SAI Steering Axis Inclination
Steering axis inclination is the inward tilt of the suspension toward the center of the vehicle. The SAI angle is created by an imaginary line drawn through the upper strut and the lower ball joint on a vehicle with struts 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, 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. It 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.