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Wheel alignment is part of standard automobile maintenance that consists of adjusting the angles of the wheels so that they are set to the car maker's specification. The purpose of these adjustments is to reduce tire wear, and to ensure that vehicle-travel is straight and true (without "pulling" to one side). There are two types of angles that should be adjusted regularly during the life of an automobile. The first type is called the 'primary angles' and second type is called 'secondary angles'. Within primary angles, there are three adjustment parameters involved on each axle in order to properly align a vehicle. These adjustments are camber, caster and toe. On some cars, there may only be the ability to adjust for one or two of the parameters.
Definition: The forward or backward tilt of the upper ball joint, or top of the strut, relative to the lower ball joint.
Purpose: Caster affects steering stability and steering wheel returnability.
Definition: Inward or outward tilt of the top of the wheel.
Purpose: Adjustment centers the vehicle's load on the tire, eliminating pull. Proper adjustment reduces camber tire wear and pulling.
Definition: The difference between leading edges and trailing edges of the front of the wheel & tire assembly, measured at spindle height.
Purpose: Minimize tire wear and rolling friction.
Definition: One wheel set back further than the other.
SETBACK IS (1) Manufacture. (Sometimes they build them this way).
CAUSED BY: (2) Collision
Definition: The relative position of the front wheels during a turn.
REFERRED (1) Toe out on turns
TO AS: (2) Turning radius
PURPOSE: To prevent tire side slip. To prevent excessive tire wear. To prevent tire squeal on turns
Definition: The direction the rear wheels are positioned in reference to the vehicle centerline.
S.A.I. Steering Axis Inclination
Definition: The angle between a true vertical line starting at the center of the tire at the road contact point and a line drawn through the center of the strut (or upper ball joint) and lower ball joint. S.A.I. is a non-adjustable angle on most vehicles. OR... The angle formed by the intersection of a line drawn through the upper and lower suspension mounting points (as viewed from the front of the vehicle) and true vertical.
I.A. (Included Angle)
Definition: S.A.I. angle plus actual camber (positive) or minus actual camber (negative) is the included angle. When camber is positive, add it to the S.A.I. angle. If camber is negative, subtract it from the S.A.I. angle. This angle is used as a diagnostic tool to determine if structural misalignment is present or suspension parts are bent.
Definition: When compared at ground level, the distance between the S.A.I. line (drawn
through the steering pivots) and the centerline of the tire tread is called the Scrub
Radius. When this line is toward the inside of the tread, the vehicle is said to have
Positive Scrub Radius. When the line is toward the outside of the tire tread, the
vehicle is said to have Negative Scrub Radius.
NOTE: Negative Scrub Radius will be found on FWD MacPherson Strut vehicles.
Directional Control Stability. • Steering Wheel Returnability. • Vehicle Load Placement.
S.A.I., I.A. and Camber can be used to locate areas of the strut system on unibodies which may have damaged or misaligned parts. I.A. (Included Angle) is used to determine if there is a damaged spindle or strut tube. The S.A.I. (Steering Axis Inclination) is used to determine if the unibody is misaligned.