The Four Basic Collision Conditions
1) Side Sway
2) Sag
Sag is the bending of the chassis out of its true horizontal position. It can cause extensive flange damage.
On
both of these points, side-sway and sagging, we mentioned flange
damage. The reason we stress this point is that the load carrying or
downward strength of a frame is in the frame rails, and the backbone of
the frame rail is the flanges. They are the most important part of the
frame rail and must be straight and repositioned to their normal
position without heat and with a minimum amount of hammering in order to
keep the original strength of the frame.
3) Twist
The vehicle’s front end rolls to one side and the rear rolls to the opposite side. The twist is held into the chassis by the cross members. To make the correction, you must look at the strongest cross members in the chassis and where they are located since they will be the ones holding the twist into the chassis. Most vehicles will have the heaviest cross members from the cab to the back of the frame. Therefore, this is the part of the frame that you will do the correcting in.
4) Diamond
A diamond in the frame is when one frame rail is moved slightly ahead of the other. This is held in by the cross members. Most vehicles today, when they are in a collision, will generally not have a diamond condition due to the construction of the chassis. A diamond condition will result if they are hit directly in the front or back on either of the frame rails or have been towed by one of the rails.
