A technician is performing a four-wheel alignment on a vehicle with a MacPherson strut suspension. The alignment machine shows a front left Steering Axis Inclination (SAI) of 10.5 degrees and a front right SAI of 10.0 degrees. The manufacturer's specification for SAI is 11.0 degrees +/- 0.5 degrees. All other angles are within specification. What is the most likely cause of the SAI discrepancy?
- AInsufficient positive caster on the left wheel.
- BA bent strut on the front left side.
- CExcessive positive camber on the left wheel.
- DIncorrect tire pressure on the left front tire.
Show answer & explanationAnswer & explanation
Correct answer: B. A bent strut on the front left side.
SAI is a non-adjustable 'diagnostic' angle. A difference in SAI between the left and right sides, especially when one side is outside specification (FL is 10.5, spec is 11.0 +/- 0.5, so 10.5 is at the very edge of spec for the low side), often indicates a bent steering knuckle, strut, or control arm. Given the MacPherson strut design, a bent strut is a very common cause of SAI being out of spec.
Why the other options are wrong
- A. Caster primarily affects steering returnability and stability, not SAI.
- C. Camber affects included angle, not primarily SAI directly, especially not causing a lower SAI.
- D. Tire pressure primarily affects camber (due to tire deflection), not SAI, which is a geometric angle of the suspension.
SAI Discrepancy Causes
Steering Axis Inclination (SAI) is a non-adjustable angle formed by the steering axis and true vertical. Discrepancies between sides or being out of specification usually indicate bent or damaged suspension components.
- SAI is a diagnostic angle, not adjustable.
- Side-to-side differences point to bent components.
- Bent strut, steering knuckle, or control arm are common culprits.
Memory trick: SAI's off-kilter, danger's near; bent metal's the culprit, make it clear.