Certain conditions may make the frame appear to be out of alignment even though it is properly aligned. Refer to Checking Frame Alignment .

The following terms may appear in various procedures involving the frame and underbody:

Section Modulus: Section modulus is a measure of the frame strength based on the height, width, thickness, and the shape of the rails. This does not account for the strength of the material used in the frame.

Yield Strength: Yield strength is a measure of the material strength from which the frame is made. This is the maximum load measure in kPa (psi) that can be placed on a material and still have the material return to the original shape.

Resisting Bending Movement: Resisting bending movement is a single measure of the frame strength that accounts for both the section modulus and the strength of the material used in the frame. This is the product of the section modulus and the yield strength.

Sag: Sag refers to a frame or a side rail that is bending downward from its original position.

Buckle: Buckle refers to a frame or a side rail that is bending upward from its original position.

Diamond: Diamond refers to the condition under which one frame rail is moved forward from, or to the rear of, its correct alignment with the other rail.

Twist: Twist refers to the condition under which the frame assembly has been twisted. Under this condition, one of the frame rails will slope upward and the other rail will slope downward.

Side Sway: Side sway refers to a side rail that is bending to the side of its original position.

Tracking: Tracking refers to the alignment of the vehicle axles with one another. A misaligned frame may cause improper tracking. If the vehicle is tracking correctly, all of the axles will be parallel to one another and perpendicular to the center line of the frame.

Web: Web refers to the vertical part of a channel-type frame rail.