Frame and Underbody Description General
Identification of Material
Notice: Some frames are constructed of heat-treated steel. Do not weld or drill these frames. The use of heat in excess of 606°C (1200°F) can weaken, cause distortion, and change the characteristics of the steel.
It is critical to identify the rail material before attempting to straighten or repair a frame. The use of any method that is not recommended for the material may result in damage to the frame.
Measure the thickness of the frame rail in order to identify the heat-treated frame. Take the measurement where there is not any reinforcement. The heat-treated rail will measure 10 mm (0.39 in) thick.
Refer to Electrode Table and Welding Table for use of the proper electrode and chamfer for welding.
Frames
The components of a frame consist of the side rails and the crossmembers. The side rails carry the load and the crossmembers stabilize the rails. The frame assembly must be stiff enough to support the load it carries and flexible enough to absorb and distribute stress.
These vehicles have straight, full-channel, side rails. The rails should be parallel at all points along their length.
In order to prevent corrosion, it is necessary to apply a batten compound to the mating surfaces of dissimilar metals, such as an aluminum battery box that is mounted on a steel frame rail.
Attach the frame components by using grade 10.9 bolts or huck fasteners. Use prevailing torque nuts and hardened washers at each end of the bolts.
The proper frame alignment is critical to the durability and the function of many other parts of the vehicle. If the vehicle has been involved in a fire, a collision, or has been overloaded, check the frame alignment. Check the alignment if improper alignment is suspected for any other reason. Refer to Checking Frame Alignment .
General Description
Proper frame alignment is critical to ensure normal life and functioning of many parts of the vehicle. If the vehicle has been involved in a fire, collision, or has been overloaded, check the frame alignment. Certain conditions may make the frame appear to be out of alignment when, in fact, it is not. If misalignment is suspected, it should be checked. Refer to Checking Frame Alignment . Several of the terms used in this section are defined below:
| • | Section Modulus is a measure of frame strength based on height, width, thickness, and shape of the side rails. It does not account for the strength of the material used in the frame. |
| • | Yield Strength is a measure of material strength from which the frame is made. It is the maximum load measure in kPa (psi) that can be placed on a material and still have it return to its original shape. |
| • | Resistance to Bending Movement (RBM) is a single measure of frame strength that accounts for both the section modulus and the strength of the material used. It is the product of section modulus and yield strength. |
| • | Sag refers to a frame or side rail that is bent down from where it should be. |
| • | Buckle refers to a frame or side rail that is bent up from where it should be. |
| • | Diamond refers to the condition where one frame rail is moved forward from, or to the rear of, its correct alignment with the other rail. |
| • | Twist refers to the condition where the entire frame has been twisted. One rail will slope up while the other rail will slope down. |
| • | Side Sway refers to a side rail that is bent to the side of where it should be. |
| • | Tracking refers to the alignment of the vehicle axles with each other. A misaligned frame can cause improper tracking. If the vehicle is tracking correctly, all axles will be parallel to each other and perpendicular to the centerline of the frame. |
| • | Web refers to the vertical part of a channel-type frame rail. |
Frame and Underbody Description Frame Components
Frame Components (Front)
