rigid foam insulation board for frame

[gazz]

one day i'd like to build an ultra light TD to tow behind a smart fortwo, max weight has to be 330 kilos i believe.

the chassis will be alluminium to keep weight down, simple rubber torsion suspension.

for the body, i was thinking instead of a wooden frame, laminated with ply for the sides and interior with insulation in the voids, how about...

That rigid foam insulation board, the stuff that usually has a foil coating on both sides, kingspan is one brand name, celotex another i think.

i was thinking of having a wooden floor board over the chassis, the wheels would be inside the body, to allow a sitting area when the beds not made,
A slab of the foam board bonded to the floor board, then cut the sides out of the same stuff, would have to have a frame around the openings and galley hatch, not sure about the roof yet, maybe a lamination of the thinner foam boards to curve it, all glued together with some construction type adeshive.

Now i know that sounds stupid, but i'm not done, once walls, roof, floor etc are built out of the foam board, i'd take it to a fiberglass place that builds plastic boats/kit cars/grp baths or what ever, where they use those spray guns on a hose that feed chopped glass mat and the resin down to the nozzle for spraying,

and i'd get the whole sub structure coated inside and out in fiberglass,

no idea how much this would cost to do, but i'm thinking the foam boards would provide some of the rigidity as well as the insulation, and the fiberglass would hold it all together and add yet more rigidity, and make it all watertight,

does this sound feesable? sort of like making a fiberglass shell body, but the mould is the rigid foam boards and they are sandwiched between the inner and outer fiberglass shells.

[Trackstriper]

gazz,

First off, welcome aboard. I think that you'll really enjoy this forum. There are lots of good folks who will be glad to help you with your ideas. Many of them have a vast knowledge base and I'm sure they will chime in on your post.

The subject of using foam sandwiched with fiberglass has certainly come up before. I think what you will hear is basically this: It won't be a light way to build. Certainly some home-built aircraft are constructed in a somewhat similar with glass skins over a foam core. But what you'll find for fiberglass will be a very controlled application of glass cloth and epoxy, perhaps vacuum bagged. The final finish probably requires a lot of primer and sanding to be presentable. The chopper gun method that you suggested is kind of a quick and dirty solution to making a product in a female mold. Check the backside of a bath tub/shower combination that was made with this method and see if that's what you want to have on either the inside or outside of your trailer. Probably not. This method is heavy for the strength provided. Working with wood or aluminum would be lighter if weight is a concern.

http://www.tnttt.com/viewtopic.php?t=7043

Regarding your frame, there have been discussions about steel vs. aluminum. Each has it own merits. A well designed steel frame would not necessarily be heavier than an aluminum frame. Aluminum won't rust but metal fatigue will be more of a problem. Welding an aluminum frame correctly will be the work of a professional welder who welds aluminum a lot. You can't have less than excellent welds on a frame. If I were to build with aluminum I'd most likely go with mechanical fasteners. Think aircraft construction....you don't see a lot of welded aluminum used structurally. Not that it can't be done if you want to. Since you have the equipment and ability to weld steel why not go that direction. There are some minimalist frames that use very little material.

http://www.angib.pwp.blueyonder.co.uk/teardrop/tear00.htm

Andrew's design library is a great place to start. Note the ultralight trailer chassis at the bottom of the page.

Hope this will be a good beginning to the discussion.

Bruce

[gazz]

thanks for that, knew there's be down sides,

i guess an alli chassis would need a lot of strengthening ribs and all that, so a minimalist steel chassis would be lighter,

if i build a body the traditiional coachbuilt motorhome way i doubt i'd get lighter... 1x1 pine frame, 3mm ply inside lining, polystyrene slab insulation between the frames, and thin alli sheet glued on the outside,

lots to think about, but i'm not planning the build for at least a year, but then again i may be bored one day and just decide to molish something out of steel, and it'll be a trailer frame