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Old 05-28-2011, 08:59 AM   #1
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Bad frame design?

Having started doing the drawings for a new frame using the existing frame for a point of reference, and including the aluminum monocoque for strength.... Errrr it looks like the original frame was about 20% under engineered.

Is this correct?
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Old 05-28-2011, 09:19 AM   #2
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I would say by more than that.
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Old 05-28-2011, 09:37 AM   #3
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That sounds about right. Care to show your work?

What is the frame? Nothing more than a place to mount the floor and a convenient attachment point for the wheels.
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Old 05-28-2011, 09:52 AM   #4
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If I went with a taller frame rail could I order axles with a lower -Deg to keep the wheels in the wheel well.

I'm thinking a talker major frame rail and possibly using boxed structural tube. Then new outriggers on that.
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Old 05-28-2011, 10:15 AM   #5
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When I did mine I took the path of least resistance.

I don't know about the boxed tube, but I went with the C5x6.7 (5 inch C channel 6.7 lbs per foot). It is standard stuff and any metal distributor will sell it.
Around here it comes in 20 and 40 foot lengths.

I used it for everything.. mail rails, cross members, and outriggers.
Total steel there was 2000 lbs if I remember correctly, and didn't include the tongue A frame thing which was reused from the old trailer.
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Old 05-28-2011, 10:51 AM   #6
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I don't know if it's really fair to speak of a "bad" frame design for this rig. After all, we are talking about a frame that has survived nearly forty years of jostling; that alone speaks in favor of the original design. (I'm speaking here as an RV park owner who has witnessed a sufficient number of of relatively new white-sided rigs whose long-term survivability is already in doubt.)


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Old 05-28-2011, 11:02 AM   #7
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The only thing bad about the design is peoples continuing desire to change it. It was designed to work with all the elements, not the strong desire to over engineer things. Leave it alone, copy it, but do not "box it in" or "beef it up" as your manly instincts are telling you to.
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Old 05-28-2011, 01:07 PM   #8
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If you feel you really have to make it stronger/tougher/heavier, you can use 5" box channel, rather than the 5" C channel it was built with. That's what Airstream did in the mid 1980's. I'd leave everything else pretty much as it is.
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Old 05-28-2011, 06:32 PM   #9
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Where could one find the wafer thin C channel that they made the chassis out of? The stuff that collapses like tissue paper when it gets rusty.

I think that it is pretty obvious that there were.. let's not say bad, but engineering design decisions that seem not so good.

Here are a couple that I have run across personally.
1) Side underbelly wrap overlapping shell. (water travels down, not up)
2) Plywood floor at doorway breaks between outriggers and there is no support under the break or under the edge (under the door). Really? Really.
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Old 05-29-2011, 07:54 AM   #10
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if you design a skyscraper using steel that is too rigid, the first good wind and it will snap right off. Engineers take these things into account. Just because it is thicker, does not make it stronger. The frame of an airstream is designed to flex and curve with the road. The typical male thought is the steel needs to be thicker and stronger. That is why many of us are not engineers, we follow instincts that are wrong.
DanielB, permission to speak freely? 1974 was a pretty dank year for just about everything built from houses to cars and everything in between. The issues you state are very common of your era. The ability of the steel to withstand oxidation has nothing to do with engineering, it has to do with a food company not wanting to spend for the good steel alloy. Hey, it was the 70's, that was when everything became disposable, even Airstreams.
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Old 05-29-2011, 08:19 AM   #11
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Quote:
Originally Posted by 62overlander View Post
1974 was a pretty dank year for just about everything built from houses to cars and everything in between. The issues you state are very common of your era. The ability of the steel to withstand oxidation has nothing to do with engineering,...
I remember having several of Ford's "biodegradable trucks" in that time. When I went to get in one every morning, I'd do a walk-around to make sure nothing vital had fallen off overnight from rust. Of course, the bright side of that is I learned how to apply Bondo...
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Old 05-29-2011, 08:59 AM   #12
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Quote:
Originally Posted by 62overlander
The typical male thought is the steel needs to be thicker and stronger. That is why many of us are not engineers, we follow instincts that are wrong.
I looked for the testosterone multiplier function when doing the finite element analysis, but was unable to find it.

I agree, build to safely suit the need within a given structure lifespan, but not allowing any safety margin for structural degradation from rust, stress, etc , and then rolling parts out of 70's chepo steel is not a blueprint for total success.

Modern A36 or A500 structural rectangular tube is flexible enough although it will have less ability to twist and fold on it's weak axis.

I was in no way putting airstream down with the title of this post. What was acceptable practice then is not always acceptable now, so when I put her back together she will have a better chance of outliving me
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Old 05-29-2011, 10:20 AM   #13
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Quote:
Originally Posted by 62overlander View Post
DanielB, permission to speak freely? 1974 was a pretty dank year for just about everything built from houses to cars and everything in between. The issues you state are very common of your era. The ability of the steel to withstand oxidation has nothing to do with engineering, it has to do with a food company not wanting to spend for the good steel alloy. Hey, it was the 70's, that was when everything became disposable, even Airstreams.
It all good. The point is that Airstream engineering is not perfect.
I chose C5x6.7 because it was the best choice that I could reasonably make. I posted many times trying to get information on what to use for the chassis and the only thing that was actually helpful was a guy that told me about C56.7 and C5x9 steel.

Instead of "don't do this and don't do that", how about "do this.."

Use this steel.. you can find it here.. this is good because..
That would be positive and helpful.
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Old 05-29-2011, 10:22 AM   #14
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You did good! C5 6.7lb is the default structural material for trailers in your weight range

BYW... anyone who is doing anything in tube frame this is a really good product

http://www.eastwood.com/internal-fra...ay-nozzle.html
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