This article is part of a 6-part series, Part 3. It covers how SAI uses the vehicle’s weight to return the steering to center and how included angle, setback, and Ackermann geometry affect tire scrub and chassis squareness. Understanding these mechanical interactions is what separates a car that tracks straight from one that fights the driver at every turn.
The Lifting Effect and SAI Tracking Force
Steering Axis Inclination (SAI) utilizes the vehicle’s own mass to provide a self-centering force and shock isolation. The reason why is rooted in gravitational potential energy and directional resistance.
The lifting effect occurs because the steering axis is tilted. The spindle is engineered to move in a downward arc as the wheels are turned from the straight-ahead position. Since the spindle is supported by the wheel assembly, which is fixed to the road, the spindle cannot move downward. Consequently, the steering knuckle is forced upward, lifting the front of the vehicle. Directional tracking is achieved because the weight of the vehicle resting on the lifted knuckles creates a constant force that pushes the spindles back to their lowest point, which is the center position, automatically returning the wheels to a straight-ahead position. Scrub radius control happens because SAI brings the steering axis centerline and the wheel centerline closer together at the road surface. This minimizes the lever arm through which road forces act upon the steering linkage.
Load Path and Steering Axis Intersection
The intersection point of the steering axis and the tire contact patch dictates how the vehicle reacts to forward motion and impacts.
The toe-in tendency is that if the steering axis centerline strikes the road inside the wheel centerline, resistance to forward motion forces the wheels to pivot inward, which is toe-in. The toe-out tendency is that if the steering axis centerline strikes the road outside the wheel centerline, resistance forces the wheels to pivot outward, which is toe-out. Shock absorption is facilitated by SAI through what is called suspension shock transfer. By aligning the steering axis closer to the wheel centerline, road shocks are transmitted through the heavy inner end of the spindle and absorbed by the suspension components rather than the tie rod ends and steering gear.
SAI Fixed Geometry and Diagnostic Limits
The engineering rationale for SAI focuses on structural integrity and reducing steering feedback, which is sometimes called kickback.
Pivot point optimization means that by angling the steering axis so the top ball joint is closer to the vehicle center than the bottom ball joint, engineers reduce the arc the tire must scrub through during a turn. This reduces steering effort and tire wear. Fixed geometry means that in most modern suspension designs, SAI is a fixed geometric relationship between the upper and lower pivot points. It is used as a diagnostic benchmark. If SAI is incorrect, it indicates structural deformation of the knuckle, control arms, or strut tower, since there is no mechanical adjustment for this angle. Symmetry means the self-centering effect only functions correctly if the SAI is identical on both sides. Unequal SAI will cause the vehicle to pull toward the side with the lower inclination regardless of camber settings.
Local Shop Note:
I was thinking about this the other day — a shop owner on US-20A in Warsaw, N.Y. told me about a job that went sideways. He was at an Automotive Seminars, Inc event, and he was telling me about a sedan that came in with a complaint that the steering wheel would not return to center after a turn. You’d turn left, and the wheel would stay off-center. Turn right, same thing. No pull, just no return.
He put it on the alignment rack and checked caster — within spec. Checked camber — within spec. Toe was good. Everything looked correct. So he started measuring SAI and included angle. The left front SAI was 12.5 degrees, and the included angle was 13.2 degrees. The right front SAI was 12.3 degrees, and the included angle was 13.0 degrees. The math worked: SAI plus camber equals included angle. But the steering still didn’t return.
He manually rotated the steering wheel lock-to-lock and watched the SAI readings change. On the right side, the SAI would spike and drop as the wheel turned. That told him the steering axis pivot point was shifting under load. He raised the vehicle and found the right lower control arm bushing was worn and allowing the control arm to shift fore-aft. That movement was changing the effective SAI angle during a turn, preventing the self-centering force from working.
He replaced the control arm bushings, re-checked the alignment, and the steering returned to center smoothly after every turn.
That one stuck with me because SAI is supposed to be fixed. If it’s changing dynamically, something in the suspension is moving that shouldn’t be. The self-centering effect relies on the vehicle’s weight lifting the knuckles during a turn. If the pivot point shifts, that lifting effect is lost. When you’ve got a no-return complaint and the numbers look good, always check for worn bushings or loose components that can shift the pivot axis under load.
Included Angle, Setback, and Ackermann Steering
Geometric Summation and Scrub Radius Control
Alignment precision relies on the mathematical combination of fixed and adjustable angles to manage tire scrub and chassis squareness.
The included angle is the sum of the Steering Axis Inclination (SAI) and Camber. This represents the actual relationship between the wheel centerline and the pivot axis. It determines the scrub radius, which is the distance between where the steering axis hits the road and the center of the tire footprint. Setback is a condition where one wheel is positioned further back on the chassis than the opposite wheel relative to the geometric centerline. This is measured as the angle between the geometric centerline and a line perpendicular to the front axle. Ackermann steering, also called toe-out on turns, is the geometric principle that allows the inner wheel to turn at a sharper angle than the outer wheel. Because the inner wheel follows a smaller radius (shorter circle) during a turn, it must pivot more sharply to prevent lateral tire scrub.
Static Settings Versus Dynamic Deflection
Static alignment settings are engineered to compensate for the mechanical deflections that occur when the vehicle is in motion.
Toe-in and toe-out compensation works as follows. For rear-wheel drive (RWD), front tires tend to pull outward, which is toe-out, due to road friction. Static toe-in is set to allow the wheels to run parallel once the vehicle is at speed. For front-wheel drive (FWD), drive torque often forces tires to pull inward. Some FWD configurations require a static toe-out setting to achieve parallel tracking under load. Steering arm geometry means toe-out on turns is achieved by angling the steering arms toward the center of the vehicle. This mechanical linkage ensures the wheels automatically diverge during steering input.
Setback Tolerance and Feathering Wear
Setback tolerance is that a difference of more than 1 degree in caster readings from one side to the other often indicates excessive setback, usually resulting from collision damage or failed suspension bushings. Feathering is a specific wear pattern where the tire tread develops sharp edges on one side of each rib and rounded edges on the other. This is a definitive indicator of incorrect toe settings and excessive scrubbing. Mechanical play means proper toe settings must account for the cumulative tolerances, or slop, in the tie rod ends, steering rack, and control arm bushings.
Included Angle as a Bent-Spindle Indicator
The engineering rationale for these measurements is to maintain the squareness of the chassis and the efficiency of the steering linkage.
Included angle diagnostics means that because SAI is usually fixed, a change in the included angle is the primary diagnostic indicator of a bent spindle or steering knuckle. Setback impact means positive setback (right front wheel behind the left) or negative setback (left front wheel behind the right) causes the vehicle to pull toward the side that is furthest back, even if other angles are within specification. Centering the steering wheel means toe is the final adjustment in the alignment sequence. It is adjusted at the tie rod sleeves to ensure the steering wheel is centered when the wheels are in the straight ahead position.
The key takeaway is that SAI, included angle, and setback work together to control steering return and chassis squareness. The 6-part series continues with Part 4.