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Old 01-23-2007, 09:57 AM   #1
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Box frame swiss cheesing

A major problem with box frames is rust from the inside out due to moisture retention.

One way of solving or preventing this would be to open a series of holes in the box frame, to allow access to the inside for painting or POR15. Also, like hotrodders already know, swiss-cheesing a frame removes excess weight without (if properly done) a loss in structural integrity.

Has anyone done this to their AS? Is it a bad idea? Whaddya think?
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Old 01-23-2007, 12:03 PM   #2
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An Airstream frame is not boxed; it's a pair of open channels. Also, the lateral members and outriggers incorporate the idea you mentioned. In fact, an Airstream frame is quite lightweight and flexible, and I think it's rather clear that the ends are held up by the shell. It's a unibody, really. That's why rear separation will cause major problems if neglected.

The rust problem is mostly a consequence of having an insulated floor and an enclosed belly. These are good things, the latter being a benefit for aerodynamics. However, moisture does get trapped. A future plan of mine is to drop the belly pan, remove the old insulation, weld repair any frame problems, and treat the frame with POR 15 before installing new insulation and replacing the belly pan.

Stainless steel frames combined with rotproof plywood would be ideal. Otherwise, fix leaks quickly, and try to rustproof the frame somehow.
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Old 01-23-2007, 12:05 PM   #3
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In the yachting world they would be called "limber holes". I think it's a great idea for someone else to try, mine is already back together.

I know that Nick Crowhurst is working on an idea for a 'car' that would be pulled through the frame tube with spray nozzles for applicating (muw?) a rust-proofing compound.
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Old 01-23-2007, 12:25 PM   #4
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Because of the AS frame construction as two running channels, limber holes would not be so great an idea. Whenever you are removing material to save weight there is a trade; weight vs. strength. The amount of true weight saved vs. the amount of frame integrity doesn't seem to work out. Good idea but the math isn't there. The weight and strain on the frame is constant along with the flexing not allowing the holes. Good idea, but not for this application.
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Old 01-23-2007, 02:51 PM   #5
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Quote:
Originally Posted by markdoane
I know that Nick Crowhurst is working on an idea for a 'car' that would be pulled through the frame tube with spray nozzles for applicating (muw?) a rust-proofing compound.
This is described in this thread:
http://www.airforums.com/forum...-rot-2607.html?

I aim to do the final spraying in February, when I get back to my base in North Florida. On my Excella, the main frame members are full box sections. Inspection with the fiber-optic scope reveals pitting corrosion, but not yet flaking, on the inside of the box sections.
Nick.
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Old 01-23-2007, 03:41 PM   #6
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Quote:
Originally Posted by nickcrowhurst
... Inspection with the fiber-optic scope reveals ...
I'm sorry, but THAT is just too cool.

Karma to you, Nick, for non-destructive evaluation.

Tom
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Old 01-23-2007, 09:51 PM   #7
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My 60 tradewind has a boxed frame on the main members as well .
from the front tongue to the rear bumper .the crossmembers have the
common oval holes as most do .

Scott
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Old 01-24-2007, 06:06 AM   #8
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Box frame swiss cheesing.

Quote:
Originally Posted by Pinecone
A major problem with box frames is rust from the inside out due to moisture retention.

One way of solving or preventing this would be to open a series of holes in the box frame, to allow access to the inside for painting or POR15. Also, like hotrodders already know, swiss-cheesing a frame removes excess weight without (if properly done) a loss in structural integrity.

Has anyone done this to their AS? Is it a bad idea? Whaddya think?
The design of AS frame is marginal at best. As someone else has pointed out, it is a uni-body, with the shell which is attached around the outer perimeter to the frame. Most of AS frames were made of formed C channel.
Cutting holes in it, would not accomplish anything since the sides of the frame are accessible once the belly pan is removed. Holes cut in the frame would cause the frame to collapse by reduced lateral integrity. The lower portion of frame would buckle allowing the frame to sag, thus distorting the shell and bringing disastrous results. In a box frame configuration, there is not much that can be done once the rust has developed. The only thing in my mind would be to dry the frame inside with forced hot air, lift the tongue of the trailer up high and pour Rigid Closed Cell Urethane Foam in liquid form, and let it expand back out through pour hole. Providing that the frame is solid with no indication of any weakness or sag, the urethane should withstand a reasonable flex. This application should slow down the corrosion by eliminating air access. We have never done it on AS frames, but such method proved very successfully on painted utility trailers, which are very rigid to begin with. Second and most troublesome issue in rustproofing the AS frames, is the top of the frame where the floor rests on it. This contact area is known to collect moisture and produce rust, because moisture always tries to bridge minute gaps between the materials, and those spots are most difficult to dry out . Unless the shell is lifted from the frame, there is no way of rustproof it. As you can see, there is a number of issues to be addressed in rustproofing. We are restoring a 1973 26' Argosy and have opted for a new Stainless Steel frame for that reason, as well as totally sealed aluminum coated plywood floor. This however is not within the reach of most AS owners. I am fortunate to have a complete fabricating shop and have the clout to purchase SS tubing for $1.50/lbs. Since I needed new axles anyway, it did not make sense to me to do a lot of useless work to a original rotted out frame, so new SS one was built. With steel frames, equalizing temps inside the belly pan to those of outside temps, is the best way to slow down the frame corrosion, and that means providing quick and ample air exchange within the belly pan so that the inside and outside temps do not differ much at any given time. It is the collision of two opposite temps on the belly pan surfaces that produces the moisture. Thanks, "Boatdoc"
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Old 01-24-2007, 05:22 PM   #9
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Quote:
Originally Posted by Pinecone
A major problem with box frames is rust from the inside out due to moisture retention.

One way of solving or preventing this would be to open a series of holes in the box frame, to allow access to the inside for painting or POR15. Also, like hotrodders already know, swiss-cheesing a frame removes excess weight without (if properly done) a loss in structural integrity.

Has anyone done this to their AS? Is it a bad idea? Whaddya think?
You are 100% correct about frames rusting from the inside out!

I think I would E-Coat (Electrocoat) the frame. This process involves dipping the frame in various baths for coating. This will coat the interior as well as the exterior. It is tough - tough enough that they use it on the Military Hummer frames. We have an operation in town that does this for a lot of RV frame manufacturers.

I would not drill additional holes!

Best Regards,
Henry
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Old 01-24-2007, 11:12 PM   #10
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Thanks all

Maybe that's the best solution (other than full stainless), the full frame acid dip. I know that they do that to classic car bodies to remove every trace of garbage from every nook and cranny.

Would powder coating reach the nether regions, or is it limited to places that are easily accessable? And, is powder coating a frame prohibitively expensive?
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Old 01-25-2007, 09:58 AM   #11
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Right on!

The acid dip is the first step in E-Coat.

Small (1/4) holes would need to be drilled in enclosed box tubing, about every 18" or so - to drain the acid and allow E-Coat penetration.

From my experience - this process penetrates the tight enclosed areas far better than Powder Coating. You are actually dipping in baths rather than trying to spray powder into areas.

I hope this helps!

Regards,
Henry
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