full frame. The rear and front subframes are connected to something called a unitized body (also known as unibody). Unitized Body Construction Most modern front-wheel-drive automobile designs incorporate a unitized body design (Figure 2). This means that the body panels and their related supporting features (like door pillars, rocker panels, and the inner aprons) are joined together into a box-shaped structure. It s this box-shaped structure that takes the place of the heavy steel members found in a full frame design. While some unitized body designs support the weight of engines and suspensions without frame attachments, front and/or rear subframes are often attached to the unitized body. Even considering the weight of FIGURE 2 Unitized body construction relies on the strength of many interconnected sheet metal and structural components.
Suspensions 5 these partial frames, unitized body construction reduces vehicle weight when compared to full frame designs. Unitized body designs are more complicated than conventional frames but give automotive designers the flexibility to better distribute the effects of an impact caused by an accident throughout the vehicle. Thus, unitized body construction can improve the overall safety of the vehicle. FRONT SUSPENSIONS TYPES AND FUNCTIONS As you ve already learned, the suspension system of an automobile must perform two primary functions: to support the weight of the vehicle and absorb road shocks. Several types of front-suspension systems can be found in modern automobiles. All front-suspension systems are variations of two basic designs: solid front-axle systems and independent front-suspension systems. In this section, we ll discuss the features and benefits of several specific systems. It s important to understand the underlying operating principles of specific suspension systems these remain constant from one model to another. The names for the specific systems differ, however, from one vehicle manufacturer to another. All types of front-suspension systems share two common components: a spring and a damper (shock-absorbing) component. The words spring and damper have more significance than is often attached to them. Figures 3A and B show a spring and damper, respectively. Each of these components is capable of resisting change by exerting a reactive force. Springs resist a change in position. This change in position can be caused by a bump in the road, for instance, that pushes the vehicle s wheel towards the car. As the wheel moves upwards, it pushes on the spring and compresses it. The spring responds to this compression by pushing back with a force that s proportional to the distance it s compressed. In other words, when a spring is compressed six inches, it ll push back with twice as much force as when it s compressed three inches. Because of this, springs are rated (with a spring constant) in terms of pounds of force per inch of displacement. Engineers design springs for specific vehicles. Only the manufacturer s specified spring should be used. A damper (shock absorber) resists changes in motion. That means that a damper works to slow a body that s moving. The force it produces to slow the moving body is proportional to the speed of the body. If two wheels (on two different vehicles) travel over the same bump, the wheel on the car that s traveling faster will rise and fall over the bump faster than the wheel on the slower vehicle. A damper will resist the faster-moving wheel with a greater force than with the slower-moving
6 Suspensions FIGURE 3 Spring and dampening components exist in every type of suspension system. A spring reacts to a change in position, with a force proportionate to the distance involved (A). A damper reacts to a motion by working to slow down the system. wheel. In real-world applications, many suspension components behave as a combination spring and damper. You ll find it useful, as you work to understand how suspension systems combine springs and dampers to provide a safe, comfortable ride. Straight Front Axles Many trucks are equipped with straight front axles (also known as solid). The axle itself is made from steel in the shape of an I beam. At each end of the axle is a pivot, called a kingpin, which attaches to the wheel-supporting spindle (Figure 4). The kingpin s design allows the spindle and the front wheels to be steered. The front-suspension system relies on leaf springs that connect the axle to the frame. There s a disadvantage to this type
Suspensions 7 FIGURE 4 Straight front axles are now found only in certain utility vehicles and truck applications. of system. When one wheel goes over a bump, one side of the axle tilts and moves upward. This affects the orientation of the other wheel, causing it to tilt. This affects the vehicle s ability to handle properly. The strength and durability of the straight front-axle design is well suited to heavy-duty trucks and some utility vehicles. The straight front-axle design is no longer found in automobiles because of the rough ride and steering peculiarities. Short and Long Arm Suspensions The automobile industry s answer to steering problems associated with the straight front axle was an independent front-suspension system. This general type of suspension system is used in almost all modern automobiles and light trucks. One such system is shown in Figure 5. In the particular design shown in Figure 5A, a steering knuckle takes the place of the spindle in a straight axle installation and provides a mounting point for the wheel. The short upper and long lower control arms have each a ball joint (Figure 5B) to support the steering knuckle while allowing it to move up and down and to be turned to steer the vehicle. As you can see, this type of system allows one wheel to move up and down to follow road irregularities without moving the opposite wheel. As a result, the car can be controlled better and will be less affected by rough road surfaces. In early independent front-suspension systems, the upper and lower control arms were the same length. This resulted in a special tire-wear problem (Figure 5C). As a wheel traveled over a bump, the upper and lower control arm and steering/knuckle
8 Suspensions FIGURE 5 Independent front suspension allows for better control and handling of an automobile. assembly rotated, causing the bottom of the tire to move in and out. This resulted in excess tire wear. The answer to this design problem was the short and long arm (SLA) suspension system. In this design, the upper control arm is shorter than the lower arm. This means that when the wheel travels over a bump, the bottom of the tire maintains a more
Suspensions 9 constant position while the top of the tire moves in and out (by some small amount). In SLA systems, the control arm is referred to as having inner and outer ends. The inner end is the one closer to the vehicle center line while the outer end is nearest the wheel. The inner ends are mounted to the frame where rubber bushings are used as a method of vibration damping. The outer arms (both lower and upper) contain ball joints like the one shown in Figure 5B. These balls form the joints between the arms and the steering knuckle. Most technicians will refer to SLA front-suspension systems simply as two-control arm or upper- and lower-control arm front-suspension systems because the equal-length control arm designs have long since been eliminated from automobiles. While the design of SLA-type systems differs from one model of automobile to another, the basic operating principles are the same. Before we discuss these basic operating principles, keep in mind that this type of two-control arm front-suspension system is primarily found on rear-wheel-drive cars and light trucks. While you may encounter an exception, you won t normally see this particular design in a front-wheel-drive automobile. However, many of the components found in the SLA system are used in other independent rear-suspension designs most often found in front-wheeldrive automobiles. In a typical SLA system like the one shown in Figure 6, each wheel assembly is mounted on a spindle that is carried on a steering knuckle. The steering knuckle is supported on the upper and lower control arms by ball joints. The control arms, in turn, are mounted to the frame or front structure of the car so that they can pivot at their inner ends. This means that the outer ends of the control arms, which are connected by the steering knuckle and wheel spindle, will move up and down as the car travels over rough pavement. At the same time, each front wheel can be turned left or right to provide steering control. The FIGURE 6 This SLA-type independent front-suspension system incorporates upper and lower control arms. While considered a modern design, it generally won t be found on front-wheel-drive cars.
10 Suspensions ball joints permit this combination of movements the flexing of the assembly to absorb road shock and the pivoting of the steering knuckle, spindle, and wheel let the driver steer the car. Because pivot points and other points of motion can wear as they move, these parts are usually equipped with grease fittings for regular lubrication. A coil spring may be mounted (Figure 7) between the spring housing on the frame or frame member and the upper control arm. Springs may also be mounted between the lower control arm and the frame. Mounting on the lower control arm is more common. On some vehicles, torsion bars are used in place of coil springs. Torsion bars, which you ll learn more about shortly, perform the same function as coil springs and may be attached to either the upper or lower control arm. As you already know, a spring resists displacement in the suspension system. In an SLA-type system, shock absorbers, mounted inside of the coil springs, provide a damping action. FIGURE 7 Coil springs can be mounted between the upper control arm and the frame but are most often designed to fit between the two control arms as shown here. You ll learn more about each of the SLA system components later in this study unit. First, however, we ll discuss other types of independent front-suspension systems, including the most commonly encountered types found on front-wheel-drive vehicles. Twin I-Beam Suspensions The twin I-beam suspension is an independent front-suspension system that incorporates some of the strengthening features of a straight (solid) front-suspension system while allowing for independent movement of the front wheels. As the name twin suggests, each side of these axles are connected (through kingpins or ball joints) to the vehicle and has its own I-beam-shaped axle (Figure 8). The outer ends spindle, while
Suspensions 11 FIGURE 8 Twin I-beam suspension designs incorporate features of both the SLA and solid axle systems. (Courtesy of Chrysler Corporation) the inner ends mount to the frame through pivot brackets. Most twin I-beam systems use a radius arm to constrain the wheel from moving forward or backward, and use coil springs as suspension components. In installations that use leaf springs, the radius arm isn t needed. In a twin I-beam installation, up and down wheel movement will cause some tire translation, but much less than is experienced with a straight axle design. This type of system doesn t handle as well as other independent front designs, and isn t regularly found in passenger car installations. It is, however, very strong and durable. For that reason, twin I-beam systems are found on many four-wheel-drive and light truck models. Torsion-bar Suspensions As learned earlier, torsion bars can be used in place of the coil springs most often found in an SLA-type front-suspension system. In the newest torsion bar systems, there are two transverse (meaning left-to-right running) mounted bars that mount to the frame on one end through pivot cushion bushings. They work through a sway bar on the other end to replace the resistance-to-displacement qualities of a traditional coil spring (Figure 9). The bar is designed to resist torsional (twisting) motion just as a coil or leaf spring resists linear (straight line) motion.
12 Suspensions FIGURE 9 In this type of independent front-suspension system, torsion bars take the place of coil springs. (Courtesy of Chrysler Corporation) Earlier designs, called longitudinal torsion bar systems, used torsion bars that ran rearward from each lower control arm. Torsion bar systems eliminate problems with spring durability and usually allow for vehicle height adjustment. You ll learn more about both of these factors later in this study unit. MacPherson Strut Suspensions MacPherson-strut-suspension systems (also called single control-arm suspensions) combine a damper/spring assembly, together with a lower control arm, to form a relatively compact, lightweight suspension system. The elimination of an upper control arm makes this a good choice for front-wheel-drive design systems. Therefore, as most newer passenger cars are front-wheel-drive, the MacPherson strut system is the one you ll most often encounter in the workplace. With MacPherson strut suspensions (Figure 10), the shock absorber, strut, and spindle are combined in one assembly. This assembly is supported by the inner fender panel at the top and the steering knuckle at the bottom. On most designs, the vertically mounted shock-absorber assembly is surrounded by the coil spring and incorporates a replaceable damper cartridge. In addition, a strut rod or sway bar is connected to
Suspensions 13 FIGURE 10 The MacPherson strut suspension is commonly used on front-wheel-drive vehicles. the lower control arm and is sometimes referred to as a track control arm or transverse link. It s critical to understand that in a MacPherson system, all spring and strut loads are transferred to the upper mounting point of the strut assembly at the fender s inner panel. Figure 11 shows how the steering knuckle/strut/spring assembly is mounted in a typical installation. Note the vibration-limiting rubber components used in the upper attachment. In this type of installation, the strut assembly actually turns as the knuckle (and the wheel) is steered. The lower strut attachment point on the steering knuckle is shown in Figure 10. In this specific design, the lower control arm is directly connected to a frame cross member. The cross member itself is part of a front partial frame (which, in turn, is connected to a unitized body). In other installations, the lower control arm is connected directly to the main (a front-to-back running) frame member (Figure 12). This direct connection is made with pivoting supports that allow the lower control arm to rotate while moving with a rising and falling steering knuckle. In the design shown in Figure 12, the top surface of the frame member will support the engine and drive train weight with vibration-absorbing mounts. MacPherson strut-type suspension also incorporates a stabilizer bar that ties the two lower control arms together through a soft (rubberized)
14 Suspensions FIGURE 11 The MacPherson strut assembly is critical in the operation and handling of a strut suspension. mounting system. Each end of the bar is connected to a lower arm using a rubber bushing/mount bolt combination. The center of the bar is clamped to the unitized body using large rubber bushings. As the wheel moves up or down, the lower control arm moves with it (pivoting about its frame mounts), and the stabilizer bar moves with the control arm. The bar s soft mounting system allows it to move with some resistance dampening or stabilizing the motion of the control arm. Note that a MacPherson front-suspension system includes a sealed wheel bearing that s part of the wheel hub/rotor/steering knuckle assembly. The bearing itself isn t serviceable. The inside of the wheel hub includes a female splined opening that accepts the male spline from the drive shaft (Figure 11). The drive shaft (often called half shaft because one connects each side of the drivetrain to a front wheel) delivers torque to the front wheels.
Suspensions 15 FIGURE 12 Control arms of an independent front suspension can be mounted on a cross member typical of unibody frame construction or to the main frame rails as shown in this illustration. Another type of strut system is most commonly referred to as a modified MacPherson system. In this design (Figure 13), the coil spring is mounted between the lower control arm assembly and a spring pocket in the frame s main cross member or unibody. The lower arm assembly is attached to the steering knuckle and the steering pivots are at the lower ball joint and the shock/strut insulator, just as in a conventional FIGURE 13 A modified MacPherson strut suspension uses a portion of the frame or cross member instead of an upper control arm as the upper coil spring mount.
16 Suspensions MacPherson system. Once again, the strut assembly is mounted to the inner fender panel. Other Front-suspension Systems Multilink front-suspension systems use the strut/spring assembly found in the MacPherson system but replace the lower control arm with a series of links. These links (Figure 14) pivot slightly when the vehicle turns, allowing for increased control and stability during cornering. This design also includes an upper link that attaches to the unitized body near the top of the strut/spring assembly, a lower link, and a third link that connects the upper link to the steering knuckle. Unlike other front strut-suspension systems, the strut assembly doesn t rotate when the tires are steered. Our discussion of front-suspension systems ends with a doublewishbone style that, in operation, closely resembles the SLA design. FIGURE 14 Upper and lower links replace control arms in a multilink suspension system. (Courtesy of Chrysler Corporation)
Suspensions 17 FIGURE 15 A double-wishbone suspension offers high performance stability in braking and cornering. Wishbone-shaped upper and lower control arms are used together with a strut/spring assembly (like the one found in a MacPherson system). Both arms are connected to the steering knuckle in a design that provides for very stable high-speed cornering characteristics (Figure 15).
18 Suspensions Power Check 1 At the end of each section of Suspensions, you ll be asked to pause and check your understanding of what you ve just read by completing a Power Check exercise. Writing the answers to these questions will help you to review what you ve studied so far. Please complete Power Check 1 now. Indicate whether the following statements are True or False. 1. 2. 3. 4. 5. 6. 7. Except for their heavier weight, unibody frames are preferred to full frames because of their superior strength. Suspension system faults can become evident through steering problems. When upper and lower control arms are found to be different lengths, you should assume that incorrect parts were installed in the vehicle. Shock absorbers act as dampers that react to and oppose motion. Solid front axles are most commonly found in front-wheel-drive suspension systems because of their versatility and handling capabilities. The ball joint is designed to handle tension loads. The upper end of the strut is welded to the inner fender to ensure a secure attachment. Check your answers with those on page 57. REAR SUSPENSIONS TYPES AND FUNCTIONS The rear-suspension system of a vehicle is designed to absorb the effects of irregular road surfaces while helping to keep the vehicle in control. The rear-suspension system must also work with the frontsuspension system to support the vehicle s weight and maintain the correct vehicle height. Damaged or worn rear-suspension components (even ones you might consider minor such as the control arm bushings) can directly affect vehicle height and, ultimately, most other alignment features.