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Old 06-29-2004, 06:30 PM   #1
Rivet Master
1973 31' Sovereign
Portland , Oregon
Join Date: May 2004
Posts: 1,245
Images: 22
Inner skins - just how structural?

A structural question...

I know that my 1973 31' Sovereign has monocoque construction where every aspect of the frame and body contribute to the overall strength. I am also aware that there have been discussions in the forums about the notion that the inner skin contributes to the overall strength. Having just removed all the inner skins on my unit (for various reasons) and observing how few rivits there were I am wondering just how much the inner skin does contribute to the overall strength. I can also understand how the rear end of an AS could indeed separate from the body if the connections from the frame through the floor to the body degrade.

So here are some questions for discussion:

1.) Is it reasonable to think that the main structural issue that the inner skin would contrubute to is keeping the ends of the frame from sagging relative to the middle?

2.) Is it also reasonable to think that this is more of an issue at the back end than at the front - in light of the fact that the front is supported by the towing vehicle or jack stand?

3.) If the above is true would it make sense structurally to add some sort of suspension cable on each side of the trailer? On each side I am picturing a vetical member (steel I suppose) perhaps 4 feet tall and centered over the wheels. I would then picture a cable with a turn-bukle for adjustment going diagonally from the top of the vertical member to the end of the rear most cross member and one to the front most cross memeber. I would then picture some re-enforcement of these cross-members to better transfer the load to the frame main channel.

Would this approach work?
Could it eliminate the need for the inner skin to be considered structural?

Could this mean I could put light-weight wood paneling (maybe even 1/8" thick) on the inside of my trailer without re-installing the aluminum first if I wanted to go for that effect?

What do you think?


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Old 06-29-2004, 07:42 PM   #2
Rivet Master
Join Date: Mar 2002
Posts: 1,474
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I am not an engineer, so keep that in mind.

My suspicion is that the inner skin adds stiffness by keeping the ribs rigidly parallel. Without the inner skin the torsional effects would allow the ribs to twist causing both fatigue problems and unwanted movement in the outer skin.

If you cut through an Airstream at, say eye level, from end to end and looked straight down into the walls you would see each pair of ribs along with the skins forming a box beam.

Having said all that, I guarantee that if you remove the inner skins and do not support the frame along its length the ends will droop.


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Old 06-29-2004, 08:06 PM   #3
Rivet Master
LOST , Hawaii
Join Date: Jun 2002
Posts: 2,193
I think it would be easier to panel over the skin. But before you get too far get a sheet of the paneling you want to use and try and form it into the curve; it is not as easy as it looks. I broke a couple, even made a buck to form them. That worked if I took a couple of days to slowly pull it down. I was worried about long term, decided against it.

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Old 06-30-2004, 09:11 AM   #4
Rivet Master
Airstream Dealer
Inland RV Center, In's Avatar
Corona , California
Join Date: Feb 2002
Posts: 16,501
Images: 1
The Airstream trailers method of construction is called "semi-monocoque."

Monocoque means "load bearing shell."

Therefore the interior metal is just as important as the exterior metal.

If in doubt, remove the interior metal, install luan paneling, travel with the coach, and watch the inside fall apart, quickly.

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Old 06-30-2004, 09:23 AM   #5
Rivet Master
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1967 26' Overlander
Huntsville , Alabama
Join Date: Dec 2003
Posts: 2,893
Images: 2
Originally Posted by Inland RV Center, In
...Therefore the interior metal is just as important as the exterior metal.

If in doubt, remove the interior metal, install luan paneling, travel with the coach, and watch the inside fall apart, quickly.
I agree.

Cleaning every square inch of my interior at one point afforded me the opportunity to examine the rivets which hold the inner skins in place. While probably due to running gear imbalance, I was surprised at how the skin had really worked on some of the rivets - They were loose, and occaisionally missing. The bad rivets were easy to identify because of the black luminum dust/dirt left by the skin stressing the rivet.

I would recommend leaving the skin in place, and putting any other desired coverings over it.

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