Frame and Underbody Description

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Proper frame alignment is critical in order to ensure the normal life and the function of many other parts of the vehicle. If the vehicle is involved in any of the following, inspect the frame alignment:

A fire
A collision
An overloaded

Inspect any condition of misalignment. Refer to Checking Frame Alignment .

The following terms are defined:

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

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

Resistance To Bending Movement (RBM): RBM is a single measure of frame strength that accounts for both the section modulus and the strength of the material used. The RBM is the product of the section modulus and yield strength.

Sag: Sag refers to a frame or side rail that is bent downward.

Buckle: Buckle refers to a frame or side rail that is bent upward.

Diamond: Diamond refers to the condition where 1 frame rail moves forward from, or to the rear of, the correct alignment with the other rail.

Twist: Twist refers to the condition where the entire frame has been twisted. 1 rail will slope up while the other rail will slope down.

Sidesway: Sidesway refers to a side rail that is bent to the side.

Tracking: Tracking refers to the alignment of the vehicle axles with each other. A misaligned frame can cause tracking. If the vehicle is tracking correctly, all axles are parallel to each other and perpendicular to the center line of the frame.

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