004-Auto Repair Technician Career Diploma

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Suspensions AUTOMOTIVE COMFORT AND CONTROL In this study unit, you’ll learn about common  automotive suspension components that make make up these systems, systems, and how the systems , the components systems and components are serviced. serviced. As you may already know, an autom au tomobi obile le’s ’s sus suspe pensi nsion on sys syste tem m is re respo sponsi nsibl blee for pr provi ovidi ding ng a com comfo fortrtable 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 system sys temss an and d the their ir com compo pone nent nt pa parts rts,, we we’l ’lll be begi gin n by di disc scuss ussin ing g how ste steer er-ing in g an and d su suspe spensi nsion on sys system temss wo work rk tog togeth ether er to pr provi ovide de a com comfor fortab table le,, 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-and pinion  pini on 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 suspension,, and the rear suspension. All automobiles have a front and rear suspension system, yet

 

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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 un  frames  unitized itized body construction 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 side appearance rungs on a ladder. These cross members connect two rails thatofare 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.

 

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FIGURE 1—Full frame designs can be grouped into three classifications: classifications: ladder (A), perimeter (B), or  X-frame (C). (Courtesy of Chrysler Corporation)

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Subframes Front and rear subframes support selected systems as full frames do,  but rear and and front subframes subframes aren’t aren’t directly directly connected connected to each each other. Rear subframes support rear-suspension components (like shocks or springs) and sometimes components. Front subframes support either the engine orrear-body 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 mem bers found found in a full frame frame design. 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.

 

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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 independe nt front-suspen front-suspension sion 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

 

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FIGURE 3—Spring and dampening components exist in every type of suspension system. system. A spring reacts r eacts 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-worl real-world d applications, many suspension components  behavee as a combination  behav combination spring spring and damper. damper. You’ll You’ll find it useful, useful, as you work to understand how suspension systems combine springs and dampers to provide a safe, comfortable ride.

Straight Front Axles Many tru Many trucks cks are eq equi uippe pped d wi with th str (als lso o kn know own n as solid). straig aight ht fro front nt axl axles es (a Thee ax Th axle le it itse self lf is ma made de fr from om st stee eell in th thee sh shap apee of an I be beam am.. At ea each ch en end d of  thee ax th axle leis isaa pi pivo vot, t, ca call lled ed a kingpin , wh whic ich h at atta tach ches es to th thee wh whee eell-su supp ppor orti ting ng spi spindl ndle (Figur (Fi gure e 4) 4). . red.” Thekin The gpin’s n’snt-sus desig de sign n al allow lows thetem spindl spi ndle e and andthe front fro nt wheel whe elsse to be “stee “st eered .”kingpi The frontfro suspe pensi nsion onssys system relie re lies s onthe leaf springs that connect the axle to the frame. There’s a disadvantage to this type

 

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FIGURE 4—Straight front axles are now found only in certain utility vehicles  and truck applications.

of system wheel goes goes over over a bum bump, p, one sid sidee of the axl axlee til tilts ts system.. When one wheel and an d mov moves es up upwa ward rd.. Thi Thiss af affec fects ts the ori orient entati ation on of the other other whe wheel, el, 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 was an independent front-suspension system. This generalfront type axle of suspension system is used in almost all modern er n au auto tomo mobi bile less an and d li ligh ghtt tr truc ucks ks.. On Onee su such ch sy syst stem em is sh show own n in Fi Figu gure re 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 upper and lower lower control control arm and steering steering/knuck /knuckle le

 

 

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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

 

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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  becau se 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-suspensio front-suspension n system incorporates  upper and lower control arms. While considered a modern design, it generally won’t be found on front-wheel-drive cars.

 

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permit this combinati combination on of movements—the movements—the flexing flexing of the  ball joints joints permit assem as sembl bly y to ab absor sorb b roa road d sho shock ck an and d the pi pivo votin ting g of the ste steer erin ing g kn knuck uckle le,, 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 inside of the coil springs, provide a damping action.

FIGURE FIG URE 7—C 7—Coil oil spr spring ings  s  can be mo can moun unte ted d be be-twee tw een n th the e up uppe perr co contr ntrol  ol  arm ar m an and d th the e fr fram ame e bu but  t  are mo most st of ofte ten n de desi sign gned  ed  to fi fitt be betw twee een n th the e tw two  o  cont co ntro roll ar arms ms as sh show own  n  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 thatt in tha incor corpo pora rates tes som somee of the str stren ength gthen enin ing g fe featu ature ress of a str strai aight ght (soli (solid) d) front-suspension system while allowing for independent movement of  the front wheels. As the name “twin” suggests, each side of these axles aree co ar conn nnec ecte ted d (t (thr hrou ough gh ki king ngpi pins ns or ba ball ll jo join ints ts)) to th thee ve vehi hicl clee an and d ha hass it itss own I-beam-shaped I-beam-shaped axle (Figure 8). The outer ends spindle, while

 

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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 than is experienced experienced with with a straight axle design.front Thisdesigns, type of system doesn’t handle as well as other independent 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 mo st oft often en fou found nd in an SL SLA-t A-typ ypee fr front ont-su -susp spen ensio sion n sy syste stem. m. In the new newest est torsion bar systems, there are two  transverse (meaning left-to-right running)  mounted bars  that mount to the frame on one end through pivottocushion bushings. They work throughqualities a sway bar the other end replace the resistance-to-displacement of aon 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.

 

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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 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-str  MacPherson -strut-su ut-suspen spension sion system systemss (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 supsup ported by the inner fender panel at the top and the steering knuckle at the bottom. Onround most designs, shock-absorber assem as sembl bly y is sur surrou nded ed by the the coill vertically coi spring spr ing an and dmounted incorp inc orpora orates tes a repla replaceabl ceablee damper cartridge. In addition, a strut rod or sway bar is connected to

 

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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 No te the vi vibr brati ationon-li limi miti ting ng ru rubbe bberr com compo ponen nents ts use used d in the up uppe perr att attachachment. 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 conne co nnecti ction on is ma made de wi with th pi pivo votin ting g su supp pport ortss tha thatt al allow low the lo lowe werr co contr ntrol ol 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 wi ll sup suppo port rt the en engi gine ne an and d dr driv ivee tra train in we weig ight ht wi with th vi vibr brat ation ion-a -abs bsorb orbing ing mounts. MacPherson MacPher son stru strut-ty t-type pe susp suspensi ension on als also o inc incorp orpora orates tes a stab stabili ilizer zer bar that ties the two lower control arms together through a soft (rubberized)

 

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FIGURE 11—Th FIGURE 11—The  e  MacPherson MacPh erson strut  asse as semb mbly ly is cr crit itic ical al in  the th e op oper erat atio ion n an and  d  hand ha ndli ling ng of a st stru rut  t  suspension.

mo moun unti ting ngbushing/mount syst sy stem em.. Ea Each ch en end dbolt of th the e ba barr is co conn nnec ecte ted d to a lo lowe wer arm ar mbar usin us ing a rubber combination. The center ofrthe isg clamped to the unitized body using large rubber bushings. As the whee wh eell mo move vess up or do down wn,, th thee lo lowe werr co cont ntro roll ar arm m mo move vess wi with th it (p (piv ivot otin ing g 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 o f the t he drivetrain driv etrain to a front wheel) delivers torque to the front wheels.

 

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FIGURE 12—Co FIGURE 12—Control  ntrol  arms ar ms of an in inde depe pend nd-ent fro front nt sus suspen pensio sion  n  can ca n be mo moun unte ted  d  on a cr cros osss me memb mber  er  typica typ icall of unib unibody  ody  frame const constructio ruction  n  or to th the e mai ain n fr fram ame  e  rail ra ilss as sh show own n in th this  is  illustration.

An Anot othe herr ty type pe of st stru rutt sy syst stem em is mo most st co comm mmon only ly re refe ferr rred ed to as a modified this is de desi sign gn (F (Fig igur uree 13 13), ), th thee co coil il sp spri ring ng is mo moun unte ted d  MacPhe  Mac Pherso rson n syste system m. In th  between  betwe en the lower control control arm assembl assembly y and a spring pocket 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.

 

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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  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. Corporation)

(Courtesy of Chrysler

 

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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).

 

 

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 Power  Pow er Check 1 At the end of each section of  Suspensions , you’ll you’ll be asked to pause pause and check 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. Except for for their heavie heavierr weight, weight, unibody unibody frames are are preferred preferred to full full frames frames because because of their superior strength. _____ 2. Suspe Suspension nsion system system faults faults can can become eviden evidentt through through steering steering problems problems.. _____ 3. When upper upper and lower lower control control arms arms are found found to be different different lengths, lengths, you you should assume that incorrect parts were installed in the vehicle. _____ 4. Shock absorbe absorbers rs act act as dampers dampers that react react to and oppose oppose motion. motion. _____ 5. Solid front front axles axles are most most commonly found found in front-wh front-wheel-dr eel-drive ive suspensi suspension on systems systems  because  becau se of their versatil versatility ity and handling handling capabilit capabilities. ies. _____ 6. The ball ball joint joint is is designed designed to handle handle tension tension loads. ____ __ ____ 7. Th Thee up uppe perr en end d of th thee st stru rutt is we weld lded ed to th thee in inne nerr fe fend nder er to en ensu sure re a se secu cure re at atta tach chme ment nt.. 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.

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