How to make a good camper shell?

[Ndmker]

So my thoughts on a good camper shell have been changing and evolving.

First I started with a steel and aluminium frame and was eager to cover it with aluminium sheet. But I resigned because of the weight of steel, the lack of flexibility of aluminium and because of the softness and weight of aluminium sheets (aluminium doesn't bounce back so every hit by a branch leaves a dent). Oh I forgot to mention that sticking together alu sheets is not so elegant and prone to create leaks. Plus it would cost me a lot of Sikaflex. In other words a metal cage covered in alu is a dumb idea, although the easiest to make.

Then I thought about epoxy resin and carbon fiber which have a perfect weight and strength, but the cost is terrible so it is not a realistic idea. Even if I ordered carbon fiber mat from China, the amount of epoxy would still create a huge cost.

Finally my thoughts went to fiberglass. My first thought were boats, which have quite thick layer of fiberglass, have good water resistance but are terribly heavy. After some thinking I realized that a boat needs to have a thick layer of fiberglass because it hits the water with great force, so all the surface needs to be equally strong. But this is not the case with a camper shell, which is in touch with air, not water. What's more, the camper shell walls cut through the air instead of hitting water head-on, as is the case wit boats. So my next step was to think of making the camper shell with a few layers of fiberglass: like making the wall only a few millimeters thick and not 2cm. But the shell would be wobbly and prone to cracking, there would be no possibility to hang anything on the walls etc.
So I've been thinking about making some kind of ribs between fiberglass layers, because making a tubular structure gives a lot of strength. I've been looking for some good materials for ribs but couldn't find anything hi-tech.

Well, XPS (extruded polystyrene) is lightweight and good as an insert which would be sandwiched between fiberglass layers. But I am afraid that the resin might melt it and it is still too soft to put a screw in it.

Eventually my thoughts stopped on wood. Making a fiberglass shell with wooden ribs in between layers. The wood would be soaked in resin so it would mean good protection against moisture, it is flexible and doesn't crack, it it relatively lightweight. The downside is that wood seems 'medieval' to me and I am not sure if it is a good building material for such reinforcements/ribs. So this is my first question.

The second questions is how to connect together the pieces of my camper shell. For example the roof with the walls. The diagram below shows my general idea of the ribs which would be covered with fiberglass. On the right I have drawn a hypothetical connection of the roof and a wall.

Should the mating surfaces covered with gelcoat of just fiberglass and resin? I will use sikaflex 221 on the mating surfaces.
Should I use only Sikaflex all along the mating surfaces? Or maybe I should also attach some plates to the vertical surfaces and fit them with screws through fiberglass to the wooden reinforcements. My thought is that Sikaflex will distribute the forces evenly along the whole frame, whereas a few plates with screws would concentrate torsional forces in just a few places. But somehow it seems odd to have my camper walls and roof together by just some sticky goo :-)

Please comment on my ideas. Maybe there are some other building materials or connection methods?

camper-shell-fix.jpg

[trailrunner]

Well, there is no reason you couldn't just fiberglass the pieces together as a solid unit. Wood is hardly medieval and when used appropriately is exceptionally strong. From my reading, usually when people build fiberglass structures they build and skin the frame with thin marine plywood. Those boats that you talk about are usually molded completely from fiberglass which is a different process and relies almost solely on the fibreglass for strength which leads to the increase in weight. this http://en.wikipedia.org/wiki/Stitch_and_glue is a lightweight technique people use to build boats. If you search around the forum you will find some builds that successfully used fibreglass and produced a relatively lightweight structure.

[herm]

wood or rigid foam is used in most boats for the stringers and anyplace anything is attached to the hull. I would stick with wood ribs as they will be easy to mount things to and provide a lot of strength and are fairly light weight. you could use basswood as some boats do to decease your weight even more.

[Treenail]

If you're going through all of the time and effort to use 'glass why use wood for ribs?

Wood is problematic in camper construction. Even the best construction methods can't isolate it from water. Water+wood=decay.

I'm an arborist and a woodworker, I know a little bit about wood decay.

Use foam blocks for ribs. You can make a foam hot cutter easily. Or use a sharp knife.

Thick wall plastic tubing can be used to form 'studs' too. Designed correctly you'll have plenty of strength in the 'glass/plastic to support interior finishings.

Another great source of design info:

http://www.tnttt.com/viewforum.php?f=55

Tom

[pods8]

So my thoughts on a good camper shell have been changing and evolving.
Well, XPS (extruded polystyrene) is lightweight and good as an insert which would be sandwiched between fiberglass layers. But I am afraid that the resin might melt it and it is still too soft to put a screw in it.

Polyester resin will melt it, epoxy won't. Browse my thread if you want to look at some epoxy/fiberglass over XPS building.

[DiploStrat]

You might want to review this thread and compare notes with the builder. He specifically chose wood/fiberglass because it allowed him to maximize his own skills, allowed complete flexibility on tank and cabinet shapes, and (and this is the point I really like) because all of the internal fittings - seats, cabinets, tanks, etc., serve to reinforce the external shell. Very elegant monocoque construction.

[Ndmker]

Many thanks for the replies. Here are some of my thoughts:

1. I have read about stitch&glue but I still think it is way too heavy. My construction is supposed to be a shield from the elements and only the ribs will perform any form of support for furniture and other stuff. I think that building the whole structure with plywood (stitch&glue) plus covering it in fibreglass makes all the work useless. My goal is to keep the weight of the vehicle under 3 tonnes and I am already starting with a heavy chassis.

2. I don't know what to do with the opinion of Treenail. I don't know why he, as an arborist, is so pessimistic about wood. Maybe there is some more knowledge, he could share with us. From what I know, almost nothing can happen to a block of wood entirely covered in fiberglass and resin. There will be no piece of wood exposed to air neither inside or outside the camper, so where could the problem be? I might as well use waterproof plywood for the ribs. I have some pieces of plywood which have been left outdoors half a year ago (rainy autumn, freezing winter, and now rain again). They are still straight and with no swelling. So sealing such plywood ribs between fiberglass looks like a perfect solution at least for 20 years. But maybe Treenail will comment, which would be great.

3. The more I think about this, the more I am willing to go for wood instead of XPS. Mainly due to strength issues. If I put a screw in XPS I can take if off by hand, but a screw in wood is solid. So any furniture I attach to XPS will actually rely only on the strength of fiberglass, but whatever I screw into wood will strengthen the whole construction even more by pressing furniture/fiberglass/wood against one another.

[Treenail]

What do you want your shell to do for you? Have you considered buying a shell and modifying?

Here's a Craigslist search tool that makes searching easy:

http://www.searchtempest.com/

If wood is kept dry or saturated with water, like dock posts under water, won't rot. The decay micro-organisms need oxygen and water to live. The best environment for the rougish fungis to live in is damp.

From what I've seen of camper construction wood will eventually get damp. If you could encase the wood in 'glass and then never break that seal you might be OK. It seems that mounting screws get set into the wood which makes an opening. Something that I had never thought about but someone on a camper-build forum shared was how a screw can act as a heat sink in the wood. This will cause tiny bits of condensation from the humidity in the air. Over time this leads to rot.

I wouldn't even consider using treated wood. More than likely you'd have to rip the wood to size. The center of some treated wood doesn't get the full dose so you'd have an opening that decay could enter. If I were using wood in this application I'd use cedar or even white or burr oak which are naturally decay resistant.

If I understand your design you're making the vertical channels as stiffeners. In that case whatever is inside won't add very much to the strength. If you used thick walled PVC you would have a material that could support some loads. The glass will too of course but it would have to be thick in order to support very much sheer load. Foam strips would add shape and glass would give the support if thats all you need.

[pods8]

3. The more I think about this, the more I am willing to go for wood instead of XPS. Mainly due to strength issues. If I put a screw in XPS I can take if off by hand, but a screw in wood is solid. So any furniture I attach to XPS will actually rely only on the strength of fiberglass, but whatever I screw into wood will strengthen the whole construction even more by pressing furniture/fiberglass/wood against one another.

If you were to use XPS I wouldn't be looking at doing ribs to stiffen things instead I'd just make the whole thing a sandwich panel. Separating two skins will take less fiberglass to get the stiffness you are looking for verse a single thickness of fiberglass. Also unless you have a mold to shape the fiberglass into you can just shape the XPS to the form you want. That's what I'm doing on my camper, but it is time consuming as a warning but aren't all these projects . I opted to embed wood in there for attachment points but I also plan to inject resin into screw holes that I do make to seal the wood fibers in the hole up the best I can (also I did use cedar in places I thought I might through bolt or have a large lag bolt in, both for rot resistance and weight savings). In general though I plan to surface mount as much as possible to fiberglass skin with adhesive/epoxy. If one really wanted to avoid wood for attachment points either buying some thicker FRP stock (or other plastic that bonds well to resin) or laying up some of your own to use as attachment points similar to wood would remove wood completely from the equation. I felt reasonably comfortable with wood because there are wood/epoxy boats out there doing just fine when they are properly constructed/maintained, neglect will ruin all the best laid plans but that is at the hands of the user.

Also I'm trying to build mine well for the long haul but devils advocate on the situation is I'll have either started a new version or overhauled it long before a previously mentioned 20yrs.

[DiploStrat]

Many thanks for the replies. Here are some of my thoughts:

1. I have read about stitch&glue but I still think it is way too heavy. My construction is supposed to be a shield from the elements and only the ribs will perform any form of support for furniture and other stuff. I think that building the whole structure with plywood (stitch&glue) plus covering it in fibreglass makes all the work useless. My goal is to keep the weight of the vehicle under 3 tonnes and I am already starting with a heavy chassis.

The assumption here is that you would be using two layers of relatively light plywood, separated by insulation of some sort. The structure would gain its strength from its complexity, not from the raw strength of any one panel. Those who work with metal frames and various plastic panels work the other way - the panels add no strength.

Thus, if you do it right, each window, etc., would actually serve as a reinforcement of the panel structure and reduce the number of ribs required. Similarly, any place a cabinet joins the wall, the cabinet framing replaces a rib. Watch the videos on Shachagra and you will get the idea. Probably the only place you would need to use heavy plywood would be the floor, and even here, I use a double structure with insulation. (And a few ribs.)

Good luck!