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Caster is the backward or forward tilt of the steering axis when viewed from the side. A tilt toward the rear at the top is + , a tilt toward the front is - . This is a steering control angle. Excessive positive caster will result in good directional control, but hard steering effort to turn away from the straight ahead position. Excessive negative caster will make steering effort very easy, but will result in excessive wandering. Caster may be adjustable on some front suspension systems. Most cars with + caster use power steering to reduce steering effort.
Camber is the inward or outward tilt of the wheel when viewed from the front. An outward tilt at the top is +, an inward tilt is - . This is a tire wear angle. Excessive postive camber will wear the outside edge of the tire, excessive negative camber will wear the inside of the tire. Camber may be adjustable at the front wheels, and also at the rear wheels on many cars - front wheel drive - and rear wheel drive with independant rear suspension. Most vehicles have a slight + camber at the front wheels to compensate for the loading force on the front of the car due to wind resistance.
Toe is the pulling in, or pushing out, of the wheels at the leading edge. If the wheels point inward, this is + toe (or toe-in). If the wheels point outward, this is - toe (or toe-out). This is a tire wear angle and is the only angle that is adjustable on all vehicles at the front wheels, and on many rear wheels on front drive cars (and rear drive cars with independant rear suspension). Most vehicles are set with a slight + toe to compensate for the deflection of the steering linkage and suspension as the car begins to roll forward.
Positive Toe
Steering Axis Inclination (SAI) is the angle formed by the rotational axis of the front suspension in relation to a vertical angle through the center of the wheel spindle. SAI causes the wheels to return to the straight ahead position after cornering, and is therefore a steering control angle. It is not adjustable, and if incorrect, requires replacement of suspension parts to correct it.
This is also known as Steering Angle, or the Ackerman Effect. When a vehicle turns a corner, the inner wheel should turn at a sharper angle than the outer wheel because it is following a smaller radius. A typical example would be an inner angle of 23 degrees, with an outer wheel angle of 20 degrees. It is a tire wear angle, and is controlled by angling the steering arms in toward the center of the vehicle. It is not adjustable, and if incorrect, indicates steering arms that are bent, and which must be replaced.
Included Angle includes SAI and Camber. These two angles should intersect at the center of the tire on the road surface for maximum steering control and tire wear life. This angle, by itself, is not adjustable - and is used by most computerized alignment machines as a method to measure SAI by subtracting Camber from the included angle.
Thrust Angle is the angle formed by the two rear wheels in relation to the geometric center line of the vehicle. If the angle points to the left (the driver's side), it is +. If it points to the right, it is - . This is a steering control angle, because the car will always travel in the direction that the rear wheels are pointed. This may be adjustable if the rear Toe is adjustable. On a fixed axle, it indicates that the rear axle is out of line and may be correctable.
Positive Thrust Angle
Set Back occurs when one front wheel is pushed back in relation to the other. This is usually not critical if not excessive, and may be corrected through body alignment, or by shifting the suspension cross-member. It is usually the result of a collision or damage caused during shipping the vehicle. Excessive set back may affect steering pull. the side that is further back is +, further forward is - .
