Steering Axis Inclination (SAI) Explained: Why It's Important

The Steering Axis Inclination (SAI), sometimes referred to as Kingpin Inclination, is a critical, non-adjustable angle of the steering system. Understanding its role is key to diagnosing handling issues, tire wear, and vehicle safety.

1. What is the Steering Axis Inclination (SAI)?

The Steering Axis Inclination (SAI) is the inward or lateral tilt of the steering axis when viewed from the front of the vehicle. This angle is engineered into the steering knuckle and suspension design. The steering axis is the imaginary line drawn through the upper and lower pivot points—typically the center of the upper ball joint or strut bearing and the center of the lower ball joint.

SAI is defined as the angle between the vertical line and this steering axis line. Along with Included Angle (IA) and Camber, SAI forms the fundamental angles of the steering system.

2. What is the Purpose of the Steering Axis Inclination?

The primary purpose of SAI is to ensure directional stability and to provide a natural, self-centering action for the steering wheel after a turn.

SAI's Effect on Vehicle Stability

When you turn the steering wheel, the SAI causes the spindle (and thus the wheel) to pivot in an arc, effectively raising the center of the vehicle's weight. This phenomenon is known as "jacking." When the driver releases the wheel, gravity naturally wants to pull the vehicle's weight back down to its lowest point, which automatically forces the wheels to return to a straight-ahead position. This gravity-assisted return-to-center feel is critical for safe and predictable driving.

3. What Measurement Does the Steering Axis Inclination Perform?

The SAI measurement itself is an angle, expressed in degrees. It primarily determines the **Scrub Radius** of the wheel.

• Scrub Radius: This is the distance between the center of the tire's contact patch on the road and the point where the steering axis line intersects the ground.
• Positive Scrub Radius: The steering axis intersects the road *inboard* (inside) of the tire's center line.
• Negative Scrub Radius: The steering axis intersects the road *outboard* (outside) of the tire's center line.

The SAI angle is factory-set to achieve a specific scrub radius (usually near zero or slightly negative) which helps modern front-wheel-drive vehicles maintain stability during braking and acceleration, especially when wheel spin or traction loss occurs on one side (like hitting a patch of ice).

4. What is the Steering Axis Inclination as it Affects Wheel Alignment?

SAI is a core component of the total alignment geometry, specifically when combined with Camber.

Included Angle (IA)

The SAI angle and the Camber angle add up to form the **Included Angle (IA)**. $$\text{Included Angle} = \text{Steering Axis Inclination} + \text{Camber}$$ The Included Angle is a fixed, non-adjustable factory specification. Technicians use this relationship to diagnose bent steering components, as it allows them to check two related angles simultaneously. If the Camber is off-spec, but the SAI (or IA) is within spec, the Camber may be adjusted. However, if the IA is out of spec, it almost certainly indicates a bent spindle, strut, or damaged suspension mounting point, requiring component replacement rather than simple adjustment.

5. What is the Relationship of Steering Axis Inclination as it Affects Steering?

SAI directly controls the steering effort and stability.

• Steering Effort: High SAI requires more effort to turn the wheel initially because the system has to lift the vehicle's weight more dramatically. This is desirable for highway stability, providing a heavier, more planted feel.
• Returnability: It provides the power for the wheels to return to center. A reduction in SAI (often due to damage) will result in "lazy" steering that fails to snap back after a turn.
• Road Feedback: The scrub radius, controlled by SAI, determines how road forces (like bumps or braking on a split-traction surface) are transmitted back to the steering wheel. Proper SAI helps minimize sudden wheel jerks.

6. How to Diagnose that the Steering Axis Inclination is Out of Spec?

Since the SAI is fixed by the component geometry, being "out of spec" almost always means a component is bent or damaged, often from an accident or hitting a large pothole.

Symptoms of Incorrect SAI:

• Poor Steering Returnability: The steering wheel does not return to the center position easily after making a turn.
• Excessive Steering Effort: The steering feels unusually heavy or stiff.
• Directional Pull: The vehicle pulls heavily to one side, especially combined with unusual Camber readings.
• Abnormal Included Angle: The most definitive diagnosis is an alignment report showing the Included Angle (IA) outside the manufacturer's specified range.

If the SAI is different from side to side, a "cross-SAI" issue exists, leading to a strong pull toward the side with the lower SAI reading.

7. What is the Procedure to Adjust the Steering Axis Inclination and What Tools are Required?

The critical thing to understand is that the SAI is a **non-adjustable** angle on almost all modern cars and trucks. It is fixed by the manufacturing geometry of the steering knuckle, strut, and lower control arm mounting points.

Procedure for Correction:

1. Professional Measurement: Use a specialized electronic four-wheel alignment machine. This machine measures the SAI angle and calculates the IA.
2. Identify the Cause: If the IA is out of specification, the technician must inspect for a bent steering knuckle/spindle, a bent strut assembly, or a damaged frame/suspension mount.
3. Component Replacement: Correction requires replacing the damaged component(s) (e.g., the bent steering knuckle) with new, undamaged, factory-specification parts.

Tools Required:

Specialized tools include a high-precision, computerized **Wheel Alignment Machine** that uses sensors (often laser or camera-based) attached to the wheels. General repair tools (wrenches, presses, torque specifications) are then needed for the removal and installation of the necessary suspension components.