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Old 01-10-2013, 10:09 AM   #29
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M2HB...I probably still don't agree with over building the frame like you do but based on your work I would definitely consider you to build a frame for me. Good looking work and definitely a lot of care taken in the welds. The only thing for me personally is I would maybe go up to 6" rather than the 5" C-Channel that I have and instead of the weak tack welds they do on outriggers and some of the box sections on the front of the frame I would have you do longer beads. those were the only areas that I saw frame "failure" on my 31' excella.

I think one of the things that we are missing is the physics of how much this trailer really moves and flexes under even normal road conditions. Now that my shell is off I push just a little bit and the amount that the shell twists 27 feet down on the other end is pretty amazing. Granted my inner skin is off but I can't produce the amount of forces and flex that the wind and the road can produce but it is pretty substantial. If you youtube any of the discovery channel (maybe natgeo) videos of high speed cameras slowed down such as a drummer hitting a cymbal or a snare drum the amount of 'activity' that you see is ten times what we perceive with our naked eyes and we never see how much force is generated over a short time period. I think that a frame that doesnt want to flex next to a shell that wants to in the wind and standard road conditions will require the same engineering that sky scrapers have at their foundation. Most of those buildings yes have a solid immovable foundations but the actual building sits on steel that is designed to bend and flex and there are rubber/hydraulic/spring systems that take a lot of the movement (up to 2' in any direction) from that solid foundation when the wind starts blowing and when earthquakes happen. I think if you're concept of a heavy duty frame were to work it would need rubber torsion spacers on the outriggers to allow the frame above to do what it wants to do irregardless of the frame below it.

Again requires real world testing but I think the idea of sky scrapers applies somewhat to this theory...

again though I applaud your work.
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Old 01-10-2013, 10:12 AM   #30
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BTW. I have a couple of test piece airstreams here in LA...we should test the theory.....
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Old 01-10-2013, 10:17 AM   #31
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Quote:
Originally Posted by perryg114 View Post
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
I think also just the point loads could be reinforce by possibly a larger plate that sits inside the C channel from inside the shell. big washers at the corners doesnt seem smart but maybe a longer plate with which to sandwich the C channel between the outriggers might work. You are definitely correct about the point loads at the outriggers but I think that is different from the overall strength of the frame...they're just weak connections
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Old 01-10-2013, 10:19 AM   #32
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Ok lets put some strain gages on everything now.

The outriggers don't do much for the shell/frame connetion but they should. They hold the floor up and that is about it.

Perry
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Old 01-10-2013, 10:44 AM   #33
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I actually think that the outriggers do nothing more than keep the shell from flexing out in the middle and possibly tipping over The front and rear plates and ends of the frame seem to be what's holding up the shell (for lack if a better term cause again the shell supports the frame) but a lot of people have chopped off outriggers after securing the inside of the frame and the shell sits on just the ends of the frame with no problem. Of course that's not an ideal tow condition but I think the outriggers have holes in them so they can somewhat breath when the shell flexes in an outward fashion
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Old 01-10-2013, 02:30 PM   #34
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To be strong, the outriggers should be an extension of the crossmembers.
If you look at the Avion frame, it doesn't have any crossmembers where the rear tanks are. This would allow the frame to flex easier. This is were I think they use the shell to keep the frame from flexing. I still would like to know the difference in the attachment areas between the Airstream and the Avion.
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Old 01-10-2013, 05:40 PM   #35
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Strong Frame = Good

Neat technical discussion. Please allow me to weigh in here....I am one of those In-Guh-Neers

First, it is a myth that Airstreams are truly "monocoque". There are too many large holes in the shell for that to be true. They are barely semi-monocoque. Monocoque is a French term that basically means "shell", as in the shell takes the load. We take it to mean "stressed skin." It doesn't. Not significantly anyway. It's marketing.

M2HB's frame looks great. It is similar to the frame I designed a few years ago for my 1977 Excella 500 that had the 4" deep flexiframe that had both sag and separation. I was going to use an 8" deep channel. My design had eight times the stiffness of the 4".

The weight doesn't go up that much, because depth of beam is what matters. The formula for stiffness is bh^3/12. So if the height doubles, the stiffness goes up by a factor of 8. You don't have to make it much deeper to see a giant result. When Airstream went from 4" to 5", the problems went away. My frame weighed about 150lbs more than the stocker, but it was eight times stronger.

How could a frame be too strong? The stronger it is, the less load gets dumped into the shell. The less load in the shell, the less you bend it. Once you make a door cut out, you can forget any monocoque action at all from there forward (for this example assuming a front door model) Now from there back, you do get some monocoque action, but not much. The window cut outs are too big.

Just do a ratio: Look at the size of the windows compared to the diameter of the shell. Compare that to the size of the windows on a 777 and compare that to the diameter of the shell. There's a reason Boeing makes their windows fairly small.

Avions do have stronger frames. It is non negotiable. You will never hear of an Avion having sag or separation. They have just as much "monocoque" action as an Airstream. But their frames are stronger. As well, they also have a suspension frame that the main frame sits on top of. So where the area of maximum bending moment occurs (over the axles), Avion has much more strength.

My '87 Avion has a 6" main frame, but with three rails rather than two. It has an 8" suspension frame under that. So in the area of maximum stress, there is 14" of frame. My Excella had 4" here, as the axles bolted onto the main frame. The difference in strength between the two is enormous.

An '87 Avion 34 footer did weigh more than an Airstream 34 footer of the same year. But the newer 34 footers are actually quite a bit heavier than the Avion. It doesn't add that much weight do deepen the frame. Corian counter tops and all of the interior acouterments add a lot more weight.

I do think the old Airstreams look nicer than Avions. I think they look better than the new Airstreams, as well. The new Airstreams look just about like my '87; at least as far as shape and window size. They are squared off more than the old ones. That is better for storage, but just not as sleek. I really like the older Airstreams, up through the mid 70's, for their lines. Even if they didn't have the room the newer ones do.

Some day I'd like to buy a pair of Excella's (as you can get the old 31 footers cheap) and chop the front 2/3 off one and the back 2/3 off another and make a 40 footer on a custom frame. I'd also put a couple slide outs in it. Then polish it like a mirror and have LongStream

In 1985, Airstream made their frames deeper by an inch. That resolved the sag and separation issue.

OK, back to the program...
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Old 01-10-2013, 06:50 PM   #36
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Jim, you and I think alike. There is no doubt that a frame member with a higher web will be much stronger. I figured that a better frame and axle setup would only increase the weight by about 300-400#. In the whole scheme of things, this isn't much, especially since you would have heavier axles to go with it.

I like your idea of adapting an old Airstream shell. I was thinking about taking a 31' AS and cutting it down to about 25' with a tandem axle setup or down to 22' with a single axle.
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Old 01-10-2013, 07:20 PM   #37
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Quote:
Originally Posted by JimGolden View Post
Neat technical discussion. Please allow me to weigh in here....I am one of those In-Guh-Neers

First, it is a myth that Airstreams are truly "monocoque". There are too many large holes in the shell for that to be true. They are barely semi-monocoque. Monocoque is a French term that basically means "shell", as in the shell takes the load. We take it to mean "stressed skin." It doesn't. Not significantly anyway. It's marketing.

M2HB's frame looks great. It is similar to the frame I designed a few years ago for my 1977 Excella 500 that had the 4" deep flexiframe that had both sag and separation. I was going to use an 8" deep channel. My design had eight times the stiffness of the 4".

The weight doesn't go up that much, because depth of beam is what matters. The formula for stiffness is bh^3/12. So if the height doubles, the stiffness goes up by a factor of 8. You don't have to make it much deeper to see a giant result. When Airstream went from 4" to 5", the problems went away. My frame weighed about 150lbs more than the stocker, but it was eight times stronger.

How could a frame be too strong? The stronger it is, the less load gets dumped into the shell. The less load in the shell, the less you bend it. Once you make a door cut out, you can forget any monocoque action at all from there forward (for this example assuming a front door model) Now from there back, you do get some monocoque action, but not much. The window cut outs are too big.

Just do a ratio: Look at the size of the windows compared to the diameter of the shell. Compare that to the size of the windows on a 777 and compare that to the diameter of the shell. There's a reason Boeing makes their windows fairly small.

Avions do have stronger frames. It is non negotiable. You will never hear of an Avion having sag or separation. They have just as much "monocoque" action as an Airstream. But their frames are stronger. As well, they also have a suspension frame that the main frame sits on top of. So where the area of maximum bending moment occurs (over the axles), Avion has much more strength.

My '87 Avion has a 6" main frame, but with three rails rather than two. It has an 8" suspension frame under that. So in the area of maximum stress, there is 14" of frame. My Excella had 4" here, as the axles bolted onto the main frame. The difference in strength between the two is enormous.

An '87 Avion 34 footer did weigh more than an Airstream 34 footer of the same year. But the newer 34 footers are actually quite a bit heavier than the Avion. It doesn't add that much weight do deepen the frame. Corian counter tops and all of the interior acouterments add a lot more weight.

I do think the old Airstreams look nicer than Avions. I think they look better than the new Airstreams, as well. The new Airstreams look just about like my '87; at least as far as shape and window size. They are squared off more than the old ones. That is better for storage, but just not as sleek. I really like the older Airstreams, up through the mid 70's, for their lines. Even if they didn't have the room the newer ones do.

Some day I'd like to buy a pair of Excella's (as you can get the old 31 footers cheap) and chop the front 2/3 off one and the back 2/3 off another and make a 40 footer on a custom frame. I'd also put a couple slide outs in it. Then polish it like a mirror and have LongStream

In 1985, Airstream made their frames deeper by an inch. That resolved the sag and separation issue.

OK, back to the program...
So Jim you're the guy I wanted to hear from. They guy who had real world experience. Can I take a stab into the wind and assume your shell to outrigger contact was beefier than a chintzy little screw and bolt through the c channel?

I will buy into the monocoque myth but it just sounds cool to use the term. You and I have or had the same rig in the excella and I'm going with 6" channel but I'm also reducing the overall weight inside my trailer. The next airstream I acquire I will be cutting it down for sure

Thanks for the practical and real world lesson frame construction.
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Old 01-10-2013, 08:05 PM   #38
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Jim, I have a 1988 Avion 34' triple axle trailer with the independent leaf spring axles. Do you have the same setup on your 87 Avion?
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Old 01-11-2013, 05:32 PM   #39
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Hi Protohyp,

Hope I didn't lecture too long. My coworkers say I go into WAY too much detail when I try to explain something

Yep, there's a little more than a little screw holding it together. Actually Avions are built a little differently. On them, the shell doesn't sit on top of the floor. The floor sits inside the shell, and they do run screws through the belt trim into the floor, but the shell is independent of the floor. As in, you can remove the floor without having to jack the shell up. Check out Veggie Bullet's thread if you haven't already.

http://www.airforums.com/forums/f417...-96039-13.html

He has some great photos of the floor replacement he's doing on his. He has one of the 1990 fifth wheels; ultra rare, and very cool. A Silver 5er!



M2HB, yep, mine has that suspension. They called it "Adjust-a-Ryde" and it was three Dexter axles cut in half, to give you six independent swing arms. Six leaf spring packs, six shock absorbers. With the right tire pressure, it rides really smoothly. Sometime in '88 they switched from that to the rubber torsion axles like Airstream uses. I prefer the older setup better because it doesn't really wear out.

I put new shocks on mine when I got it, along with sanding and repainting the entire frame and suspension. With new tires, I run about 60psi in it and it does great. Tires run cool, ride is smooth and stuff doesn't get bounced around. It's a good setup.

The older ones used Moryde walking beam type suspension, which is supposed to be even better. At least, it's stronger. Not sure if it rode any better. The Adjust-a-Ryde was supposedly cheaper than the Mor-ryde (I know I'm spelling that wrong each time....), and the Alko rubber axles were cheaper yet. With Fleetwood at the helm, they kept trying to find ways to build them cheaper. To the best of my knowledge, '89 was the last year for the 34 foot travel trailer, and 1990 was the last year for the 5er (I think they only made them in 89 and 90).

I'm happy with mine. Wish it had a slide out, but other than that we like it.

I do think the idea of a Super Stream would be cool. I saw one of those railroad bunkhouse models...that was a long rascal!

cheers,
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Old 02-19-2013, 12:36 PM   #40
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I lost this thread and am trying to find it again. I am hopeing this will bump it further up my subscribed thread list.

Perry
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Old 02-19-2013, 07:09 PM   #41
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MOR/ryde "Smooth Glide" on AVION ca. 1968-1976. Similar to a walking beam suspension (as used in logging and offroad commercial trucks).

STREAMLINE used a center beam channel to distinguish itself from SILVER STREAK after the owners parted ways and set up a new company.



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Old 02-24-2013, 12:33 PM   #42
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chassis/fram

My frame is rusted out. I am thinking of replacing it with a enginered pultruded fiber glass frame. Any thoughts?
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