Designing a Subframe for a pickup / Ute Camper Build

Hi all.
I am planning to build a camper that will be permanently mounted on my Toyota T100 with a pass through similar in theory to the Maltec land cruiser campers. The camper will be attached to the truck cab and move together as one.

My general plan is to build a subframe out of steel square tubing that will be attached to the frame using 6 urethane body mounts as well as being tied into the rear body mounts of the cab. Then build the camper on top of this frame out of 2 inch XPS foam board laminated with 3/16 inch marine grade plywood inside and out. Then a layer of 6oz fiberglass cloth on the outside. The camper will be attached to to truck cab with Sickaflex or Korapop or ? adhesive. Interior cabinetry, etc. will be primarily plywood glued and screwed.

I have some experience working with all of these materials, but am not an expert at any of them. I will be learning to weld for this project and hopefully improving my other skills as well.

Questions:
1.What size steel square tubing should be used for the subframe?
- I've seen everything from 1x1 to 2x6 and rarely see wall thickness mentioned in people's write-ups. The camper's footprint will be 6'2" wide and 8'8" long and likely be 1500+ lbs when loaded.

2.Will 6 polyurethane body mounts (equivalent to the ones that hold the rear of the cab) be appropriate?
- My thinking is that I want the cab and camper to move together, so using the same mounts throughout would work well, apposed to hard mounting or using the pivots and springs that I've seen used on bigger builds?

3. What should the first layer of material on top of the frame be?
- I've seen plywood used often, but am concerned with it's durability when exposed to water, heat, mud, etc.

4. will a camper made of XPS and 3/16" plywood hold together for many 1000s of miles off pavement or does it need a frame? (talking about a frame supporting the walls and roof, not the subframe)
- Ive read threads on the subject, but can't come to a conclusion. People seem to disagree without saying why. Anyone with experience, please chime in.

Thanks for the help.
 
Sounds like a great project. Good luck!

With moving the cab and camper together... that's a big box to stop flexing. I have a lot of movement between my cab and camper and mine is tiny. Maybe a flexible rubber pass through might be better? I've seen these on trucks.

My top panels are simply glued together, plenty strong enough with the right adhesive, no upper frame. I've also added external 2 inch aluminum angle over the high stress join areas with more adhesive, this is probably overkill but it doesn't move.

Read the full specs on adhesives, get a strong one, some also need 2mm thickness to work. I'm using teroson ms 939 but there's plenty of others. Dow seems to be recommended a lot.

If the plywood is well adhered to the foam it's very strong.

Sent from my SM-T825 using Tapatalk
 

Joe917

Explorer
I would avoid using any steel tube in a camper box. Steel will create frost bridging that can be avoided with a composite build that will be as strong or stronger than steel.
 
Sounds like a great project. Good luck!

With moving the cab and camper together... that's a big box to stop flexing. I have a lot of movement between my cab and camper and mine is tiny. Maybe a flexible rubber pass through might be better? I've seen these on trucks.

My top panels are simply glued together, plenty strong enough with the right adhesive, no upper frame. I've also added external 2 inch aluminum angle over the high stress join areas with more adhesive, this is probably overkill but it doesn't move.

Read the full specs on adhesives, get a strong one, some also need 2mm thickness to work. I'm using teroson ms 939 but there's plenty of others. Dow seems to be recommended a lot.

If the plywood is well adhered to the foam it's very strong.

Sent from my SM-T825 using Tapatalk
Thanks for the input.
What vehicle is your camper on?
What did you build your subframe out of?
 
I would avoid using any steel tube in a camper box. Steel will create frost bridging that can be avoided with a composite build that will be as strong or stronger than steel.
I'm currently only planning on using steel for the subframe. a layer of XPS foam and plywood will make up the floor of the camper on top of the subframe.
Will this still cause "frost bridging" ?
What would you recommend using to build the subframe?
 

Joe917

Explorer
I just reread your build description. For steel subframe you are talking about the floor only, not the walls. Perfectly acceptable, My error I was reading it as steel in the floor and walls. I would consider buying composite panels rather than trying to create them yourself. A properly assembled box made of composite panels will not need any extra re-enforcement.
 
This is what I have mocked up, but I'm starting to think it is overkill. What do you think?

yellow = 2x3 inch
red = 2x2
blue=1x2
green= 1.5 inch angle iron

I'm thinking 1/8" wall on all tubing.
The total floor plan will be 104x75 inches.
The closest corner in the image is the passenger side front and has a step down where the door will be.
The angle iron is where the wheel wells will be.

subframe-2.png
 

tanuki.himself

Active member
have you added up what all these materials are going to weigh? its sounding pretty heavy to me.....

if possible i would go thinner materials but deeper cross sections - the ability to resist bending and flexing increases dramatically as you increase the distance from the centre line to the outer surface
 

shortbus4x4

Expedition Leader
have you added up what all these materials are going to weigh? its sounding pretty heavy to me.....

if possible i would go thinner materials but deeper cross sections - the ability to resist bending and flexing increases dramatically as you increase the distance from the centre line to the outer surface
This with some well placed gussets. I think you have too many pieces in there which will add lots of weight. Go look at some flat beds on commercial trucks and model you bed after them.
 
Ok. I managed to find a couple of smaller camper builds (not on unimogs or canters or other such trucks) and got some info on their subframes. They are WAY smaller then the subframes on the bigger trucks that I was previously trying to "scale down". Confirming the advice I was given here.
Here's the reworked plan:
Yellow=1x3" square tubing 1/8" wall
Blue=1x2" square tubing 1/8" wall
Green=1.5" angle iron

I've also found contradicting opinions on mounting the subframe. I've seen people use bodycount bushings as I plan to use, but I've seen anywhere from 6 to 2 mounting points being used for similar sized campers. The logic on one side of the fence being more mounting points is stonier and distributes the load on the tuck fame. The other side of the fence says fewer mounting points allows for more moment between the truck frame and camper frame and less torque being applied to the camper.
Any thoughts?
subframe-3.png
 
still looks really heavy.
OK.
Would you suggest thinner walled tubing or small tubing or fewer crossmember?
the floor will be a sandwich of .5" plywood- 1"XPS- .5" plywood adhered together with epoxy. I haven't found any good information on how wide of a gap is acceptable between supports when using this kind of composite/sandwich, so I'm not sure how many supporting beams need to be in the subframe.

Thanks for the input.
 
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