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Old 01-09-2013, 12:02 PM   #1
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The Perfect AS Frame Design

With all the reports of the 22ft Airstream Frame failures and the other frame related problems, I thought I would take a stab at designing the ultimate frame.

The current frame design is not very good. The outriggers are not well connected to the shell and they have very little strength. The shell and the frame don't really support each other that well.

What would work much better would be a perimeter steel channel that is attached to the aluminum C-channel at the bottom of the aluminum skins. This C-channel would follow the contour of the floor of the trailer bottom all the way around. There would be outriggers or ribs that go from one side of the trailer to the other. There would also be two main beams in the same location as the current frame design. The frame outer C-channel would be connected to the skin bottom C-channel with bolts every few inches. The two main beams would not have to be as strong as the current design because the new frame would actually be structurally attached to the shell. The outriggers would be connected around the perimeter and would not bend when loads are applied to them. Bending the steel channel to the shape of the trailer would be the hardest part. It would also change the contour of the side and corner wraps to some extent. I suppose you could bridge the current outriggers together with cross braces as close to the walls as possible and maybe add more outriggers. This type of frame would be strong enough to support the shell even if the back section rotted off. I will make some concept drawings when I get a chance.

Those of you replaceing an entire frame might as well replace it with something much better and something that actually has some basic structural design ideas incorporated into it.

Perry
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Old 01-09-2013, 12:25 PM   #2
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Draw a picture.
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Old 01-09-2013, 12:40 PM   #3
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Wat about the axles?
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Old 01-09-2013, 12:42 PM   #4
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Originally Posted by perryg114 View Post
With all the reports of the 22ft Airstream Frame failures and the other frame related problems, I thought I would take a stab at designing the ultimate frame.

The current frame design is not very good. The outriggers are not well connected to the shell and they have very little strength. The shell and the frame don't really support each other that well.

What would work much better would be a perimeter steel channel that is attached to the aluminum C-channel at the bottom of the aluminum skins. This C-channel would follow the contour of the floor of the trailer bottom all the way around. There would be outriggers or ribs that go from one side of the trailer to the other. There would also be two main beams in the same location as the current frame design. The frame outer C-channel would be connected to the skin bottom C-channel with bolts every few inches. The two main beams would not have to be as strong as the current design because the new frame would actually be structurally attached to the shell. The outriggers would be connected around the perimeter and would not bend when loads are applied to them. Bending the steel channel to the shape of the trailer would be the hardest part. It would also change the contour of the side and corner wraps to some extent. I suppose you could bridge the current outriggers together with cross braces as close to the walls as possible and maybe add more outriggers. This type of frame would be strong enough to support the shell even if the back section rotted off. I will make some concept drawings when I get a chance.

Those of you replacing an entire frame might as well replace it with something much better and something that actually has some basic structural design ideas incorporated into it.

Perry
You may be on to something innovative,there is definitely a demand for improved frame function and strength.(necessity breeds invention)
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Old 01-09-2013, 12:57 PM   #5
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It is the same concept as building a foundation for the floor of a house. You start with a perimeter of wood and then fill in the interior with joists and then you attach the wall to the perimeter. The way Airstream built the frame is like putting the perimeter 4 feet inside the walls then put the house walls on top of the plywood floors.

Perry
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Old 01-09-2013, 02:20 PM   #6
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Who needs axles?!?!


I think Andy has stated many times that axles and running gear minimize damage to any part of airstreams save for hailstorms, trees, rocks etc. I'm sure there are a few who do take care of their AS's and still experience frame issues but the majority are probably those that tow for years and only replace brakes and tires as standard maintenance. There are plenty of ways to prevent frame failures but I would like to hear from those that did everything they're supposed to do from balance running gear to proper axle replacements etc that still had frame failure
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Old 01-09-2013, 02:22 PM   #7
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One phrase that shouldn't be in an airstream owners vocab is " well I didn't know that!"
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Old 01-09-2013, 02:48 PM   #8
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Quote:
Originally Posted by perryg114 View Post

The current frame design is not very good. The outriggers are not well connected to the shell and they have very little strength. The shell and the frame don't really support each other that well.

Perry
I'm not saying this idea does not have merit. And I'm not saying just because this is the way it has always been done is the way to go.

But with a circumferential frame a failure of a cross member would be a total failure leaving that crossmember hanging by one end. With the traditional frame a cross member failure is only a partial failure, confined to either the end or the middle with a large percent of the load still supported by the frame.

With a traditional frame the axles are directly supported by the frame. With a frame that goes around the circumference the axles would need to be supported by some other indirect method, unless a new type of axle was designed for the purpose.

I would ad this disclaimer. Please don't get the idea that I am some kind of engineer or in any other way know what I am talking about.
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Old 01-09-2013, 03:11 PM   #9
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Having built equipment trailers on and off for the last 38 years, I can tell you that the main frame rails need to be where the axles mount. You want the distance from the mount, on the axle, to the hub face to be as short as possible because it reduces the leverage on the axle beam.
I would use structural channel instead of rectangular tubing for a few reasons. It is cheaper, it is strong, and it is easy to paint for corrosion resistance. You can't get into rectangular tubing to paint it. The other thing I don't like about rectangular tubing is that it has a tendancy to warp more. Structural channel is a little harder to "fit" because of the angled flange, but once you are used to it you can make the cuts quickly.
I would use structural channel cross members and outriggers. I would use thin angle iron (1/8") to bring the cross members up flush with the top of the frame. A person could run a perimeter angle iron, for additional strength. Building a frame isn't hard, it is just time consuming, once you know what you are doing. The key is to have a good jig to make it easy to build.
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Old 01-09-2013, 03:32 PM   #10
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I'm not sure if equipment trailers share monocoque construction but I firmly stand behind Andy that over strengthening the frame is asking for trouble. Perimeter reinforcement might work but trust that your shell will do all the work and maintain your trailer in all aspects is the best preventative medicine IMHO
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Old 01-09-2013, 03:55 PM   #11
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It is my understanding that a somewhat flexible subframe is what keeps the skins from twisting and kinking. Building a strong frame and then attaching it to the unchanged "bubble" seems like a problem.

JD
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Old 01-09-2013, 04:03 PM   #12
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It is my understanding that a somewhat flexible subframe is what keeps the skins from twisting and kinking. Building a strong frame and then attaching it to the unchanged "bubble" seems like a problem.

JD
I would think it is the other way around.

If the frame is rock solid there is nothing to cause twisting and kinking on the coach.( I suspect Inland Andy may differ) But the frame would be heavier. I think the ability of the moncocque to stabilize the frame allows a lighter frame, reducing the overall weight. Airstream frames are quite light and you would not want to use them for anything without additional support. In this case provided by the shell itself.

Maybe we are both right.

I once owned a 62 Overlander that went around the world with Wally. In India a 2nd pair of rails were added in preparation for the almost impassable roads in Afghanistan. No sign of any coach damage.

Again, just my amateur impression.
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Old 01-09-2013, 04:31 PM   #13
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Everytime some states that a stronger frame is not needed, they usually state that a stronger frame would ruin the aluminum shell. I have yet to have any one of them answer one simple question. How did Avion do it without ruining their shell?

If I had a perfect 22' shell, I would have a frame built for it in about a week. It would take another 3 weeks to receive the ordered axles, install all the mounting tabs for the tanks, etc, and paint it. I seriously doubt that I would have an issue with the trailer when I was done.

No, equipment trailers are not Monocoque construction. They need to hold the entire weight of the backhoe, or whatever equipment is going on them, and they need to be able to run off paved roads to construction sites without falling apart. In that world, there are no excuses.

I would also consider building the trailer with two Flexiride Torsion axles built with an equalizer like a leaf spring trailer works. I think that I would end up with the best of both worlds. One of the advantages to the rubber torsion axle is the axle beam can be engineered to be part of the frame structure. In this case, it wouldn't be but it would have a lot more wheel travel and each axle wouldn't have to be overrated.
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Old 01-09-2013, 04:46 PM   #14
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I do see the points above.

As for heavy equipment trailers, the frame does carry all the weight. And, they weigh as much as the cargo a lot of the time. They do flex, and they crack and break.

I am not saying the AS frame cannot be improved. The folks that build them are in it to make money and costs play a huge part in how the trailers are built.

However, I do think that a if one bolts the bubble hard and fast to a backhoe trailer, one should not be surprised if a trip down a rough road leaves a kink or two on the shinny part.

Regards,

JD
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