RV Shell Construction: Aluminum, Steel or Composite?

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I am planning on purchasing a new 26' bed gooseneck flatbed trailer and then build a custom RV on top of the trailer. I want to purchase the trailer rather than welding it myself because of safety reasons. The welds will be under considerable stress from the road and the pros are better equipped for that then I am as a hobbyist. Once I have the flatbed, the question will be how to build the rest of the shell. A little bit about me, I am a hobbyist, not a professional welder. Professionally I am a computer programmer. My hobbies are Blacksmithing and Fabricating of which I am far better at blacksmithing but then I do more of that. In fact I plan to take advantage of that to hand forge all the drawer pulls and other ornamental hardware in the trailer. When it comes to welding I do some TIG though I dont have a machine personally and I do a lot more MIG welding and I am decent at it though not a pro of course. (I have a couple issues I am working with but that is for another thread). So back to the trailer shell, Essentially I have several options on building the shell: Aluminum Tubing Frame on top of Aluminum Trailer: The pros of this are corrosion resistance and light weight but the cons are that it will be significantly more expensive for the gooseneck as well as for the actual 2" x 1/8" wall tubing (3.37 a foot). Since the tubing will be enclosed in the wall and not getting moisture unless something is wrong, I am not sure I am gaining much on the shell. The actual trailer base, though, will have exposure and could gain from being aluminum. Given that I am going to put 30k worth of appliances, furniture and other stuff in there, I wonder if I should worry aobut the base cost. On the other hand an aluminum Featherlite is going to cost me 15k which is a huge initial expense I cant spread out. Steel Tubing on Steel Trailer: Corrosion of the base trailer is a major CON to this one but cost is a major PRO. I can get a Steel gooseneck off auction for 2 to 5 grand and the tubing will be much cheaper per foot. In fact, perhaps I could downgrade from the 1/8" wall to 1/16th for the same strength as the aluminum. What do you think? 2x2x0.125 is 2.31 per foot and the 16 guage is 1.40 per foot. I dont think corrosion should be an issue because the tubing will be in the wall but if I want to use aluminum siding I will need to isolate them dilectrically and that could be a pain. Aluminum Tubing on Steel Trailer: I dont see where I would gain much except weight. The wall should be water tight and with steel covered with a protectant, it should fare just as well as aluminum inside. As for weight like I said I can perhaps use less steel than aluminum for the same strength. Steel Tubing on Aluminum Trailer: Here I gain the corrosion resistance on the base trailer and lose only weight in the camper shell. However, aluminum also flexes more than steel and so that could be a problem structurally when designing the camper shell. Again the trailer will set me back a lot more than the steel one. Composite Steel / Wood on Steel Trailer: In this design the steel would form the major structural pieces but I could screw in 2x2 Wood framing members to add extra support and provide nailing surfaces for putting things on the wall. This would reduce the weight and probably not compromise strength. The trailer would probably be more cell phone freindly. To build this I would build the walls on the ground then run the screws through the outside of the tube and into the wood member underneath. I would only worry about compromising the strength with the screw through the tube. So can anyone add light to my quandry here? I would appreciate any and all generously offered opinions.
Reply:You can use galvanized steel tubing and reduce the corrosion issue to a great degree versus plain steel tubing.  You can also have the steel trailer frame sand blasted and galvanize flame sprayed, which protects the trailer frame quite well.  This will be much cheaper than building the whole trailer from aluminum.Remember that plain steel tube, even when kept painted on the outside, will still rust from inside out, due to condensation inside the tube.  Using pre-galvanized tube resists this rusting from the inside out.  Then you only have bare steel where the welds are.  Coat the welded areas with cold galvanized spray before priming and painting the frame and the only area not protected is the inside wall at the welded areas.  For complete galvanized protection, you would have to have the whole frame hot-dip galvanized after welding, which is not very practical (and may be impossible) with something as large as a trailer frame.  Yes, you can use thinner gauge steel tube than otherwise equal diameter aluminum tube and end up with similar strength characteristics.  2:1 on aluminum vs steel material thickness is in the ballpark for equal strength.I would not do any framing or siding with wood.  You're just asking for rot later on down the road, should you have any kind of leak.  There are always leaks on rv trailers eventually.  Designing the trailer with as few holes/protrusions through the roof as possible would be a good thing.  Of course, wall holes/protrusions will leak as well.  Anywhere there's an exterior siding/sheeting seam, or a window, plumbing or appliance vent, access door, clearance light, etc, any kind of hole going through the wall or roof, is a possible place for water intrusion.  It will get in there eventually.Another way many camper/trailer manufacturers are building them is out of fiberglass, like a boat.  They build a top half tub and a bottom half tub and then join them together as one big shell, then cut holes through it for vents, windows, doors, etc.  There are no exterior siding/sheeting seams to leak with this type of construction, so at least half the battle against leaks is won by virtue of construction method.Last edited by DesertRider33; 10-21-2010 at 04:47 PM.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:Trust me when I tell you as an RV owner that fiberglass sounds great until you get into it. If even one little kid puts a hole in the wall, you have to replace the entire wall. They sound great on the surface but the reality is that fiberglass is more of a gimick. If I used 6061 Aluminum sheets for siding, I can easily still block leaks but without the need to replace the whole wall if damaged. Galvanized is an idea though we would have to wear paint masks when welding to avoid zinc poisoning and that isnt so cool. I don't know perhaps C Channel or I beam would be better then you dont have to worry about the inside. I don't know what you lose in strength doing that.
Reply:C-channel or I-beam for the upper frame would be way too heavy.  You do want to tow the thing down the road with a pickup, not a semi tractor, right?It's no big deal to wear a mask under your hood while welding galvanized.  Welding outdoors, if you can keep your head on the upwind side of the weld, most of the smoke goes the other way.  Lots of folks here have a phobia about welding galvanized, but to tell you the truth, it's just another metal I weld on a regular basis, no big deal.  Galvanized is welded all the time every day all over the world.  If you do use 16ga galvanized tube, .035 fluxcore self shield is a good wire to weld it with.  The berries don't stick to the galvanized nearly as well as they stick to plain cold rolled, knock off easily with hammer, if you leave the galvanized un-ground before welding.  If you grind it off before welding, which is a big waste of time anyways, then the berries stick to the ground area and don't want to knock off easily with the hammer, so then it's get out the grinder with sand paper wheel...On the fiberglass, I think you're thinking of the thin Filon sheeting they use in place of corrugated aluminum.  I'm talking about actual molded fiberglass, like boat hull.  Look at Bigfoot campers, and a few others that use fiberglass clam shell construction.  Holes are not easily poked through it like thin Filon and if it does get dented or cracked in a minor collision, you can repair it like any other fiberglass repair of a boat or surfboard.Last edited by DesertRider33; 10-21-2010 at 05:40 PM.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:Another option with fiberglass would be glass over a plywood core like is used with some boats. The epoxy/glass seals and waterproofs the unit, as well as adding strength. The plywood acts as a shear wall. If you want to skip the marority of worries about rot with plywood, get form ply or marine grade plywood. The 1/2" high density form plywood we use is stronger than 3/4" and is extremly rot resistant, even without any fiberglass overlay. We're talking about panels that stay outside in the weather for years, in many cases wet between the panels from rain and almost never dry, yet the plywood doesn't rot or delaminate like standard plywood will in only a couple of weeks. It's not cheap or light weight. A sheet of 1/2" weighs more than standard 3/4" by quite a bit..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Again the problem with fiberglass is that of repair and weight. It is much heavier than aluminum and if you need to repair it, that is a total PITA. As for marine grade plywood, you will have rot problems. Trust me that an RV is not a home. It is subject to what amounts to an earthquake whenever it is pulled. The RV dealers use a vacuum bonded structure that I couldn't replicate in my garage even if I didn't think it was a cheap crappy method. Really aluminum vs steel is the real question here. Aluminum is much more expensive but after looking at some steel trailers I am beginning to think it is worth it.
Reply:Here's a guy that has done what you are thinking. It looks good and he has a lot of data on the costs etc.http://www.chadsprojects.com/RV_Homemade_Trailer.asp
Reply:The question mostly boils down to money versus weight.  How much are you willing to pay to save weight and is the weight savings worth the cost.  The cost is not just the material.  Aluminum is more expensive to weld (filler and gas cost more, plus more gas used) and the equipment to weld it with is more expensive as well, if you don't already own it.  The learning curve for welding aluminum is also longer compared to steel, in both mig and tig processes.  You will need to do a good amount of practicing on scrap before tackling the project.The other consideration is corrosion resistance.  In my opinion, galvanized, primed and painted, or powder coated, steel will be about as corrosion resistant as the aluminum.  Aluminum does corrode eventually, it's not stainless.  The aluminum should be kept painted or otherwise sealed, especially if being operated on salted roads or near the ocean.  For ultimate corrosion resistance (and highest project cost), build it out of stainless...The next issue is strength/durability.  The way I see it, the welded steel frame, although heavier, has greater durability for the long run in a constant dynamic force application.  Aluminum eventually begins cracking with fatigue stress, sooner than steel gives out.  That said, there are many rv truck campers and some rv trailers built from welded aluminum tube frame and there are many trailers, outside the rv type, built with aluminum chassis, many in heavy use applications.Either way, regardless of the material choice, this will be a very interesting project and I hope you take lots of pics and give us updates as you go along.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:Buy an old B29 bomber, DC 3 or similar plane. Cut and paste as necessary. You get a hollow that is water proof, and very light, not to mention strong. There has to be a few old planes in the boneyards available for cheap!!!And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:Originally Posted by RojodiabloBuy an old B29 bomber, DC 3 or similar plane. Cut and paste as necessary. You get a hollow that is water proof, and very light, not to mention strong. There has to be a few old planes in the boneyards available for cheap!!!
Reply:Those old planes were built out of aluminum if I recall correctly and they seem to have lasted.
Reply:The old fuselage from a boneyard turned trailer body trick eh?  Or how 'bout parking a quonset hut on top of it?You just have to answer the tough questions for yourself. What do you want from the trailer? What will you settle on? We had a customer who wanted mirror finish stainless on a ramp door. It was hung in place with contact cement of some sort. What a blinding rig to get stuck behind on the freeway, huh? I have wondered the same things, as I have a possible similar project in mind. Sheets on box trailers are usually flat and riveted to the studs. They could be galvanized or aluminum, I am not sure. I would have to check them with a magnet. I have no love for filon, it looks nice and smooth, but it's heavy and labor intensive to repair. I would not build the framework from aluminum myself, simply because I hate aluminum work.  But I could. Most RV's are framed from 1x2 wood and staples. I suppose it is just frugality that drives the cheese factor up so high. Those wavy looking aluminum siding strips that run horizontally and inter-lock actually do work really well in the real world. Even though it is not as clean in appearance as flat sheets, replacement strips are readily available and they are fairly simple to replace if needed. It does take 2 or 3 people to install them, and they are extremely long and awkward during installation though. And how about the roof?  Attached ImagesCity of L.A. Structural; Manual & Semi-Automatic;"Surely there is a mine for silver, and a place where gold is refined. Iron is taken from the earth, and copper is smelted from ore."Job 28:1,2Lincoln, Miller, Victor & ISV BibleDanny