Section 3.3: Advanced Alignment Geometry: SAI, IA, and TOOT

Advanced Alignment Geometry: SAI, IA, and TOOT

While camber, caster, and toe are the primary adjustable alignment angles, several diagnostic alignment angles are used to identify bent suspension components, structural frame damage, or steering linkage defects. These diagnostic angles include Steering Axis Inclination (SAI), Included Angle (IA), Toe-Out on Turns (TOOT), and Setback.

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Steering Axis Inclination (SAI)

Steering Axis Inclination (SAI) is the angle formed between a true vertical line and the steering axis (the line running through the upper and lower steering pivot points), when viewed from the front of the vehicle.

• On MacPherson strut suspensions, the steering axis runs from the center of the upper strut mount through the lower ball joint.
• On double-wishbone systems, the axis runs from the upper ball joint through the lower ball joint. SAI is always an inward tilt at the top. It is measured in degrees (°) and is typically between 8° and 15° depending on the vehicle design.

Purpose of SAI

• Directional Stability and Returnability: Because the steering axis tilts inward, turning the wheels forces the spindle to move downward. Since the wheel cannot push into the ground, this downward motion actually lifts the front body of the vehicle. Gravity acting on the vehicle's weight forces the wheels back to the straight-ahead position, aiding steering wheel returnability and straight-line stability.
• Scrub Radius Reduction: SAI pivots the tire contact patch closer to the center of the tire footprint. The distance between the steering axis projection on the ground and the centerline of the tire footprint is the scrub radius. Minimizing scrub radius reduces steering effort and minimizes the feedback of road shocks (such as bumps or potholes) through the steering wheel.

SAI is built into the spindle assembly and suspension mount locations and is not adjustable on most vehicles. An incorrect SAI reading indicates a bent component.

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Included Angle (IA)

The Included Angle (IA) is the angle formed between the steering axis (SAI) and the centerline of the wheel (Camber). It is a calculated value used to diagnose structural damage.

Included Angle (IA) = SAI + Camber

(Note: Camber must be added algebraically. If camber is positive, IA is larger than SAI. If camber is negative, IA is smaller than SAI. For example: SAI = 12°, Camber = +1.0° -> IA = 13°. If SAI = 12°, Camber = -1.0° -> IA = 11°).

                     \  |  /
                      \ | /
                       \|/
                        X  (Included Angle)
                       / \
                      /   \
          Steering Axis   Wheel Centerline
             (SAI)           (Camber)

Diagnostic Significance of SAI and Included Angle

Because the included angle is determined solely by the physical angle of the steering knuckle (spindle) snout relative to the ball joint or strut mounting points, the Included Angle will only change if the steering knuckle itself is bent. Technicians use the relationship between Camber, SAI, and IA to isolate bent parts:

• Camber is out of spec, but SAI and IA are within spec: The steering knuckle is normal. The issue is a misadjusted cam bolt, a shifted subframe/cradle, or a bent control arm/strut tower.
• Camber is out of spec, SAI is out of spec, but the Included Angle (IA) is correct: The steering knuckle is normal. The bend is in the strut shaft, the upper strut mount tower, or the control arm.
• Camber is out of spec, SAI is normal or out of spec, and the Included Angle (IA) is incorrect: The steering knuckle is bent and must be replaced.

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Toe-Out on Turns (TOOT) / Turning Radius

When a vehicle makes a turn, the inner wheel must travel along a smaller radius (a tighter circle) than the outer wheel. Therefore, the inner wheel must turn sharper than the outer wheel. This is known as Ackermann steering geometry. To achieve this, the wheels must toe-out during a turn. This angle is called Toe-Out on Turns (TOOT) or the turning radius.

How It Works

Ackermann geometry is built into the steering knuckles by angling the steering arms (where the tie rods attach) inward toward the center of the rear axle. When the vehicle steers:

• The steering arm of the inside wheel pulls the tie rod end further inward, increasing its steering angle (typically to about 22.5°).
• The steering arm of the outside wheel turns the wheel less (typically to about 20°).

Symptoms and Diagnostics of Incorrect TOOT

If the turning radius is incorrect, the tires will scuff sideways during cornering.

• Symptoms: Squealing tires during normal, low-speed cornering and rapid, feather-like wear on the outer edges of both front tires.
• Causes: A bent steering arm (knuckle arm) or a bent tie rod. TOOT is non-adjustable. If the alignment rack indicates that the steering angles are unequal when turned 20 degrees, the technician must inspect for a bent steering arm on the side that is out of specification and replace the steering knuckle.

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Setback

Setback is the distance that one front wheel is positioned further back from the vehicle's transverse centerline than the other wheel. Setback is measured in degrees or linear units (inches/millimeters).

• Positive Setback: The right front wheel is further back than the left front wheel.
• Negative Setback: The left front wheel is further back than the right front wheel.
• Causes: A front-end collision, hitting a curb at speed, or a bent lower control arm. It can also be caused by a shifted engine cradle or subframe.
• Diagnostic Tip: Front setback can cause the vehicle to pull to the side with the setback. If setback exceeds 0.5 inches (or 0.5 degrees), the technician should check the subframe alignment and inspect the lower control arms for bends.