LukeH
Adventurer
Oh dear oh dear oh dear.
Your reactions to certain posts indicate that despite the title of the thread, what you’re actually asking is “is there someone out there who will validate my structure choice?”
It certainly won’t be me; your construction choice is based on false assumptions on maintenance and repairability; you have personal reasons too, but I won’t go into the questions on attitude. The method you describe is perfect for restoring an antique gypsy caravan, or a wartime railway carriage, but not for a camper that you’re going to really use.
I apologise if this offends you, try to make it to the end.
Victorian is right. DZLToy is right, even if he’s brought in his personal feelings which have obviously upset you.
And the RVs in question rarely see the kind of vibration that an Expo camper sees.
Your reactions to certain posts indicate that despite the title of the thread, what you’re actually asking is “is there someone out there who will validate my structure choice?”
It certainly won’t be me; your construction choice is based on false assumptions on maintenance and repairability; you have personal reasons too, but I won’t go into the questions on attitude. The method you describe is perfect for restoring an antique gypsy caravan, or a wartime railway carriage, but not for a camper that you’re going to really use.
I apologise if this offends you, try to make it to the end.
Victorian is right. DZLToy is right, even if he’s brought in his personal feelings which have obviously upset you.
Actually the price is the only reason, although the “it’s always been done like that, who the hell are you to change it” attitude has played a role.
And the RVs in question rarely see the kind of vibration that an Expo camper sees.
One more key point between the wood vs aluminum since he brought it up, an aluminum joint will fatigue and break well before a similar wood joint will.
QUOTE]
That’ll be why even Morgan have stopped using it, and also why the latest Boeing 777 has abandoned aluminium in favour of WW2 style canvas wood and wire construction.
Bad engineering is responsible for so many of these blanket statements that hinder progress. I didn’t know the latest mountainbikes were made of wood either. Come on.
But before I get onto glass fibre which, although it’s been around for over sixty years now, is obviously far too “new technology”, let’s have a look at your concept, its construction and its repairability (or not):
You have a base frame in steel, your foundations if you like. You have lots of short lengths to cut and weld together. As good and talented a welder as you may be, the engineer’s approach is to consider a structural weld as the start of a crack. At each weld you weaken the crystalline structure of the metal by heating it to near melting point and cooling it too rapidly. I doubt you have a heat treating oven nearby so they will remain weak points.
That’s not too much of a problem depending on the rest.
I don’t suppose you’ve found how to weld steel to wood, so you’ll either be bolting/screwing or bonding. If you’re bolting you waste 90% of the wood’s load capacity by concentrating the load on a small threaded bar; which you will then render inaccessible by skinning and adding furniture.
Let’s hope you’re bonding, in which case you’ve gone to the trouble of making up a nice socket into which the strut goes, and you’ll fill it with Sikaflex 252 or equivalent (the same stuff you’d use to stick panels together if you were to opt for a sensible construction).
So now your frame is Sika’d together, let’s think about covering it. Now we have a problem because the size you cut your aluminium skin is going to depend on the season, temperature and hygrometry of where you’re doing your build.
Yes indeed that’s why ALL those old wood framed campers and slide ins have folds along the length (I’ve owned two), it’s because wood changes, moves, expands with humidity.
And the nature of the grain and the way wood grows means there’s NO WAY you’ll be able to master every movement in your frame.
Unless you opt for a nice thick chequer plate style skin which will hold everything in place.
Let’s think of that option; you Sika up every frame member and stick the skin onto it. I’m sure you wouldn’t be foolish enough to want to screw the panels on without any bonding, all those holes to waterproof, all that movement of the wood underneath, with a screw in it trying to split it…
You’ve put the roof on and sealed along the edges; only the frame is constantly trying to move the roof up or down depending on where you’re on holiday; impossible to keep that edge watertight. So you need edging strips, which you’re going to bond on, with NO screw holes to leak and split the upper spars. The Sika will enable the frame to move the roof a tiny bit without breaking the seal, you hope.
With the frame held rigidly by thick flat plates the expansion and contraction of the wood has to go somewhere so don’t be too surprised if the sides bow ever so slightly in and out according to where you’re camping etc.
Your steel foundation is totally incompatible with the thick aluminium plate, the two metals have different coefficients of thermal expansion, so how you join them is again critical. ALL mechanical methods involve a hole just down where you don’t want one; All mechanical methods create an electrical connection between the two metals (I’ll get to that problem in a moment) and will work loose with the expansion and contraction of the different metals.
By a long measure the best means of attaching steel to aluminium in this circumstance is with a nice thick layer of Sikaflex, ensuring electrical isolation, free thermal expansion and a good seal. The absolute reference of a build blog for this method is Rob Gray’s WOTHEHLLIZATs 1 & 2; I very strongly recommend you read them.
About the whole electrical problem; you wand to go to cold places, unless you have spray foam insulation and miraculous sealing powers humid air inside WILL get to the skin, and into the steel tube where it can condense on the rivets, bolts or screws. The various metals form a battery with the help of water and your foundation will rot from the inside.
But that’s ok because you weren’t going to screw the skin on anyway. However the condensation will run down the inside of the skin and sit on the steel frame that forms your foundation. The puddles of water will still make the electrical connection between steel and alu, and the movement of the insulating panels will eventually wear through the paint on your steel bits, much slower if you Sika’d them in place but still…
After a few years use you’ll have little white pock marks around the bottom, just at the level of the top of the steel tube; corrosion from the inside. There are of course some pretty fancy zinc epoxy paints you can use to try and ensure that it doesn’t happen in your life time but it will happen eventually.
Great, it’s built and you’ve glued the insulation plates in where you can (where there’s not wood getting in the way); now you can skin the inside and put in your furniture. It weighs a hell of a lot because you’ve gone for the thick skin option, after all you wanted it to stand up to the beating of trail driving. The wood is a little more tolerant of you trying to split it by sticking screws in because it’s being held together by the Sika against the skin.
But what happens if you ding it?
You hit something and damage one of the wooden struts. You could strip out the furniture, rip out the interior skin and try to bang out the dent from the inside I suppose, but dismantling furniture isn’t really a field repair situation. And to get to that strut, so well glued you’ve got to do some serious damage to the finish.
So let’s try from the outside; but the Sika doesn’t peel. Somewhere in his blog Rob Gray points out that if you position a panel wrong with Sika, you’ve lost the panel because you will DESTROY it getting it off. SIKA is permanent if you’ve prepared the surfaces correctly. So to get at that strut in the roof or on the side you have to bend and ruin the skin in such a way that you really wouldn’t get it back in place to continue your holiday without leaks.
Heaven forbid your foundation frame cracks or bends, imagine having to peel the skin away (bent and wrinkled now) to weld the steel, only you’ve got the foam insulation and the inner skin to move out of the way; with foodstuffs and furniture sitting just inside the skin. And of course you paid particular attention NOT to run the power, gas and water lines around the bottom edge of your camper, because then you’d have to move those too!
In conclusion the idea that this obsolete construction method is field repairable is a total myth. If you bang it, the only way to get home without leaks is either tons of rigger’s tape or to drape a glass fibre mat over the hole and paint it with resin.
Which turns out to be how you would field repair a fibre glass construction anyway.
Now onto the panel version:
Your shape requires ten panels, that’s it.
OK the raw material might just cost more than a couple of bits of wood from your local DIY store, but if you add up ALL the bits and bobs you’ll need, tooling etc. I’ll wager that the cost comes out very close. Remember thick aluminium sheeting doesn’t come cheap either.
You don’t have to use custom made panels either; my box is going to be made out of panels that I’ve recovered from a damaged frozen goods semi-trailer that I found in a breaker’s yard, for very little money. Here it is:
I’m sure America has frozen goods delivery vans, that may have crashed, from which you could cut the box and trim it to size. Or
In fact the only advantage your construction has is that you can spread the cost gradually over the whole of your build period. Which raises the other point:
What are you worth?
The hundreds of hours your assembly costs you would be much better spend sorting out the inside, or actually travelling.
But you already know how to work with it!!!
If you’re confident enough to embark on a wood build, you obviously know which end to hold an electric saw, and you can handle a sealant gun. All you need is to know how to clean the join area and you have all the skills. You might, however, need a friend or two to help manhandle the panels into the jig you’ve just built.
If you don’t have any friends I’m sorry; but you may still need someone to help handle the alu skin panels.
Here’s a build that should help give you an idea.
And I can assure you that even if the bonding process squeezes out a lot of wasted Sika, you’ll still use less than with a wood and skin old skool build
Once upon a time you were unfamiliar with a welding torch too, and a mouse and all the fangled gadgets they have in modern cars, you’re never too old to learn new stuff.
And heavy, and slow, and uneconomical (does that sound like a superpower I know? ;-) )
So let’s ding it.
A great big branch has scraped down the side leaving an ugly gash.
With a can of builders foam you can fill the gaps where the foam’s been torn out. Once it’s dry you can cut it flush with the side. Sand the side and the foam so that it’s slightly recessed. Drape glass mat over the gash and imbibe it with resin. When it’s set you can go on.
Field repair done, and waterproof, with fewer tools too.
When you’re home you can sand it down and paint it up.
Yet again, my apologies if you take this as a personal affront, it is not meant as such.
I’m a consultant mechanical engineer, my masters thesis was on steel-composite race car chassis design, I’m passionate about expedition campers; Usually I’m pretty quiet about these design questions even if it grates to see post war construction methods maintained when the “new” methods are already very very tried and tested. Don’t get me wrong I’m not knocking all the builds that I see here on the forum, they’ve been done with passion, time and fantastic energy.
But that energy and time could have been much better spent elsewhere.
It’s just that your thread has inspired me to finally open the floodgates.
I guess I’d better duck now ;-)
