[evldave]

I might have slightly different perspective than most on here. I've actually used the foam on my surburban, mostly for sound deadening, but it does have some very good structural capabilities.

Here's what I did and what happened. I filled the entire rear panels of the suburban with spray foam (the area between the inner and outer walls). I think it took 25-30 cans total (this was the really good UL-listed fireproof stuff, minimal expanding). It completely re-inforced the rear of the truck - much more sturdy, plus it made the sheetmetal so tight I could hit it with a baseball bat and wouldn't make a dent.

But, would it work on the rails? I don't know but my now ex-girlfriend ran her truck into the rear of the suburban on the sheet metal above the rear wheelwell (where it was full of spray foam). She bent her (steel) front bumper and quarter panel, and I had absolutely no damage to the suburban. Since she used the bumper and hit the sheetmetal, I figured it was the spray foam re-enforcing the sheetmetal that made the whole area bulletproof.

If you do end up doing it (can't hurt) and it hits a rock, let us know if it works.

Here's a pic of the bumper and the sub. You can kinda see the wrinkles in the subs sheetmetal where the spray foam actually expanded out the side of the rig. The bumper bent back about 8-10", enough to push the wheel well into the tire.