Steering Axis Inclination is the inward tilt of the steering axis when viewed from the front. It is usually not adjustable, which makes it one of the most useful diagnostic values on an alignment report.
SAI belongs to the steering pivot, not to the wheel.
It is calculated during the caster sweep.
SAI and camber combine to form included angle.
A side-to-side difference can help locate structural or component damage.
Defining the steering axis
On a short-long-arm suspension, the steering axis runs through the upper and lower ball-joint centers. On a MacPherson-strut suspension, it runs through the upper strut mount and lower ball joint. When viewed from the front, the upper point is normally closer to the vehicle centerline than the lower point, creating inward inclination.
The steering axis is an imaginary line, but its position controls scrub radius, steering effort and how the body rises or falls slightly as the wheels are turned.
How SAI is measured
Like caster, SAI is calculated from wheel-angle changes during a steering sweep. The aligner observes how camber changes as the wheels turn and uses the measured steering angle to solve the geometry. Stable targets, free turn plates and a smooth sweep are essential.
Relationship to included angle
Included angle combines SAI and camber for the same wheel. Because SAI represents the pivot and camber represents the wheel, comparing them helps separate a bent wheel-supporting component from a displaced suspension mounting point. The exact diagnostic interpretation depends on suspension design and the direction of the changes.
| SAI | Camber | Included angle | Possible interpretation |
|---|---|---|---|
| Changed | Changed opposite direction | May remain near expected | Pivot position or mounting shift. |
| Near expected | Changed | Changed | Wheel-supporting part or camber adjustment. |
| Changed | Changed same diagnostic direction | Changed significantly | Bent or displaced suspension components. |
| Both sides unusual | Both sides unusual | Check conditions | Ride height, loading or measurement procedure. |
Diagnostic use
SAI is particularly useful when camber is outside range on a vehicle without camber adjustment. Compare left and right SAI, included angle, setback and ride height. Inspect the strut, knuckle, ball joints, control arms, subframe and body mounting points in the direction indicated by the complete pattern.
Do not use SAI alone to identify a specific failed part. It narrows the search by showing that the steering pivot relationship differs from the expected geometry.
Why SAI is normally not adjusted
SAI is created by the location of structural mounting points and steering pivots. Changing it normally requires moving or replacing parts rather than turning a normal alignment adjuster. If a repair procedure allows subframe repositioning, SAI may move as a result, but the goal is restoring component location rather than “setting SAI” directly.
Best practices
- Confirm the correct vehicle and ride-height condition.
- Repeat the sweep when SAI is unexpected.
- Compare both sides rather than relying on one number.
- Interpret SAI together with camber, included angle and setback.
- Inspect structural and pivot components before using correction parts.
Frequently asked questions
Is SAI the same as kingpin inclination?
They describe the same basic front-view inclination of the steering axis. SAI is the common modern alignment term because many vehicles do not use a physical kingpin.
Can SAI cause tire wear?
SAI itself is mainly diagnostic. A condition that changes SAI may also change camber, toe or steering behavior, which can contribute to wear.
Why is there no SAI specification for some vehicles?
Not every manufacturer publishes every diagnostic angle. Side-to-side comparison and related geometry can still provide useful information.
Technical content reviewed for TreadPlus Learn v1.0 · Updated July 16, 2026
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