Foam strength for truck cap trailer?

[DrewsBrews]

I've got a fiberglass truck cap to use as the top half of my build... The rest of it will essentially be a stand-in for a truck bed on a trailer frame not unlike the few other similar builds I've seen. It had been suggested that I try foam for the short side walls (~24").. However the forces of a cap on top would be different than the standard uniform, one piece shell that the foamy design typically incorporates.

Any thoughts on how I might go about constructing foam walls in a way that would support a ~200lb Cap and all the various forces that it would apply to the walls? Or would I be better off using standard stud and skin style construction?

[KennethW]

Run 6 cables to tie the cap to the trailer (or threaded rod).

[KCStudly]

Glue 1x2's or 3's (whichever matches the bottom flange on the truck cap) to the top and bottom of your foam walls and then wrap the canvas (or fiberglass) around them when you skin it. Then screw to the trailer and screw the cap on. More smaller fasteners would be better at providing structural unity than just a few larger ones.

I would use at least 1-1/2 thick foam. It will be very sturdy and light weight at the same time. Skin the inside of the foam with 1/4 underlay ply and it will be a fortress. No need for cables or long threaded rod, but you can use them if it makes you feel better. I guarantee you will realize that they are superfluous before your build is done.

If you plan to have a bed platform or storage bins, tie those into the lower walls for added strength; you probably won't need it, but if you are putting something like that in anyway, you might as well have it pulling double duty and put it to work as gusseting.

The trick will be either using a cap with a built in/OEM rear entry door, or, if a std. rear glass lift gate style, removing the glass and fitting your own door insert in such a way that it is sealed up.

[DrewsBrews]

Thanks guys I just might try it!

Initially I thought of a raised bed that would fold futon style with storage underneath and along the slide rails, but that would require extra overall height to the roof that I don't want.. not to mention the extra weight, time and expense to build.

I've also thought of angling the sides inward to the floor, mirroring the top, giving a hexagonal front/rear profile and reducing the surface area against the air while towing. Though that is doing battle in my head with vertical sides, which are easier to build and give more floor space.