Construction for a traditional shape travel trailer...

[SteveW]

... how about skinning the inside with a fabric. Same structural panel just flipped inside out.

Would it have the same strength? Seems that a canvas exterior would have good tensile strength (being stretched around the exterior), but wouldn't it lack the benefit of that stretching on the interior? The foam itself resists compression very well, and would work well keeping the forces in balance with the canvas. Would the canvas on the interior get a substantial boost in rigidity from the epoxy?

[atahoekid]

... how about skinning the inside with a fabric. Same structural panel just flipped inside out.

Would it have the same strength? Seems that a canvas exterior would have good tensile strength (being stretched around the exterior), but wouldn't it lack the benefit of that stretching on the interior? The foam itself resists compression very well, and would work well keeping the forces in balance with the canvas. Would the canvas on the interior get a substantial boost in rigidity from the epoxy?

I'd think that a structural panel provides strength from both sides... I'm not an engineer, so I may be assuming something that is incorrect but if someone knows one way or another I'd like to know...

[KCStudly]

Depends on how it is being loaded.

A simple beam supported at both ends and loaded with a single weight in the middle will result in compression of the fibers on the "top" and tension in the fibers on the "bottom". The stress (load per unit area) on the extreme fibers will always be the highest, basically due to leverage about the center of gravity of the piece in question, and the further apart these fibers are the higher the Moment of Inertia, or ability to resist bending for a given strength of material. Kind of like a lever; the further out on the lever there is less force required to resist a given torque load, and therefore the fibers either don't need to be as strong, or they can take a bigger load.

So if your concern is that you want to be able to stand on the roof of your foamie, then you probably would want a material that is good in tension on the ceiling, and one that is good in compression on the roof. For the same materials, if you want them to handle bigger bending loads in the long axis, place the fibers further apart (thicker). If the walls supporting your roof are further apart (6, 7, 8 ft wide); thicker.

If, however, you are building a tanker to haul 500 pounds of beer in bulk form, then the pressure is coming from the inside and you want the tensile member on the outside and the compressive member on the inside. (Or maybe you plan on leaning up against a wall as a back rest, or standing on your floor.)

Bunk beds or a hammock? Tension on the inside of the wall from the weight of the person sleeping trying to pull the wall in, compression on the outside.

Of course this is all just static theory. The reality of a simple foamie construction is that the sock not only provides an exterior skin, it also is a means of fastening the main panels together at the seams. And the geometry of this joint makes it far easier to apply effectively from the outside (it is easier to apply tension pulling the joints together by stretching the fabric over an outside corner, than it is to cram it into an inside corner, which by the nature of the application method would be tending to separate the joint. When looking at the big picture, at some point the cocoon, or overall shell construction becomes the other side of the stressed member (assuming that there are at least some shear walls (bulkheads, structural cabinet frames, roof spars, front and rear walls, etc.) spanning between the side walls.

My thought is that fiberglass weave and epoxy become more monolithic when cured (better in both tension and compression) than cotton fabric and glue or Latex, so might work equally well on the inside. Whereas the glue and cloth on the inside is doing more to protect the soft foam from local damage due to incidental contact from the occupants. Again, my perspective here is for a small simple teardrop structure. Larger panels will need stronger tensile and compressive qualities for both the interior and exterior skins (or the panels must be thicker) because the spans are greater and the loading can be expected to be more diverse, coming form either direction, or both.

But again the reality is, who wants to be stuck inside a little box applying epoxy and glass (ick). That and the warm appearance of wood is the reason I plan to use thin plywood on the inside and canvas/glue/paint on the outside (but I am not building a big camper, nor an ultra light).

Just my ramblings on the topic, take them for what they are worth.

[atahoekid]

the warm appearance of wood is the reason I plan to use thin plywood on the inside and canvas/glue/paint on the outside (but I am not building a big camper, nor an ultra light).
.

Yup, I agree there, but I am opting for fiberglass/epoxy

[GPW]

No Rules eh !!! Build it like you want ... The more diversity we have with our Foamie builds , the more information we can gather that may (or may not ) ultimately affect the future Foamie design parameters...

KC , my humble experience comes from years of making primitive bows ... out of wood , like the Indians did... Tension and compression was always the first consideration , certain woods/materials being better at one or the other ... certain design profiles being better at handling the stresses induced by bending ... For certain designs where the tension side was under considerable stress, I’d sinew the tension surface (like a skin on a foamie ) as the sinew now did most of the tension work and made the stretched wood close to the surface safe as it was a more elastic material able to stretch/handle to the tension loads ... keeping the wood fibers from separating/lifting , the first sign of impending breakage ..
And then there was all those years of Foamie model airplane design , where the same factors applied to the flying surfaces ... a wing spar being Very similar to a bow ... and working with Foam ...

Thicker Foam for larger trailers does seem like the BEST idea ... it is the lightest component in the whole system , so thicker would have little effect on the overall weight , and a tremendous affect on the strength, as you’ve stated ^

And since we’re dealing with a Lightweight design , a little “overbuilding” seems prudent and acceptable ... within reason , of course ...

Funny , the RV folks consider anything under 4000lb. as “Ultralight" ...

[eaglesdare]

well i only have one comment about putting fabric/canvas on the inside. i never finished mine at all. my walls are all exposed foam on the inside. i do think the roof part is covered, but thats about all. so i am not sure what all covering the inside does or does not do.
that did not help at all did it.

[GPW]

Covering the inside is not totally necessary on a well built Foamie (like Eagles) , but does add an extra bit of strength ... and is “cosmetic “ ...

[SteveW]

On a tangent, I've read a lot about difficulty with installing the canvas on the ceiling. Since these things are so light, anyone tried flipping the cabin over (once the outside skin was complete) and working the ceiling upside down? Seems it would make the installation quite a bit easier. Work with gravity rather than fight against it.

[KCStudly]

GPW wrote

Covering the inside is not totally necessary on a well built Foamie (like Eagles) , but does add an extra bit of strength ... and is “cosmetic “ ...

I agree, for a small teardrop, but for a larger structure like the OP was asking about, where the spans are larger and smaller loads in both inward and outward directions become more significant, then having a strong membrane on the inside is probably a necessity.

Speaking of going thicker while reducing weight, how about hollow core foam walls and ceiling? Kind of like this foam wing cross section.


Eagle, No worries. Your using yours!!! I have a friend that is a doer. He will make something three times to get it right before I'm done designing the first one, but you know what, even though he spent more effort and material making his, he still got done before I did (sometimes I never get off the drawing board ).