camber, caster

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M

odern steering andsuspension systemsare great examples of solid geometry at work. Wheel align-

ment integrates all the factors of steer-ing and suspension geometry to provide safe handling, good ride quality and maximum tire life.

Front wheel alignment is describedin terms of angles formed by steeringand suspension components. Tradi-tionally, five alignment angles arechecked at the front wheels—caster,camber, toe, steering axis inclination(SAI) and toe-out on turns. When wemove from two-wheel to four-wheelalignment, we add setback and the ve-hicle thrust angle into the equation.Rear wheel camber and toe also mustbe checked for a four-wheel alignment.

Tire Wear &Directional ControlCamber, toe and toe-out on turns aretire-wear angles. If they ’re incorrect,the tires will wear unevenly and fasterthan normal. Because camber is relat-ed to steering axis inclination, SAI alsocan be considered a tire-wearing angle.Caster and setback are not tire-wearangles unless extremely out of specifi-cation. All alignment angles are direc-tional control angles, which meansthey affect steering and vehicle control.

Troubleshooting a complaint about vehicle handling, ride or vibration starts with knowing what a particular align-ment angle does and how all the angles work together. Before you plunge intosystem diagnosis, take a moment tothink about the system operating princi-

ples. Therefore, let’s review these basicalignment angles with an eye towardtypical complaints and troubleshooting.

CasterCaster is the tilt of the steering axis of each front wheel as viewed from theside of the vehicle. Caster is measuredin degrees of an angle. If the steeringaxis tilts backward—that is, the upperball joint or strut mounting point is be-hind the lower ball joint—the casterangle is positive. If the steering axis tiltsforward, the caster angle is negative.Caster is not measured for rear wheels.

Caster affects straightline stability and steering wheel return. High posi-tive caster makes the front wheels want to go straight ahead. A normalamount of positive caster provides sta-bility and makes the steering wheelstraighten out after turning. On theother hand, positive caster increases

the effort needed to turn the wheel.Power steering allows the use of morepositive caster than would be accept-able with manual steering.

Too little caster can make steeringunstable and cause wheel shimmy. Ex-tremely negative caster and the relatedshimmy can contribute to cupped wearof the front tires. If caster is unequalfrom side to side, the vehicle will pulltoward the side with less positive (ormore negative) caster. Remember this when troubleshooting a complaint of vehicle pull or wander.

CamberCamber is the tilt of the wheel from true vertical as viewed from the front of the vehicle. Like caster, camber is measuredin degrees of an angle. If the tire ap-pears to tilt outward at the top, the cam-ber angle is positive. If the top of the tiretilts inward, the camber angle is negative.

BY KEN LAYNE

BASIC WHEEL ALIGNMENT

43June 2002

Caster CamberCaster Angle

Frontof Car

Ball JointCenterline

True Vertical (0°)

Camber Angle

True Vertical

(0°)

WheelCenterline



Zero camber—a perfectly vertical wheel and tire—causes the least tire wear. Positive camber causes the outertread of the tire to wear more than theinner tread; negative camber has theopposite effect. Engineers design smallamounts of positive or negative camberinto vehicle suspensions to aid handlingand steering. Normal camber angleshave little visible effect on tire wear, butextreme camber causes noticeably ab-normal tire wear and shortens tire life.

Positive camber, like positive caster,

affects straight-ahead stability andsteering wheel return. As the vehicleturns, the outside suspension tends torise on the wheel because of positivecamber. When the wheel returns tostraight ahead, the vehicle’s weightpresses down on the steering axis andhelps straighten the wheel.

Negative camber resists the tendency of the tire to slip sideways during cor-nering. It also can increase steering ef-fort. Most cars and light trucks are de-signed with positive camber, but many

race cars and some high-performancestreet vehicles have negative camber.

Rear wheels usually have zero cam-ber, but some independent rear sus-pensions are designed with someamount of (usually negative) camberangle. If front camber angles are un-equal side to side, the vehicle pulls to- ward the side with the greater positivecamber. Unequal rear camber also canaffect vehicle handling.

ToeToe is how the wheels are aimed, as viewed from above. A pair of front orrear wheels aimed inward at the for- ward edges have toe-in; wheels aimedoutward have toe-out. The toe anglefor front or rear wheels is measured infractions of an inch, millimeters orfractions of a degree.

Zero toe— wheels aimed straightahead—causes the least tire wear. Ex-treme toe-in or toe-out causes feather-edged wear across the tire tread. Toomuch toe-in wears the outside treadedges, with feathered edges on the in-side of each tread row. Too much toe-out has the opposite effect.

Front wheels are usually toed in onrear-drive vehicles and toed out onfront-drives to compensate for changesin the steering linkage and tires whenthe vehicle is moving. When the vehi-

44 June 2002

Toe

Front of Car

Zero Toe

Toe-In

Toe-Out

Toe ChangeTie RodToo Short

Outer Tie RodEnd Too High

Tie RodEnd Travel

Tie RodEnd Travel

SuspensionTravel

SuspensionTravel

Toe-Out On Turns

Centerline of Right Front Wheel

Turning Center

Centerlineof Left

Front Wheel

Centerline ofRear Wheels

20°18°



cle is moving, toe decreases (or disap-pears) because the wheels straightenout under acceleration and the steer-ing linkage flexes slightly.

Toe change, or bump steer, occurs

when a steering tie rod is the wronglength or is installed at the wrong an-gle. The outer tie-rod end moves upand down as the suspension compress-es and extends. If tie-rod length or an-gle is incorrect, it pulls or pushes thesteering arm and aims the wheel in anew direction. The driver feels this when the steering wheel jerks to oneside as the car goes over a bump or dip.

Toe-Out on TurnsToe-out on turns also is called turningradius, or the Ackerman angle. As a ve-hicle turns, the outside front wheelturns at a lesser angle than the inside wheel. This causes the front tires to toeout during cornering.

Some amount of toe-out on turns isnecessary because the outside wheelmust turn on a larger radius than the in-side wheel. If wheel turning angles were equal, the outside tire would scuff as it tried to turn on a shorter radius.

Toe-out on turns is designed into a vehicle’s steering geometry and mustbe equal in both right and left direc-tions. Toe-out on turns is not ad- justable. Angles that are unequal fromside to side or out of specification arecaused by vehicle damage.

Steering Axis InclinationSteering axis inclination (SAI) is the tiltof the steering axis from vertical as viewed from the front. It’s an angleformed by a line through the centers of the lower and upper ball joints of thestrut mount. Like caster, SAI affectssteering feel and stability. In a suspen-sion that doesn’t have much caster,high SAI can provide solid steeringfeel and stability.

SAI plus the camber angle form what’s called the included angle. If cam-ber is positive, the included angle isgreater than SAI; if camber is negative,the included angle is less than SAI.Knowing the SAI, camber and includedangle can help you diagnose problemsin the steering knuckles and suspension.

the thrust angle, but the rear suspen-sion design may not allow toe correc-tion. If a thrust angle can’t be elimi-nated, align the front wheels to thethrust line rather than to the vehicle’sgeometric centerline. Aligning thefront wheels to the centerline withthe rear wheels driving along a differ-ent thrust line can result in any or allof the following: a crooked steering wheel, incorrect front camber andtoe while moving, accelerated tire wear or pull.

45June 2002

Thrust AngleThe thrust angle is the angle betweenthe vehicle’s geometric centerline andthe direction in which the rear wheels are aimed. If the rear wheelspoint straight ahead, the thrust lineand the geometric centerline are thesame, and there is no thrust angle. When a vehicle is being drivenstraight ahead, the rear wheels steerit along the thrust line, so a zerothrust angle is ideal.

Adjusting rear toe should correct

Steering AxisInclination (SAI)

Included Angle

0°Steering Axis

InclinationSteering

AxisCenterline

Vertical

0°

IncludedAngle

SteeringAxis

Inclination

Camber

Thrust Angle Setback GeometricCenterline

ThrustLine

ThrustAngle

RightWheelSetBack

Measuredin Inches

BehindLeftWheel



SetbackSetback is a condition in which one wheel on an axle is in front of or be-hind the other wheel in relation to thechassis. Setback is actually designed in-

to some vehicles, such as old Fordtrucks with twin I-beam front axles,but incorrect setback most often re-sults from collision damage. Extremely uneven caster also can cause setback atthe front wheels.

Ride HeightStrictly speaking, ride height is not analignment angle. It can, however, affectother angles, especially caster. Many manufacturers specify ride height mea-surement positions for alignment ad- justment. Ride height often is measured

at the bottom of a front or rear rockerpanel or the top of a wheel well. Be-cause vehicle body panels are not themost accurate data points (with the pos-sibility of collision damage), ride height

is more accurately measured frompoints on the suspension or frame.

Many trucks are fitted with a variety of wheel and tire sizes, suspension op-tions and lift kits. Therefore, truckmanufacturers often specify differentcaster angles for different ride heightson the same basic truck model.

Start With theCustomer ComplaintCustomers rarely complain about “anincorrect Ackerman angle,” “too muchcaster” or even worn control arm bush-

ings. Their complaints usually identify the effects of steering and suspensionproblems rather than the causes. Own-er complaints that relate to wheelalignment generally relate to poor han-

dling, hard steering, vibration or tire wear. The first step in diagnosing aproblem is to determine the category into which it fits. A careful road testfollowed by a thorough inspection willget you off to a proper start.

A Final Word:Check the TiresBefore you begin your road test, take aclose look at the tires. Most cars andlight trucks should have four tires of the same size and tread pattern. Atleast the pair on each axle should bethe same size and have approximately equal wear patterns.

Next, check the tire pressures. Oc-casionally, you’ll find a tire that’s over-inflated, but probably two-thirds of thetires rolling down the highways today are underinflated. Bring all four tiresto their recommended inflation pres-sures. Most car owners will be sur-prised by how a simple tire pressurecheck can solve problems like vehicle wander, brake pull, hard steering and aspongy ride.

During your road test evaluation andalignment measurements, rememberthe basic purposes of alignment anglesand how they all work together to main-tain vehicle motion balance.

Visitwww.motor.com todownload a free copy of this

article. Copies are also availableby sending $3 for each copy to:

Fulfillment Dept., MOTOR Magazine,5600 Crooks Rd., Troy, MI 48098.

46 June 2002

Answers to PreviousAssessment Quiz

(Battery Operation & Testing, Apr. 2002)

1-C 2-C 3-D 4-C 5-B

6-C 7-D 8-B 9-A 10-C

Ride Height

Front Rear

MeasureDistance

camber, caster

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