2A-2 Frame And Bumpers; R/V And P-Model Frames; Description; Diagnosis Of The Frame - GMC G Service Manual

2A-2 FRAME AND BUMPERS

DESCRIPTION

The models covered under this part of the section
include R/V, and P. The service information for the G-model
will be found in the underbody portion of this section.
Proper frame alignment is important to assure normal
life and functioning of many other parts of the vehicle. If the
vehicle has been involved in a fire, collision, or has been
overloaded, there is cause to check the frame alignment. If,
for any other reason, alignment is suspected, it should be
checked. A procedure for this is described later in this sec­
tion under "Checking Frame Alignment."
It is possible that certain conditions make the frame
appear to be out of alignment when, in fact, it is not. These
conditions are also described under "Checking Frame
Alignment."
Terms used later in this section are briefly defined
below:
1. SECTION MODULUS — This is a measure of the
strength of a frame, 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.
2. YIELD STRENGTH — This is a measure of the
strength of the material from which the frame is made.
It is the maximum load measured in kPa (psi) that can
be placed on a material and still have it return to its
original shape.
PROBLEM
Sag 1. Loads greater than the frame is designed to
2. Uneven load distribution.
3. Abrupt changes in section modulus. (For a brief
4. Improper body, or accessory, mounting:
— Holes drilled in the flange of the frame rail.
— Too many holes in the web section of the rail.
— Holes in the web section which are too close
— Four or more holes in the same vertical line of
— Welds on the flange, particularly across the
— Cutting holes in the rail with a torch.
— Cutting notches anywhere on the rails.
5. A fire involving the vehicle.
6. A collision involving the vehicle.
7. The use of equipment for which the frame has

R/V AND P-MODEL FRAMES

DIAGNOSIS OF THE FRAME

POSSIBLE CAUSE
carry.
definition of section modulus, see the "Descrip­
tion" at the beginning of this section.
to each other.
the rail web.
flange or along its edge.
not been designed or reinforced.
3. RESISTANCE TO BENDING MOMENT (RBM) —
This 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 sec­
tion modulus and yield strength.
4. SAG — This refers to a frame or side rail that is bent
down from where it should be.
5. BUCKLE — This refers to a frame or side rail that is
bent up from where it should be.
6. DIAMOND — This refers to the condition where one
entire frame rail is moved forward from, or to the rear
of its correct alignment with the other rail.
7. TWIST— This refers to the condition where the entire
frame has been twisted. One rail will basically slope
up while the other rail will basically slope down.
8. SIDESWAY — This refers to a side rail that is bent to
the side of where it should be.
9. TRACKING — This refers to the alignment of the
vehicle axles with each other. A misaligned frame can
cause improper tracking. If the vehicle is tracking cor­
rectly, all axles will be parallel to each other and per­
pendicular to the center line of frame.
10. WEB — The vertical part of a channel-type frame rail.
CORRECTION
1-7. Straighten and reinforce the frame as de­
scribed later in this section. See
"Straightening Frames" and "Reinforce­
ments."
GMTB-0813-2L