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Old 07-08-2013, 02:59 PM   #1
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Frame Skin Flex Question.

Well I've been lurking on the forum for a few weeks now. I will starting the restoration of my 70 ambassador in a couple months. Will be doing a frame off I suspect. I will be posting pics in my album later today so everyone can see the pics themselves. But for 1000.00 bucks I couldn't pass it up LOL.

My question I want to purpose to the experts is this. I plan on doing a spray on expanding foam for the insulation in the wall and floor. Now I know from reading it sounds like A.S. did not do this because there chassis is designed to "FLEX". So my goal is to eliminate the chassis "FLEX".

I believe the most flex is over the axles along the length of the unit. If I was to weld a 1/4" steel strap 4-6" wide down the length of the frame would this not alleviate the "FLEX"? I'm thinking 2 on the top one down the middle on the bottom.

I work in a cabinet shop that has C.N.C. capability so I will be making a 1/4" route into the bottom of the wood sub-floor. I also have another Idea on the wood sub-floor to alleviate the Rot issue. I plan on making an airstream my grandchildren can camp in with there family.

Please let the ideas fly on this one. And if this is discussed in a another post please provide me the link so I can check it out.

Thanks
Jonathan
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Old 07-08-2013, 03:55 PM   #2
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I believe I read a post from Andy @ InlandRV regarding the flexing of the Airstream frame and shell...by design. If you eliminate the flex, you will be increasing the amount of jarring that your frame and shell experience as you tow it down the road. This jarring will eventually loosen rivets, pop rivets....and generally shake the heck out of the Airstream. That was my take on it, anyway...
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Old 07-08-2013, 05:29 PM   #3
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An Airstream flexes mainly because the connection between the shell and the frame is weak. The only real attachment points are at the front and back of the shell. As a result of the weak connection between the shell and the frame you get alot of flex. This is not a good thing. I recommend a perimeter frame with the outriggers connected to each other along the perimeter. This would allow you to put bolts every few inches and would make the structure very rigid.

Perry
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Old 07-08-2013, 05:30 PM   #4
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Expanding foam is not the best in HMO I prefer prodex which is a better insulation for rediant heat. Foam board with reflective side for the belly.
These are just my opinions.
Cliff
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Old 07-08-2013, 05:34 PM   #5
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Put your strapping on the sides of the frame members to get rid of the flex, not on the top and bottom. Think of trying to bend a flat piece of steel over your knee. Now put it on edge and try to bend it. The major flex is up and down, not side to side. That is why I would weld the flat reinforcements to the sides of the frame.
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Old 07-08-2013, 05:46 PM   #6
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Originally Posted by ventport View Post
Put your strapping on the sides of the frame members to get rid of the flex, not on the top and bottom. Think of trying to bend a flat piece of steel over your knee. Now put it on edge and try to bend it. The major flex is up and down, not side to side. That is why I would weld the flat reinforcements to the sides of the frame.
I thought of that also but then I thought with the cross braces it would create sort of a stress skin type application. Think a slinky yeah the old toy lol.. Then stretch it and weld a rod down the top and bottom. You now have a ridged tube.
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Old 07-08-2013, 10:03 PM   #7
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Sooo... Let's go back where this thread started. You are not bothered by the flex that all Airstream's have, by design, or by mis-design, it is your desire to put expanding foam in as an insulation that has you worried about flex???

Well, it seems like I have read a few posters who talk of using expanding foam, but don't remember any concerns about flex. I have read arguments against expanding foam in automotive applications because it tends to trap moisture and aggravate the resulting corrosion. That would be my concern. I have to ask: what difference is flex going to make with expanding foam?

I would caution against trying to remove the flex from an airstream frame. If you look around very much, you will see all kinds of explanations of the "semi-monocoque" design of the trailers, but more importantly, I have seen examples of problems down the road, that may be the result of a replacement frame that is built too heavy/rigid, that causes the shell to buckle/oilcan at temperature, etc.

My advice--don't fix a problem you don't have. Try to keep your frame's design as close to original as possible, and maybe it will last another 40 years.
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Old 07-08-2013, 10:16 PM   #8
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Sooo...
My advice--don't fix a problem you don't have. ... and maybe it will last another 40 years.
I agree with this part...IMHO, spraying foam into the shell/frame is the beginning of the end for your 'Stream. Nobody in the future will be willing to dig out the mess to replace a snapped rivet or dented panel. Wiring issues? never mind, they are hopelessly encased in foam. Need to replace the AC drain line...oops...no way.

I tend to have a different view on the flexing stuff...'designed to flex' is completely different than 'designed to allow flex'. Airstreams are designed so flexing doesn't cause failure. The shell's cross section actually squishes and gets wider as it bounces down the road. If you are danged determined to add foam, pick something that allows for that.

I think spray foam is tossing $1 at a $.10 problem
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Old 07-09-2013, 07:39 AM   #9
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First thing that comes to my mind is trapping leaks.... your AS will leak. Spray foam just mencapsulates it and wont be able to fix and it will just sit wet...

I have mixed thoughts on flex level. I put some reinforcing to stiffen up a bit because it just seemed very weak to me, but wasnt trying to make rigid. Got new running gear to keep the ride soft.
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Old 07-09-2013, 09:03 AM   #10
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Thanks for all the replies. Perhaps I will just do some frame repair a little reinforcement and go with the foil bubble insulation and a ridged foam glued in. I will NOT put the std insulation that it has now back in.

As far as leaks I find it hard to believe you cannot get these leak free. I have had cars that have brush on automotive seam sealer in them from the 60's and its holding up fine. A couple of these have been 60's stock drag racers without solid cages. These cars have endured 30+ years of extreme chassis flex. This is what I plan to brush onto the seams from the inside.

I guess if I want to make it that ridged I should get a storage container and put wheels on it.

I will be changing the floor plan around to a rear bedroom. I cant wait to dig into this.

I will also throw out my plan for the flooring and see what everyone thinks of it on my next thread.
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Old 07-09-2013, 09:14 AM   #11
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You can get them leak free but it takes a while. They take constant maintenance to stay leak free.

Perry
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Old 07-09-2013, 09:14 AM   #12
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I dont think the problem is that you cant make these leak free, rather once you make it leak free, it will leak again somewhere else. I'm amazed at all the different places mine is leaking. I just have empty/ bare shell on now so everything is exposed. Surprised how many rivets leak. My guess is that its just a bit more "loose" from 40 years of flex??? Its always something though, so accessibility is key.
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Old 07-09-2013, 09:21 AM   #13
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On the strapping, it is the top and bottom of the beam that carries most of the load. The taller the crossection of a beam the stronger it is. It is called moment of inertia. That is why most of the metal in an I-beam is at the top and bottom. The 1/4" ribbon of metal is overkill as far as weight. I am not sure when Airstream started beefing up their frames but some time in the 70's they started using a box beam or two C-channels inside each other welded together. If you have single C-channel frame railer consider adding a second one facing the first one.

The rigid foam won't hurt anything and it will flex a little. You want the kind that solidifies hard so you can trim it. I used rigid RMAX foam board and silver duct tape to install insulation. This has the silver radiation barrier made into it.

Perry
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Old 07-09-2013, 10:43 AM   #14
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On the strapping, it is the top and bottom of the beam that carries most of the load. The taller the crossection of a beam the stronger it is. It is called moment of inertia. That is why most of the metal in an I-beam is at the top and bottom. The 1/4" ribbon of metal is overkill as far as weight. I am not sure when Airstream started beefing up their frames but some time in the 70's they started using a box beam or two C-channels inside each other welded together. If you have single C-channel frame railer consider adding a second one facing the first one.

The rigid foam won't hurt anything and it will flex a little. You want the kind that solidifies hard so you can trim it. I used rigid RMAX foam board and silver duct tape to install insulation. This has the silver radiation barrier made into it.

Perry

I think the foam board it the rout im going to take.
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