70 Section 1 Introduction to Collision Repair
Copyright Goodheart-Willcox Co., Inc.
Frame rails and cross members are strong. The
thickness and weight of the frame make it resistant to
force. When impact forces greater than this resistance
act on the frame, the frame rails tend to transfer these
forces through the vehicle. The frame will usually
distort in the area of a pivot point. Many full-frame
vehicles are not designed to collapse and absorb
impact forces. A unibody vehicle, on the other hand, is
designed to collapse so the vehicle absorbs the impact
forces, protecting the passengers. Some full frames
have notches or crush zones that absorb impact
forces.
Front Impact
The bumper on a full-frame vehicle is bolted
directly to the frame, so the frame is generally one of
the first items affected in the event of a collision. For
example, consider a pickup truck that has taken a hit
on both rails. If the impact force is greater than the
resistance of the frame, the frame will be damaged.
The impact force will travel through the frame until the
force is dissipated. Depending on the frame design, the
force may cause the frame rails to move up or down. In
one type of truck, the frame rails move upward. If the
impact force is great enough, the upward movement
of the frame can cause the engine to hit the cowl. See
Figure 4-24.
The cross members or frame step-in may act as a
pivot point for the impact force. The impact force may
cause distortion or buckling just ahead of the pivot
point. See Figure 4-25.
The damage caused in this collision would be a
change in frame height and length. The change in
frame height is called kickup when the frame moves
up and sag when the frame moves down. The change
in frame length is called mash.
If the impact is between the frame rails as shown in
Figure 4-26, the frame rails move toward each other.
The lateral movement of the rails is called sidesway.y y
If the impact is on one rail, only the impacted rail will
move back. The other rail does not move back, but it
may move toward the damaged rail. After this type of
impact, the cross members and rails no longer meet
at right angles. See Figure 4-27. This type of damage
is called diamond because the frame rails and cross d
Goodheart-Willcox Publisher
Figure 4-24. The impact caused the front of the frame
to move upward. Notice the door/fender gap, tight
at the top, wide at the bottom and the position of the
wheel in the wheel opening.
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Figure 4-23. This frame has arches at the front and rear suspension mounting areas.
Arch