Def'n of truck's "frame rails"?

Which parts are the “frame rails” in the photo on the link below? Only the two beams which run in parallel, the length of the truck? Or does the term “frame rail” include the beams that run at 90 degrees, i.e. the cross members too?

The reason I’m asking is b/c the repair manual says the truck’s aux fuel tank is located “between the frame rails” when it is actually located not in the middle, but on the side of the truck. It is between two cross members though.

Or maybe they mean that while it is on one side of the truck, it is still technically between the long-axis frame rails?

I’m of the opinion that the frame rails run lengthwise . . . whereas the cross members run horizontally

Not only that, but that’s how our fleet’s documents, forms, literature, etc. refer to the components

And that’s how I’ve seen it labelled in countless textbooks, service manuals, etc.

you thinking of buying the kit and fabricating a truck . . . ?!


I would consider anywhere inboard of the frame rails to be considered the same as between the frame rails as opposed to outboard of the the frames rails. So my question is your auxiliary tank inboard or outboard of the rails?

I’ve always considered that any frame parts (even unibody, critical reinforced “frame” components where engines, suspension, etcetera, attach) are frame members, which is the broad family name for these parts.

I consider frame rails to be front-to-rear frame members on cars which have separate frames (think parallel railroad rails). The cross-wise members I call cross members act as the ties between the frame rails.

I’d say a tank mounted anywhere within frame members on a vehicle with a separate frame would necessarily be mounted between frame rails.


aux fuel tank is different than the main fuel tank