Section 6.5: Wheel Alignment Geometry & Correction

Wheel Alignment Geometry & Correction

Wheel alignment is the process of adjusting the angles of the steering and suspension components so that the wheels are perpendicular to the ground and parallel to each other. Proper alignment ensures directional stability, predictable handling, and maximum tire service life. The three primary alignment angles are caster, camber, and toe.

Primary Alignment Angles

1. Caster: The forward or backward tilt of the steering axis when viewed from the side of the vehicle.
   • Positive Caster: The steering axis tilts toward the rear of the vehicle. This creates a self-centering force that improves straight-line stability and steering returnability. Most modern vehicles are designed with positive caster.
   • Negative Caster: The steering axis tilts forward. This reduces steering effort but makes the vehicle wander at high speeds.
   • Caster Pull: Caster is not a tire wear angle. However, if there is a side-to-side difference in caster (cross-caster), the vehicle will pull to the side with the least positive caster (or most negative).
2. Camber: The inward or outward tilt of the wheel when viewed from the front of the vehicle.
   • Positive Camber: The top of the wheel tilts outward.
   • Negative Camber: The top of the wheel tilts inward. Modern performance vehicles use slight negative camber to improve tire contact patch during hard cornering.
   • Camber Pull and Wear: If camber is unequal side-to-side (cross-camber), the vehicle will pull to the side with the most positive camber. Excessive camber causes rapid wear on one side of the tire tread (camber wear).
3. Toe: The difference in distance between the front and rear edges of the tires on the same axle.
   • Toe-in (Positive Toe): The fronts of the tires are closer together than the rears.
   • Toe-out (Negative Toe): The fronts of the tires are further apart than the rears.
   • Tire Wear: Toe is the most critical tire wear angle. Even a slight toe misadjustment causes rapid, characteristic feathering wear across the tire tread. Incorrect toe-in wears the outer edge of the tread blocks, while toe-out wears the inner edge.

Secondary Alignment Angles

These angles are measured to diagnose bent components or structural damage.

1. Steering Axis Inclination (SAI): The angle of the steering axis from vertical when viewed from the front. It is a non-adjustable design angle that helps center the steering and provides steering returnability.
2. Included Angle (IA): The sum of SAI and Camber (if camber is positive) or the difference (if camber is negative).
   • Diagnostic Value: If camber is incorrect but SAI is correct, the steering spindle or knuckle is bent. If both SAI and camber are incorrect by the same amount, the strut tower or lower control arm mounting location is shifted or bent.
3. Thrust Angle: The angle between the vehicle centerline and the direction the rear wheels are pointing.
   • A crooked rear axle creates a thrust line that deviates from the centerline. This causes the vehicle to 'dog-track' (drive sideways down the road) and forces the driver to hold the steering wheel off-center to drive straight.
4. Turning Angle (Ackermann Angle): The difference in steering angle between the inside and outside wheel during a turn. The inside wheel must turn at a sharper angle because it travels a smaller radius circle. This is determined by the steering arm geometry and is non-adjustable.
5. Setback: The distance that one wheel center is behind the other on the same axle, indicating a bent frame or control arm.

Pre-Alignment Inspection Procedures

An alignment should never be performed on a vehicle with worn suspension or mismatched tires. Follow this checklist:

1. Tires: Ensure tire inflation pressures match placard values. Verify tire sizes, tread depths, and wear patterns are similar on the same axle.
2. Suspension and Steering Play: Raise the vehicle and inspect ball joints, tie rod ends, control arm bushings, steering gear, and wheel bearings for play. Replace worn parts before proceeding.
3. Wheel Runout: Mount alignment targets. Perform a rolling compensation to calibrate the alignment sensors and compensate for wheel runout.
4. Ride Height: Measure ride height. If the vehicle is sagging due to worn springs, the alignment angles will be incorrect.
5. Trunk Load: Remove any heavy temporary cargo. The vehicle should be at its curb weight.

Alignment Adjustment Sequence

Always perform adjustments in this specific order:

1. Rear Camber: Adjust rear camber to specification.
2. Rear Toe: Adjust rear toe. This establishes the thrust angle.
3. Front Caster: Adjust front caster. (Caster is adjusted before camber because caster adjustments often alter camber).
4. Front Camber: Adjust front camber.
5. Front Toe: Set front toe while locking the steering wheel in the straight-ahead position. Adjust both tie-rod sleeves equally to maintain steering wheel centering.

Correction Methods

• Eccentric Cam Bolts: Commonly used on control arm mounts or strut-to-knuckle bolts. Rotating the bolt shifts the position of the control arm or knuckle.
• Shims: Inserted behind the rear wheel hub assembly or between the control arm pivot shaft and frame.
• Strut Slotting: In some MacPherson strut systems, the strut mounting holes can be slotted using a grinder to allow camber adjustment.