Rear End Sag Not Separation

[airstream74]

I have a 31' 1974 Excella 500. It sags from just aft of the rear axle. I am in the process of removing the interior to replace the floor and repair the frame. Only the bathroom is left and I should have it out by the weekend and then I can start on the repair.

I have searched the forum and found a couple of references. They talk about rear end separation, but that is not my problem. My frame is slightly bent just aft of the rear axle. This damage may have been caused when the previous owner made a trip from Alabama to Alaska.

Someone mentioned a TSB (technical service bulletin) and an upgrade kit that could be purchased from Airstream to repair this condition.


Has anyone else had this problem? How did you go about repairing it? Has anyone purchased this upgrade kit from Airstream -- can you give me information about how it worked out? Does anyone have any information on this to help me out?

Thanks for any help you can give.

[JimGolden]

Airstream got a little "wild" with the weight savings and made the frames on the 70's trailers a little "light". There's a kit out there to beef the frame up in this area. Some guys say it fixes it, others say it doesn't.

Later on they made the frames deeper.

My own 31' shares this malady. I plan to make a new frame for it with a deeper and much stronger section. But if you don't want to go to that extent, you can add doubler plates to stiffen your's in that area. There are some photos on this board of the factory setup. Or, you could design your own. Bottom line is that the frame isn't stiff enough as the factory sent it out back then, and you need to increase the stiffness.

If you do some searching, you can find some posts on here that show photos of the factory fix. As well, it'd not be so hard to design one.

If you'd like help desiging a doubler, shoot me an email and I'd be glad to help. But, a lot of guys say the factory fix is a pretty good one. It's worth looking at that one too.

Best of luck,

[airstream74]

Thanks for the help.

I have been searching but have not found the photos yet. Got any hints on what key words to search under.

[JimGolden]

Check this thread out.

http://www.airforums.com/forum...g+repair+plate

It doesn't have the photos, but it's a good starting point. Those photos are on here, it'll just take a bit of searching to find it. Inland RV might actually sell them, I'm not sure. As I recall it was a bolt on thing. I think people called them "elephant ears" or something like that.

I believe the general consensus is that these plates will fix the immediate problem, but you also need to address other issues that caused the marginal strength frame to be pushed over the edge. Biggies are things like balanced running gear (Inland RV's website gives a very good description of how to do this), and weight distribution internally. Things like don't carry your blacksmith tools in the back, don't tow with full black and gray tanks, etc. Basically keep the back as light as you can, and have the running gear balanced, so the rear of the trailer isn't like a fishing pole bouncing up and down.

That info is all on here, I just didn't have time at lunch here to find it for you. Take a look and if you still can't find it, let me know and I'll dig some more for you. But I'd do searches on "rear sag", "ears", elephant ears", "frame plates", things like that. Also check out Inland RV's site. That should get you pointed in the right direction.

Good luck!

[kposey]

PizzaChop has dealt with this problem. You might want to do a search on the members list to find his web pages.

[drag'nwagon]

Airstream74

My '74 31' sov has the elephant ears on it, which is a sheet metal patch/ cover where the unit was repaired at the factory. I can send you pictures if you would like. The elephant ears were installed after the factory (usually Jackson Center from what I have heard here) has cut out the sheet metal at the rear lower corners to re-attach the floor to the frame. That being said my unit does not have separtaiton, but I do have a bit of sag.



Dave

[airstream74]

Thanks for the information everyone.

JimGolden - Been searching but so far the only photos I have found were the ones from PizzaChop. If you happen to run across the others please let me know.

dragn'wagon - yes, send the pictures, like to see everything I can before I start.

Thanks again everyone -- and if anyone else has any information or photos, please let me know.

[wahoonc]

Airstream 74,
If the frame is actually bent you will need to correct that no amount of floor repair or rebolting is going to cure that. I have attached a couple of pictures of what I am aware of being frame issues. They were corrected with a stiffener plate that was available from Airstream. Mine has it installed and I am assuming by either the factory or a dealer. BTW these pictures are not of my unit they are some that I came across when researching the issues with the longer 70's units.

Aaron

[Inland RV Center, In]

Originally Posted by wahoonc

Airstream 74,
If the frame is actually bent you will need to correct that no amount of floor repair or rebolting is going to cure that. I have attached a couple of pictures of what I am aware of being frame issues. They were corrected with a stiffener plate that was available from Airstream. Mine has it installed and I am assuming by either the factory or a dealer. BTW these pictures are not of my unit they are some that I came across when researching the issues with the longer 70's units.

Aaron

Your cracked frame and axle mounting plate is a beautiful example of what happens when the running gear is not properly balanced.

That type of damage happens with many many miles of towing.

Repairing the frame and axle mounting plate, and then adding the frame stiffeners, is a great help.

However, in time, all three can again do the same thing.

We have seen that happen many times.

Bottom line again, is that the running gear must be reasonably properly balanced, or else.

Andy

[airstream74]

Well, back to work on my 31" Excella. The interior is completely out. Interior side panels removed, and have started taking out the floor sections.

Thinking of boxing the frame rails like an old street rod. Wondering if adding the strength to the frame could possibly cause any damage to the shell?

Still pllanning to install the frame reinforcing kit from Airstream. As I understand it, this only reinforces the area between the axles on my trailer. The plates the axles are bolted to appear to be wavy and it looks as though a kink or separation might be forming at the rear most point where the left plate is attached to the frame. The weld appears to be broken at this point and a small pocket is forming between this plate and the frame.

Can anyone give me insight into possible problems or benefits from boxing the frame?

If boxing is a good idea, where should it extend to and from. For example, from the front to rear, from center of axles to the rear, etc., where?????

Any comments would be appreciated before I jump into this mess any further.

[JimGolden]

Boxing the frame will help make it stronger, though not as much as deepening it would for the same amount of additional weight. Deepening it, however, would require some other changes (new belly pan, have to do something about the banana wrap,etc.)

The stiffer the frame, the less it will flex, and the less load you dump into the aluminum shell. I've heard it said that too stiff a frame is bad, but I disagree. If the frame were infinitely stiff, that'd be great, provided the suspension works correctly. It's the suspension's job to take out the bumps. So, a stiff frame is a good thing, with a properly set up suspension (aka not too high a spring rate, correct shock damping).

I'm currently enrolled in a welding class and am having a ball. My plan is completely new frame with several key design changes from OEM. A lot of work, but it'll be fun

Good luck with your Excella!

[airstream74]

Thanks for replying, it's nice to have someone to bounce things off of.

Changing the depth of the frame seems too drastic (my wife can't believe I said that - she thinks the total destruction I have already done to the interior was too drastic).

Got any ideas on on far I should go with boxing the frame (just forward of the axles or all the way to the front)?

Plan on using 1/8th of an inch to box the frame with and to add a few more outriggers.

Any other thoughts or comments?????

[JimGolden]

Most of the problems seem to be just forward of the axle mounting plate, and then basically the whole way to the rear (although mainly just aft of the axle mounting plate). You might just box from the back bumper to about 4 feet ahead of the frontmost edge of the axle mounting plate. Although if I were doing it, I'd probably just do the whole thing.

The stress is greatest around the axle plate ends and decreases as you go toward each end (front and back).

If a guy wanted to really build an efficient frame, he'd take some flat plat that's about 9" deep right between the front and rear axles, say make it 9" deep the whole way along where the axles will mount (say about six feet of length at 9" deep), then taper it to about 5" deep at each end. Then weld the flanges onto this tapering web. You could even punch lightening holes in the web.

However, that's an awful lot of work to go to to save probably 50-75lbs. Much easier and cheaper to just use an off the shelf available cross section.

It'll be a fair amount of work to box the frame as you've described. What will you do about the crossmembers? Will you cut them out, shorten them, then reweld them to the boxed in areas?

You've got a good idea. I think it'll be a good amount of work; maybe as much as making a new one, but at least you won't have to pull the shell.

Take care,

[airstream74]

You are right, the worst spot is just aft of the axle mounting plates.

The suggestion about using a 6 foot long 9 inch deep plate spanning the axles is essentially what I thought the airstream reinforcing kit did. Is that true, do you know?

In regards to boxing the frame, I had planned to buy 20 foot lenghts of 1/8 x 5 inch mild steel (hot roll) and cut to fit between the cross members on the frame rails notching them to fit the cross members, and welding them to the cross members. Would this not work?

Also, did I read somewhere that boxing would be stronger if it were not welded solid but rather stitched -- weld a few inches, skip a few inches, weld a few inches, etc. (this makes no sense to me, I can't understand how it could be ), is this true?

I will try to take some pictures over the weekend and bore you with these too. Maybe you will see something I don't see.

Thanks again.

[Inland RV Center, In]

Originally Posted by airstream74

You are right, the worst spot is just aft of the axle mounting plates.

The suggestion about using a 6 foot long 9 inch deep plate spanning the axles is essentially what I thought the airstream reinforcing kit did. Is that true, do you know?

In regards to boxing the frame, I had planned to buy 20 foot lenghts of 1/8 x 5 inch mild steel (hot roll) and cut to fit between the cross members on the frame rails notching them to fit the cross members, and welding them to the cross members. Would this not work?

Also, did I read somewhere that boxing would be stronger if it were not welded solid but rather stitched -- weld a few inches, skip a few inches, weld a few inches, etc. (this makes no sense to me, I can't understand how it could be ), is this true?

I will try to take some pictures over the weekend and bore you with these too. Maybe you will see something I don't see.

Thanks again.

CAUTION..

Remember that the pricipal method of construction is called "monocoque."

It means a load bearing shell. It also means that it is supposed to flex to some degree.

Beefing up the frame excessively, could restrict that flexing and in turn cause a number of other problems.

Andy

[TomW]

Originally Posted by Inland RV Center, In

...Remember that the pricipal method of construction is called "monocoque."...Beefing up the frame excessively, could restrict that flexing and in turn cause a number of other problems.

I thought it was "semi-monocoque". But either way I can't come up with a problem that would be caused by excessively beefing up the frame other than eating up one's load carrying capability.

Outside of that, what other problems might possibly occur?

Thanks,
Tom

[Inland RV Center, In]

Originally Posted by TomW

I thought it was "semi-monocoque". But either way I can't come up with a problem that would be caused by excessively beefing up the frame other than eating up one's load carrying capability.

Outside of that, what other problems might possibly occur?

Thanks,
Tom

Semi-monocoque is correct.

Flexing of the shell softens the impact of a sudden shock, such as hitting a bump with a tire.

The more rigid the shell or it's frame, the greater that same impact will have.

The greater the "impact shock" the greater chance of metal fatiguing to the frame as well as the shell.

A good example is the wings of old piston engine aircraft wings compared to the new type wings on commercial jets. The old style bouced around the sky even from the smallest turbulence. Not so with the modern jets. The wings flex which minimizes the shock to the fuselage.

Same is true with an Airstream trailer. It must flex to minimize the shock of impacts. To make it rigid, either the frame or shell, should in due time show cracks in the frame as well as the shell.

It would also, and probably in short order, start shearing rivets.

A soft cushion is the ideal. Worn out torsion axles, bad shocks, unbalanced running gear all take a toll on the shell and frame. Same will be found true with excessive rigidity.

Andy

[TomW]

Originally Posted by Inland RV Center, In

...It would also, and probably in short order, start shearing rivets...

"Probably"? In an earlier post, I got the impression you had seen specific issues due to a well meaning individual strengthening the frame

The greater the "impact shock" the greater chance of metal fatiguing to the frame as well as the shell.

The extra beefyness proposed by the proposed, mild steel mod would counter that. I am still trying to visualize why the shell would be affected MORE than usual.

A good example is the wings of old piston engine aircraft wings compared to the new type wings on commercial jets. The old style bouced...Not so with the modern jets.

ALL aircraft wings on aircraft of size still flex within the realm of visual perception. If no flex occured, the wings would shear off the fuselage in short order.

I am open to the possibility that beefing up the frame affects more than the load carrying capability, but specific case examples of bad results would be nice to hear about.

Tom

[JimGolden]

Hey guys,

Let me pose a question here: Which Airstream would last longer (all else being equal), one that's sitting immobile or one that's being hauled over railroad cross ties going humpty bumpty all along?

OK, so the answer is obvious, but here's the deal: If the suspension is working properly, and you have the correct spring rates and the correct damping rates on your shocks, then the stiffer the frame the better. The idea is that the wheels move up and down but the frame and shell keep on cruisin' nice and smooth. You don't want the frame/shell to be flexing around, you want them to move in a straight line. It's the wheels that you want to do the vertical moving.

If the spring rates were too high, then I would tend to go along with what some of the other guys have said about beefing up the frame too much might make it too stiff to where it cannot flex to absorb shock. However, in my opinion, that is true only if the trailer has suspension that is too stiff.

Now, we get into complications with springing it for a heavy load and then pulling it with light loads. So that is a big no no.

However, if the suspension rates are correct, I would think that a frame of infinite stiffness would be fine because the shell would not see any flexing at all then, and so there would be less stress on the rivets.

Anyway, if you do stiffen your frame, look at your spring rates. Don't buy 20,000lb axles for a 7,000lb trailer.

[airstream74]

Well, as you can tell I haven't posted the pictures yet. I have taken them, but just haven't figured out how to post them. Will work on that again within the next few days.

thanks guys, hang in there with me, I need all the help I can get.

airstream74