Hi, I’m going to be undertaking a project very soon, I have an LDV convoy van, a cab and chassis van. I am going to build my own coachbuilt motorhome on the chassis, however i used to own an aluminium cladded motorhome and have decided against these construction methods due to damp problems and also repairing damaged panels. I am going to use sandwich panels, foam core with fiberglass skin either side, vacuum bagged to keep the weight down. However, I’m not sure what foam to use, and what weight and weave of cloth to use. I had in mind polystyrene foam, as in the uk i can buy it very cheap from most bulding supplies in 8’x4’ sheets, and i would have to glass it with epoxy. I would like to do it in 25mm thick foam, guessing the only reason the older motorhomes had 50mm thick walls was to allow for the timber framework. i’m sure with composite construction i will be using that i wouldnt need such framework, the fiberglass and foam all taped on the joins etc would be rigid enough… so am i ok to think this would work ok? please tell me if its not as i do not want to get halfway through my build or worse my holiday and suddenly realise its wrong!! so main question, would eps foam and epoxy with no extra framework be sufficient, and second, what lay up would i need to use?? thanks if anyone can help!!!
Although we’ve never built a camper this way I think it would be best to try to replicate the construction method that was used previously since it was proven to work. Like you said, it wouldn’t be fun to find out this doesn’t work half way through your holiday so…better safe than sorry. I know that in the old fiberglass motorhomes they used fiberglass mat and in some areas they would use a knytex or vectorply type materials to add reinforcement. As long as you have a sound framework to build from you should be good with using these materials. Usually the body is constructed in a similar way to a boat and then attached to the structure so think about that too.
hi there, yes i thought about laying it up in a mould, as i would a boat, but i think due to the size it would be easier to bond the panels together in place. and i have researched caravan construction and there are some being made without framework, just foam and fiberglass sandwich panels bonded on edges. i guess internal furniture adds to the strength of the structure too…
While I have not done anything like this, our other facility has made several types of vehicle bodies and add-ons. What they did was a foam core box (5 sided), using several layers of 24oz and other glass skins, and then bonded the roof on as a separate panel. From what I understand, as long as you give a solid composite shell some shape, it wil be very strong (verses, just 6 flat panels together…this had some curves, and wheel well shapes, that add to the stength)
Also, I believe they used internally sparred foam panels (1.5" thick), something you might try to replicate. Do this by cutting the foam in sections, and adding 1-2 layers of glass in the cuts, wrapping it around one end of the foam (say, 2-3cm wrap on the top and bottom)…this will give the structure internal support.
Besides that, if you are just replacing the flat panels with composites, You should be able to get away with 2 layers of 24oz glass per skin. Weave doesn’t matter much really…If you can get a multi-axial stitched fabric, that would be ideal. if you go with a lighter cloth, you can lay the panels up with each skin having 0, 90, and 45 deg placements, giving the panel equal strength.
Also, make sure you close off the ends of the panel, and add hardwood/metal inserts to any place things will be bolted too. You can’t bolt to foam cored composites easily 
So I guess if i was to use 50mm foam, extruded polystyrene, and a few layers of bi-axal either side, and made a typical motorhome style shell, with crestomer bonded joins and taped them, and made the internal furniture out of veneer faced nidacore or something similiar, and bonded them to the shell, i would end up with a very rigid motorhome body wouldnt i? also its a bit of a daft question but its been a while since i did any boat building, but will i have to do the whole layup in epoxy because of the typ of foam i will be using? or is there some sort of polyester resin that wont eat into the foam? or an alternative cheap boat building foam i could use? i’m just thinking it could get really pricey doing the whole thing in epoxy… and also when i laid up the panels i could for the internal walls lay down wood veneer instead of gelcoat to give me a nice interior finish?
Balsa is relatively cheap, PU foam is cheap, EPS foam is even cheaper (but do buy a heavier version than 1 lb/ft3)
There are polyesters which are EPS safe.
I have a friend that did this. He used 25mm , 60psi EPS , and thin ply 3mm or 5mm and then v-bagged glass outer skin and inner skin. His was to go over a ute or pick up 4x4.
Simple design… Large pannels… 2x side , roof , back , bed over the cab. You get the idea.
Came out light and super strong and under his target of 1000kg. he also had inner bulk head’s from timber that where glass in and over and joined to the internal storage and cupboards to make then part of the structure.
At memory high load part where made from 2 pieces of 25mm eps.
Glass/ply/eps/ply/eps/ply/glass. Couldn’t bend them.
Also inner join’s where from carbon to help to take the flex out of the join. Also his was all done from epoxy.
Tim
ok so after a look for some eps foam in the uk at a higher density than the standard building insulation material, and no luck, i’m thinking maybe a polyurethane foam… any boatbuilders out there who can recomend a core foam that would be suitable? and also suppliers and prices would be handy… i remember when i used to work at a boatyard we made all the internal furniture from this green foam that was really strong, and we glassed it with polyester so i guess it was some kind of pu foam… i think it came in green and grey,different colours being different densities…
What you saw was PVC foam. Strong, but more expensive.
You found no EPS. Did you contact all companies on this list: http://www.eps.co.uk/
what is the brand name for that pvc foam? just want to check prices and stuff… and cheers for the link i’m checking that out now.
Airex, Divinycell, Klegecell, Core-Cell (SAN foam, but does not matter)