Where it all began. Old rusty frame from a 1972 pop-up camper trailer. Ready for weldin’ and grindin’.
After mods it looked like this, almost ready to paint. I lengthened the frame by welding tubes to the back, and welded chunks of steel angle to the sides for a slight increase in width. I moved the axle rearward as well, to maintain the correct location ratios. I have not drive tested the re-done trailer yet, but the distance between axle and hitch appears to be good for a stable non-whipping tow. Trailer math here for anyone interested.
CAD model showing the floor plan. The blue is the original trailer frame, and the red parts are the portions I welded on. It ended up a bit different in real life, but remains similar.
The floor is the only part of this trailer that is susceptible to water damage (it’s the only major wood component). To mitigate the risk I used 3/4 inch marine grade plywood that’s painted with several coats of thinned polyurethane. I also painted the bottom with exterior house paint, and glued the whole thing to the trailer frame with waterproof construction adhesive (it’s also attached with bolts). After the trailer exterior is finished I’ll spray undercoating underneath as another layer of protection.
As I was finishing up the trailer frame I began work on the operable drop floor. It’s built out of xps foam and wood, with plywood added to the top and bottom to create a sort of sandwich construction. I covered the whole thing in canvas and glue to increase the durability and waterproofness.
I built the door first, in order to get some practice with the foam and PMF construction. The drop floor construction makes the trailer wall an odd height (~5ft), so a custom door was necessary (I otherwise would have bought one from the RV salvage yard where I got the windows and a few other parts). It’s built out of foam with a minimal wood frame. This makes it exceptionally light and well insulated.
Adding the wood-glue and canvas combo to the exterior of the door.
Door with one side canvased and painted with primer.
Foam scattered everywhere. Hopefully this isn’t what the highway looks like after the first test drive!
The walls are constructed of several pieces of foam with wood ribs laminated in between. This adds a smidge of strength, while still keeping wood to a minimum, to increase the water resistance and reduced weight. The angles of the hexagonal footprint add strength as well. I will be using this on rough dirt roads, during ski touring access, so the stronger the better.
After the wall is glued, we fill divots left by various stages of the construction, and sand smooth in preparation for layering with the canvas and glue PMF.
Attaching walls to the trailer floor. The walls are attached to the floor with Gorilla Glue, which is fairly strong and totally waterproof. Once the exterior canvas is applied, it will wrap around the bottom of the trailer floor, adding more strength to the wall-floor bond (no wish to glance in the rear view and see the camper box flying away like a bounce house in a tornado). The sheets of plywood visible in the photo are temporary braces. Due to the flexibility of the foam as well as the hexagonal floor plan, squaring the walls to the floor proved to be challenging. I used everything from trigonometry to “eyeballing” during the process.
The bed is built with 1×2 wood, to keep it light. The frame is glued and screwed to the foam walls, which adds a significant amount of strength and stiffness to the entire structure.
I use a few different methods for cutting the foam. One of the best tricks is to use a smooth edge diamond cutting blade in a circular saw. Since there aren’t any teeth it doesn’t create much dust, but cuts easily; the friction of the blade melts through the foam.
The wheel wells are made from galvanized steel flashing. The visible pulley and cord is part of the drop floor system that raises for ground clearance while accessing locations such as ski touring trailheads.
Louie Dawson earned his Bachelor Degree in Industrial Design from Western Washington University in 2014. When he’s not skiing Mount Baker or somewhere equally as snowy, he’s thinking about new products to make ski mountaineering more fun and safe.
17 comments
Great looking project Louie. When it comes to trailer towing tongue weight is important but proper toe-in is the most critical part of having a nice towing trailer.
Just curious what are you going to use for heat?
Cool project! I’m excited to see this drop floor in action. That’s an inventive solution.
If you aren’t already, may want to protect your pulley system that raises and lowers the floor. As is, it looks like it’ll get blasted by road dirt/salt/water/etc. Either way, curious to see how that thing works out. It’s an interesting alternative to a pop-up roof. The major downside seems to be that the mechanism needs to be stronger and stiffer since you stand on it, unlike a pop up roof. Hopefully there is more to it than I can see in the pictures. If not, it’ll be like standing on a swing.
I echo some of Richard’s concerns from part 1. I’ll add that the skin to foam bond is important to the strength of the structure, and that bond can fail gradually over time without it being visibly obvious. About the best you can do is tap on the surface with something hard like a quarter, kind of like how you’d knock on a flake while climbing to see how well it is attached. My point is that relying on that bond is sketchy in the first place, but worse, it could get sketchier over time and fail without warning. A skeleton or outer shell would reduce this risk considerably.
I hope you’ll also consider some test runs down a deserted road at higher than the planned operating speeds to proof load your structure. Obviously no one wants this to happen: https://www.youtube.com/watch?v=FBEuXeny7pA
Anyway, sorry to be just another armchair QB. Props for putting in the hard work and good luck!
Hey, bring on the QBs! I’m sure it’s helpful for the builder. Lou
Yea I’m all for any advice, keep it coming. The drop floor is going to be a bit different in its final form. Mainly I’m going to replace some of the rope with steel cable. The floor has rubber feet on it. When I’m parked on fairly level ground, the floor will just sit on the ground when it’s in use, so it won’t swing around. Even when I do hang it from the ropes, it doesn’t swing around too much, when it’s cranked up next to the floor (in storage mode) it doesn’t move much at all.
Hi Louie
Mr Negativity here–
Myths abound.
“Marine Plywood” The plywood that was sold to you is Ocoume — one of the most rot-prone species of Mahogany. It is pressed together with water resistant glue and is generally free of voids on the interior— not a bad product all in all. But it certainly isn’t suitable for unprotected use in the PNW. At least when I lived there it does rain some times? So as you drive down the first gravel road a weak coating like house paint will be sand blasted off in the first 100 miles. Then while driving down the freeway it will receive a firehouse blast of water that will thoroughly saturate the wood. The rot will probably start where wood meets steel frame.
Bottom line is that the bottom of the plywood floor needs a nice thick layer of fiberglass– Minimum of 10oz woven cloth and epoxy resin to protect against rocks, sand, and moisture.
All fiberglass is not created equal. “Fiberglass” trailer panels are blown out of a a gun using chopped strand and polyester resin. If you want resin to penetrate a wood surface and form a waterproof coating you use epoxy resin. And if you want strength you use woven or unidirectional fiberglass stabilized by resin.
If it were mine I’d unbolt the foam cabin with its floor, lay it on its side, and properly waterproof the exterior of the bottom before doing anything else.
Looks like it’s too late because the floor is glued to the frame. A heavy coating of bed liner might do the trick!
The canvas looks great, but was there a great deal of cost/labor savings over using an EPS compatible epoxy and 3.5oz fiberglass?
Having seen how easily and quickly that type of foam burns, I would be worried about fireproofing. Its also highly toxic when it burns…
The foam by itself burns pretty easily. Once it has the canvas/glue/paint coating it gets more fireproof. I’m also going to put a bunch of metal flashing on the interior in the kitchen area.
I’ve flame tested this XPS foam as well. It’s perhaps dangerous when unprotected. It behaves much differently than wood, the foam is more of a fire “contributor,” while XPS does burn more readily than some other foams (see videos below), the wood is what seems to keep things going. Once enclosed the XPS behaves quite a bit differently, much less scary. Bear in mind that nearly any commercial “camper” is also made with some sort of at least somewhat flammable foam sandwiched in flammable materials. For example, our slide-in truck camper is all wood and carpet surrounding the kitchen area, nothing fireproof. And the walls are indeed foam sandwich. I’m a big fan of fire proofing the kitchen area, using aluminum or steel flashing metal. I did that in our mountain cabin, and it sounds like Louie will do so in his pull-behind. Escape windows and fire extinguishers are key as well. As is a tool known as the brain.
https://www.youtube.com/watch?v=lpzXogsNACo
https://www.youtube.com/watch?v=VPoY6nsh1Vs
https://www.youtube.com/watch?v=8NC79e0oztM
Lou
Were there any thoughts to using Plyiso Board?
R factors seem to be about the best and comes with a metal skin.
Bill, I’m not familiar with Plyiso, I’ll check it out. One thing everyone should remember about campers and tiny houses, with such small square footage all you really need in the way of insulation is enough to prevent condensation on the walls and ceiling during cold weather. I frequently see tiny house builds, for example, that obsess on insulation that’s clearly overkill. Lou
Bill, oh, “Plyiso Board” is what we’ve always called “Thermax” around here. Some is said to have fire retardant, and the foil skin is a good thing. It’s expensive, and not necessary when buried in a wood frame wall since once a fire starts with the wood, it’ll burn anyway. I’ve tested it. Lou
Do you mean “polyiso” as in polyisocyanurate foam?
Hey Louie – you’re likely already past this point, but another blog I read is the timeline of a boat build out your way. He’s just putting cabin roof on – similar technique to what you’re using but slightly different materials,… thought you might find it an interesting comparison. Scroll way, way, way to the bottom for roof construction – https://bischoffboatworks.com/bish-the-build/
Wow Kevin, thanks for the link! Lou
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