... with the advantage going to the designers & builders familiarity with the material. (craftsmanship wins!!!!)
Well, I'm a mechanic first, a carpenter and metalworker tied for second. I've built whole projects from scratch more with wood, but made small random repairs and minor fabrications more with metal. I feel more familiar with steel than anything, followed by alu and wood about equally. I have zero experience with fiberglass and other composites, and next to zero with adhesives (those that work, anyway).
1. 3 inches of insulation is over kill for such a small interior volume & adding 2, 3 or 4 inches to the interior space would be much more useful. You are right to deal with thermal bridging...that's the biggest problem compared to keeping warm in such a small enclosed space. This assumes it will have some sort of heater & you are comfortable (cool with???) sleeping bags at night. I always found cold floors the main problem so maybe more insulation there - down booties go a long way but if you can deal with the concept some sort of floor radiant heat is great. Here's a good post on camper insulation from an excellent & well documented build -
http://www.expeditionportal.com/forum/threads/48351-out-with-the-old-in-with-the-new
Might be overkill, but it won't add much weight (or that much cost, either) and with how I want to avoid thermal bridging with the frame thickness I have, it's just about necessary. I'll stick with it. I intend to take it to places where it might not be overkill after all; the target temperature I've used for thermal calculations is -40. I don't have nearly as much thermal mass as most campers, either, so I have to rely on insulation more.
2. "crumple zone" - that's an interesting idea...1" of low compressive foam sheathed with .019 alu would provide a degree of protection - decreasing the impact damage to the frame as you suggest. Soft mounts to the truck frame would do the same (assuming the shell is stiff enough to transfer the impact load to the mounts...) I'm leery of spending too much design energy on narrow failure modes - designing for 1-5mph impacts is a wonderfully complicated business! Personally on such a small camper with minimal side overhangs I think you might be overthinking the theoretical problem. BUT it's a great experiment.
I'd approach the problem more from a "ease of repair" angle - since I can't control the conditions of a future accident I would use a construction method I can repair - that's my idea of "control". From that perspective alu framed builds as you've sketched are more difficult to repair compared to composite or wood builds - I may be talking into my hat on that one because I've never repaired a alu frame build, while composites & wood are my natural element. The point being the material you have the most experience & affinity for is the right material for you.
I hear you, and this is something for me to chew on, because I've worked some with alu, but it's not my most familiar material. My only big problem with steel is weight, and the increased likelihood of ever relying on impact-resistant strength if I have a lot of weight up high.
So how would you fix a bent alu internal frame with the skin all deformed & the interior wood deformed or debonded?
Good question! As "field" repairs go, fiberglass is about the only reasonable material, it seems: a small vat of resin and a roll of the material. Everything else requires carrying spare panels. But I have zero experience with fiberglass. The idea is that minor damage would just bounce off or make pound-out-able dents in the skin, major damage wouldn't make limping home impractical (one wall destroyed, but the frame is intact and therefore so are the other walls), and catastrophic damage... is catastrophic damage no matter how I build it. In the first scenario, I drill out rivets or unscrew screws, pound out the dents, and put the panel back on, perhaps sans the outer inch of foam (which gets replaced later). Apart from lack of successful experience with adhesives, I don't plan on using them much if at all, specifically to make disassembly easier for repairs. In the second scenario, I can repair a wall with locally-sourced plywood, steel, random composite, or even planks of wood if I have to, and the structure would stay intact enough that I can then make it to a facility where I can rebuild as much of the camper as necessary. In terms of amount of disassembly, I don't see wood being any easier. Welded steel would be marginally easier (cut out the bent section and butt-weld in a new one), but steel that's thick enough to weld is too heavy to use as general frame material. Riveted steel - which can be thinner - seems no easier than riveted alu.
Back to "crumple zone" - pods8 has an on going build and ran some tests on foamular 600 foam - which gets to the role compressive strength plays when asking insulation to perform structural duties.
Post#22 -
http://www.expeditionportal.com/forum/threads/58926-POD-Homebuilt-foam-core-fiberglass-skin-pop-up-camper-build-thread/page3?highlight=pods8
I'd suggest bonding your alu to your insulation and seeing what impacts do. The weaker the compressive strength the greater the skin deformation. So you have to choose between a tougher skin with more forces being passed on to adjoining structural elements or a softer skin that might bend & crease enough to ask for occasional tedious cosmetic remediation.
Good homework for me. With three inches of insulation, I can afford to have the outer layer be less than optimal thermally, if the benefit is greater strength.
Some screenshots of the new design:
2x2x1/8" alu square tube frame where strength (floor) and impact resistance (edges and overcab area) are required; 2x1 wood everywhere else, mostly to keep the skin from flexing too much; 1/16" alu skin, riveted to the alu frame and screwed to the wood frame. The skin is not shown except in the cross-section, but it's very much structural, keeping the walls from becoming parallellograms, especially around the wheel arches. Interior design can remain somewhat of an afterthought, as far as structural integrity goes.
My concern with using wood for skin stiffening is that it has much lower thermal expansion than the skin it supports. Otherwise I'd use aluminum angle stock; 1x1x1/4" is slightly heavier, a lot more expensive, and at my guess neither as stiff nor as strong as 2x1 wood. Is this materials mismatch likely to become a problem?
. The mounts are tentatively soft rubber or even hydraulic-filled motor mounts, and the idea is so that the camper can move a lot, relative to the truck frame, without ever having to flex. But the big question is this: should I consider crash-worthiness as "flex", for the purposes of materials used, or can I get away with assuming the camper will be stiff, and that one big impact (rather than a lot of twisting and vibration over time) won't tear it apart much more than a steel structure?
