The main parts of the frame are the side rails and the crossmembers. The rails carry the load and the crossmembers stabilize the rails.

Types of frame misalignment can be divided into five groups. The five groups are sag, buckle, diamond, sidesway and twist.

A misaligned frame rail may have moved forward from, up or down from, or to the side of where it should be. These possibilities must be investigated.

The easiest way to check frame alignment is with gauges made for this purpose. Detailed instructions are normally supplied with the gauges at the time of purchase. Therefore, instructions for gauge use are not given in this manual.

It is possible to check certain portions of the frame alignment without the proper gauges. The procedure is described later in this section.

Whether alignment is checked with or without gauges, the vehicle must be parked on a level surface.

Certain conditions call for preliminary checks before actually checking the frame. Suspension or axle problems may make it appear that the vehicle frame is out of alignment. If an axle has shifted, "diamond" or "sidesway" may appear to exist when, in fact, they do not. A weak spring may make the vehicle appear to have a twisted frame.

A visual inspection of the top and the bottom flanges of each rail may reveal the specific area where the sag or buckle exists. In the case of sag, wrinkles may appear on the top of the upper flange; such wrinkles are definite evidence of sag.

Wrinkles on the bottom of the lower flange are definite evidence of buckle.