Your thoughts on Removable 5x10 body on HF 4x8 frame

[iamroot]

Here's my goals:

1. Removable, so I still have a trailer I can use for hauling heavy or dirty stuff
2. Low drag and lightweight.
3. Can tolerate living outside in the PNW (coldish wet winters, but warm dry summers)

I need to make a decision because summer break is coming up fast and I tend to dither around too long in the planning phase.

I'm thinking I should install weld nuts and bolt through timber from above into the frame. (I have marmoleum (real linoleum) that I plan to lay loose on the inside as my floor.) But here are the concerns in general I want to solve:

1. Don't trap moisture or be built entirely of moisture-immune material
2. Insulate so the floor isn't too cold in winter
3. Rubbing/damage between the body and the frame

Body-on-frame trucks solve #3 by having the body sufficiently-self-supporting and mounted on a handful of rubber bushings to the frame, #1 uh, by rusting out after 10 years (well, back in the day...), and don't do #2.

The floor could be a SIP (XPS with luan on top and bottom), and if this was a permanent installation, I think that could be done because I could use flexible caulk along the top of the frame and hopefully hold up. But I want it to be removable, which probably means bolts, which means point loads, which I would expect a luan/XPS/luan alone would not be strong enough.

I saw Tony said he's successfully done thick plywood on the bottom with epoxy and fiberglass cloth, and that is regular boat construction and I would expect it to hold up physically, but if there's ever a leak from above during winter storage, it'll be entirely destroyed.

40% of aero drag is undercarriage and wheels, so I want to have a smooth belly under the frame. Coroplast seems ideal for this because it's immune to water, cheap, smooth, and impact resistant.

Okay, that's all said, what do you all think of these assemblies:

Coroplast -- trailer -- TPU (or similar) flat gasket -- latex paint -> luan -- XPS foam except timber over frame members -- luan -- marmoleum

The luan--foam/wood--luan sandwich would use subfloor adhesive to make it a SIP, which would be expected to carry the wall load horizontally 7" or so into the frame.

-or-

I weld on another rail 7" out on each side, then
Coroplast -- trailer/infill fiberglass insulation -- TPU (or similar) flat gasket -- latex paint -> 3/4" plywood -- marmoleum


Both plans are vapor permeable up and down. I am worried slightly about coroplast holding moisture in, but I planned to leave a vent along the sides so that hopefully doesn't happen, or maybe add a solar fan to circulate air under when it's not being used. There will be gaps so liquid water can flow out. I could substitute mineral wool in if it turns out the fiberglass gets wet or somebody else has had that happen to them.

I could spring for okoume plywood or something else for the bottom of the SIP, if people think my plan is likely to fail for that reason alone. Or maybe you think no plywood will hold up, and I should be the wood framing for the entire load itself without relying on the torsion box. Or maybe I should use angle steel inset into foam and no wood at all! (I can weld). Sorry, I'm just stuck in analysis paralysis. Please help!

(as far as length, I'm extending the coupler forward at least 2 feet with 2x3x0.12" steel)

 

[KCStudly]

Um, not sure I understand. Perhaps a sketch or two would help. Coroplast trailer? Flat gasket?

What trailer will you start with? If a flat deck harbor freight or tractor supply kit, then your deck and "side door" options are more flexible. But if you are starting with a UT with side rails then you are probably looking at a rear entry scheme (unless you want to have a storage "basement" and have the sleeping platform raised and side doors able to swing open over the side rails... but this would conflict with desire to maintain low aero drag). Don't over complicate the removable cabin concept; incorporate adequate blocking for D-rings and cargo strap the box to the trailer. 360 degree seal the box then add "consumable" hard runners that reduce the friction between box and deck, and allow air to flow between to drive out moisture. Better yet don't make it removable; it won't ever be as practical to do as you may be envisioning. My 2 pence.

 

[iamroot]

Coroplast trailer?

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Corrugated plastic attached on the bottom of a regular flat HF 4x8 utility trailer, no side rails, only thing above the frame is wheels and fenders.

Also

https://youtu.be/F7n6ZX2gmKM

is a video about installing it.

Flat gasket?

add "consumable" hard runners

Instead of hard runners, I thought to use something slightly rubber to protect the cabin.

cargo strap the box to the trailer.

I'm thinking you're making this suggestion with the rails idea, but... I could build ratchet straps into the body, but it would still be a point load.

It's true though that I haven't figured out how to get it off once built. But I want to leave the possibility of version 2 and reusing version 1 for the kids in the backyard.

 

[KCStudly]

Rubber won't slide easily. Maybe a hard plastic like Delrin. Oak would be cheaper and more available.

I picture a small raised platform, or "parking deck", about the same height as your trailer deck, back up to it, unstrap, and then drag or winch/come-along the cabin onto the parking deck. Parking deck could be as simple as a bunch of used pallets, scrounged, and some ply or boards

Sure, there are other elaborate schemes. One builder inserted reinforcements for attaching removable jacking legs that reached out past the trailer like buttresses on Notre Dame. Can't remember how he attached the cabin to the trailer, but any method that requires a person to crawl on the ground and reach under to undo fasteners, or lift the mattress to get to them from the top, would require a high degree of accuracy to get the holes to line up and more physical effort to execute... several more excuses to seldom, if ever, take the cabin off. Also holes bring up the water intrusion thing.

I know you said no electrical, but these things have a way of developing "project creep". If you strap it down there is no question that it is cargo. If you bolt it down then the question of clearance lights could come into play.

 

[iamroot]

One builder inserted reinforcements for attaching removable jacking legs that reached out past the trailer like buttresses on Notre Dame.

I remember this, I think it can be strong enough because the mount points don't have to tolerate people inside or driving 60mph, just lift and set down.

Rubber won't slide easily. Maybe a hard plastic like Delrin. Oak would be cheaper and more available.

Hmm, I wasn't thinking sliding, I'm more worried about rubbing between the frame and body. I don't want to rub off the paint. Luckily if I use TPU, it's coefficient of friction is fairly low.

would require a high degree of accuracy to get the holes to line u

Hmm, the machinist in me says locating pins would be important. I'm thinking two holes in the trailer -- add on plate into opposite corners with a hole (so the trailer is still flat when body removed) --and two big tapered wood pins extending downward to align it as it's lowered. Or if it's slid on, vertical pins located on the sides near the bottom.

Actually, sliding isn't an option due to wheels. Lowering is the only option. Well.... I suppose the wheels could be removed?

I know you said no electrical

I didn't, I'll have lights.

 

[Tony Latham]

I saw Tony said he's successfully done thick plywood on the bottom with epoxy and fiberglass cloth, and that is regular boat construction and I would expect it to hold up physically, but if there's ever a leak from above during winter storage, it'll be entirely destroyed.

I've never advocated for fiberglassing the bottom of a teardrop. Epoxy soaks into the wood and platices it. The glass isn't needed.

I fiberglass the rest of the cabin, and using my methods, water doesn't get inside the cabin, galley, or touch the wood. It'll never rot or, for that matter, be damaged by moisture.

Be cautious about adding a layer of plastic to the bottom (such as the "extreme duty" product you mentioned), since water can wick between the wood and the material.

I construct the cabin off the chassis, and loading it onto the frame is a chunk of work I wouldn't want to do twice a year. It's a lot easier to have two trailers.

Tony

 

[iamroot]

I've never advocated for fiberglassing the bottom of a teardrop. Epoxy soaks into the wood and platices it. The glass isn't needed.

I apologize for the mistake. Thanks for the correction!