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# 6.2.4 Wheel Alignment Theories

Information related to wheel alignment
by

## Ivan W. Anderson

on 12 May 2014

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#### Transcript of 6.2.4 Wheel Alignment Theories

The left illustration shows that the steering axis inclination angle is determined by drawing a line through the center of the upper and lower ball joints. This represents the pivot points of the front wheels when the steering wheel is rotated during cornering. The right illustration shows that the steering axis inclination angle is determined by drawing a line through the axis of the upper strut bearing mount assembly and the lower ball joint.

Included angle on an SLA-type suspension. The included angle is the SAI angle and the camber angle added together. If the camber angle is negative (–) (tire tilted inward at the top), the camber is subtracted from the SAI angle to determine the included angle.

Incorrect front toe does not cause a pull condition. Incorrect toe on the front wheels is split equally as the vehicle is driven because the forces acting on the tires are exerted through the tie rod and steering linkage to both wheels.

Incorrect (or unequal) rear toe can cause tire wear. If the toe of the rear wheels is not equal, the steering wheel will not be straight and will pull toward the side with the most toe-in.

As the spindle rotates,
it lifts the weight of the vehicle due to the angle of the steering axis.
(Courtesy of Hunter Engineering Company)

Caster is not a tire-wearing angle, but positive caster does cause changes in camber during a turn. See below. This condition is called camber roll.

Road Crown Effects Most roads are constructed with a slight angle to permit water to drain from the road surface. On a two-lane road, the center of the road is often higher than the berms, resulting in a road crown.

(c)

(b)

(a)

a) Zero thrust angle. (b) Thrust line to the right. (c) Thrust line to the left. (Courtesy of Hunter Engineering Company)

Cradle placement not correct on a front-wheel-drive vehicle. This can be caused by incorrectly installing the cradle after a transmission, clutch, or engine replacement or service.

An accident that affected the frame or cradle of the vehicle and was unnoticed or not repaired

The causes of setback include the following:

For best handling, the included angle should be within 1/2 degree of the SAI of the other side of the vehicle.

If the included angles are equal side-to-side, but the camber is unequal on both sides, the SAI must be unequal.

Cradle placement. If the cradle is not replaced in the exact position after removal for a transmission or clutch replacement, the SAI, camber, and included angle will not be equal side-to-side.

Included angle is an important angle to measure for diagnosis of vehicle handling or tire wear problems. If the cradle is out of location due to previous service work or an accident, knowing SAI, camber, and included angle can help in determining what needs to be done to correct the problem.

Toe on the front of most vehicles is adjusted by turning the tire rod sleeve as shown.
(Courtesy of John Bean Company)

(b)

(a)

Setback is the angle formed by a line drawn perpendicular (at 90 degrees) to the front axles.

SETBACK

The included angle is determined by
the design of the steering knuckle,
or strut construction.

INCLUDED ANGLE

Included angle on a MacPherson-strut-type suspension. (Courtesy of Hunter Engineering Company)

The included angle is the SAI added
to the camber reading of the front wheels only.

Feather-edge wear pattern caused by excessive toe-in or toe-out.

Many vehicle manufacturers specify a slight amount of toe-into compensate for the natural tendency of the front wheels to spread apart (become toed-out) due to centrifugal force of the rolling wheels acting on the steering linkage.

Excessive toe-in and the type of wear that can occur to the outside of the left front tire.

Feather-edge wear is also common, especially if the vehicle is equipped with nonradial tires.

Front-wheel-drive vehicles
have greater than 9 degrees
SAI (typically 12 to 16 degrees) for directional stability.

The SAI/KPI angle of all vehicles ranges between 2and 16 degrees.

It also helps center the steering wheel after making a turn and reduces the need for excessive positive caster.
The greater the SAI, the more stable the vehicle.
The SAI causes the spindle to travel in an arc when the wheels are turned. The weight of the vehicle is therefore used to help straighten the front tires after a turn and to help give directional stability.

The SAI provides an upper suspension pivot location causing the spindle to travel in an arc when turning, tending to raise the vehicle.

Wheel Alignment Theories
6.2.4
Bent, damaged, worn suspension and/or steering components or tire problems
can cause this.

Most alignment diagnosis is symptom-based diagnosis. The problem with the alignment is determined from symptoms such as excessive tire wear or a pull to one side of the road.

Pull Defined as a definite tug on the steering wheel or pull toward the left or the right while driving straight on a level road.

Symptoms

A pull is usually defined as a tug on the steering wheel toward one side or the other.

A condition where constant steering wheel corrections are necessary to maintain a straight-ahead direction on a straight, level road is called wander.

Wander is an unstable condition requiring constant driver corrections.

Worn suspension and/or steering components are the likely cause of this condition. Incorrect or unequal alignment angles such as caster and toe, as well as defective tire(s), can cause this condition.

WANDER
The vehicle will pull toward the side with the most positive camber.

Camber is measured in degrees or fractions of degrees.

Camber can cause pull if it is unequal side-to-side. A difference of more than 1/2 degree from one side to the other will cause the vehicle to pull.

Camber can cause tire wear if not correct.

Excessive positive camber causes scuffing and wear on the outside edge of the tire.

Excessive negative camber causes scuffing and wear on the inside edge of the tire.

CAMBER

Excessive positive camber and how the front tires would wear due to the excessive camber.

Excessive negative camber and how the front tires would wear due to the excessive camber.

Positive camber tilts the tire and forms a cone shape that causes the wheel to roll away or pull outward toward the point of the cone.

If camber angles are different from one side to the other, the vehicle will pull toward the side with the most camber.

Negative camber creates a pulling force toward the center of the vehicle.

The steering axis is defined as the line drawn through
the upper and lower steering pivot points.

Zero caster means the steering axis is straight up and down.

This is also called 0 degrees or perfectly vertical, as shown at right.

Measured in degrees or fractions of degrees

Caster is the forward or rearward tilt of the steering axis in reference to a vertical line as viewed from the side of the vehicle.

Positive (+) caster. (Courtesy of Hunter Engineering Company)

Negative (–) caster is seldom specified on today’s vehicles because it tends to make the vehicle unstable at highway speeds. Negative caster was specified on some older vehicles not equipped with power steering to help reduce the steering effort. (Courtesy of Hunter Engineering Company)

CASTER
Vehicle weight tends to lower the spindle, which returns the steering to the straight-ahead position.

If caster is excessively positive, the vehicle steering will be very stable (will tend to go straight with little steering wheel correction needed). This degree of caster helps with steering wheel returnability after a turn.

Caster is a stability angle.

Vehicles with as many as 11 degrees positive caster usually use a steering dampener to control possible shimmy at high speeds and to dampen the snap-back of the spindle after a turn.

If the caster is positive, steering effort will increase with increasing positive caster. Greater road shocks will be felt by the driver when driving over rough road surfaces.

If caster is negative, or excessively unequal, the vehicle will not be as stable and will tend to wander (constant steering wheel movement will be required to maintain straight-ahead direction). If a vehicle is heavily loaded in the rear, caster increases.

As the load increases in the rear of a vehicle, the top steering axis pivot point moves rearward, increasing positive (+) caster. (Courtesy of Hunter Engineering Company)

Slots
Strut rods

Shims
Eccentric cams

Caster can cause pull if unequal; the vehicle will pull toward the side with the least positive caster. The pulling force of unequal caster is only about one-fourth the pulling force of camber. It would require a difference of caster of one full degree to equal the pulling force of only 1/4-degree difference of camber.

Caster is not adjustable on many vehicles.

If caster is adjustable, it is changed by moving the lower or the upper pivot point forward or backward by means of the following:

NOTE: Caster is only measured on the front turning wheels of the vehicle. While some caster is built into the rear suspension of many vehicles, rear caster is not measured as part of a four-wheel alignment.

Caster should be equal on both sides; however, if caster cannot be adjusted to be exactly equal, make certain that there is more caster on the right side (maximum 1/2-degree difference) to help compensate for the crown of the road.

As viewed from the top of the vehicle (a bird’s eye view), zero toe means that both wheels on the same axle are parallel, as shown:

Toe is the difference in distance between the front and rear of the tires.

Toe Measured in fractions of degrees or in fractions of an inch

Toe
Toe is also described as a comparison of horizontal lines drawn through both wheels on the same axle, as shown here.

Total toe is often expressed as an angle. Because both front wheels are tied together through the tie rods and center link, the toe angle is always equally split between the two front wheels when the vehicle moves forward.

Toe-in, also called positive (+) toe. (Courtesy of Hunter Engineering Company)

If the front of the tires is closer than the rear of the same tires, then the toe is called toe-in or positive (+) toe.

If the front of the tires is farther apart than the rear of the same tires, then the wheels are toed-out, or have negative (–) toe.

NOTE: Some manufacturers of front-wheel-drive vehicles specify a toe-out setting to compensate for the toe-in forces created by the engine drive forces on the front wheels.

Toe-out, also called negative (–) toe. (Courtesy of Hunter Engineering Company)

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