Alignment: Description and Operation

GENERAL INFORMATION
Front wheel alignment refers to the angular relationship between the suspension attaching parts, the wheels, and the ground. Proper alignment is needed for efficient steering, good directional stability, and to prevent abnormal tire wear. The most important factors of front wheel alignment are wheel toe-in, wheel camber, and axle caster.

CASTER
Caster is the tilting of the front steering axis either forward or rearward from the vertical. It is measured in degrees of tilt. A rearward tilt is called positive caster and a forward tilt negative caster.

Incorrect caster may result from damaged control arms and other related front suspension parts. Also, uneven or spotty tire wear are signs of unequal caster and camber settings:

CAMBER
Camber is the inward or outward tilting of the front wheels from the vertical. When the wheels tilt outward at the top, the camber is positive. When the wheels tilt inward at the top, the camber is negative. The amount of tilt, measured in degrees from the vertical, is called the camber angle.

If camber is extreme or unequal between the wheels, improper steering and excessive tire wear will result. Negative camber causes wear on the inside edge of the tire, while positive camber causes wear to the outside.

TOE-IN
Linear sum toe-in is when the distance between the front wheels is less at the front of the axle than at the rear of the axle.

Angular toe-in is the angle of the front wheel centerlines, relative to the intersection of the forward projected vehicle center line.

The actual amount of toe-in is normally a fraction of a degree. Toe-in is measured from the center of the tire treads or from the inboard side of the tires. The purpose of toe-in is to ensure parallel rolling of the front wheels and offset any small deflections of the wheel support system that occurs when: the vehicle is rolling forward. Excessive tire wear and unstable steering are a result of incorrect toe-in. Toe-in is the last angle to be set in the front wheel alignment procedure. Toe-in is the most critical factor related to tire wear.