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UNIT ONEUNIT ONESTEERING SYSTEM DESIGNSTEERING SYSTEM DESIGN

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TABLE OF CONTENTSTABLE OF CONTENTS

LESSON ONE- INTRODUCTION TO STEERING SYSTEMS.......................................................2HISTORY OF STEERING AND SUSPENSION........................................................................4DIRECTIONAL STABILITY.....................................................................................................5CASTER .............................................................................................................................7CAMBER ..........................................................................................................................10TOE..................................................................................................................................14

Toe-Out On Turns ...........................................................................................................16GEOMETRIC CENTERLINE ................................................................................................18THRUST ANGLE................................................................................................................19PARALLELISM AND CENTERLINE STEERING ....................................................................21TREAD CENTERLINE.........................................................................................................22RELATIONSHIP OF ALIGNMENT ANGLES..........................................................................23

LESSON TWO - POWER NON-RACK-AND-PINION SYSTEMS................................................24INTRODUCTION.................................................................................................................26PARALLELOGRAM STEERING SYSTEMS..........................................................................27PARALLELOGRAM STEERING SYSTEM COMPONENTS ....................................................29

Pitman Arm....................................................................................................................29Idler Arm........................................................................................................................29Centerlink (Relay Rod or Draglink)....................................................................................30Tie Rod..........................................................................................................................31Recirculating-Ball Steering Gear.......................................................................................32

LESSON THREE - POWER RACK-AND-PINION SYSTEMS .....................................................33RACK-AND-PINION STEERING...........................................................................................34

Rack-and-Pinion Steering Gear ........................................................................................37SPEED PROPORTIONAL VARIABLE ASSIST .....................................................................38REAR STEERING ..............................................................................................................40

Passive Rear Steering (Multi-Link)....................................................................................40Passive Rear Steering (Toe Link)......................................................................................41Mechanical Rear Steering................................................................................................43

GLOSSARY..........................................................................................................................45


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LESSON ONELESSON ONEINTRODUCTION TO STEERINGINTRODUCTION TO STEERING

SYSTEMSSYSTEMS

TERMINAL OBJECTIVESuccessful completion of this Unit’s enabling objectives(technical competencies) will allow you to meet theIntegrated Curriculum Standards (ICS) listed in the rightmargin.

ENABLING OBJECTIVESUpon completion of Lesson One, you should be able to:• Identify terms and definitions associated with steering

systems.• Define the characteristics of liquids• Identify the fundamental laws of hydraulics and conclude

how they apply to the operation of a power steeringpump.

ICS

007Chemical and PhysicalProperties

101Basic Physics

102Mechanics and Forces

155Steering andSuspension Systems


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KEY TERMSYou will see the following key terms used throughout this lesson.You may also refer to the glossary at the back of this book fordefinitions of these terms.• Camber• Caster• Directional Stability• Geometric Centerline• Lead• Parallelism• Pull• Steering Axis• Thrust Angle• Toe


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HISTORY OF STEERING AND SUSPENSIONThe first steering and suspension systems were developedcenturies ago. Horse drawn buggies used springs to help smoothout the road surface, if a road even existed. The buggy was steeredby pulling the reins of the horse, causing the horse to pull the buggyinto a turn. The front axle pivoted on a pin that allowed the frontwheels to turn in the desired direction. A rod and lever wereattached to the middle of the axle to assist the driver in turning andholding the front wheels straight. Early horseless carriages alsoused this design but it was not very accurate and it caused thewheels to wear rapidly.

In the early 1800’s Rudolf Ackerman designed a steering systemthat incorporated angled steering arms and knuckles. This allowedboth front wheels to turn in their own path or arc. The inner wheelturned at a sharper angle, which allowed both wheels to turn at thesame pivot point, minimizing tire wear. Over the years manyimprovements have been made to the vehicle’s steering systems.

Today, there are many variations to the basic steering andsuspension systems, independent suspension systems haveevolved from simple means to cushion a jolt or shock, into highlyengineered systems for the best ride quality, directional control andease of handling. Additionally, most steering systems are powerassisted, and some are power assisted proportional to vehiclespeed. Even with all of the variations, the basic concepts still applyto all systems.


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DIRECTIONAL STABILITYDirectional stability is needed to keep vehicles going ion a straightline or in line with the direction of the steering wheel. Steering andsuspension systems are closely related, and in most cases, aredependent upon each other. The relationship between thesesystems is important to the operation and performance of thevehicle. Each system relies on the other to perform adequately.Wheels and tires also impact the performance of the steering andsuspension systems. Any changes or modifications to thesesystems will affect the ride and handling of the vehicle.

The steering system allows the driver to direct the movement of thevehicle. It must provide a means for proper handling, gooddirectional control, and stability. The most common front steeringsystems are the parallelogram and rack-and-pinion steering systems.


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The suspension system supports the weight of the vehicle and anyload that is placed into it. It also smoothes the ride for passengers,holds the wheels in position, and keeps them in contact with theground. Without a suspension system, the vehicle would becomeunstable as it goes over bumps and dips in the roads. At higherspeeds, road variations would cause the steering or braking systemsto lose their effectiveness and the variations would be transferredthrough the car to the passengers.

Vehicles do not steer or brake well if the tires are not contacting theground properly. The suspension system keeps the tires on theground so the contact area is properly aligned. Variations areabsorbed so that the steering and brake systems can work asdesigned.

Tires should follow the shape of the road and adapt to variations.Ideally, the tire should rise when it encounters a bump or “jounce”.When a tire encounters a dip or hole, it should go down into the dipwhile still supporting the vehicle at close to its original height. Thisaction is referred to as rebound.


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CASTERCaster is the forward or backward tilt of the steering axis whencompared with a true vertical line. The steering axis is the line thewheel turns around, when the wheel is turned to the side.

Caster is positive if the axis is leaning rearward. Caster is negative ifthe axis is leaning forward. It is zero when the steering axis isstraight up or down. Caster is measured in degrees. Most vehicleshave a small amount of positive caster.

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Caster gives the front wheels the ability to return to the straight aheadposition after a turn. Caster also provides directional stability.The directional stability resulting from caster can be compared to ashopping cart. The front wheels have caster which makes them tendto track straight ahead. If caster was set properly and you gave theshopping cart a shove, it would go off in a straight direction. Try it!

When a wheel is turned out, the spindle lowers and raises thevehicle. When a wheel is turned in, the spindle raises and lowers thevehicle. When the wheels are released from a turn, the weight of thevehicle helps move each spindle back toward the mid-point until theload is equal on both front wheels.

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A difference in caster between one side and the other of more thanhalf of a degree may cause a pull or lead toward the side with theleast positive caster

High caster settings have advantages and disadvantages. Theadvantages are greater directional stability and increases the frontwheels ability to return. The disadvantages are steering requiresmore effort, road isolation is reduced, and tire wear on turns isincreased.

High positive caster can also cause the wheels to return to centervery fast. A steering dampener is used in some high casterapplications to reduce the speed at which the wheels return tocenter. Some vehicles use a steering dampener to reduce theeffects of having a large amount of positive caster.

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CAMBERCamber is the inward or outward tilt of the wheel when comparedwith a true vertical line.

Camber is positive when the top of the wheel is tilted out. Camber isnegative when the top of the wheel is tilted in. It is at zero when thewheel is vertical (straight up and down). Front wheels usually havesmall positive camber.

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Positive camber helps give the front wheels directional stability.Positive camber projects a portion of the vehicle weight onto a pointnear the inner wheel bearing. By contrast, negative camber projectsa portion of the vehicle weight onto a point near the outer bearing.

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When a wheel hits a bump, the spindle pivots at the spot near theinner wheel bearing where the weight is projected so the movementtransmitted to the suspension is small. More force (a larger bump) isneeded to move the wheel away from the straight-ahead position.

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With negative camber, the spindle pivots at a spot near the outerwheel bearing. As a result, the movement transmitted to thesuspension when the wheel hits a bump is larger. Less force isneeded to move the wheel away from the straight-ahead position.This may reduce directional stability. It may also result in excessiveroad shock, a reduction in ride quality, and increased wear on theouter wheel bearing.

Positive camber improves road isolation, ride quality, and directionalstability, because more force is needed to move the wheel awayfrom the straight-ahead position.

Rear camber, on the other hand, usually has a negative specification.This is done to improve cornering, directional stability, and tire treadlife.

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TOEToe is the difference between the front and rear edges of a set oftires. When the wheels are parallel to each other, toe is zero. Whenthe front edges of the tires are closer together, the tires are toed-in,and toe is positive. When the rear edges are closer, the tires aretoed-out, and toe is negative. Toe is specified in degrees or inches.

Toe specifications are usually provided in the Service Manual. Tofind the toe for an individual wheel, divide the specification by two.

During high-speed driving conditions, the wheels should have nearlyzero front toe, to minimize tire wear. Excessive positive toe (toe-in)will scuff the outside of the tire and wear down the outside shoulder.Excessive negative toe (toe-out) will scuff the inside of the tire andwear down the inside shoulder.


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Toe-Out On TurnsToe-out on turns is the angle that the outside front wheel followsthrough a turn. When a vehicle turns left, the right tire turns through alarger circle (longer radius). The outside tire does not turn as sharpas the inside tire, and the inner wheel is always ahead of the outerwheel through the turn.

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The steering system is designed with the steering arms angled toturn the wheels at the correct toe through the turns. Toe-out on turnsis usually designed so that if straight lines were drawn through thesteering arms, they would intersect at or near the middle of the rearaxle. This principle is called “Ackerman’s Geometry”. Toe-out onturns is considered a non-adjustable alignment angle. However, itshould be checked during alignment. If out of specification, it mayindicate bent steering or suspension components.

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GEOMETRIC CENTERLINEThe vehicle’s geometric centerline is formed between the centerof the front wheels and the center of the rear wheels. The geometriccenterline passes through the midpoints of the front and rear wheelspans or track widths. The geometric centerline could also be drawnthrough the midpoint of the front and rear axles. The geometriccenterline is used as a reference to align toe on all four wheels

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THRUST ANGLEThe thrust line is the direction the rear wheels are pointing. If therear suspension is not damaged and the rear toe is properlyadjusted, the thrust line and the geometric centerline of the vehicleare the same.

The thrust angle is the difference between the thrust line and thegeometric centerline. A thrust angle to the right is positive. A thrustangle to the left is negative. Thrust angle is measured in degrees.

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When the trust angle is too large, problems can result, including dog-tracking and an off-center steering wheel. When the thrust angle isexcessive, the driver has to turn the steering wheel to one side tokeep the vehicle moving straight ahead. Now the vehicle goesslightly sideways along the road. This sideways movement is knownas “dog-tracking”. Excessive thrust angle is one of the primarycauses of a steering wheel that is not centered.

Even on vehicles where rear toe is not adjusted, thrust anglemeasurements provide important diagnostic information. Forexample, if the left rear toe in and the right rear toe out, on a vehiclewith a fixed rear axle, it is possible that the axle has shifted or aframe rail has been damaged.

A thrust angle also exists when the individual toes of the rear wheelsare not equal. For example, if the left rear wheel is pointing straightahead and the right rear wheel toes out two degrees, the thrust angleis one degree to the right.

Note: Thrust Angle = (Left Toe – Right Toe) / 2


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PARALLELISM AND CENTERLINESTEERINGParallelism refers to the wheels tread centerlines being parallel tothe geometric centerline. When rear toe is adjusted, the thrust lineshould be parallel with the geometric centerline. The steering wheelis set straight and the front toe is adjusted to the thrust line, which isnow the centerline. When the vehicle moves, the front tires assumea parallel direction with the rear. If toe is correct on the rear, the fronttires will follow a parallel path with the rear, creating centerlinesteering.

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TREAD CENTERLINEOn a vehicle that has front and rear wheels equally wide apart, thetread centerline is a line from the midpoint of the front tire tread tothe midpoint of the rear tire tread on the same side. It should beparallel to the geometric centerline.

If the tread centerline is not parallel to the geometric centerline, across-member may not be positioned right, or the cradle may beshifted to the side.

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RELATIONSHIP OF ALIGNMENT ANGLESCaster, camber, and toe are the alignment angles which are usuallyadjustable. Many vehicles today also have rear camber and toeadjustments. It is important to note that alignment angles affect eachother. For example, a vehicle is being serviced because the camberangle of the left front wheel is out of specification. The camber isnegative, meaning the top of the wheel is leaning in toward the centerof the vehicle. An adjustment is made to correct the condition. Theadjustment brings the bottom of the tire inward. This causes thedistance to change between the two front tires. As a result, thedistance becomes smaller and the toe setting changes.

Changes like this can also occur when making other adjustments. Itis possible, however, to intentionally adjust more than one angle at atime by knowing how one angle can affect another. It is importantthat you check and align all the adjustable angles so one correctiondoes not cause misalignment of another. The order in whichalignment angles are set is important. Always refer to the properService Manual for the correct alignment procedure.


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LESSON TWOLESSON TWOPOWER NON-RACK-AND-PINIONPOWER NON-RACK-AND-PINION

SYSTEMSSYSTEMS


ENABLING OBJECTIVESUpon completion of Lesson Four, you should be able to:• Identify the fundamental laws of hydraulics and conclude

how they apply to the operation of non-rack and rack andpinion power steering gears.

• Explain the terms friction, force, inertia, lever, gear ratios,momentum, reduction, overdrive, speed, work, torque,and power and how these science terms apply toautomotive steering and suspension systems.

• Identify the components of the integral non-rack andpinion power steering gear and explain system operation.

ICS


042Math Formulas

101Basic Physics



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KEY TERMSYou will see the following key terms used throughout this lesson.You may also refer to the glossary at the back of this book fordefinitions of these terms.• Centerlink• Idler Arm• Parallelogram Steering System• Pitman Arm• Recirculating-Ball Steering Gear• Tie Rod


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INTRODUCTIONTwo main types of steering systems are used on today's vehicles -parallelogram and rack-and-pinion. These systems are designed todo three things:• Hold the wheels exactly in the direction the driver requires• Isolate road shock (kick-back or jerk) while still offering some

feedback to inform the driver of changing road conditions• Multiply the driver's effort

Most vehicles are steered through the front wheels. As the driverturns the steering wheel, the movement is transferred through thesteering system to the front wheels. The rear wheels follow the frontwheels through the turn.

The amount of force required by the driver to turn the front wheelsdepends on many things, but primarily the weight of the vehicle andthe speed the vehicle is traveling. The steering on a bicycle isrelatively easy due to the light weight of the bicycle and driver. Aperson can easily turn a bicycle wheel with the leverage created inthe handlebars; however, a car or truck is much heavier.


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PARALLELOGRAM STEERING SYSTEMSIn a parallelogram steering system, all the steering linkageconnecting points on the right side of the vehicle are parallel to thoseon the left side.

The graphic below shows how the linkage is parallel up and down.

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This system offers many advantages by allowing the motion on oneside of the vehicle to be duplicated on the other. The pivot pointscan also be aligned with pivot points in the suspension system. As aresult, the steering motions are transmitted to the front wheelsindependent of suspension status (jounce or rebound). Additionally,the turning angles are the same for both front wheels, allowing fortoe-out on turns, regardless of the wheel's vertical position. Bothrecirculating ball and rack-and-pinion steering gears are designed tooperate in a parallelogram.


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PARALLELOGRAM STEERING SYSTEMCOMPONENTS

Pitman ArmThe Pitman arm is attached at one end to the steering gear's sectorshaft. The other end is connected to the centerlink (sometimesreferred to as a relay rod or draglink). The Pitman arm is the leverthat converts the rotary motion of the sector shaft into side-to-side(lateral) motion. The Pitman arm is securely attached to the sectorshaft by splines, so that any movement of the sector shaft istransmitted to the Pitman arm and centerlink.

Idler ArmThe idler arm is attached to the opposite end of the centerlink fromthe Pitman arm. It moves in the same plane as the Pitman arm,which keeps the linkage parallel. It usually bolts to the vehicle'sframe. It supports and guides the outer end of the centerlink throughthe same path (arc) as the Pitman arm.

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Centerlink (Relay Rod or Draglink)The centerlink moves to the left and right under the vehicle as thesteering wheel is turned. The centerlink is connected between thePitman arm and the idler arm. The connections between the Pitmanarm and idler arm are usually ball and socket type so the centerlinkcan swivel and move through various angles. Tie rods are alsoattached to the centerlink with similar ball and socket type joints. Asthe centerlink moves, it causes the tie rods to move the wheelassembly.

Draglinks are a variation of the centerlink and are used on manymodern day trucks and sport utility vehicles. This design places thePitman arm in proper alignment with the wheel assembly and ofteneliminates the need for an idler arm.


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Tie RodThe tie rod connect the centerlink or draglink to the wheel assembly.This causes any centerlink movement to be transferred to thewheels. Tie rods have ball studs (ball and sockets) on both ends toallow a full range of motion, due to the wheel's vertical and turningmovements.

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Recirculating-Ball Steering GearThe recirculating-ball steering gear was developed in 1923. Thisdesign eliminated much of the friction associated with earlier gearstyles. It has since become one of the two most widely usedsteering gears. The steering gear consists of a worm gear, which issupported on each end by ball bearings in a housing. The wormgear has a spiral groove cut into it. A ball nut fits over the worm gearshaft but does not contact it. The nut also has a groove, whichcorresponds to the groove on the worm shaft.

The groove between the gear and the nut is filled with steel balls,which are recirculated back into the groove as the worm and nutmove. The ball nut converts the rotational motion of the gear intolinear motion. The ball nut transfers its motion to a sector gear andshaft. As the ball nut moves along the worm shaft, the sector gearon the cross shaft is rotated.

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LESSON THREELESSON THREEPOWER RACK-AND-PINIONPOWER RACK-AND-PINION

SYSTEMSSYSTEMS


ENABLING OBJECTIVESUpon completion of Lesson Four, you should be able to:• Explain the terms friction, force, inertia, lever, gear ratios,

momentum, reduction, overdrive, speed, work, torque,and power and how these science terms apply toautomotive steering and suspension systems.

• Identify the components of the power rack and pinionsteering system and explain system operation.

• Identify the components of a typical electronicallycontrolled automotive steering system and explainsystem operation.

KEY TERMSYou will see the following key terms used throughout thislesson. You may also refer to the glossary at the back of thisbook for definitions of these terms.• Rack-and-Pinion Steering System

ICS


042Math Formulas

101Basic Physics



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RACK-AND-PINION STEERINGMany vehicles use a rack-and-pinion steering system. Rack-and-pinion systems offer several advantages over parallelogramsystems:• Saves space• Weighs, costs less• Provides responsive steering

While rack-and-pinion systems have these advantages, rack-and-pinion systems are only efficient on light weight vehicles (if powersteering is not included).

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Many vehicles have power steering to assist the driver. Powersteering decreases the effort required by the driver through the useof hydraulics.

A pump driven by the engine creates hydraulic pressure that isapplied through a control valve as the driver turns the steering wheel.Hydraulic pressure is then applied to one of two pistons to assist inmoving the steering linkage left or right.

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Some vehicles are equipped with variable assisted power steeringsystems. The principle of operation is similar to the traditional powersteering system, but the amount of hydraulic pressure applied iselectronically controlled. For example, most vehicles are equippedwith a Variable Assisted Speed Proportional Power SteeringSystem. This system incorporates an electronic steering controlmodule that varies the power assist according to vehicle speed. Thevariable assist reduces steering effort at low speeds and increasesthe effort at high speeds to provide better feedback to the driver.

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Rack-and-Pinion Steering GearThe rack-and-pinion gears are enclosed in a housing that is locatedbetween the front wheels. The pinion gear is attached to the lowerend of the steering column and meshes with the rack gear.

This gear combination converts rotary motion directly into linearmotion. A tie rod is attached to each end of the rack by means of aball and socket. The other tie rod end is attached directly to thewheel assembly with ball and socket joints, which are often referredto as tie rod ends.

Rack-and-pinion assemblies are sealed at each end with a rubber orplastic bellows (boot) to keep dirt out. Rack-and-pinion assembliescome in different gear ratios and should only be replaced with thesame ratio as originally equipped. Most current rack-and-pinionassemblies are non-serviceable. Refer to the Service Manual forrack-and-pinion replacement procedures.

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SPEED PROPORTIONAL VARIABLE ASSISTA variable assist speed proportional power steering system isavailable on some front-wheel drive cars. The system increases lowvehicle speed power steering assist during parking and low speedturns. During faster vehicle speeds the system pressure (assist) isreduced based on vehicle speed sensor input. A Solenoid ControlModule (SCM) and solenoid control valve reduce the assist toprovide a firmer and more stable ride.

Pressure is regulated by a torsion bar much like a Saginawrecirculating ball steering gear. The SCM provides less returnpressure against the reaction disk at low speeds causing thereaction disk to move up; this reduces the torsion load (steeringeffort). By modulating return pressure with the solenoid, the systemdecreases assist as vehicle speed increases.

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The system provides full assist for "evasive maneuvers" at higherspeeds. To test the system, you can test drive the vehicle or use aScan Tool to enter false vehicle speed inputs to check if the systemis changing the amount of assist.

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REAR STEERING

Passive Rear Steering (Multi-Link)In the passive rear steering system, the suspension uses thevehicle's weight and the forces induced on the suspensioncomponents to slightly change rear toe angles during turns. Theconfiguration of the toe control arm, the lower lateral arm, and thetrailing arm force the inside rear wheel into a toe-out condition duringa turn. The outside wheel toes-in. This provides passive rearsteering and, therefore, improved cornering without a large numberof additional components like mechanical or hydraulic rear steeringsystems.

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Passive Rear Steering (Toe Link)Some vehicles use a toe link rear suspension. The suspension isactually an SLA suspension using upper and lower arms, with theknuckle mounted on ball joints. The knuckles remain stationary whenthe vehicle is turned, but the toe links (tie rods) adjust rear toe as thesuspension goes through jounce and rebound.

As the spindle arc changes during jounce and rebound, the tie rodpivot point moves. This causes toe to be altered, much like an SLAsuspension changes camber because of the different arm lengths.The toe link sets toe similar to tie rods on other front suspensionsystems. The system improves vehicle response and does notrequire a large number of additional components like mechanical orhydraulic systems.


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Mechanical Rear SteeringSome manufacturers use a mechanical linkage to help steer the rearwheels on a vehicle. The mechanical rear steering system uses ashaft turned by the front rack to control a gear box in the rear. Thesystem is designed to steer the rear wheels in the same direction asthe front wheels when the steering wheel is turned between centerand about one third of a turn off center in either direction. This allowsthe rear wheels to help turn when maneuvering at driving speeds.

The rear steering gear turns the rear wheels in the opposite directionas the front wheels when a vehicle is performing low speedmaneuvers, such as parking. The steering system alters the rearwheel direction as the steering wheel is turned from about one thirdof a turn and beyond.

During an alignment, this system requires the rear toe be set first,like all other four-wheel alignments. The rear toe aligns the thrustangle and centers the rear steering gear. If a steering wheel is off-center after alignment, the rear steering is affected because the frontand rear steering gears are not synchronized (centered together).


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GLOSSARY• Camber – Tilting of the top of the wheels from the vertical; when

tilt is outward, camber is positive.• Caster – Tilting of the steering axis forward or backward to

provide directional steering ability.• Centerlink – A link used to connect the idler arm to the Pitman

arm in a parallelogram steering system. The link transferssteering motion to vehicle tie rods.

• Directional Stability – The tendency of a vehicle to follow astraight course without excess effort on the part of the driver.

• Geometric Centerline – A line formed from the front to the rearin the middle of a vehicle. Tread centerlines should be spacedevenly from this point to maintain proper tracking.

• Idler Arm – A component used to transmit steering forces froma centerlink to a tie rod in parallelogram steering system.

• Lead – Pull to a specific side of a vehicle. The vehicle turns inone direction when not controlled by the driver.

• Parallelism – Rear wheels should be parallel to the vehiclecenterline. If not, the thrust angle (rear toe) is incorrect.Parallelism causes the front tires to proceed in the samedirection as the rear tires to make a vehicle go straight. If reartoe/thrustline is off, a vehicle will dog track.

• Parallelogram Steering System – A linking arrangement whichallows the wheels to maintain the correct steering positions injounce or rebound. Linkage components take the shape of aparallelogram, with equal length Pitman/idler arms and steeringarms. All components are parallel to one another from one sideof the vehicle to the other.


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• Pitman Arm – An arm used to transfer rotational movement ofthe sector shaft to linear movement of the centerlink in aparallelogram steering system.

• Pull – Turning to a specific side of a vehicle. The vehicle turnsin one direction when not controlled by the driver.

• Rack-and-Pinion Steering System – A steering system thatuses a pinion gear connected to the steering column shaftthrough a coupling to turn a long rack gear. The rack has tie rodson each end which move the steering arms, knuckles andspindle in the direction desired.

• Recirculating-Ball Steering Gear – A steering gear whichuses a worm gear to drive a ball nut through recirculating balls.The recirculated balls reduce friction between the worm and ballnut. Commonly used with parallelogram steering systems.

• Steering Axis – The angle formed between the line or axisthrough which the steering knuckle rotates and true vertical.

• Thrust Angle – Angle formed by the average of the rear wheeltoe settings. Thrust angle propels a vehicle body in the directionof the thrust line. Improper thrust angle causes dog tracking.

• Toe – The amount, in inches or millimeters, that the front of thefront wheels point inward.

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