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Old 01-09-2013, 04:46 PM   #15
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I somewhat agree but I think between curtis wright, spartan, avion and airstream they all had their own engineers and designers that thought along the same lines but probably didn't come to a lot of similar conclusions. Obviously it takes real world experience. I think Daniel B (might be mistaken) did his 31' frame using C5 x 6.7 channel which is structural and weighs a ton over 31' length. Once his trailer is done and he puts 10,000 miles on it will we have an answer or a good idea on a super strong and rigid trailer. I'm in shell off mode right now and my 100 lb girlfriend steps on the rear corner of my 31' frame and she brings the opposing corner up almost 2". That's how much flex was designed into the frame and yes it seems rickety but man for 35 years I'd say it held up pretty well
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Old 01-09-2013, 04:58 PM   #16
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Check out page 9 of this link. r carl posted it on another thread. I don't know why Avion could do it and Airstream can't.

http://my.execpc.com/~drg/1972_Avion_Brochure.pdf
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Old 01-09-2013, 05:10 PM   #17
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It's kinda like saying if Andriod can't do it that way then why can't Apple? Or visa versa. They're all phones meant to make phone calls and access the web and take pictures etc. I don't think the world works on the principle of everybody should do it the same way. But why are there more Airstreams on the road than Avions if Avion did it right? Maybe Airstreams are like Apple...not the best out there but really cool to own.
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Old 01-09-2013, 05:22 PM   #18
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Btw don't mind me and my logic. I'm an architect and we are 100% wrong 90% of the time
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Old 01-09-2013, 05:28 PM   #19
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Check out page 9 of this link. r carl posted it on another thread. I don't know why Avion could do it and Airstream can't.

http://my.execpc.com/~drg/1972_Avion_Brochure.pdf
I checked it out, especially the chicks. There are a couple of descrete shots of the girls bathing. As expected no guys in the bathroom. Sure am glad the 70s are over.

Avions do have their pros and cons. My impression is they were very substantial and significantly heavier than Airstreams.
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Old 01-09-2013, 06:05 PM   #20
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It's kinda like saying if Andriod can't do it that way then why can't Apple? Or visa versa. They're all phones meant to make phone calls and access the web and take pictures etc. I don't think the world works on the principle of everybody should do it the same way. But why are there more Airstreams on the road than Avions if Avion did it right? Maybe Airstreams are like Apple...not the best out there but really cool to own.
There is a lot of truth to what you say. Airstream was in business since 1936. (?) They probably made many times more trailers than Avion. They probably live off of their ruputation.

Avion was in business (on a much smaller scale) from 1955 to 1990 (Fleetwood stopped making the aluminum skin trailers).
When was the last time you heard of an Avion having a frame break? I have never seen or heard of one breaking. Just looking at the picture, I can tell you that the odds are that it will never break during the life of the trailer unless it rusts out.
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Old 01-09-2013, 06:09 PM   #21
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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.
I am an engineer, but don't take my answer as definitive, either. Without actually crunching numbers, any engineer's opinion is only that, opinion.

Frame doesn't support the axles. Axles support the frame. Minor quibble, pay it no mind.

Part of the problem with a perimeter frame is this:

Side-to-side, if your factory-stock trailer is eight feet wide, and the longitudinal frames are four feet apart (for simplest math; I haven't measured a trailer to see the exact spacing), then for ease of analysis, you could draw a line down the center of the trailer, and say that all loads on the left side are transferred to the left frame member, and all loads to the right of the centerline are transferred to the right framing member, and all loads on the centerline are evenly split between the two. In real life, it's not that simple, of course, but again, we're not crunching numbers here. I'm just illustrating a point.

There's an engineering term called "moment arm". A load of 100 pounds applied one foot from your support creates a moment arm of 100 foot-pounds. A load of 100 pounds 2 feet away creates a 200 foot-pound moment. so, in your standard trailer, a 100 pound load right on the centerline provides a moment of 50 pounds (half to each frame) at 2 feet from each frame, for 100 foot-pounds to each frame.

For non-engineers, moment arm is also referred to as torque. A torque wrench is the perfect example of the principle. The force you apply to the torque wrench, times the length of the wrench from your hand to the bolt, equals the torque.

With a perimeter frame, your supports— the longitudingal frames— are eight feet apart, and all loads are somewhere in between the two frames. A 100-pound load right on the centerline provides a (50 pounds ◊ 4 feet =) 200-foot-pound moment to each frame. So, by making the frames twice as far apart, the frames have to support double the torque or moment. Your frames would have to be huge and heavy compared to the frames on a factory-stock trailer, to support the same load.

Now, you could do it a bit different, and put in THREE longitudinal frames, one on each side, one on the centerline, and then your frames could conceivably be the same size as the originals. But you're still adding weight (for the third frame) without actually improving the load-bearing capacity of your trailer.

Now, if your trailer was gutted, and all you had to support was the skin and ribs, you might get away with it. But you've got concentrated (point) loads, including people, appliances, holding tanks, all somewhere between your perimeter frames, and all proiding torque loads on your perimeter frame.

The placement of the frames in your trailer might have been developed by trail and error over the last eighty years. It might have been precisely engineered on a computer in the past five years. Who knows? But it's a safe bet that the location of the frames is probably within a few inches of the ideal spacing for distributing the weight of trailer and contents to the frame.

That's not to say that the metal channels used for the frames are the ideal size and shape, mind you…
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Old 01-09-2013, 06:30 PM   #22
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I checked it out, especially the chicks. There are a couple of descrete shots of the girls bathing. As expected no guys in the bathroom. Sure am glad the 70s are over.

Avions do have their pros and cons. My impression is they were very substantial and significantly heavier than Airstreams.
Here are the weights of the 1972 Avion trailers.

31' 5320# curb weight 750# hitch weight
28' 4770# curb weight 680# hitch weight
25' 4425# curb weight 650# hitch weight

It doesn't appear that the Avion is much heavier than an Airstream. As I said in an earlier post about the additional weight required to make a decent frame, they really aren't that much heavier.
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Old 01-09-2013, 06:39 PM   #23
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Going by memory here. The 1972 Avions were made when Avion was a family business, the Cato brothers. It was after Fleetwood took over they got heavy. But Excellas got heavy too!
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Old 01-09-2013, 07:11 PM   #24
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Ok here is a concept drawing. The axels attach just like they do now on the inner frame rails. Steve was beefing up the corners of his trailer which is the same concept. The walls are attached to a steel C-channel that goes around the perimeter of the trailer and ties the walls and frame together as a single unit. The current design ties the frame directly to the shell in only a few places at the front and back and at the ends of the poorly supported outriggers. With this design you don't need to have the floor between the shell and the frame.

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Old 01-09-2013, 09:19 PM   #25
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That is basically what I described except I recommended angle iron on the perimeter to tie the outriggers together and to give a large surface area to attach the shell. I like angle iron because it is easy to bolt to. I would probably put a rubber strip (maybe 1/8" thick) between the angle iron and the aluminum "C" channel for seperation. I think that would make a great isolator.
Remember, the main frame rails need to be as close to the hub face as reasonably possible to put less strain on the axle beam (you don't want it so close that you have clearance issues). I don't think Avion needed the center rail for most of their trailers, but it was needed on my 1988 34' triple axle because it uses independent suspension leaf spring axles that pivot from the middle.
I prefer to use 6" structural channel (depends on trailer length) with 3" structural channel cross members centered in the 6" channel. I would then use 2"x2"x1/8" angle iron to make the top flush with the top of the 6" rail. This gives you an extra strong cross member with a good area to bolt to.
So who has a perfect shell that they don't need?
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Old 01-10-2013, 12:30 AM   #26
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I'm not an engineer and I'm not trying to get off topic here but my classic moho is built on a very heavy, compared to a trailer, truck frame. The longitudinal rails are also closer together because of the dual rear wheels and there are no "outriggers" as in a trailer. What it has is 2" square tube cross members 8' long, every 24" to form a ladder type frame that sits on top of the truck frame. The aluminum shell and plywood floor attach to the square tube ladder frame. I don't know how such a ladder frame system might work in a trailer but it seems to work in the moho. Again just another system to think about.
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Old 01-10-2013, 07:03 AM   #27
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The truck chassis is a step in the right direction. Tieing the ends of the crossmembers together and bolting the shell to those cross members every few inches will make the whole thing 10 times stronger. You could probably accomplish the same thing with an extra thick bottom C-channel to attach the walls to. The stuff that Airstream uses is way too thin to handle any point loads.

A perimeter frame could probably be lighter if some basic finite element analysis were done. A perimeter frame will actually allow the shell to stiffen the frame and vice versa. You want no movement (shear) of the shell relative to the frame. Shear stresses are how loads are transmitted between the shell and the frame. If there is a lot of slop in those connections, the frame and the shell are pretty much acting independently of each other. The way the Airstream is built there is a weak connection at the front and the back that is loaded in tension against thin sheet metal C-channel. This does not work as anyone who has worked on a rear end separated trailer can see as the bolts start pulling through the C-channel.

Perry
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Old 01-10-2013, 08:17 AM   #28
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The truck frame concept puts the trailer axle way to far away from the hub face. The trailer frame only has a few inches of a variable where it can work. As stated earlier, it needs to be as reasonably close to the hub face as possible. The truck frames have to clear the Dual rear wheels, and the leaf spring mount rides on the outside of the frame.
I recently built a frame to use with a pickup truck bed. The standard width is 38" outside to outside. This is way to narrow for a travel trailer.

Below are some pictures. You will notice that I had to build a mount for the axle that moved it out 6" and down 6" to have a place to mount the axle. Notice how close the hub face is to the axle mount. This is absolutely necessary, especially on the heavier axles. This particular axle is a 7000# Flexiride and I can guarantee that it will not break under normal or even hard usage. I would build an AS trailer frame very similar except the frame rails would be further apart and there would be 3" structural channel outriggers tied together with thin wall angle iron. There would be thin angle iron to make the outriggers and crossmembers flush with the top of the frame rails. I place my crossmembers 24" apart.









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