Rear End Sag

[JimGolden]

Hi Everyone,

Been reading a bunch on here about rear end sag but I'm confused. What exactly is it? Is it the frame deflecting downward and buckling the aluminum shell, or is it plywood floor rotting out and sagging inside the coach?

My 1977 Excella '31 apparently had the chocks kick up or something and it's bent the belly pan up behind the axles. The lowest piece of moulding looks like it's running down hill toward the rear, but the upper two piece of moulding look straight as an arrow front to rear. Surely the shell didn't stretch. As well, jumping on the back bumper shows no gap so I don't think it's got separation. I think the lower moulding angle is an optical illusion type thing caused by the bent belly pan.

So, what exactly is rear sag and how do you check for it? As well, how do you fix it?

If it is truly the frame, would you jack up the back to a little bit above horizontal and then weld doubler plates onto the outside of the main frame longitudinal, maybe extending the doubler five feet past the axle mounting plate?

Please advise:

Thanks,

Jim

[SilverSurfer]

I have that problem too! Oops, I mean my trailer does! Mine is separated between the rear bumper and the shell. You can see the wood from the bathroom floor through the separation, and it looks rotten. I can't tell you how to fix it since I haven't done it yet, but there are lots of posts regarding it on here. Good luck!

[Silvertwinkie]

Using the search button above, type in the words "rear end seperation" and you will find many links that discuss in detail the issue and the suggested repair.

[Action]

The floor actually supports the outside shell panels. Should the floor rot enough the shell will sag. This occurs frequently in the rear bath models. Part of that is due to the weight the floor supports is also the commode.

Repair any leaks, from roof or pipes to keep water off of the floor. Keep the floor dry and it will not rot. Repair the rot as soon as it's found to lessen the impact.

This is one of the great examples of an oz of cure is worth a pound of fix.

>>>>>>>>>>>>Action

[dwightdi]

The usual circumstance is the frame bends on the longer units with rear baths and gray water tanks in the 70's. This caused a gap between the shell and the floor. Airstreams answer was to increase the number of bolts holding the shell to the frame to allow the shell to support the frame. It wasn't till 1984 they decided to increase the frame size from 4 inches to 5 inches. In the mean time they sold a kit to bolt a stiffener plate onto the web between the wheels to increase the strength and help relieve the bending problem that was occuring there.

[JimGolden]

OK, so if I'm understanding this right, there are two things going on.

First, the floor rots out and allows the banana wrap to sag downward somewhat (I guess 3/4 inch is the most it possibly could) and you get a warp in the shell.

Second, the frame is long and spindly and cantilevered out there a good ways, so it actually can deflect downward.

I did talk to Andy about this awhile back and shame on me for not fully understanding it at the time. But as I recall, he told me that the monocoque basically supports the frame and that separation occurs from leakage and the bolts rusting through allowing the frame to bounce downward independantly of the shell. So in other words, the moncoque shell holds the frame up, rather than the frame keeping the shell from coming down. But that is separation, which is a different issue. Or is it? Are the two interrelated? Andy said that out of balance wheels/hubs cause the long cantilevered rear of the trailer to shake like a fishing pole, which makes total sense. Combine that with 100lbs of mountain bikes and stuff hung off the bumper along with a 13 foot moment arm from the axle to the bumper and you have a 1300 ft-lb moment force beating on the back end and that messes them up.

So maybe I've resolved my own question here: It looks like they're separate issues yet maybe inter-related (sag and separation). If you get the separation problem, you then get the sag.

Looks to me like the fix would be to remove the belly pan, put a jack under the back, figure out how much static sag you'll get from just the weight, and then jack the rear up slightly above horizontal and weld on a doubler plate to beef up the frame. Then when you let the jack down it settles to a static resting position of straight. The greatest stress would be just aft of the axles. It'd be easy enough to get the same strength using a doubler plate as you would from going to a 5" deep channel section. It's a bit of work to do this, but not that much money. You could buy the steel for less than $100. Just have to weld it on. Even that's not so bad, worst thing is removing all the stuff to get into the main longitudinals. Looks like I'll probably do it as preventative medicine though.

Thanks everyone!

[Action]

I might be wrong and as I understand it (and from what I have seen on my unit) is the frame supports the floor, and the floor supports the walls.

Any issues with the frame or floor and it will show up in the walls/skins (inner and outer)

>>>>>>>>>>>>>>Action

[markdoane]

There is some truth in the idea that the body of airstream trailers holds up the frame.

If you consider the frame to be a beam, it is supported at two points, the hitch coupler and the axle. This leaves the back end free to bounce up and down like a diving board. I know this to be true because I watched it bounce when I took the frame down the road to be welded.

When you put the body on, it stiffens the beam tremendously. Now the back end of the beam is restrained and can't bounce up and down. It acts like a cantilever truss bridge.

That's a happy condition unless the connection between the body and the frame is weak. That happens when there is a leak in the rear causing the floor to rot and the bolts to rust. Or if it the connection was poorly concieved and manufactured in the first place.

In my '59 Tradewind, the only connection at the rear was a thin piece of aluminum channel that was screwed to the floor and riveted to the skin. As soon as the rear got wet the connection was lost.

Or it can happen if the original design is faulty, as expecting a 4" frame to hold up a 30 ft trailer. Or the end of the beam has too much weight on it, such as mounting a spare tire on the rear bumper, ot putting a greywater tank in the rear.

In order of importance I would rank repairs as follows:

1. Check for a cracked or broken frame, behind the axle mounting plate.

2. Make sure the connection between the frame and the body at the rear is secure. Do the rear separation 'bounce test'.

3. Balance the running gear and replace the axle if the arms are in 'positive' territory.

4. Remove any added weight such as spare tires and bicycle racks from the rear bumper. Don't store boxes of books and your blacksmithing tools in the rear locker.

5. Only after doing the above would I consider adding doublers, and then only if it was a long trailer with a 4" frame.