Ideas for lightweight cab over bed board?

[Boonie]

In a separate thread I am describing my camper build, but since only the people following that thread would answer this question, I decided to start a new thread to involve the whole FWC community.

My camper is a 1981 Grandby, I can tell that the cab over bed board was replaced and was replaced with 3/4" plywood. I think that was the same as original, but I need something lighter. Not only will I be replacing the original 23" overhang , but in my build I am increasing the overhang to 34 1/2" which will increase the weight of the already heavy bed board by 50%. A little less than that since I am narrowing the camper from 80" Grandby to 69" Eagle" and changing sleeping East/West to North/South.

What I am thinking is interlacing a 38" slide out with the 38" top bed (it does extend 3 1/2" past the 34 1/2" overhang) which will give me a 76" long bed.

Here is the idea of interlacing: Pushed together 38" pulled apart 76". If I made it out of 1"X1" aluminum (where would a person working on a FWC get that idea) spaced 12" apart, with the bottom (exterior) covered with say 1/4" aluminum clad plywood and the top covered in 1/4" plywood. Cushions attached directly to the top of either the fixed part or the slide out and cushions with intregal 1/4" plywood placed on the other. The space between the "fingers" would be filled with 1" ridged insulation.

OK, engineers, wannabe engineers, and of course everyone who has an opinion or experiences, what do you think?

 

[riz]

Sounds like a lot of work and lot of trouble and a lot of expense when a piece of 3/4 inch plywood would work just as well and weigh what... 12 ounces more?

 

[Boonie]

riz,

Quick fact check: 3 sheets of 1/4"x38"X69" = 37.6 lbs. Lowe's 3 sheets 1/4" Birch ply = $58.46 + $30 aluminum
2 sheets of 3/4"x38"X69" = 69.8 lbs. 2 sheets 3/4" Birch ply = $89.96

Savings of 32.2 lbs. Same cost

There are two ends of the WTW spectrum. Some people keep putting stuff in there campers until they realize that like Quint in "Jaws"; "We're going to need a bigger boat". I'm on the end of the spectrum that thinks about what I put into my camper trying to keep it as light as possible and am willing to put in the effort to accomplish that.

Thanks for the thought.

 

[Vic Harder]

In a separate thread I am describing my camper build, but since only the people following that thread would answer this question, I decided to start a new thread to involve the whole FWC community.


What I am thinking is interlacing a 38" slide out with the 38" top bed (it does extend 3 1/2" past the 34 1/2" overhang) which will give me a 76" long bed.



Here is the idea of interlacing: Pushed together 38" pulled apart 76". If I made it out of 1"X1" aluminum (where would a person working on a FWC get that idea) spaced 12" apart, with the bottom (exterior) covered with say 1/4" aluminum clad plywood and the top covered in 1/4" plywood. Cushions attached directly to the top of either the fixed part or the slide out and cushions with intregal 1/4" plywood placed on the other. The space between the "fingers" would be filled with 1" ridged insulation.

OK, engineers, wannabe engineers, and of course everyone who has an opinion or experiences, what do you think?

Interesting idea, with a worthy savings in weight. Some questions I would ask:
- what keeps the aluminum fingers in place?
- won't the ends of the fingers abrade the 1/4" ply from the inside every time you slide it together?
- where is the strength coming from if none of these cross pieces are actually one piece anymore?
- won't the aluminum conduct the heat/cold a lot more than the wood does now?

 

[Boonie]

Vic,

Great questions.
#1 The dual tubes that hold the "fingers" in place would be fixed to the front overhang c-channel and at the rear to the cross member that replaced the front wall. The "fingers" slide out similar to the stock bed slide out, riding on the support rails on each side.
#2 Yes they would, so the ends of the "finger" tubes would be swedged in (like the tip of a ski), but I would need to look at some type of protection of the plywood.
#3 same answer as #1
#4 Yes, but only the 1" strips where the tubes are rather the entire surface. The space between each set of the dual "fingers" would be 1" ridged insulation.

Thanks for the helping me think through this idea.

 

[cwdtmmrs]

It is creative, but I too am not sure it is worth the weight savings. I weighted a piece of 3/4" maple plywood and came up with 63.7 lbs. My camper's bed board is made from lumber core plywood which would be significant lighter than that. I am not sure how many feet of tubing you will use, but by your description and photos, I guess at least 20 pounds with 1"x1" x .063 wall tubing @.3 lbs per foot. Insulation is another couple of pounds. If you attach the plywood to the tubing with screws or even adhesive will add another pound. Your long cross piece on the slide out supported on only the outer sides will not have any strength and will buckle the 1st time you got into bed. You may save 10 pounds and probably less, I remember seeing honeycomb boards that might be a better option.

cwd

 

[Beach]

If you go the plywood route. You might look at fir plywood, light than pine, more stable than pine(doesn't warp and twist nearly as bad as pine), IMO better looking when finished. I once used 5/8 instead of 3/4 when building a boat cabin interior to save weight.

 

[nikonron]

You can also skeletize the 3/4" plywood by drilling mutiple holes in it to reduce weight. Ron

 

[cwdtmmrs]

I would checkerboard the underside about of the pull out with a router and a fixture.That would leave a smooth surface on top.

cwd

 

[ntsqd]

Component geometry always trumps bulk when light weight strength is the goal. Look at truss bridges, their weight vs. strength ratio is really high. Or ponder a fabricated wood "I" beam vs. a glue-lam or solid beam. Same thing strength to weigh ratio of the fabricated beam is far higher than the glue-lam or the solid.

The structure supporting the cab-over bed has two challenges. It needs to support the two occupants who are more or less static; and it needs to support that part of the camper when going down a rough road - which is everything but static. Given the typical layout of these campers most of the fore-aft loads are carried by the sidewalls of the over-hang and translated back into the rest of the camper structure. The under-bed parts need only support the bed loads across from wall to wall.

The way that an "I" beam works is that the top flange is in compression and the bottom flange is in tension, the web only serves to maintain the spacing between the top and bottom flanges. Essentially this a 'linear' application, but the same principle works as a flat panel. the top sheet is in compression and the bottom sheet is in compression. What resides in between them is of no consequence except that it can not be compressible. Something like one of these panels: http://www.pacificpanels.com/ (no experience with them, searched "honeycomb panels" & took the top link) could be ideal for the under bed structure. As a bonus depending on the materials used I suspect that its insulative properties could be quite good.

 

[Boonie]

cwd,
Thanks for pointing out that I did not add the weight of aluminum which according to my wife's kitchen scale would add 6.4 pounds, so my weight savings would drop to 26 pounds. (I definitely will not use maple, two partial sheets would put me at around 100 lbs.!)
Ron,
One of my earlier thoughts was to "skeletize" a sheet of 1/2" then laminate a 1/4" piece on top with the aluminum sheet on the bottom filling the "holes" with 1/2" insulation. That helped the top bed part , but I would still have the pull out section which I could do the same thing.
Thorn,
I think the above laminated concept does what you mention.

Again your thoughts and questions help me think through this.

 

[Boonie]

Thorns comments on the physics of the load leads me to comment that I am able to support the entire overhang section from the cab because my camper is permanently mounted. I do question the torsion factor of overhang to cab if I did permanently mount it. Oh well, another thread.

Please note my Avatar to get an idea of what my camper looks like.

 

[ntsqd]

I wasn't fully understanding the laminating concept. It looked like it was more about building a telescoping bed platform than anything else.

I don't see the weight loss of pocketing the plywood being worth the effort. There are better ways to end up with a strong enough, light solution. With not much support (3 stringers of 1/2" plywood on edge laid width-wise of the bed) our bed lays on a section of 1/4" plywood and I can't say I've ever felt it deflect under my or our combined weights. This tells me that 1/4" with reasonable support will work and with better support you might be able to use 1/8" or thinner. The skins on those panels that I linked are typically less than 1/8" and good luck getting one of those bend or deflect.

With a 4 door JK (?) you're getting close to the length where designing in some relative motion allowance in the extreme areas won't hurt you.

 

[Wallowa]

Thorns comments on the physics of the load leads me to comment that I am able to support the entire overhang section from the cab because my camper is permanently mounted. I do question the torsion factor of overhang to cab if I did permanently mount it. Oh well, another thread.

Please note my Avatar to get an idea of what my camper looks like.

Interesting discussion...Question: What do you mean by "I am able to support the entire overhang section from the cab because my camper is permanently mounted."

What does "permanently mounted" mean? No vehicle bed but bolted directly to the vehicle frame?

Thanks,

Phil

 

[Boonie]

Thorn,
My original idea was a telescoping bed. I am glad you used that term because it better describes what I proposed. Others were suggesting other ways to achieve a lightweight bed board. "Sheletizing", or checkerboarding, as I took it were creating the honeycomb center like the link you offered. In each of these suggestions the telescoping feature was lost. I did not follow your lightweight suggestion, so I would appreciate it if you would explain what you have in greater detail, but it seemed that it also would loose the telescoping ability.
Phil,
Check out my build thread "'81 Grandby on a '91 Jeep YJ - New build" and I think you will get a better idea of what my camper looks like and where I could attach the overhang to the roof.

 

[cwdtmmrs]

Boonie,

If I understand the telescoping bed correctly, the stationary "bed board" will be will be a sandwich construction of 1/4" plywood, 1" x 1"x 1/16" wall tubing and 1/4" plywood on top. This will all be bonded or screwed together and the forward edge will be attached to the "C" channel of the cab over and the rear section will rest on top of the cross piece which has been relocated somewhat to fit your jeep build. The internal aluminum rafters/floor joist will be doubled up with a 1" gap to provide a track for the single rafters that will be part of the sliding section assembly. You also stated that these rafter/joist will be space 12" apart. If this description is correct. then I have no doubt that the stationary bed board section will be more than strong enough. The problem is the sliding section will just be fingers welded only on one end to a 1"x1" tube that will span the 69 inches across the camper. This part of the assembly cannot have any sheeting attached to it or it will not telescope. It is just the skeleton (like a fish backbone with ribs) which will have a cushion with a 1/4" plywood bottom simply resting on top. With the bed extended, that piece of tubing must carry all of your weight when you get in and out of bed. I think it will bend like a pretzel. No one else has commented on this, but focused primarily on the fixed bed board.

Also if all of the above is accurate, then your weight numbers just do not add up. I do not know where you came up with a 100 lbs for maple plywood. Before I posted before, I actually weight a 2'x 5' x 3/4" piece of maple plywood I hade in the shop (I have a woodshop) on a UPS scale and it weighed just shy of 17.5 lbs or 1.75 lbs per sq. ft. You stated that your build needed 2 pieces 38"X 69" or 18.2' x 1.75 lbs =31.86 lbs per piece. As I posted earlier, there are lighter plywoods available. You also posted that all of your aluminum tubing for the project will only weight 6.4 lbs. I do not think that is possible since you will need 15 pieces 38" long for rafters/joist (3 pieces for 5 tracks/fingers). 2 more 38" pieces if you do both outsides of the slide out and a 69" piece for the span. That is 715 inches or 59.58 feet @ .28 lbs per foot =16.68 pounds. When you add it all up. I do not think the big weight savings are there and I will bet money that the slide out section will not have the necessary strength. My 2 cents, I am sure you will do whatever you think is best.

cwd

 

[Boonie]

cwd,

I am grateful that you actually took the time to understand and visualize what I am proposing and you are correct. I would also add an appropriately sized "head board" to the connecting stringer (69" span) of the joists/rafters similar to the stock pullout, maybe a 1x4. My aluminum weighed 4.5 oz for a 26" piece. You can do the math from there. You may be assuming a heavier wall thickness, which is actually a great idea for the pullout section and maybe an option to add strength to it. Or maybe doubled 1" x 1"?

I misunderstood your maple weight measurement as being one 4x8 sheet, sorry.

Please keep the thoughts coming!

Boonie

 

[PokyBro]

Boonie,

Staying with your idea of a telescoping platform, here is a quick sketch of what I might imagine it looking like.

The number of aluminum stringers over the cab might very from what I've drawn, but the principle is what I'm trying capture.

If you find single 1" tubes are sufficient to carry the weight, on the roughly 3 feet of overhang, then the slide portion might look like the drawing. I think if you welded up some rectangular sections, for the slide out, to fit between your over cab portion, and welded them to a flat plate on the pull out, and attached an oak 1"X 3" to the flat plate spanning the width of the camper, with a notch to ride on the slider board on each side wall, it would provide enough strength everywhere needed. You would want the welds to be on the inside corners, and not on the top and bottom, so the welds would not interfere with the slide movement.


Using a rectangular box arrangement would keep everything aligned because welded aluminum tubing is fairly rigid. I think single tubes might be strong enough if spaced closely enough, but if there is too much flex, then as you mentioned, doubling up with two one inch tubes together, should do. Additionally, a box arrangement would also provide something for your plywood overlay to rest on between the over cab stringers when the slide pulls out.

Your weight figures would need to be considered, as well as the costs of material and welding. Again, for the sake of this discussion, these are just my thoughts, for whatever they're worth, in support of what you started out with, in your mock up description and photo.

One additional thing to consider, is thinking also about how you would seal everything up from outside moisture when you get to that phase.

Good luck with is as you continue on. For all those who have done a build out of any amount, part of the fun is coming up with ideas, and figuring out a way to make them work. Sometimes an original plan fails, but that's okay, the final product looks and functions great. I certainly had plenty of things I had to attacked in different ways on my build. I hope all the ideas from everyone helps you with something that works for you!

Poky





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[ntsqd]

I hadn't made the leap to telescoping, I was thinking purely about the cab-over portion only. I would not want a telescoping section if at all possible. I would consider other means, starting with a fixed bed first. The constant shuffling of stuff is bad enough without throwing a moving bed section into the mix.

 

[Boonie]

Another great thread " what do you constantly move around and how do you resolve it?"