Yet another camper homebuild thread - fibreglass subframe

[tanuki.himself]

Short version - should i use pultruded fibreglass beams to build the floor and subframe of a hardside cab-over camper with cleated, bolted and glued joints?

long version - i'm in the design phase for building a slide in cab-over camper

design criteria

• hardside 3 season
• demountable - 30-40% time only, normal size european vehicle
• 5' (1.50m)north-south bed, proper independent sprung mattress
• internal cassette toilet/shower
• below 7' (2.15m) height & width - standard UK garage door for storage
• single vehicle for ferries/shipping - Norway, Greece, southern Africa
• tow a small trailer - motorcycle, extra weight

solution

• Nissan Navara king cab/Ford Ranger supercab - 1100kg, 1.800m loadbed, circa 2014 vintage, euro 5 engine
• slide in cab over, jacks/legs - based on Lance 865, Northstar Laredo layouts
• factory made styrofoam, or home made nidacore panels for walls/roof/internals - we've all seen plenty of these being made in this forum

The big question is the subframe. Most people seem to go with steel or aluminium. I can't weld steel well, I can't weld aluminium at all, people complain about thermal breaching using either, I have no tools for working metal, nowhere big enough to work inside on the whole thing and i'm very limited on weight by the available trucks.
Also, this is my project - i don't want to pay other people to make it for me, or to outsource as little as possible.

So, can I make the floor and wall subframe from fibreglass beams? Pultruded beams are lightweight, strong, rot resistant, thermally insulating, and should expand at the same rate as the wall sheathing.
I can work them with my existing woodworking tools by just adding carbide or diamond blades - circular saws, router, jigsaw, 125mm angle grinder.
Cleat and bolt is recommended by the manufacturers for jointing - i can make internal and external cleats easily from CSM glass and resin, incorporating stainless steel square nuts into internal fastenings, use countersunk screws where a flush finish is needed, and resin glue everything as a bonus,
It should be a quick build, i can work in the shed to make cleats and then just bolt it together outside on the truck.
100x50mm longitudinal box sections, or I beams to cantilever out of the back of the tub by 750mm for the shower/toilet. Cross members/noggins can be overlain with woven fibreglass tape and resin top and bottom to add strength
Should i make the whole floor and then add the wall supports as L shapes, or make the wall supports as Z shapes and bolt to front and rear floor crossbeam where the leg mounting points will be?
Its a thick floor but its not dead space - i can incorporate fibreglass water tanks into the floor between the beams for weight distribution over the tub, and a small grey tank under the shower for regulations compliance.

Any reasons why this can't work? or better ways to build it?

edit: modified attachments to show as thumbnails

 

[Freebird]

Looks like a very interesting build. I’ve got no advice to offer, unfortunately, but I certainly am interested in you project’s progression.
Please document it on here for our enrichment.

 

[tanuki.himself]

Will do - hopefully its still several months away from starting as i'm still on the road in central and south America, but if things go to plan i will have about a 6 week window at the end of this trip where i literally have nothing else to do so want to be able to work on this quickly and have all my ideas ready to go. And if plans change, it may be started sooner.....

 

[boxcar1]

Flex will be the largest problem you face with this design. Flex = leaks long term. If you have a few months. Take a welding class. Cheap , not terribly technical and well worth the effort. People who complain about thermal breaching don't understand the tech. It's a non issue if designed and built correctly.

 

[tanuki.himself]

Flex will be the largest problem you face with this design

could you elaborate please? Do you mean in the beams, the joints/cleats or the overall frame structure?

 

[boxcar1]

The unit in it's entirety. A chassis is the foundation of the unit. A bolted together glass structure will flex. This will cause the entire rig to flex. That's what causes the majority of failures in the RV industry. Windows, doors , vents , corners and joints loose there seals due to flex. Not trying to rain on your idea. Just trying to prevent pain down the road.

 

[tanuki.himself]

The unit in it's entirety. A chassis is the foundation of the unit.

OK, Understand where you are coming from. In this case the design works a bit backwards. The superstructure will be dual side fibreglass sheathed foam or hollow core material. Each panel effectively acts as a torsion box, and panels in all 3 planes will act to brace each other. Edge joints will have 60mm+ of glue along their entire edge, and may be reinforced with additional fibreglass tape internally - i will be guided on that by the panel manufacturers. Overall the structure will be extremely rigid.

In this case the purpose of the fibreglass beam frame covered here is to support the weight of the superstructure where it extends outside the tub/loadbed and to transfer the load into the tub, so the superstructure will actually stiffen and brace the subframe.

 

[tanuki.himself]

Great material. Not inexpensive. Had you researched just how much it will weigh compared to other materials?
My experience with pultruded profiles, its awfully heavy stuff.

I think the overall subframe comes in a little over 70kg, plus however big i make the cleats. Not the lightest, but given how much i am hanging over the back of the tub i want it strong, especially where the beams transition from support to cantilever and its being bounced up and down. And of course rot and corrosion proof. I'm dealing with a couple of UK suppliers, one of which charges about a 15% premium for the profile over what it would cost me to buy the fibreglass and resin per kilo, so i didn't think that was too bad for getting a highly engineered product compared to trying to make my own by wrapping foam cores....their product uses mostly longitudinal fibres rather than chopped strand or woven mat and achieves a very high glass to resin ratio. I could probably achieve a similar profile using layers of unidirectional tape or even loose roving but i think the pultruded would still be stronger and more uniform, and a lot quicker and cleaner