Steering and suspension alignment is the process of correctly positioning the steering and suspension components to each other and to the vehicle’s frame. Five basic front end geometric angles, common to all vehicles, perform two basic functions:
1. They position the steering axis which controls directional stability and handling.
2. They position the wheels so that tires obtain maximum road contact with minimum tire wear.
The five basic angles are: Caster, Camber, Steering Axis Inclination (SAI) angle, Toe, Turning Radius. Caster and SAI position the steering axis of each wheel. Camber, Toe and Turning Radius position the wheels.
SAI: Steering Axis is an imaginary line through the upper and lower ball joints (pivot joints) on short & long arm suspensions. On strut suspensions, the steering axis passes through the lower ball joint and the upper end of the strut. Steering Axis Inclination is the angle of the inward tilt of the steering axis and is measured in degrees from true vertical. SAI provides good driving and handling characteristics through directional stability and weight projection. Directional Stability is the tendency of a wheel to straighten from a turned position and remain straight. As the wheel is turned, the spindle swings on a downward arc and the weight of the vehicle forces the spindle to return to the highest point in the arc, returning the wheel to a straight ahead position. Weight projection is the projection of the vehicle’s weight to a point within the road contact area of the tire. The weight projection point at the road surface is the point where the wheel pivots for turning. SAI is determined by a true plumb line, placed at the center of the wheel at the point of road contact, and the projected line created by the strut or the upper and lower ball joints. The two lines will intersect at a point just below the road surface on most vehicles, but on some front wheel drive vehicles the point of intersection will be just above the road surface. The distance between the projected line and the vertical line at the road surface is called scrub radius. Scrub radius is positive when the projected line is inward of the true vertical line at the road surface and the point of intersection is below the road surface. Positive scrub radius forces the front wheels to toe out when the vehicle is in motion. This is usually found on rear wheel drive vehicles. Negative scrub radius places the projected line outward of the true vertical line and the point of intersection is above the road surface. This forces the front wheels to toe in to provide stability when braking on front wheel drive vehicles. SAI is a directional control angle. The point of intersection is designed by manufacturers to provide a pivot point for the front wheels when cornering. SAI is not adjustable on most vehicles. SAI can be affected by loose, worn or damaged suspension parts or by frame damage.
CASTER: Caster is the position of the steering axis from true vertical when viewed from the side. The caster angle is formed by the steering axis and a true vertical line passing through the spindle. The purpose of caster angle is to provide directional control stability for the front wheels to travel a straight course with minimal effort. Proper caster angle also helps to return the front wheels to a straight ahead position after a turn. Caster is adjustable on most short/long arm suspensions on passenger cars and light duty trucks to compensate for normal wear in suspension and steering. Positive Caster is the backward tilt of the steering knuckle at the top. This places the vehicle point of load ahead of the point of contact at the road surface, which provides good directional control. Negative Caster is the forward tilt of the steering knuckle at the top. This places the point of contact ahead of the point of load, which provides easier steering at slower speeds. Negative caster does not provide good directional control stability. The vehicle must instead rely on its SAI angle and wide tires for directional control. Caster is a directional control angle and can be used offset road crown on most short/long arm suspensions. Unequal caster causes the vehicle to pull to the side with the least positive caster. Unequal Caster Effect – to offset road crown, the top left steering knuckle is always leading the right; the right side has more positive caster. Too much positive caster out of factory specs causes hard steering and excessive road shock and shimmy. Too much negative caster causes instability at high speeds. When caster is out of factory specs, the vehicle should be checked for loose, worn or damaged suspension and steering parts before alignment. FRAME DAMAGE also affects caster angle.
CAMBER: Camber is the inward or outward tilt of the wheel at the top, measured in degrees from true vertical. Proper camber angle provides directional control stability by placing maximum tire tread in contact with the road surface under all driving conditions. Camber is adjustable on most domestic cars and light trucks. Positive camber is an outward tilt of the wheel and Negative camber is an inward tilt of the wheel. Camber is a directional control angle. Unequal camber causes the vehicle to pull toward the side with the most positive (outward) camber. Camber can be used offset road crown on most short/long arm suspensions. Improper camber settings can cause uneven tire wear and wheel bearing wear.
Included angle is the sum of the camber and SAI angles and should be within ½ degree from side to side. On a unibody vehicle, the two angles that tell the most about the condition of the suspension are SAI and Included Angle. If SAI is off, either the vehicle structure or a suspension mounting part is bent or out of proper specs. If the included angle is off, there is a bent or damaged suspension part. SAI and included angle should be within tolerance before any other alignment adjustments are made.
TOE: Toe angle is the distance between the wheels when measurement is taken at the front and rear of the wheels. The purpose of toe angle is to compensate for tolerances in steering linkage and the effect of positive/negative scrub radius when the vehicle is in motion. When the front of the wheels are closer together than the rear of the wheels, they have a toe in angle. When the front of the wheels are farther apart than the rear, they toe out. Toe in is recorded as positive toe angle and toe out is negative. Toe measurement is the distance from the center line of the vehicle to the front of each wheel. Toe can be measured in inches, decimal degrees or millimeters. Toe will become a tire wearing angle if not set within factory specifications. Early indication of toe tire wear can appear as a feather edge or scuff on the edge of the tire tread surface. Toe tire wear can also be found on rear tires as a cupping, feather edge or smooth edge on the tire tread surface. Too much toe in will cause the feather edge to point in while toe out will cause the feather edge to point out. Toe angle is adjustable on the front wheels on all vehicles and is adjustable on the rear wheels of many vehicles. Toe is adjusted by turning the threaded sleeve or rod which adjusts the length of the tie rod. Variation from factory specs is usually caused by worn or bent suspension parts or changed in caster/camber settings. Toe angle can also be affected by body structure or frame damage.
TURNING RADIUS: Turning radius angle is the angle formed by the amount the front wheels toe out on turns. The purpose of turning radius is to steer the vehicle through turns with minimum tire wear. A non adjustable angle built in by the manufacturer, turning radius is the angle you should check any time you have an excessive toe reading or if there is body/frame damage to the front of the vehicle. This prevents covering up bent parts with a toe adjustment.
The amount of turning radius for each vehicle is established by the wheel base and tire width. Turning radius is controlled by the angle of the steering arms to the center line of the wheel. Each steering arm has its own arc of travel as the front wheels are turned. Turning causes the inside wheel to turn more sharply than the outside wheel, setting up a “toe out on turns” effect. This toe out effect aids in steering through turns with minimal tire wear. When turning, the inside wheel will always have the greater angle, because the inside wheel must turn sharper. Turning radius is measured in degrees by reading a plate degree indicator. This is a plate which is placed underneath each of the front wheels and is gauged to show the angle of the wheels as they are rotated. Turning radius will become a tire wearing angle if it is not within factory specifications. Early indication of turning radius tire wear will appear as a feather edged tread or a scuff on the edge or side of the tire tread. Extended periods of turning radius tire wear will appear smooth on the edge or side of the tire tread. On any vehicle, when turning radius is not within factory specifications, the technician should always check for a bent steering arm, bent tie rod or bent center drag link. Since turning radius is a non-adjustable angle, the only way to correct a misalignment is to replace any bent parts or repair the damaged frame or body structure. An excessive toe angle can also affect turning radius.