Hybrid Foam Teardrop Build With Plywood Skeleton

[Chefsonora]

Hey everyone, Im Brett. I've been planning this concept in my head for years. Wanted to build some sort of teardrop camper and because im a glutton for punishment, I've incorporated some difficult design elements (that I partly blame on the Foamie influence for ease of structure).
I'm starting with the standard 4'x 8' trailer and sticking to the single sheet plywood as my size limit(high and wide). My standout feature is a sort of bullet nose cone capped with a large dome skylight. That comes back to meet the body with coved front and top edges (15" radius). Flat or slightly sloping roof with rear hatch and galley. I kept it straight to incorporate a bunk for my 2 daughters (7 &5). Incorporating a lot of modular/ adjustable elements. It will have a plywood skeleton (mostly 1/2", some areas with 1/4" glue laminated on) with stringers notched together with the coved shaping ribs (of 1/4" plywood) all walls and ceiling will be 2" purple foam. Coved areas will be blocked in with chunks of foam and gaps will be filled with expanding foam to bond it all together, then shaped to the curves. All PMF skin inside and out.
Bunk will be supported by ribs tied together with stringers. It will be a kind of canvas sling between tubular aluminum struts running on tracks of t-slot aluminum track embedded in plywood and backed by an aluminum plate with riv-nuts. Galley will be modular/ removable, tying in to the same t-slot track on the sides as well as on the floor. Floor will be a foam sandwich of 1/2" plywood, with the PMF wrapping under the edge. All plywood will be treated (edge and face) with penetrating epoxy with fiberglass at stress points. All mounting points through the floor will have polyurethane bushings embedded in and isolated/ sealed with epoxy. Frame will have strips of epdm rubber to reduce friction/ wear.
Roof vent will be framed in between roof ribs and have a built up flange to avoid leak potential. Building a tongue box to tie in with the curves that will have battery & propane). Balancing that with weight to the rear, 3 section galley with fridge/cooler, water jug and a pull out counter/stove area. Adding external t-slot track at the corners for a attachable kick leg table, galley bag hanger, potentially shower/curtain attachment. Planning on t-slot roof racks to tie in a vestibule tent and solar. Planning on primarily 12volt system for lights, fan and charging with power hub behind the tongue box.
I probably missed a couple of things, but that's what im working towards. If anyone has experience with the hybrid ply skeleton/ foam build any tips on bonding it all together would be greatly appreciated! Wish me luck!

 

[Chefsonora]

Me and ChatGPT worked up some rendering ideas.

 

[Tony Latham]

Interesting concept. There are two things that caught my attention. First, the suicide doors. Sooner or later, you'll drive off with the door unlatched. Not a big problem if the hinge faces forward.

But more importantly, an adult and two kids in a 4' wide camper.
That may work for now, but someone once told me that kids grow up fast. Pull out a tape measure and look at what 48".

A 5' wide trailer will give you an extra mile.

Quick question: No sheathing, inside or out?

Either way, you and your little guys are going to have fun.

Tony

 

[Chefsonora]

Interesting concept. There are two things that caught my attention. First, the suicide doors. Sooner or later, you'll drive off with the door unlatched. Not a big problem if the hinge faces forward.

But more importantly, an adult and two kids in a 4' wide camper.
That may work for now, but someone once told me that kids grow up fast. Pull out a tape measure and look at what 48".

A 5' wide trailer will give you an extra mile.

Quick question: No sheathing, inside or out?

Either way, you and your little guys are going to have fun.

Tony

No sheathing, just PMF inside and out with canvas. Size wise I'm doing this as a proof of concept for a first build, planning on a larger one down the road. I figure its a good way to try out my modular system ideas without overextending my capabilities and budget. The rounded corners would get tricky and it's my design step before I commit to fiberglass. I'm glad there are so many epoxy options these days, I can still smell the polyester resin my parents used on our kit boat when I was a kid!

 

[Chefsonora]

And the suicide doors is one of my many little arguments with ChatGPT. It's only one door, not suicide and a small hatch. Thanks for the input!

 

[Tom&Shelly]

I'm glad there are so many epoxy options these days, I can still smell the polyester resin my parents used on our kit boat when I was a kid!

Know just what you mean! My Dad made radio-controlled airplanes with polyester resin.

Tom

 

[Chefsonora]

First cuts made for the framework! The hoops are for the radius/ coved corners that will tie in to longitudinal stringers. Will be reinforcing in areas for the bunk bed (will be a sliding system of bars across with a canvas sling between) it will slide on a t-slot track and lock into place. Galley will also tie in to the t-slot, with some in the floor as well(making the galley modular/ removable as well.)
Will also be beefed up in the upper area where a roof rack will go

 

[Chefsonora]

Stitching together the Harbour Freight folding trailer. Took the caster mount piece of angle iron and drilled and bolted across the break in the frame where it folded. Had to cut the end foot off of the angle as well as trim a little over an 1/8" off the cross member to accommodate the angle iron piece. Will be welding all joints once completely assembled.
By my calculations to hit the 60/40 weight/ axle placement I moved the mount back 3-1/2".
Will be adding a center spine of 2" x 2" x 1/4" square tube down the center, adding about 8-12" in the front to attach the hitch to and terminating at the stern with a section of 2-1/2"x 2-1/2" x 1/4" as a hitch for a bike rack(or stern support for long things on the roof rack)

 

[Chefsonora]

Got the frame to the trailer mostly assembled (waiting for the 2" x 2" square tube to come in for the center beam/ extended hitch). Going to weld all the joints and have a receiver in the rear for a bike rack/ cargo basket.

Laminating some of my horizontal rib strips and got my big router bit set to start testing my coved edge joinery. Once I get the trailer together I can go get my sheet goods picked up!

 

[ghcoe]

Two things.
1. Don't weld the on the Harbor Freight trailer. The welding will make the metal brittle around the welded edge. They are welded at the factory, but those welds are known to break. That is why you need the 2"x2" spine to relieve the extra strain on the tongue that happens when moving the axle back.

2. For better strength in the trailer frame take the two center cross members and bolt them inside the side frame rails to attach the front and rear frame structures together. This will strengthen the frame much better. If you need a cross member there a 2"x4" piece of wood notched on the ends to fit will work great. You would not think that the frame will flex much as that joint, but it does. Especially if you plan on using weight on the rear with the hitch idea that you are planning. I though the 2"x2" would strengthen the frame plenty but I was wrong. I never used the rear receiver for anything either.

 

[Chefsonora]

Thank you! You just saved me a bunch of grinding and smokey welding(from the paint). I did brace across the folding joint using the angle iron provided for the caster mount (notched inside the crossmember to fit inside the side rails) and bolted the 2 crossmembers together.

Did you weld the spine in? I was considering more of a sleeve with a flange to bolt through the crossmembers. Your front tongue section looks very similar to what I had in mind (I was thinking of just extending the 2" x 2" forward enough to attach the hitch to, not a full removable reciever on the front)

 

[ghcoe]

Thank you! You just saved me a bunch of grinding and smokey welding(from the paint). I did brace across the folding joint using the angle iron provided for the caster mount (notched inside the crossmember to fit inside the side rails) and bolted the 2 crossmembers together.

That probably still won't be enough. I just used the spring perches and bolted the two halves together at the cross members. I started to get a weird buckle/delamination above that area on my foamie on the door side. On closer examination I found that the frame was flexing on that joint and over time it transmitted into the structure of the wall. If you add a cargo/bike carrier back there it will magnify the effect even more. The problem is that the spring perch, similar to what you did but under the frame, helps to keep the frame from flexing upwards but does nothing to keep it from flexing downwards. That is why my wall started to have issues. Those cross bars fit nicely into the outer channel of the trailer and tie the two pieces together very nicely. They transfer the flexing force across the whole side of the frame not just that small portion at the joint.

Did you weld the spine in? I was considering more of a sleeve with a flange to bolt through the crossmembers. Your front tongue section looks very similar to what I had in mind (I was thinking of just extending the 2" x 2" forward enough to attach the hitch to, not a full removable reciever on the front)

I bolted the spine by drilling through the square tube and the crossmembers. I used a 3/8" bolts with washers and a nylock nuts. I added the receiver to the front so I could change out the hitch to a pintle style hitch if needed for off road use. It is also useful in case I need to lengthen the tongue for some reason. Also, I take out the hitch all together when I leave the trailer alone. Keeps people from easily stealing it.

 

[Chefsonora]

I do like the anti-theft aspect of the front receiver. I guess I'll roll the dice with my current brace setup. I have spotted a set of independent torsion axles on FB marketplace for $250 (2000#), but I'd have to figure out some sort of mounting plate for them. At that price I'm tempted to get them for another build....

 

[Chefsonora]

Starting to get exciting around here! I got tired of having to wait for my truck to work before I could go get my supplies, so I bit the bullet and had them delivered (got the 2" foam & plywood). Got the spine 95% complete and ready to bolt down (made the mistake of welding on my receiver ends before I did the brackets, so I had to weld them in situ). Any tips before I drill and bolt it all down?

Also wondering about a good method to bolt on the rear stabilizing jacks? I was thinking some channel steel diagonal on the rear corners.

 

[Chefsonora]

Got the spine installed (mostly, need some bigger bolts)
still debating on axle placement because I may incorporate an integral tongue box ahead of the front bulkhead (utilizing the gained tongue length/ spine support. )
Heading over to the other property to gather tools and supplies (going to turn an old radial arm saw into a pin router) for more precise control doing kerf cuts and recessing in the foam. Gotta poke around and see if I've got a pair of some decent rims to put some better tires on.
Got my epoxy ordered so I can start sealing up all the wood.

Forgive my rambling/ free association trying to remember all I need to get at the other property, lol.
Oh, total length with the spine/ tongue extension is at 14'.
Feel like im making some progress!

 

[Chefsonora]

Trailer is fully assembled (just need to install the safety chain). The HF stabilizers are definitely overkill, but they'll get the job done! Got the last of my ribs cut and laminated. Definitely much less of a hassle gluing up after dark rather than in the beating California sun! Still waiting on my epoxy, but I'm ready to start on the base/floor. Finally going to start having a shape! Found a source/ plastic fab shop relatively nearby that will blow a Polycarbonate bubble for my dome skylight for pretty reasonable $ 250 for a 24" diameter x 12" tall dome.

 

[Chefsonora]

Scarf joints anyone? Decided to extend the nose by 1', so to stretch the floor I decided to try my hand at scarf joints. Never tried it before built a scarf jig with some 3/4" plywood, cut a 12° bevel then glued and nailed it together added a rail/guide for the skilsaw to run on, waxed it up and let 'er rip (pun intended). Seemed to work pretty good! Clamped and curing now. Excited for the next phase of actually starting the cabin and seeing some actual shape! Trailers maiden voyage today to go to the lumber yard for foam and more plywood. Tracked great and didn't bounce around even with a light load!