SAI Steering Axis Inclination
Steering axis inclination is the inward tilt of the suspension toward the center of the vehicle. The SAI angle is an imaginary line drawn through the upper strut and the lower ball joint and another true vertical line that runs through the center of the tire. The line runs through the upper and lower ball joints in a conventional suspension system.
Unlike SAI, the caster angle is adjustable. SAI is always positive, and except for heavy applications, the caster is positive, as well. Caster is the forward or rearward tilt of the strut as viewed from the side of the vehicle. Too little would cause the wheel to shimmy, and too much caster would make it hard to steer. Positive caster keeps the tire tracking behind the imaginary line created by the angle. Positive caster and the SAI angle provide for greater stability and better steering wheel return to center.