Reinforcing Camper Frame - Need Structural Advice

[reverse01134]

I pulled the trigger on what I thought a great idea: converting a commercial aluminum cap into a camper... and now I'm starting to understand why this isn't a popular path in DIY camper communities. Before I go any further, I should admit that I have no engineering background and no experience building a DIY camper. That said, I've learned a lot through research and in hindsight, I'm not sure if I would've still purchased the ARE DCU cap if I knew what I know now.

It seemed like a brilliant idea that I could purchase a plain aluminum shell with a full height vertical rear door under $3k and customize however I wanted. I was planning to add RV windows and mount gear like awning, propane tank, Rotopax, etc on the exterior walls. Unfortunately, the shell is made of thin aluminum tubing, the 0.035" outer aluminum skin add no structural strength and the frame welds are pretty poor. Forget about all the stuff I wanted to mount, I'm not even confident the camper can handle long term off road use as-is.

At this point, the camper has been delivered and I have to live with it. As long as I can improve the structural integrity of the shell, I think I can still turn it into a solid camper for my needs. I don't plan to put much load on the camper itself beyond some lightweight gear. All interior features - bed platform, drawers and countertop - will be mounted to the truck bed. Other than possibly adding solar panels later, nothing will go on the roof. The camper is mostly meant to function as a hard-sided "shell".

However, I do need enough rigidity for the shell to handle rough terrain. And that's where I'm looking for some structural engineering guidance. I'm not sure which part of the frame structure will experience the most stress under torsional loads. I assume the weakest points are the welds. I plan to add gussets and shear panels in the most critical areas but want to avoid overstiffening it.

I asked ChatGPT for FEA style analysis after providing photos and all the background details - materials, dimensions, etc. It suggested the followings:
- Add shear panel to the front vertical wall using 16 gauge aluminum sheet.
- Add gussets to the 2 front roof corners.
- Add diagonal braces to each side of the rear door opening.

Does this seem like the right approach? Any suggestions would be really appreciated.

 

[DaveInDenver]

For what's it worth I put a similar topper on a pickup years ago and asked what the roof rack weight rating would be for the factory installed racks.

This isn't the exact one but it was essentially the same thing the manufacturer was offering. There were 3 points of contact, the two on the sides and one in the middle.



They told me the capacity was 400 lbs if evenly distributed. That might be true if it was very well distributed but I can't confirm this since I opted to do something different. I've seen some contractors with a lot of stuff on them. Seen some guys standing on top of their ladder racks, too. My camper was not a commercial one, just the cheapest thing I could find, so the tube walls were probably very thin compared to yours.

I put Yakima Side Loaders mounted so they connected to a horizontal frame member that ran from the front to back just below the roof line. The sides were flat and the roof on mine curved and this was the common tie between the two frame sub-assemblies, if that makes sense. I used carriage bolts and Nyloc bolts with Yakima's matching inside plate to sandwich the frame element.

I ran this roof rack for 6 years with bikes and a basket of stuff over all sorts of forest service and BLM roads and didn't develop any weirdness, cracks or bolts loosening. I'd guess that was around 80 to 100 lbs of cargo plus maybe 20 or 30 lbs in the rack itself. And some of that mass had a decent moment to it, being a couple feet high in bike trays on top. If the weight is held close to the frame it'll stress the structure less.

When you think about this way that means it could support significantly more than double it's own weight. The cap itself I bet weighed maybe 75 lbs, if even that much.



My point would be these caps are light but reasonably strong as long as you spread the loads into the frame. The skin is just there to keep things dry and a little bit more secure (although don't get complacent, someone who's even slightly motivated can rip it open with a screwdriver or even a sharp rock).

This construction is essentially how most RVs and truck campers are made. It you tore the inside walls off you'd see basically the same thing. The tube walls and skin might be thicker and more care in workmanship for the price, though.

For example, this is a Four Wheel Campers frame before it's skinned and the interior trimmed out.



That said adding some reinforcement to the structure and backing up the welds probably isn't a bad move. You will run into the material strength of the tubes themselves at some point. The welds probably aren't that bad and my first thought wouldn't be they are the weak points.

Also don't necessarily fall into a trap thinking stiffer is better. If you make one part of the frame more rigid you may move stresses. I doubt the manufacturers do FMEA on them but who knows, now that it's a cheap option package with CAD they might. In any case they've been making these style caps for many decades so there's knowledge built up. You don't need super thick wall tube, it gains its strength based on geometry.

Think of a beer or pop can. The walls are paper thin but a full sized human can stand on it without buckling, as long as there's no dents. They could make a beer can wall even thinner from a structural standpoint, the limitation is you could cut them with a fingernail if they got much thinner. It's a durability limit. Or even your truck itself. The sheet metal is extremely thin in the body and it gets it's strength from the placement of the folds and the ladder frame under it. I bet the walls in your topper aren't any thinner than your truck body.

So it is with the tubes in your topper frame. They might seem thin but they are likely plenty thick to do the job. It's a question of durability and how much abuse it will take. Meaning if you are hitting the structure sliding ladders in and out, hitting with a fork lift forks at the lumber yard, throwing spools of cable in then the tubes need to be thicker just to stand up to that. They wouldn't need to be thicker for the topper structure as such.

What I'd consider is maybe a thicker skin. I don't know specifically the shape of yours but if it hasn't a lot of curves on the skin that would be easy and reinforce the whole thing uniformly, especially if they didn't use any structural glue to bond the skin to the frame originally. It wouldn't have to be all that thick to gain a lot. Even a 0.032" thick aluminum skin is going to add strength if it's well fastened (glued and riveted) to the frame. So going to a 0.050" or 1/16" (0.063") skin would toughen that thing right up. It's a cost trade-off (same with you as the manufacturer).

The one thing you want to do in any case is put an interior on it. The skin on these toppers gets really hot in the sun, heats up the inside and is unpleasant if you touch it. Fitting insulation in the voids and a nice looking heavy fabric makes a huge difference in comfort and quiets them down a lot.

I don't think you've made a bad decision at all.

 

[Fenderfour]

Some views outside might be helpful.

I don't think I'd try to weld anything in. .035 aluminum will require TIG and someone why knows what they are doing. I would want to see some of their work before handing over my project as well. Lots of folks can TIG weld aluminum, not everyone can weld thin aluminum.

I would try to avoid putting any holes in the frames for all the reasons DaveInDenver said. Holes in flexible aluminum structures can lead to cracking and failure.

That pretty much leaves glue, which sounds crappy, but modern PU adhesives are pretty amazing, especially when prepped and applied properly. @Peter_n_Margaret have their entire habitat box glued to a frame. It gives me the heebie-jeebies, but it's been working for them for years.

My first inclination would be to remove the exterior skin and re-glue all of the contacts between frame and skin. This will add the skin as an integrated structural member, which will increase rigidity and toughness, and help with shear loads. Large open areas (along the side and top) are likely to oilcan (deform slightly) and may cause additional noise. Gluing will reduce this, but not prevent it.

I would add a doubler anywhere I planned to bolt something on. This could be another bit of Aluminum or a bit of ACM, and make it quite a bit bigger than the holes needed to mount. Glue it on the backside of the mount using a good quality adhesive and follow the directions. If you can span between two of the frames. This will spread the load out across the skin and help transfer it to the frame.

 

[reverse01134]

For what's it worth I put a similar topper on a pickup years ago and asked what the roof rack weight rating would be for the factory installed racks.

This isn't the exact one but it was essentially the same thing the manufacturer was offering. There were 3 points of contact, the two on the sides and one in the middle.

View attachment 909513

They told me the capacity was 400 lbs if evenly distributed. That might be true if it was very well distributed but I can't confirm this since I opted to do something different. I've seen some contractors with a lot of stuff on them. Seen some guys standing on top of their ladder racks, too. My camper was not a commercial one, just the cheapest thing I could find, so the tube walls were probably very thin compared to yours.

I put Yakima Side Loaders mounted so they connected to a horizontal frame member that ran from the front to back just below the roof line. The sides were flat and the roof on mine curved and this was the common tie between the two frame sub-assemblies, if that makes sense. I used carriage bolts and Nyloc bolts with Yakima's matching inside plate to sandwich the frame element.

I ran this roof rack for 6 years with bikes and a basket of stuff over all sorts of forest service and BLM roads and didn't develop any weirdness, cracks or bolts loosening. I'd guess that was around 80 to 100 lbs of cargo plus maybe 20 or 30 lbs in the rack itself. And some of that mass had a decent moment to it, being a couple feet high in bike trays on top. If the weight is held close to the frame it'll stress the structure less.

When you think about this way that means it could support significantly more than double it's own weight. The cap itself I bet weighed maybe 75 lbs, if even that much.

View attachment 909516

My point would be these caps are light but reasonably strong as long as you spread the loads into the frame. The skin is just there to keep things dry and a little bit more secure (although don't get complacent, someone who's even slightly motivated can rip it open with a screwdriver or even a sharp rock).

This construction is essentially how most RVs and truck campers are made. It you tore the inside walls off you'd see basically the same thing. The tube walls and skin might be thicker and more care in workmanship for the price, though.

For example, this is a Four Wheel Campers frame before it's skinned and the interior trimmed out.

View attachment 909519

That said adding some reinforcement to the structure and backing up the welds probably isn't a bad move. You will run into the material strength of the tubes themselves at some point. The welds probably aren't that bad and my first thought wouldn't be they are the weak points.

Also don't necessarily fall into a trap thinking stiffer is better. If you make one part of the frame more rigid you may move stresses. I doubt the manufacturers do FMEA on them but who knows, now that it's a cheap option package with CAD they might. In any case they've been making these style caps for many decades so there's knowledge built up. You don't need super thick wall tube, it gains its strength based on geometry.

Think of a beer or pop can. The walls are paper thin but a full sized human can stand on it without buckling, as long as there's no dents. They could make a beer can wall even thinner from a structural standpoint, the limitation is you could cut them with a fingernail if they got much thinner. It's a durability limit. Or even your truck itself. The sheet metal is extremely thin in the body and it gets it's strength from the placement of the folds and the ladder frame under it. I bet the walls in your topper aren't any thinner than your truck body.

So it is with the tubes in your topper frame. They might seem thin but they are likely plenty thick to do the job. It's a question of durability and how much abuse it will take. Meaning if you are hitting the structure sliding ladders in and out, hitting with a fork lift forks at the lumber yard, throwing spools of cable in then the tubes need to be thicker just to stand up to that. They wouldn't need to be thicker for the topper structure as such.

What I'd consider is maybe a thicker skin. I don't know specifically the shape of yours but if it hasn't a lot of curves on the skin that would be easy and reinforce the whole thing uniformly, especially if they didn't use any structural glue to bond the skin to the frame originally. It wouldn't have to be all that thick to gain a lot. Even a 0.032" thick aluminum skin is going to add strength if it's well fastened (glued and riveted) to the frame. So going to a 0.050" or 1/16" (0.063") skin would toughen that thing right up. It's a cost trade-off (same with you as the manufacturer).

The one thing you want to do in any case is put an interior on it. The skin on these toppers gets really hot in the sun, heats up the inside and is unpleasant if you touch it. Fitting insulation in the voids and a nice looking heavy fabric makes a huge difference in comfort and quiets them down a lot.

I don't think you've made a bad decision at all.

Thank you for sharing your thoughts based on the experience with your camper and your observations. Even weight distribution is definitely key. Perhaps the way ladder rack wraps around the camper could even provide some additional structural support.

I do like the aluminum tube framed construction - just like 2x4 walls in a house, it allows for easy insulation (thermal bridge will reduce insulation effect but I don't think that's a big deal unless you are in extreme arctic conditions) and gives studs to anchor anything you want to mount. What my camper lacks compared to FWC or OVRLND camper is the durability as you mentioned, that comes from thicker walled tubing and the structural support provided by proper shear panels.

While my camper wouldn't experience construction environment type of harsh abuse, the torsional stress from off camber situations on trails is my biggest concern. I definitely like the idea of using the outer skin as shear panel to add structural support.

Some views outside might be helpful.

I don't think I'd try to weld anything in. .035 aluminum will require TIG and someone why knows what they are doing. I would want to see some of their work before handing over my project as well. Lots of folks can TIG weld aluminum, not everyone can weld thin aluminum.

I would try to avoid putting any holes in the frames for all the reasons DaveInDenver said. Holes in flexible aluminum structures can lead to cracking and failure.

That pretty much leaves glue, which sounds crappy, but modern PU adhesives are pretty amazing, especially when prepped and applied properly. @Peter_n_Margaret have their entire habitat box glued to a frame. It gives me the heebie-jeebies, but it's been working for them for years.

My first inclination would be to remove the exterior skin and re-glue all of the contacts between frame and skin. This will add the skin as an integrated structural member, which will increase rigidity and toughness, and help with shear loads. Large open areas (along the side and top) are likely to oilcan (deform slightly) and may cause additional noise. Gluing will reduce this, but not prevent it.

I would add a doubler anywhere I planned to bolt something on. This could be another bit of Aluminum or a bit of ACM, and make it quite a bit bigger than the holes needed to mount. Glue it on the backside of the mount using a good quality adhesive and follow the directions. If you can span between two of the frames. This will spread the load out across the skin and help transfer it to the frame.

Very much appreciate the suggestion, and fully agree that adhesive alone can provide enough strength to hold the entire camper together. In fact, I was so surprised when I learned that those expedition campers are built from sandwich panels only glued together! Even the interior walls and cabinets are assembled with glue.

One thing I didn't mention in the original post is that the aluminum frame tubing has a 0.05" wall thickness. 0.035" for the skin. That 0.05" is still very thin compared to other campers that use 1/8" wall tubing. I wouldn't mount anything relying solely on the tubing but if I end up mounting awning, etc, I plan to reinforce the area with 1/8" plates on both the interior and exterior walls and bolt through all the layers. Maybe it's overkill but for piece of mind.

I really like the idea of securing all the contacts between frame and skin, and even replacing the skin with thicker material. The biggest hurdle would be reinstalling the rubber corner molding. I don't know if it's reusable once removed, and if not, whether I can source the same material. Aluminum angle could be used instead of the rubber molding to finish the corners. But this starts to expand the project scope into a full tear-down which feels too overwhelming. Perhaps I can slip the VHB tape into the gap between the skin and the frame wherever accessible, hoping I can still achieve 80-90% of the full effectiveness.

Another alternative I can think of would be adding inner skin, though permanently gluing it in place would block access behind the panels for future maintenance or repairs to insulation or leaks.

 

[rruff]

However, I do need enough rigidity for the shell to handle rough terrain.

You absolutely don't want it to be rigid if your truck's frame is C channel.

The shell will surely support itself and whatever weight ARE says is safe to mount on the roof. They've been in business for a long time and have a good rep, so they could give you decent advice on how to do that.