wheel alignment terms.pdf

7/21/2019 Wheel Alignment Terms.pdf

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3/9/2015 Wheel Alignment Terms

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Topics Included:Front Camber

Effects of Positive Camber

Eff ects of Negative Camber

Road Crown and Camber

Causes of Camber Changes

Front Cone Effect

Front Caster

Caster Definition

Caster Effect

Front Caster Effects

Effects of Positive Caster

Effects of Caster on Tire Wear

Front Toe

Toe Definition

Measuring Toe

Effects of Toe

Rear Camber

Rear Toe

Geometric Center Line

Thrust Line Front Setback

Toe Out On Turns Rear Setback

Steering Axis Inclination (SAI)

SAI Definition

Effects of SAI

Included Angle (IA)

I/A Definition

Measuring Procedures

Steering Axis Inclination Charts

Short Long Arm (SLA)

MacPherson Strut

Twin I-Beam

Mono Beam

Scrub Radius

Scrub Radius Definition

Squirm

Applications in Suspensions

Ball Joint Inspection

Rear Camber

Rear Camber is the inward or outward tilt of the top of the tire/wheel assembly from truevertical. If the top of the tire/wheel assembly is Tilted Inward, it has a Negative Camber. If thetop of

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the tire/wheel assembly is Tilted Outward, it has a Positive Camber. If the tire/wheelassembly is straight up and down on a true vertical line, the Camber is measured at Zero (0°). Rear camber is not adjustable on most rear wheel drive vehicles.

These vehicles are built with zero camber setting and are strong enough not to flex or bendunder normal load. Most front wheel drive vehicles have a manufacturers specification callingfor a slight amount of rear camber, usually a small amount of negative camber for cornering stability. If themanufacturers specification allows, a setting of 0° to -.5° is preferred for tire wear and ride

stability. If rear camber settings change, most vehicles can be adjusted by using an aftermarketype of adjustment, such as shims, cam bolts or bushings.

Rear ToeMeasuring rear toe using Geometric Centerline

Toe-Out (Negative Toe) is a condition where the front of the wheel is farther from thegeometric centerline than the rear of the same wheel. Toe-In (Positive Toe) is a conditionwhere the front of the wheel is closer to the geometric centerline than the rear of the samewheel.

Measuring rear toe using Total Toe

Toe-Out (Negative Toe) is a condition where the distance between the front of both wheels ona common axle are farther apart than the rear of the same wheels. Toe-In (Positive Toe) is acondition where the distance of the front of both wheels on a common axle are closer together than the rear of the same wheels.

Toe can be expressed in degrees, fractions or decimal inches. Do Not confuse degrees withinches when selecting your method of adjustment. Rear toe adjustment is the most criticalfactor regarding tire wear, mileage, and handling.

Geometric Center Line

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The Geometric Center Line of the vehicle is established by connecting a line between thetheoretical midpoint of the front spindles and the theoretical midpoint of the rear axle.

Rear Setback

Rear setback is a measurement referencing the rear wheels to an imaginary line perpendicular to the geometric centerline of the vehicle, and is measured as an angle. If a vehicle has rear setback, one rear tire/wheel assembly is further back from this imaginary line than theother.

Some causes of rear setback may be from frame, chassis, and rear cradle mis-alignment dueto collision. If the vehicle has a setback condition, the vehicle may pull to the opposite side of the setback.

Thrust LineThrust line is determined by bisecting the rear total toe. To bisect the rear total toe, lines thatare parallel to the tire/wheel assembly are drawn until they intersect. Another line that startswhere the geometric centerline and rear axle intersect, is drawn to the intersection of thetire/wheel lines. This line is the Thrust Line. When you have a thrust line to the left it isconsidered Negative and when it is to the right it is considered Positive.

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Thrust Angle

Thrust angle is the angle created between the geometric centerline and the newly createdThrust Line.

Front Camber

Camber is the inward or outward tilt of the tire/wheel assembly. This angle is measuredfrom a true vertical line, i.e. perpendicular to the ground. A tire/wheel assembly that is tiltedoutward at the top is considered to have Positive camber. While a tire/wheel assembly tiltedinward at the top, displays Negative camber. For a zero setting, the tire/wheel assembly is inthe exact vertical position or perpendicular to the ground. To rephrase, if the top of thetire/wheel assembly is tilted inward towards the engine, it has a negative camber. If the top of

the tire/wheel assembly is tilted outward from the engine, the camber is positive.This is Negative Camber This is Zero Camber This is Positive Camber

Effects of Positive Camber

Slight positive camber results in a dynamic loading that allows the tire to run relativelyflat against the road surface. Positive camber also directs the weight and shock load of thevehicle on the larger inner wheel bearing and inboard portion of the spindle rather than theoutboard bearing. Positive camber in moderation results in longer bearing life, less likelysudden load failure, and as a side benefit, easier steering. Excessive positive camber wears the

outside of the tire and can cause wear to suspension parts such as wheel bearings andspindles.

Effects of Negative Camber

Variations in negative camber can be used to improve the handling of a vehicle. A settingof 1/2° negative on both sides will improve cornering without affecting tire life greatly. Thisnegative setting compensates for the slight positive camber change of the outside tire due tovehicle roll, thereby allowing a flatter tire contact patch during cornering. Excessive negativecamber wears the inside of the tire and similar to positive camber, it can cause wear and stresson suspension parts.

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Road Crown and Camber

A crowned road means that the outside/right hand side of the lane is lower than the leftside of the lane. This improves the drainage of the road but adversely affects vehicle handling.Road crown must be compensated for in alignment settings because a vehicle driving on acrowned road leans to the right, causing some weight transfer to the right, and the camber changes slightly more positive. This combination creates a pull or drift to the right. Mostalignment technicians adjust the vehicle with a slightly more positive camber, usually 1/4°, on

the left to compensate for the road crown. This slightly more positive camber will not cause anoticeable pull when driving on a flat road. However, if camber is unequal from side to side witha difference greater than 1/2°, the vehicle will pull to the side with the most positive camber. If the specifications allow, 0° to ±.5° is usually best for tire life and vehicle handling.

Causes of Camber Changes

Always consult a ride height specification book prior to beginning alignment. If out of specification, attempt to correct. Changes in ride height from factory specificationsaffect camber. As a vehicle ages, the suspension has a tendency to sag. The weight of the

vehicle can cause springs to weaken. Springs can also be damaged by excessive vehicleloading or abuse. Another factor to consider is sagging of center support or crossmember.Modifications to the vehicle such as raising or lowering the suspension or changing thetotal weight of the vehicle can also affect camber.

Front Cone Effect

When a tire/wheel assembly is tilted it creates a condition called the "cone effect".Thisangling of the tire/wheel assembly creates an imaginary cone that rotates in the direction the

wheel is angled. The apex of the cone is created by the intersection of two lines: 1) the groundand 2) a line projected through the centerline of the spindle to the ground. The shape of thecone is then defined by the third line from the top of the tire to the intersection of 1 and 2. Thewheel attempts to pivot around this intersection point. A positive cambered tire/wheel assemblywill roll away from the center of the vehicle. Conversely, a negative cambered tire/wheelassembly will roll towards the center of the vehicle. If the vehicle is traveling on a flat, level roadand the amount of camber offset is the same on both front wheels, the cone effects, althoughopposite, will offset each other and the vehicle will travel in a straight line. A maximum side toside variation of ± .5° is recommended.

Front Caster

Caster Definition

Caster can be defined as the forward or rearward tilt of the projected steering axis fromtrue vertical, as viewed from the side. This line is formed by extending a line through theupper and lower steering knuckle pivot points. For vehicles with front control arms, visualize theline extending through the upper and lower ball joints. On strut equipped vehicles, the line

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extends through the lower ball joint to the center of the upper strut mount. Caster is alwaysviewed from the side of the vehicle.When the upper pivot point is rearward of the lower pivot point, caster is positive. If the upper pivot is forward of the lower pivot point, casteris negative. When the two points are straight up and down from each other, the caster is zero.

A maximum side to side variation of ±.5° is recommended on most vehicles. Caster is NOT anormal tire wearing angle and is used as a directional control for stability and steeringreturnability.

Front of Vehicle Caster Effect

Caster effect is necessary so that the load of the vehicle is "carried" through the steering axisline formed on the upper and lower pivot points. Positive caster gives a vehicle directionalstability because the tire is being pulled along by the load which is projected in front of thecenter of the tire contact area. This causes a vehicle with positive caster (point of load ahead of

the point of contact) to be harder to steer away from the straight ahead position. With Positivecaster, road surface variations have a minimal effect on the tire, the tire will continue to gostraight. When a tire has a Negative caster condition, where the projected steering axis pointof load is behind the tire point of contact, a vehicle will have a tendency to be easier to steer butwill lack directional stability. A vehicle with negative caster is affected by any road surfacevariation such as small road irregularities or bumps. With the point of load pushing the tirealong (negative caster), any bumps or road irregularities which are encountered have atendency to immediately affect directional stability and vehicle handling.

Front Caster Effects

Effects of Positive Caster

Vehicles usually have some positive caster specified since this promotes directional stability,however, excessive positive caster can cause two problems. The first is that excessive caster will cause a high level of road shock to be transmitted to the driver when the vehicle hits abump, etc. The second problem is that a tire/wheel assembly with positive caster has atendency to toe inward when the vehicle is being driven. If one side has more positive caster than the other, this causes it to toe inward with more force than the other side. This will cause alead or pull to the side with least amount of positive caster.





Effects of Caster on Tire Wear

When set with a substantial amount of caster, the spindle travels in a vertical arc, causing it tomove up and down and raise and lower the wheels as the steering wheel is turned. Because of this, camber changes occur. With a high amount of positive caster, the camber changesthat occur, especially at low speeds in tight turns, cause the tires to show wear on their

shoulders. In high speed cornering, the vehicle tends to continue straight ahead when thesteering is initially turned. Due to this, and the amount of camber change that takes place whena spindle travels through its arc of travel, the shoulders of the tires on a vehicle may scrub andwear. When a left turn is made at a fairly high rate of speed with a vehicle which has positivecaster, the caster of the left front wheel changes toward positive but the momentum of thevehicle is in a straight ahead direction. This causes the inside of the left front tire to scrub as itis turned. Just the opposite effect takes place on the right wheel as the vehicle is turned left athigh speed. The right front wheel's camber will go negative but the outside edge of the tire isscrubbed because of the vehicle's momentum to go straight. On some vehicles setting caster more than +2.5° will cause scrub problems.

Front Toe

Toe Definition

Toe relates to the difference in the distance between the front of the tires and the rear of thetires on the same axle, or to the vehicle centerline. Toe-in, or positive toe, is defined as the

front of the tires being closer together than the rear of the tires. Toe-out, or negative toe,is when the rear of the tires are closer together than the front of the tires. Zero toe iswhen the tires are parallel to each other.

Measuring Toe

Toe on an individual tire/wheel assembly is understood to be the difference between thedistance of the front and rear of one tire in reference to the vehicle centerline. Since mostalignment specifications show toe as total toe, i.e. both wheels, it is important to understand twopoints: (1) 1/2 of the specified total toe should be applied to each front wheel. (2) a minus(

) sign would actually indicate a toe-out setting as being specified. It is important to note thatalthough toe has historically been measured as a distance in fractions of an inch, and thendecimal inches, it is becoming more common for vehicle manufacturers to express toe indegrees. The idea is that the angle, rather than an arbitrary distance, determines the side slip ofthe tire and the resulting scrub of the tread. This should not be affected by the tire size, butrather should be constant for a given measurement. Most alignment equipment displays toe-outas a minus (-) and toe-in as a positive (+).

Effects of Toe

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Excessive toe increases tire scuffing and results in tire wear and drag on the vehicle.Excessive toe-in, or positive toe, increases scuffing on the outside of the tire.Excessivetoe-out, or negative toe, increases scuffing on the inside of the tire, and in some casescan cause a darting or wandering problem. Bias or bias-belted tires will commonly show afeatheredge or saw-tooth toe wear pattern across the entire tire tread area. Any tire wear pattern caused by a toe condition can be further affected by an excess camber condition andmay result in irregular wear patterns.

Toe Out On Turns

When a vehicle is turned, the inner front wheel must toe-out more than the outer wheel.The inner wheel must turn this tighter radius to avoid scrubbing. This is also known as theAckerman effect. Viewing the vehicle from the top as it is turning, the front wheels should turnon two different radii.

Toe Out On Turns, also known as TOOT, is built into the front steering arms and is not

adjustable. Before checking toe out on turns, make sure that all alignment settings are withinmanufacturers specifications. If using degree marks on the turn tables, make sure that thetire/wheel assembly is centered on the tables, this will reduce erroneous readings. To check toeout on turns, steer the wheels to the left so that the inner wheel is at 20°, the out wheel shouldbe less than 20°, optimal reading is 18°. Repeat the test in the other direction, and determine if there are any problems be comparing the manufacturers specifications. A variation fromspecifications indicates damaged steering arms and one or both arms should be replaced.

Front Setback

Front setback is a measurement referencing the front wheels to a line placed perpendicular tothe vehicle centerline. This line would be parallel to a line drawn through the centers of thespindle. If a vehicle has setback, one front tire/wheel assembly is farther back from thisimaginary reference line across the front of the vehicle than the other.

Positive setback indicates that the right front wheel is setback further than the left. Negativesetback refers to the left front wheel being further back than the right. Front setback can be

checked during a normal alignment, and is used to diagnose collision damage or cradle mis-adjustment. If the cradle is adjusted incorrectly, or damage is present, it is not unusual to alsosee a reduced positive caster reading on the side with the setback condition. Excessive setbackcan cause an alignment pull to the side with the setback. If the rear axle is positioned correctlyand all other parts and systems of the vehicle are in good working order, a setback conditionwill also create different wheelbase measurement side to side.

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Steering Axis Inclination (SAI)

SAI Definition

The angle between the centerline of the steering axis and vertical line from center contact areaof the tire (as viewed from the front). SAI is typically not adjustable, but deviations fromspecification can indicate vehicle damage. A maximum variation side to side of ± 1.0° may also

indicate vehicle damage. This topic is covered in detailed charts later.

Effects of SAI

SAI urges the wheels to a straight ahead position after a turn. By inclining the steering axisinward, it causes the spindle to rise and fall as the wheels are turned in one direction or theother. Because the tire cannot be forced into the ground as the spindle travels in an arc, thetire/wheel assembly raises the suspension and thus causes the tire/wheel assembly to seek thelow (center) return point when it is allowed to return. Thus, since it has a tendency to maintainor seek a straight ahead position, less positive caster is needed to maintain directional stability.

A vehicle provides stable handling without any of the drawbacks of high positive caster becauseof SAI.

Included Angle (IA)

I/A Definition

Included Angle is the combination of SAI and camber. Viewed from the front, the included angleis SAI plus camber if the camber is positive. If the camber is negative the included angle is SAIminus camber. If a side to side variation greater than ± 1.5° exists, check for vehicle damage.

Angle + Camber = Included Angle (I/A)

Measuring Procedures

SAI should always be measured after you have adjusted the camber and caster to the proper specifications or as close to the specifications as possible. Check for worn suspension parts.SAI is best measured with the front wheels off the ground, brakes applied and alignment

equipment leveled and locked. Raise the vehicle underneath the lower control arms but, do notrelax the suspension. Not raising the vehicle from the turntables can cause the control armbushings to move when wheels are turned, resulting in an inaccurate reading. Always refer tothe manufacturers SAI specifications and measuring procedures. If the vehicle has a solid frontaxle, the measurement can be taken with the wheels on the turntables.

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Steering Axis Inclination

Troubleshooting Charts

Short Long Arm (SLA) Chart

SAI CAMBER INCLUDEDANGLE

PROBLEM AREA

More thanSpecs

Equal toSpecs

Less thanSpecs

Spindle/Knunckle or Upper Control Arm and/or Control Arm Mount

Less thanSpecs

Equal toSpecs

More thanSpecs

Bent Lower Control Arm and/or Lower ControlArm Mount

Equal toSpecs

More thanSpecs

More thanSpecs

Spindle/Knuckle Assembly

Less than

Specs

More than

Specs

Equal to

Specs

Bent Lower Control Arm

Less thanSpecs

More thanSpecs

More thanSpecs

Spindle/Knuckle AssemblyBent Lower Control Arm

Equal toSpecs

Less thanSpecs

Less thanSpecs

Spindle/Knuckle Assembly

More thanSpecs

Less thanSpecs

Equal toSpecs

Bent Upper Control Arm

MacPherson Strut Troubleshooting Chart

SAI CAMBER INCLUDED

ANGLE PROBLEM AREA

Equal toSpecs

More thanSpecs

More thanSpecs

Bent Spindle

and/or

Strut Body

More thanSpecs

More thanSpecs

More thanSpecs

Strut Tower IN at Top and Spindle or Strut Bent

Less thanSpecs

More thanSpecs

Equal toSpecs

Bent Control Arm or Strut OUT at Top and BentSpindle or Bent Strut Body

Less thanSpecs

More thanSpecs

Less thanSpecs

Bent Control Arm or Strut OUT at Top and BentSpindle or Strut Body

Less thanSpecs

More thanSpecs

More thanSpecs

Bent Control Arm or Strut OUT at Top and BentSpindle or Strut Body

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Equal toSpecs

Less thanSpecs

Less thanSpecs

Bent Spindle and/or Bent Strut Body

Less thanSpecs

Less thanSpecs

Less thanSpecs

Strut Top or Bent Control Arm and Bent Spindleor Strut Body

More thanSpecs

Less thanSpecs

Equal toSpecs

Strut Tower IN at Top

Twin I-Beam Troubleshooting Chart

SAI CAMBER INCLUDED ANGLE PROBLEM AREA

More than Specs Equal to Specs More than Specs Bent Spindle

More than Specs Less than Specs Equal to Specs Bent I-Beam

More than Specs Less than Specs More than Specs Bent I-Beam and

Bent Spindle

Less than Specs More than Specs Equal to Specs Bent I-Beam

Mono Beam Troubleshooting Chart

SAI CAMBER INCLUDED ANGLE PROBLEM AREA

Equal to Specs More than Specs More than Specs Spindle Bent Downward

Less than Specs More than Specs Equal to Specs Axle Housing Bent Downward

Less than Specs More than Specs More than Specs

Spindle Bent Downward

and

Axle Housing Bent Downward

Equal to Specs Less than Specs Less than Specs

Spindle Bent

UpwardMore than Specs Less than Specs Equal to Specs Axle Housing Bent Upward

More than Specs Less than Specs Less than Specs

Spindle BentUpward

and

Axle Housing Bent Upward

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Scrub Radius

Scrub Radius Definition

Scrub radius is the distance at the road surface between the tire line and the SAI lineextended downward through the steering axis. The line through the steering axis creates apivot point around which the tire turns. If these lines intersect at the road surface, a zero scrub

radius would be present. When the intersection is below the surface of the road, this is positivescrub radius. Conversely, when the lines intersect above the road, negative scrub radius ispresent. The point where the steering axis (sai) line contacts the road is the fulcrum pivot pointon which the tire is turned.

Squirm

Squirm occurs when the scrub radius is at zero. When the pivot point is in the exact center of the tire footprint, this causes scrubbing action in opposite directions when the wheels areturned. Tire wear and some instability in corners is the result.

Applications in Suspensions

MacPherson strut equipped vehicles usually have a negative scrub radius. Even thoughscrub radius in itself is not directly adjustable, it will be changed if the upper steering axis pointor spindle angle is changed when adjusting camber. This is the case on a MacPherson strutwhich has the camber adjustment at the steering knuckle. Because camber is usually keptwithin 1/4° side to side, the resulting scrub radius difference is negligible. Negative scrub radiusdecreases torque steer and improves stability in the event of brake failure. SLA suspensionsusually have a positive scrub radius. With this suspension, the scrub radius is not

adjustable. The greater the scrub radius (positive or negative), the greater the steering effortand the more road shock and pivot binding that takes place. When the vehicle has beenmodified with offset wheels, larger tires, height adjustments and side to side camber differences, the scrub radius will be changed and the handling and stability of the vehicle will beaffected.

Ball Joint Inspection

Test ball joints for radial (horizontal) play by grasping the tire at the top and bottom andattempting to move the ball joint in and out. Also, test load carrying ball joint for axial play(vertical) by lifting with an appropriate tool (pry bar). If any loose or bent ball joints are detected,they should be replaced. Do Not mistake wheel bearing play for ball joint wear.

For suspensions with coil springs resting on the lower control arm lift as shown below.

This method will properly unload ball joint for inspection. For suspensions with coil springs resting on the upper control arm, lift as illustrated at right.This will properly unload the ball joint for inspection.

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