The Perfect AS Frame Design

[Protagonist]

Originally Posted by Lumatic

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…

[M2HB]

Originally Posted by Lumatic

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.

[Lumatic]

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!

[perryg114]

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.

[M2HB]

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?

[Smartstream]

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.

[perryg114]

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

[M2HB]

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.

[Protohyp]

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.

[Protohyp]

BTW. I have a couple of test piece airstreams here in LA...we should test the theory.....

[Protohyp]

Originally Posted by perryg114

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

[perryg114]

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

[Protohyp]

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

[M2HB]

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.

[JimGolden]

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...

[M2HB]

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.

[Protohyp]

Originally Posted by JimGolden

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.

[M2HB]

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?

[JimGolden]

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,

[perryg114]

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