Frame Rail Sectional Dimensions

[JimGolden]

Hi Everyone.

Does anybody happen to know the actual measurements of the frame dimensions of the 70's Airstreams? By this, I mean, what are the dimensions of the channel section of the main frame?

My '77 Excella has the 4" deep frame. But, what is the flange width and what is the thickness of the section?

For you engineer types, I want to calculate the moment of inertia of the frame. So I need the web thickness, the flange thickness, and the flange width. I know the height of the rail is 4". I just need the other dimensions.

I do know that the axle mounting plate is 0.190" thick on my trailer. I figure that was 3/16" plate (should be 0.1875) and the variance is due to tolerances. I mic'd it with a digital caliper. But, I've not removed the belly pan yet and so can't get at the main channel to mic it.

I am working on the design of a doubler to beef the frame up. I'm wanting to know by what degree I will enhance the stiffness of the main frame, and so would like the dimension of the basic channel.

Can you help me?

Thanks!

Jim

[dwightdi]

Airstream sells a doubler plate. You could likely get you to give you the thickness that they used if you called them or one of the servicing dealers.

[ZoomZoom]

Jim:

I'll mic a plate tomorrow and post the thickness. Airstream refers to it as an axle re-enforcing plate.

Greg

[markdoane]

Jim,

I don't have a 70's, and the frame on my 1959 is rectangular tubing. But, I would assume the frame is standard C4×5.4 (4" channel, 5.4 lbs/ft), and you can look up the I's on this chart: http://www.engineersedge.com/standar...properties.htm

[wahoonc]

Originally Posted by ZoomZoom

Jim:

I'll mic a plate tomorrow and post the thickness. Airstream refers to it as an axle re-enforcing plate.

Greg

Greg,
I will be interested to see what you mic at...I am guessing somewhere in the 14-16 gauge range (.0747-.0598) I have the plates on mine, but have not really paid attention to them yet

Aaron

[markdoane]

Originally Posted by JimGolden

. . .I do know that the axle mounting plate is 0.190" thick on my trailer. I figure that was 3/16" plate (should be 0.1875) and the variance is due to tolerances.
Jim

Jim,

This is likely 6 gauge, steel is not manufactured in fractional inch.

6 ga. is 0.1943" +/- .0090".

[JimGolden]

Thanks guys.

I checked out that engineers edge link. Good stuff on there, makes it easier than calculating it by hand. I wrote a little spreadsheet to do it. Been awhile since I did by hand like that. Did the sum of the I + ay^2 thing. My numbers were pretty close to what they had on there so I think I did it right.

One thing of interest; if you look at the 5" section at 7.2lbs per foot so that it works out to about the same web and flange thicknesses as the 4" section, the moment of inertia is nearly double what it is for the 4" frame. I think that would pretty much cure any saggy issues.

I was thinking of welding on some .19" thick plate fore and aft of the axle mounting plates, maybe 8 feet on either side. This would then give me a smooth surface down the side of the frame. I'd then weld a .25" thick doubler onto this and maybe stop it a foot short of the end of the .19" plates. Something along these lines anyway... This would get the flexural stiffness up there where it should be.

The other option is build a new frame entirely, but man is that a lot of work. If I were to go that route, I'd pull out all the stops and probably use a 10" deep I-beam section, .19" thick, with 4" flanges. That'd give me an I value of around 33, as opposed to 3.8 like it is now. I'd never have to worry about it bending, the sag and separation problems would be gone, and I could mount my KLR-650 (340lbs) on the back bumper and not worry about it.

I'm hoping the stock frame is good enough to just beef up though. I figure it will be. I see minimal signs of sag from the outside. I put a laser along it and the one side seemed to have more droop than the other, but neither was bad. Curbside showed none at all, street side just a little. When I rip the belly pan off, I'll find out for sure.

Greg, did you get a chance to mic your factory reinforcing plates? I'm curious to know what Airstream did to fix it.

Thanks guys!

Jim

[ZoomZoom]

Yes, it mics at .175" Pretty thick huh? Here's a picture of it.

Greg

[JimGolden]

Greg, Thanks. I appreciate it.

How exactly does this mount? Does it go on the inside or the outside of the frame? Do you bolt it on or weld it?

Thanks again for your help.

Jim

[wahoonc]

Jim,
I can answer that question.....it goes on the outside behind the wheels, mine is bolted on. Apparently done by a dealer some 25+ years ago.

Aaron

[Jim & Susan]

Aaron & Jim, I'm not and engineer and didn't play one on T.V., but I think this is the answer to a question that I have been asking incorrectly in a couple of different threads.

Is this item intended to stiffen the frame BEHIND the axles? There is a lot of sway and wiggle to the frame AND shell of the trailer behind the axles. I currently have the trailer stripped down to the frame (floor is out). And I'm trying to find a way to stop or reduce the frame wobble/wiggle/sway aft of the wheels/axles before I put the floor back in.

I've read a lot about "elephant ears", gusset additions and so on, back in the back part of the trailer, but none of the things I'm looking at seem to fix the movement back there OR prevent "sagging". I only want to do this once, so I'm looking for the correct answer.

Thanks, guys, for the info.

Jim

[wahoonc]

Okay Jim,
I have done massive amounts of research...well a whole bunch anyway The 70's vintage long Airstreams (28'-31') rear bath had a couple of issues, one was frames that would crack near or behind the rear axle mount, that is what the plates Greg has pictured below are for. The elephant ears were a separate repair to stiffen the rear end where the shell was separating from the frame and floor at the extreme rear. Apparently due to the weight distribution and tankage this separation was more common in rear bath models. Also running gear condition and balance plays a large part in this too. Andy has posted good insight into the repair and stiffening of the rear frame area. The frames on an Airstream are only a part of the total support equation, the floor and the actual shell also play important and probably equal roles. You can't take just one piece of the equation...I have not had my 31 footer down to a bare frame, so I can't tell if what you are experiencing is normal or not, but I would expect the frame to be a bit on the flexible side, as long as all the metal, out riggers and welds are solid, it is the way Airstream designed it. Add the plywood and reattach the body the way Andy described and you should be good for another 25+ years.

Aaron

[wahoonc]

Jim,
Just to help you out here are a couple of pictures of what the frame cracks look like, these are behind the wheels, typically near the rear axle. In extreme cases you can see a bulge in the body of the trailer right behind the wheel arches, and some cases the frame will be completely broken.

Aaron

[wahoonc]

Originally Posted by Jim & Susan

Aaron & Jim, I'm not and engineer and didn't play one on T.V., but I think this is the answer to a question that I have been asking incorrectly in a couple of different threads.

Is this item intended to stiffen the frame BEHIND the axles? There is a lot of sway and wiggle to the frame AND shell of the trailer behind the axles. I currently have the trailer stripped down to the frame (floor is out). And I'm trying to find a way to stop or reduce the frame wobble/wiggle/sway aft of the wheels/axles before I put the floor back in.

I've read a lot about "elephant ears", gusset additions and so on, back in the back part of the trailer, but none of the things I'm looking at seem to fix the movement back there OR prevent "sagging". I only want to do this once, so I'm looking for the correct answer.

Thanks, guys, for the info.

Jim

Jim,
I am not an engineer either...but I did stay at HI Express the other night Before you yanked the floor out, did you put jacks under the frame to stabilize it? And how much of the floor do you have out? And how much of the interior panels?

Aaron

[dwightdi]

I am a mechanical engineer. The prior posting shows a very signifigant crack and it would be very unsafe to be on the road with anything like this. This crack could be a fatique crack from lots of miles of rough road or less likely from travelling with unbalanced tires. It also could have started from a single high overload from a big bump with the holding tanks full. Once the first crack starts on the top of the channel, the continuation of the crack can come at much lower loads because the stress concentration at the tip of the crack and the greatly reduced effective moment of inertial (an engineering term) caused by the loss of effect of the upper portion of the C channel. I have also seen frames that were merely bent between the wheels without cracks. These frames might be babyied along for a number of years with just the elephant ears fix without failing completely on the road.

[wahoonc]

Originally Posted by dwightdi

I am a mechanical engineer. The prior posting shows a very signifigant crack and it would be very unsafe to be on the road with anything like this. This crack could be a fatique crack from lots of miles of rough road or less likely from travelling with unbalanced tires. It also could have started from a single high overload from a big bump with the holding tanks full. Once the first crack starts on the top of the channel, the continuation of the crack can come at much lower loads because the stress concentration at the tip of the crack and the greatly reduced effective moment of inertial (an engineering term) caused by the loss of effect of the upper portion of the C channel. I have also seen frames that were merely bent between the wheels without cracks. These frames might be babyied along for a number of years with just the elephant ears fix without failing completely on the road.

dwighti,
The frame cracks were caused by a design flaw when the Airstream "engineers/designers" undersized the frames in an attempt to keep the weight down, or they did not think through the load calculations. I have seen trailers with bent frames and no rear end separation and I have seen trailers with the strengthening plates installed that had rear end separation as well as trailers that had been well cared for with neither issue. The frame stiffening plates pictured below were the factory fix for the weak frame issue. The elephant ear repair was a separate repair issue. Are the two related possibly but they are two different issues and have to be dealt with separately.

Aaron

[dwightdi]

The body separation from the frame is due to failure to hold these two subsystems together. They were designed to be held together by bolts between the lower aluminum channel (which is riveted to the skin) and the floor and frame assembly. The "elephant ears" fix increases the bolting strength and spreads the stress over a larger area of the channel. The forces that cause the separation are usually the bending of the main frame at the rear axle or between the axles or a fatique crack developing in the frame (as shown in a prevous posting). The installation of the elephant ears modification allows the body of the unit to become a better stress carring element in the design and lowers the stresses that need to be carried by the frame. The bolt-on frame strengther plate allows the frame to better resist the stresses and (in therory) reduce the stresses to the point that you never exceed the fatique strength of the material in the upper frame section and the fatique cracks will never occur. The fatique strength of steel is much lower than its yeild strength and that is why you do not see any yielding around the crack until the last minute when the last vestages of the frame fails and the unit collapses. (I used to design and test high performance boat motors and we had these kinds of problems more frequently than we desired.) Aircraft designers and aircraft maintenance personnel frequently have this problem. They replace whole areas on the planes to keep them in the air for 30 years. To them, it is a matter of economics because the planes are very expensive. For an old Airstream with fatique cracks,(in my opinion) it is most likely time to retire it to a permanent home as a summer cottage by the lake.

[Jim & Susan]

So, to noodle this out, there are two basic problems with this era trailer. One problem is that the frames have a tendency to fail aft of the axles (for various reasons, including metal fatigue, rust and so on). The second problem is that the upper shell has a tendency to separate from the frame itself.

Problem number two can be corrected by reattaching the shell to the frame using additional bolts and steel plates between the U-channel that is riveted to the shell and the actual steel frame in the trunk area between the frame C-channel members.

Problem number one, what I believe to be at the base of the issue here (my novice opinion) is more dicey. The basic “fix” here is to beef up the frame aft of the axles. There is a lot of unnecessary movement back there which I believe leads to shaking, wiggling, and up and down movement which in turn causes bolts to loosen, rivets to pop, steel frames to twist and bend, etc. In addition, the weight back there tends to push the very back of the trailer toward the ground. Everything begins to pull apart. So, the $64,000 question is how to strengthen the frame here?

One option is to purchase the axle plate mentioned by Zommzoom, above. This, apparently, bolts to the frame C-channel on the outside of the channel aft of the axles. Another option would be to weld a vertical plate between the upper and lower “flange” on the C-channel inboard on each rail.

Ok, y’all, tell me where my thinking is wrong.

Jim

[dwightdi]

The factory fix for the problem (installing a stiffener plate bolted to the exterior of the frame) is a cost effective solution that does not intail taking the whole thing apart. Welding a plate in the inside of the C frame to make it into a complete box frame can only be accomplished by removing the belly pan and means welding on the frame which anneals the existing metal and usually reduces the strength of the material. I believe the original frame is made from a cold rolled material (which increases its strength). If it were a hot rolled product it would not be as bad but still, the quality of the fix would dramatically depend on the quality of the weld and welder. Having welded on a few frames myself, this is not an easy talent to learn. Airstream would be placed in a poor position of defending the quality of dealer installed welds. It is not a good idea. The bolts in the back, allowing the shell to better take part of the load, is also a good low tech field partial fix.

Whether you think the problem is: Airstream's fault for undersizing the frame or the users' fault for abusing the product too much, or unrealistic user expectations of having a trailer that is usable for 50 years, is a matter of viewpoint. I am a frequent user. Sometimes you can not control the quality of the roads which you get into (such as road construction). The Airstream image of doing these round the world caravans has likely lead to unrealistic view of what these trailer can take. If you look closely at the pictures you will see the tow vehicles and trailers are frequently not exactly stock units. If you read the actual chronicals you will see not all units completed the treck.

If I knew I was expecting to use my trailer to do a large amount of rough road travel, I would buy a smaller unit with less overhang and a 5 inch frame and four wheels built after 1984.

[JimGolden]

On the welding issue...

I used to work for a major crane company and we made the booms out of 100ksi yield strength steel, sometimes even stronger material. We welded everything together and always took the plate strength as 100ksi (or whatever it was). We never reduced it due to welding messing up the plate strength. Never had a failure.

I can't imagine the frame of an Airstream being made out of anything exotic. It's probably just plain old 36ksi steel like they make car frames out of. As such, I wouldn't think welding to it would hurt it at all. Granted, the doublers contribution to enhanced structural rigidity would be dependent upon the quality of the welding to make the load path, but you'll have that with anything that you change.

But you make a good point: it would be much easier to just sell the trailer and buy a newer one. I'm torn with what to do. I really like this model. All these mods are just a patch. The real way to "fix" it is to make a new frame and just transfer the shell over. That's a lot more work than I was planning on.

I am trying to talk my wife into a new mig welder for Christmas though