CJ Trowbridge

Building a Moving Cabin

ds for framing. Then I covered everything in flex seal which is basically bed liner. This is definitely overkill but I want to be sure I don’t have anything to worry about.
The rear wall swings up to serve as the door, and is supported by a pair of gas pistons. I used Autocad to figure out where to mount them based on the open and closed lengths to get the angle I wanted when the door is open.
I used pressure treated fir 2×6 deck boards with three inch deck screws for the framing, and then I also added steel brackets to every joint, and the same 3/4″ pressure treated plywood on the outside.

The framing is a little wonky because of the way I’m doing the mattress. In this photo you can see there is a frame around the front, and then on the front face where the front wall will go. I’m planning to add an air conditioner to the front wall, so there will be another cross member on the front.
There is also a frame around the midpoint so that the roof panels can be mounted at the seam. Then there is a third frame to support the end of the mattress platform. You can see the three cross-beams which are also mounted with both three-inch deck screws and steel brackets.
I am planning to add sheets of styrofoam to the voids between the studs and then cover the inside walls with nicer-looking plywood.
Note that there is another sheet of plywood in the bottom just for storage. It will become the back wall/door. Also that random board laying across the beams is just hanging out, not permanently placed there.

Making the front wall was a little complicated because I had a lot of extra wood and I didn’t want to buy another $60 sheet of this plywood so I did the same thing as the base and cut and glued several 3/4 sheets together to form a 1.5″ thick front wall. This is on the ground drying in the photo.
While the glue dries, I decided to apply the first coat of white flex-seal to the side and roof panels. I used a roller to coat it all over, and then used a spray can to make sure any little crevices are well sealed. I will probably end up doing a total of two coats to the entire outside.

Here you can see all three walls up, with the roof panels laying on top. The first coat of flex seal is now on.
I need to pick up some more spray-on flex seal to fill in the seams before the second coat goes on.
Lastly because I’m using a five-foot wide trailer, there is an extra foot of space on the roof next to the four-foot solar panels. This means there is room for a vent with a built-in fan! I used a jigsaw and cut out a hole, then it was a simple process to screw the vent down and seal the edges with flex seal.

Air Conditioning

I got the only Air Conditioner that Home Depot had at the time, which ironically was the same one Will Prowse tested to run off solar. It’s rated for 6,000 BTUs which is extreme overkill and should be quite efficient for the tiny space. I love that it has a loud but low-wattage fan. A loud fan is a must have for me if I’m going to get any sleep.
I used a jigsaw to cut out a box in the front wall to fit the air conditioner. I also made sure to leave enough room underneath the AC for the toolbox. Next I will add wood at angles to make the overall shape of the front of the trailer more aerodynamic.
I screwed the air conditioner’s frame into the wall on all sides, and then I used Loctite sealant/ adhesive to fill in the gaps around the AC unit.

Back Door With Digital Lock

I wanted to have the entire back wall open as a door for several, reasons. First it’s a nice big open space for loading cargo and materials into the trailer for transit. Second it’s much cheaper than installing the typical doors people use. I cut a hole in the 3/4″ plywood to mount the digital lock directly onto the 2×6 frame on the inside of the door.

This slides perfectly into the frame of the trailer, with adhesive weather stripping forming a seal all around the opening. I also installed a pair of gas pistons to lift the door and hold it open. This was tricky and took a lot of trial and error to get them positioned perfectly. Overall I’m very happy with the way the door turned out.
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After I painted the back door, I mounted the license plate and a white board. This is a burner tradition so that people can leave messages and notes. I can also use it to post messages during transit.
One thing to note is that the license plate must have a light. So I mounted this light and wired it into my main power supply so that it’s just always on. This is helpful for navigating the area in the dark and finding the buttons on the digital lock.

Initial Performance Data

Mileage

I drove the trailer to a remote hot springs trip. It was just about 500 miles round trip, and I climbed from 50 feet of elevation to 8700 feet and then came back down. This feels like a very comprehensive test of the kind of mileage I can expect.
As you can see the Moving Cabin is not very aerodynamic, and yet I got about 22 mpg which is SO MUCH HIGHER than what RVs get. I was surprised it performed so well, but after reflecting on these numbers, I think a big part of it is the fact that I’m driving 55 mph with the trailer rather than my usual 75-80 mph.
A big part of the reason I decided to build this rather than buy an RV is mileage, but also the fact that my car is one of the most reliable cars ever sold so it’s not going to be breaking down in the back country like an RV might.

Thermodynamics

During the summer months, I left the AC on for a whole weekend (with the eco mode turned off) in order to test the power consumption in a typical summer camping situation (92F). To be fair, I would never leave the AC on if I was not inside sleeping. It averaged 175 watts per hour across the whole weekend. This is pretty high, but with all the solar it should be fine.
Based on similar work published by Will Prowse, it seems logical to expect about a 50% decrease in power consumption once the trailer is well insulated. Since I will only be using it about a third of the time, it also seems reasonable to expect a further 2/3 decrease in daily consumption. Therefore I estimate that once insulated, I will average about 1200 watts per day for the AC during warm months. The heater is rated at the same wattage.

Jacks

I had been debating whether or not to add jacks to the trailer, and as I walked around on it installing the bed, I noticed a line which I could not cross without it tipping over backwards. So I decided to add jacks.
The electric jacks I found need a 2.5 inch hole which seems fine going through a 2×6 but I decided to stack two 2x6s just to be safe. Since the trailer is five feet wide, I cut the 2x6s to six feet, meaning there will be a 6″x6″ square sticking out each side. I cut holes here to mount the jacks. I bought a set from harbor freight but these are the closest equivalent I could find online.

I cut the beams to size and wood glued them together, then deck screwed them together in eight places. I used three 120mm bolts to mount the beam to the trailer’s structural cross-member just behind the tires. This should be perfect for making sure it won’t shift around or tip over once it’s set up.
I tested the design by pushing the structure around once it was up in the air and it all seems very sturdy. Before I installed the jacks, I jumped on the ends to see if there was any give, and it was all rock solid. These jacks are each rated for more than double the weight of the trailer so I think it will work perfectly.

I used some locktite to attach a high precision bubble level to each jack’s base as well as to the trailer’s tongue. This makes it easy to level everything out. I ran power from each jack to my main 12v distribution panel. (see below) I also attached an inline breaker so the jacks don’t receive power if they don’t need it.
The trailer’s frame wiring includes a special ground line I ran to all the lights and frame segments because the manufacturer just expected the ground connection to magically travel through painted joints which obviously doesn’t work. So when I wired the jacks, I also connected the ground from the 12v distribution panel to the ground line I had run for the lights and frame. This is one of only two places where wires run through the exterior of the trailer. I drilled two tight holes for the wires, then stabled them on both sides of the wall and filled the holes with locktite. I will also add a coat of clear rubber over that once it dries. This should form a very good seal while also preventing static problems in the low voltage system that could otherwise arise. I will probably add some kind of grounding strap in the future. There are going to be a lot of mixed voltage systems with thousands of watts of batteries inside, so we want to avoid ground isolation as much as possible in order to prolong the life of the electronics.

Woops!

So I made several errors in my design based on a mistaken assumption. On this trailer, the wheels are not halfway down the bed. They are more like 2/3 of the way to the back of the bed. This means the torque moment on the tongue is basically maxed out at a proper 60/40 weight distribution. I designed the layout to be 60/40 with a centered axle so too much of the weight was forward of the axle. Also, most of the heavy items which will eventually be in the rear are not yet installed. (Ie, the fridge and other appliances.)
Adding jacks behind the wheels and lifting the trailer off the tires to level it means that the fulcrum moves even further back. I remember seeing the level on the tongue showing a different lateral angle from those on the jacks and feeling like I must have installed it crooked. In fact, the tongue beams were torquing.
Combine that with the fact that the tongue jack comes pre-assembled with some play, and therefore exerts a lateral torque on the already stressed tongue beam and you can guess what happened when I stepped on the tongue to install something on the roof. The tongue beam twisted where the jack was mounted, and then bent 90 degrees out at its center to release the tension.
I removed the toolbox from the front and the cargo from the inside before lowering the jacks. It was still pretty heavy to lift the trailer, but I was able to do that and leverage the tongue jack to bend the beam back into place. I used a car jack to lift the tongue at the front. Without the weight of me standing on it plus the cargo, plus the longer fulcrum, the trailer was suddenly content to stand on its own. But, I knew the beam that had bent was fucked.

You can see the steel that buckled where I bent the beam back into place. Luckily, the beam was the only part of the trailer that was actually damaged.
I called the manufacturer and they sent me a replacement beam as well as a new cross member for the tongue. It wasn’t very hard to replace the beam, and I added the new cross member to the bottom of the two beams so that there were now two cross members between the beams.

One of the lag bolts for the tongue jack bent when it hit the ground, so you can see I also replaced that with another bolt. I have been very careful to frequently check the tongue weight to make sure it is easy to lift, and that the jack is not tilted at all.
I will probably not be using the rear jacks to lift the trailer off the ground unless I have a flat tire to change. I think they will basically only serve as theft prevention tools as this point.
The second cross member really ought to be included in the normal design of the trailer. There is basically no way this beam could have failed with a second cross member in place. The bottom side bent out, and that couldn’t happen if it was designed like this with two of them, rather than with just one on the top. The part number for the cross member describes it as “spare tire mount.” This seems like such a stupid design. I will have to be careful not to break it again.
But all is well now!

Insulation

Back at home, I decided to run some tests to see how effective the insulation was at reducing the energy requirements. First, I put temperature sensors both inside and out to log the uninsulated temperature curves and compare the percent kelvin difference.

First let’s look at the data about the thermal performance with it just sitting there with no heater running. As you can see above, the lines are basically the same shape. In fact, with inside/outside correlation of 96% and a beta of 1.0. There is obviously a difference between the two spaces, but they track precisely together. The goal of insulation is to make the inside line as flat as possible. A beta of 1 is basically the worst possible beta for insulation. This means it’s literally not working at all. So we have a lot of room for improvement. Let’s see how low we can get that beta!
Now let’s see how much energy it takes to keep the space inside warm without insulation. My heater and AC are both 500 watts so it should be a roughly equivalent test in terms of the calorimetry. I ran the heater inside and measured how much power it needed to maintain temperature. At 55 degrees outside and 70 degrees inside, it took 200 watts to maintain the temperature for an hour. This is roughly equivalent to the AC’s power consumption as expected.

Let’s Insulate!

Since I framed the trailer with 2x6s, there is a 1.5″ gap between the exterior walls and the future interior walls. (2x6s are actually 1.5″ x 5.5″) This is the perfect depth for sheets of R-6 polyisocyanurate. It simply needs to be cut to size and put into place. Next, nice sheets of plywood will cover the interior walls.

As you can see, despite even colder temperatures, the interior temperature stays much flatter with insulation. We now see an inside/outside correlation of 95% with a beta of just 0.77. This is a huge improvement over the uninsulated values, from just an inch of polyiso insulation.
I ran the same experiment again once the insulation was installed. I ran the heater for an hour when the external temperature was 55 degrees. Heating the inside space up to 70 degrees took just 120 watts, a reduction of 40%. Hopefully when it’s warm out, I will see similar efficiency improvements with air conditioning.
As a more realistic experiment, I ran the heater all night while sleeping and set it at 60 degrees. It took just 380 watts for seven hours or about 45 watts per hour. This was on a night with a low of 36. At that rate, my 1700 watt hours would let the heater run for five days without any solar or other recharging at all. Very exciting results!
I recently published a great deal of research on how to effectively moderate the indoor temperature when camping. One of my findings was that thermal mass batteries are among the most effective techniques for warming the cool hours or cooling the warm hours. In this case, it simply means adding several gallons of water in the storage area under the bed to get these benefits.
During my recent hot springs trip, I did not use the heater despite it being just 11 degrees outside because of how effective the insulation was at keeping it warm inside the camper. I was worried that my gallons of water would freeze inside the camper, but they never came close.

Power Systems

This section is very complex so it gets its own post. Check it out here!

Heat

I had already purchased the cheapest ceramic heater I could find, but I doubt I’ll need that in such a small and well-insulated space. It uses about the same 500 watts as the AC, though neither seems likely to be on more than a 20-30% duty cycle with how small and efficient the space is.
I have also had many people recommend the Mr Heater which could come in handy if I decide to go ahead with my tentative plans to eventually build a portable sauna. I will probably buy one of these as a backup heat source just in case.
I should probably add that I have a lot of camping gear already, including cold weather gear. If you’re going to be camping in cold climates, make sure you are prepared to survive.
My experience so far has been that it stays very warm inside even if it’s below freezing outside, even without heating. So it’s not clear that this will ever be used but I have it in case I need it.

Other Accessories

Water

I have done a lot of experimenting with various methods for making my own water. I plan to upgrade my current pump to something with a little more volume.
I’ll probably get a nice multi-stage filtration system as well. If I do that, I’ll likely also add an outdoor shower and some kind of large potable water holding tank. (Possibly something wide and flat underneath the bed of the trailer)

Toilet

I purchased one of the top rated portable toilets, and a small popup tent to serve as an outhouse. I intend to store these in the trunk of the car rather than the trailer, so as to keep any scent far away from the living spaces.
I will miss having the bidet from home… :[

Moving Theater

Because the side of the moving cabin is a large blank white wall, it makes the perfect projection screen. I predict this will be a fun way to connect with others and share in things like the baby yoda show.

Radio

My original design included a radio mast which attaches to the camper, providing LTE, Wifi, VHF/UHF, SDR, etc. I eventually decided to make this a separate structure which sets up next to the camper.
I am still ironing this project out and I will do a follow-up post once it’s ready.

360 Surveillance

I installed a birds eye view surveillance system. This means I can see everything all around the camper on one simple screen.
I’m using this box to split the video signal from the 360 box so that it can go to a small screen as well as a video capture card. I’m using motion on a Raspberry Pi to automatically record video clips whenever motion is detected in any direction.
I’m planning to eventually add object recognition and tagging for the videos, and using a neural net to check whether there are any people who are unknown.

cjtrowbridge

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