building underbed storage w Coosa?

[calameda]

I’m planning under mattress storage in the cabover of my TC camper. Essentially will be the typical design of a shallow partitioned box accessed by raising a strut supported panel under the mattress. Would really like to minimize weight, so am considering coosa. Does anyone with hands on coosa experience have joinery tips to share? I’ve read it doesn’t take biscuits well. Seems most use butt joints with t-nuts and through bolts, but I’d like more rigid joinery options if possible. And is it recommended to augment joints with sikaflex or other adhesive?

And are there other light rigid structural panel materials worth consideration for box construction? ideally a material that is easy to work with std woodworking tools.

 

[g_m]

"Does anyone with hands on coosa experience have joinery tips to share? I’ve read it doesn’t take biscuits well. Seems most use butt joints with t-nuts and through bolts, but I’d like more rigid joinery options if possible. And is it recommended to augment joints with sikaflex or other adhesive?"

I've worked with the low density Coosa in 1/2" thickness (20 pounds per cubic foot). Any fasteners, biscuits or other, will only be useful for alignment before applying fiberglass tape and epoxy.

 

[simple]

A napkin drawing of how your going to lay it out would help for anyone offering advice.

I'd consider bonding in thin wall aluminum rectangular tubing for the support/partitions and then a honeycomb core FRP panel for the lid.

 

[matt_devon]

Is the idea to make the box removable or will you be bonding the partitions (basically stringers in the boat world) directly to the camper? If bonding to the camper, then butt joints between the verticals and the floor using cabosil thickened epoxy would work well. Add fillets and fiberglass if it feels weak. If it is going to be removable then a frame made with aluminum extrusion or press fit profile topped by a skeletonized plywood bed panel would probably be cheaper, lighter, and less labor intensive than building a full box using composites.

 

[calameda]

"Does anyone with hands on coosa experience have joinery tips to share? I’ve read it doesn’t take biscuits well. Seems most use butt joints with t-nuts and through bolts, but I’d like more rigid joinery options if possible. And is it recommended to augment joints with sikaflex or other adhesive?"

I've worked with the low density Coosa in 1/2" thickness (20 pounds per cubic foot). Any fasteners, biscuits or other, will only be useful for alignment before applying fiberglass tape and epoxy.

Thanks for the knowledge. Will definitely reinforce butt-joint corners with FG tape and epoxy as you suggest. Seems other folks like the Korapop 225 for butt joints (w mechanical fasteners for alignment). Any chance you compared epoxy to korapop?

Somehow my initial search missed this excellent thread:

Lightweight Cabinet Materials - Plywood/Coosa Alternatives

Hello! We are beginning our Unimog camper build which has a composite box on the back. The composite box was our first foray into the world of composites and it now has me thinking about our cabinet construction. I can weld and was planning on welding together aluminum angle and skinning it with...

forum.expeditionportal.com

I also realize this may be wrong spot for this discussion. Should I move to "DIY Expedition Camper/Van, Resource, Modification”?

 

[calameda]

A napkin drawing of how your going to lay it out would help for anyone offering advice.

I'd consider bonding in thin wall aluminum rectangular tubing for the support/partitions and then a honeycomb core FRP panel for the lid.

Basically this (copped from a camper vendor):

The storage space will be relatively shallow, just 4 inches. To save weight I’d prefer to avoid a separate bottom layer to the box. Intend to attach outermost sides and front to the FRP pultrusions in the cabover.

There will also be side storage “bins” on either side of the bed (the mattress is N-S) as in this the sketch from top:


Like the idea of a honeycomb lid, if it will be suitably stiff. I’ve never worked with the stuff. How do you suggest attaching to a stainless piano hinge? And what do you use to “edge band”?

 

[rruff]

And are there other light rigid structural panel materials worth consideration for box construction? ideally a material that is easy to work with std woodworking tools.

You can buy sheets of PP honeycomb core with FG laminate on both sides from Carbon Core. Price is reasonable ($172 for 1/2 inch) and freight shipping was $350 I think, last I bought something from them. https://www.carbon-core.com/product/composite-panels-sheets/

 

[calameda]

Is the idea to make the box removable or will you be bonding the partitions (basically stringers in the boat world) directly to the camper? If bonding to the camper, then butt joints between the verticals and the floor using cabosil thickened epoxy would work well. Add fillets and fiberglass if it feels weak. If it is going to be removable then a frame made with aluminum extrusion or press fit profile topped by a skeletonized plywood bed panel would probably be cheaper, lighter, and less labor intensive than building a full box using composites.

Thanks for the suggestions. Although I love wood cabinetry and have a lot of experience with solid wood joinery and ply, at this point I am heavily prioritizing weight, thus the Coosa. And while I don’t intend to make it removable per se, my preference is not to permanently bond to the cabover floor, just to keep open options for future modifications.

What sorts of Al extrusion do you think would work? 80/20? or something else?

 

[calameda]

You can buy sheets of PP honeycomb core with FG laminate on both sides from Carbon Core. Price is reasonable ($172 for 1/2 inch) and freight shipping was $350 I think, last I bought something from them. https://www.carbon-core.com/product/composite-panels-sheets/

Thanks! will check this out.

What are preferred joinery methods? and edge treatments (cosmetic)? Any how to links?

 

[matt_devon]

Thanks for the suggestions. Although I love wood cabinetry and have a lot of experience with solid wood joinery and ply, at this point I am heavily prioritizing weight, thus the Coosa. And while I don’t intend to make it removable per se, my preference is not to permanently bond to the cabover floor, just to keep open options for future modifications.

What sorts of Al extrusion do you think would work? 80/20? or something else?

These would probably be sufficient for a bed platform: https://www.grainger.com/category/material-handling/structural-framing-systems/press-fit-framing-systems?brandName=80/20&filters=brandName

I ordered polypropylene honeycomb core panels from Carbon Core last year, total shipping to a residential address came out to something like $400. I do think the honeycomb panels would be lighter and stiffer than coosa for the lift up panel. Edge treatment can include bonding strips of wood (like my bed platform below)


or filling the core, rounding the edges, and glassing over the edge like this:

 

[rruff]

What are preferred joinery methods? and edge treatments (cosmetic)? Any how to links?

Unfortunately I haven't used it! I only bought foam from them.

The edges are open, exposed PP honeycomb. It's the same as Nidacore which is used on boats, so maybe you can find some tips on how that is finished.

If you look at some of the build videos from Styromax in Oz, you'll see that they join their styrofoam core panels with no reinforcement at all on the edges. Just a straight cut on one side through the skin and core that the other panel fits into. Then they fill the joint with glue (some PU variety), and lay a radius bead of the same glue on the inner corner and that's it.

Also Peter & Margaret says his panels were built the same way and he's had no structural issues in many hard miles.

https://www.youtube.com/@StyroMAXInsulatedPanels/videos

 

[g_m]

Seems other folks like the Korapop 225 for butt joints (w mechanical fasteners for alignment). Any chance you compared epoxy to korapop?

I have not. Only G2 and West System epoxies. I have some Coosa in the shop right now so was able to snap a picture that may help to inform your decisions (and yes, I need a manicure). The 20lb variant I have experience with is variable density with "harder" skins on the surface with a visibly higher density of fiberglass reinforcement compared to the lower density polyurethane matrix at the core. This may not be apparent to those who have not actually used Coosa. Counterbored T-nuts, for instance, might not be a good idea without crush collars. A pan head screw with washer might hold where a flat head screw may not etc.. It's a wonderful material but demands thought to exploit its strengths while avoiding potential weaknesses.

 

[calameda]

These would probably be sufficient for a bed platform: https://www.grainger.com/category/material-handling/structural-framing-systems/press-fit-framing-systems?brandName=80/20&filters=brandName

I ordered polypropylene honeycomb core panels from Carbon Core last year, total shipping to a residential address came out to something like $400. I do think the honeycomb panels would be lighter and stiffer than coosa for the lift up panel. Edge treatment can include bonding strips of wood (like my bed platform below)

Thanks much! I’ll have to seriously consider the Al frame option. I can see the shipping on these panels, particularly if you need only a couple, is killer. I’ll hunt around for local sources.

Unfortunately I haven't used it! I only bought foam from them.

The edges are open, exposed PP honeycomb. It's the same as Nidacore which is used on boats, so maybe you can find some tips on how that is finished.

If you look at some of the build videos from Styromax in Oz, you'll see that they join their styrofoam core panels with no reinforcement at all on the edges. Just a straight cut on one side through the skin and core that the other panel fits into. Then they fill the joint with glue (some PU variety), and lay a radius bead of the same glue on the inner corner and that's it.

Also Peter & Margaret says his panels were built the same way and he's had no structural issues in many hard miles.

https://www.youtube.com/@StyroMAXInsulatedPanels/videos

That Nidacore is crazy expensive. (wondering out loud: is this what AT Overland uses for the Aterra shell? how do they get that down to 1200 lbs dry, fully equipped?) And the Styromax joinery technique if I understand you, is sorta like a dado joint in wood? I’m amazed this works with foam core panels.

I have not. Only G2 and West System epoxies. I have some Coosa in the shop right now so was able to snap a picture that may help to inform your decisions (and yes, I need a manicure). The 20lb variant I have experience with is variable density with "harder" skins on the surface with a visibly higher density of fiberglass reinforcement compared to the lower density polyurethane matrix at the core. This may not be apparent to those who have not actually used Coosa. Counterbored T-nuts, for instance, might not be a good idea without crush collars. A pan head screw with washer might hold where a flat head screw may not etc.. It's a wonderful material but demands thought to exploit its strengths while avoiding potential weaknesses.

Thanks, I appreciate your warnings about fasteners. and thanks for the photo.

Good stuff from everybody, so much knowledge in this place! I’ve got a lot to review and chew on here.

 

[rruff]

That Nidacore is crazy expensive. (wondering out loud: is this what AT Overland uses for the Aterra shell? how do they get that down to 1200 lbs dry, fully equipped?) And the Styromax joinery technique if I understand you, is sorta like a dado joint in wood? I’m amazed this works with foam core panels.

These CC panels aren't heavy, or expensive. The wonder is that AT achieve a decent insulation (or do they?) given that the PP core is poor in that respect.

Kinda like a dado I guess, that goes all the way through one skin and the core, leaving only one skin left. The joint that I'm referring to is on the edge, not in the middle of a panel. Yes, I'm amazed that it holds together also... considering how much people worry about edge joining on this forum... but these companies have been building panels and campers for a long time, and I suspect it works fine.

BTW, I recall P&M mentioning that he made some interior cabinets and walls in his rig by just butting one to another in a T junction... with the glue of course, and a bead on each side.

 

[andy_b]

I am a little late to this and it seems like you’ve gotten a lot of good ideas for using Coosa for your build. In case you’re not fully committed, here is an alternative: 80/20:



I made our bed into a lifting platform over a storage area. Ikea slats instead of solid panel for the bed base. The nice thing about 80/20 is that it is infinitely adjustable/revisable. You can cut it with wood chop and table saws. No need to mess with FG, resin, or epoxies.