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Old 11-26-2021, 09:49 AM   #1
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1968 24' Tradewind
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Does frame flex have a function?

I know that it is normal for the frame to have flex that is offset by the shell connection. My question is: is shell flex functional in some way? Would there be disadvantage to a frame that flexes less?
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Old 11-26-2021, 10:58 AM   #2
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1958 26' Overlander
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I am of the opinion that less flex of the frame isn't a negative. However, how you achieve it may have negative issues not related to flex.

Example: Stronger frame rails with less deflection over length could achieve it, but the down side would be increased weight overall.

I decided to try a different modification. On my 58 Overlander I have 2X4 frame rails that have plenty of flex over 26'. As I disassembled to pull the shell I found damage to a number of outriggers, not just from rust. I didn't like the fact that the only support for the shell channel, between outriggers/frame rails was the plywood subfloor. To me that means that any load/stress the shell receives is only through the contacts points at the outriggers/frame rails. I wanted to find a way to distribute that load/stress more evenly. I built a square tube (1.5X1.5X.125) perimeter 'oval' to weld on the frame. It allows the shell channel to have support throughout its footprint. I believe it also spreads the load on the outriggers.

My original purpose started as a way to remove the subfloor from the 'sandwich' between the shell and the frame. Keep in mind my channel is just U shaped. It doesn't the additional C shape to slide the subfloor into.

Here's the frame with the perimeter support after it was painted.

Click image for larger version

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Here's a later photo, the frame is upsidedown on the rotisserie after the subfloor is installed inside the perimeter support.

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Although I didn't do it for frame rigidity reasons I also replaced all the crossmembers from the axle back with 2X4 tubing. I had to move a number of these from their original position to allow installation of BW, GW, FW tanks. I wanted a larger attachment face on the crossmembers for tanks supports and access panels (part of the belly pan) for all the plumbing. The supports/access panels are bolted to the frame with rivnuts installed in the frame rails and crossmembers.

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The black rectangles are the BW & FW tanks supports. The blue rectangel is the FW tank supports. The green rectangles are the access panels for plumbing/valves/pumps.

I suppose these crossmembers provide more torsional rigidity than the originals, but that wasn't my primary purpose.

I be interested to see if any engineers out there feel my theory is incorrect. I suppose no one will know for sure until I'm long gone and a future owner does a shell off.
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Old 11-26-2021, 10:59 AM   #3
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Less is best. The design of the AS does not like "flex".

semi-monocoque...relating to or denoting aircraft or vehicle structures combining a load-bearing shell with integral frames.
Definitions from Oxford Languages

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Old 11-26-2021, 11:46 AM   #4
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I agree with the posts above. Less flex would probably be better, however, these trailers don't appear to suffer from what little flex there is once the shell is married to the frame - they last a long time. I really like the easy entry and low height overall of Airstream trailers which is partially due to the semi-monocoque construction.
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Old 11-27-2021, 09:05 AM   #5
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Yes, frame flexibility has a function. Consider this: the frame is essentially flat, made of steel, and shaded by the shell. The shell is thin, continuous aluminum roasting in the beating sun. When materials are heated, they expand. Steel and aluminum do not expand at the same rate (nor are the shell and frame exposed to the same amount of heat). So on a hot day, that shell is going to expand. If the frame does not have adequate flex, then the expanded shell is going to have to go somewhere, and that is when the shell starts to "oilcan."

I have read at least one thread on these forums where just that happened. The owner over-beefed the rear part of the frame, leaving it less flex than it needed, and now anytime he parks the trailer in the sun, he gets a dent in the roof.

The same thing is an issue in wood working. If I build a table out of solid wood, then I can expect the table top to expand more across the grain than along the length of the grain. As a result, I must attach the top to the base using a method that will allow this mobility, or expect to find the top start to warp, or the base joints to be forced apart.
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Old 11-27-2021, 09:38 AM   #6
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Quote:
Originally Posted by Belegedhel View Post
Yes, frame flexibility has a function. Consider this: the frame is essentially flat, made of steel, and shaded by the shell. The shell is thin, continuous aluminum roasting in the beating sun. When materials are heated, they expand. Steel and aluminum do not expand at the same rate (nor are the shell and frame exposed to the same amount of heat). So on a hot day, that shell is going to expand. If the frame does not have adequate flex, then the expanded shell is going to have to go somewhere, and that is when the shell starts to "oilcan."

I have read at least one thread on these forums where just that happened. The owner over-beefed the rear part of the frame, leaving it less flex than it needed, and now anytime he parks the trailer in the sun, he gets a dent in the roof.

The same thing is an issue in wood working. If I build a table out of solid wood, then I can expect the table top to expand more across the grain than along the length of the grain. As a result, I must attach the top to the base using a method that will allow this mobility, or expect to find the top start to warp, or the base joints to be forced apart.
Flex's function...to loosen rivets and separate panel seams.😂

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Old 11-27-2021, 10:14 AM   #7
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Hi

If you go to a totally rigid frame (car / truck / trailer / van) the ride is absolutely awful. It's no different than taking out the springs and shocks. Folks have done it and regretted it .....

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Old 11-27-2021, 10:19 AM   #8
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Originally Posted by ROBERT CROSS View Post
Flex's function...to loosen rivets and separate panel seams.😂

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Old 11-27-2021, 10:28 AM   #9
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So, after having done several Airstreams decided to try something different and got a '46 Spartan Manor 25', very similar to most Overlander. Very interesting to see the significant differences in construction from a company that actually built airplanes and was flying high at the end of WW2. Spartan was not trying to build a travel trailer but a mobile home but seems the early shorter ones can be very usable coming in at 3700 lbs. There are many detail differences but I'll focus on the equivalent to the "C" channel. It is a very thick "u", about 3/16 that goes unbroken around the trailer except for the doors. Where needed there are splices of the same material that are buck riveted together with at least a dozen rivets. The bottoms of the ribs insert into the channel and are riveted in.
There are many more ribs and every part is riveted together along with the stringers. Ribs are thinner being folded all-clad .062. Especially due to the very beefy "u" channel I would think this body would flex much less. It sits on a frame very similar to a '70's Airstream. Very Interesting. Better? Maybe or not.
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Old 11-27-2021, 02:43 PM   #10
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Ask any trailer builder. If a frame is too rigid to flex it "will" crack.
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Old 11-27-2021, 05:46 PM   #11
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Ask any trailer builder. If a frame is too rigid to flex it "will" crack.
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We know they flex, try closing the door with too much stabilization down.👎

It's always a question of what's too much, how much is too much, or should I put a bicycle rack on the back of my trailer?

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Old 11-27-2021, 06:27 PM   #12
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Quote:
Originally Posted by Boondocker View Post
I know that it is normal for the frame to have flex that is offset by the shell connection. My question is: is shell flex functional in some way? Would there be disadvantage to a frame that flexes less?
What doesn't flex, cracks if enough pressure is applied. There have been some attempts with aluminum frames which flex a lot less than steel and are much lighter (and don't rust); for example, the Camplite trailers which had (have?) full aluminum frames (see picture), but there are lots of complaints about frames cracking.

You can reduce flex if you add steel, but then you add weight. So the whole thing is about finding the sweet spot where flex is not excessive (thus the importance of not over stressing the frame with excessive weight distribution pressure) while keeping the frame weight within bounds, since the market is asking for lightweight trailers. Its not just about Airstreams, even semi-trailer flatbeds are designed to flex when carrying very heavy loads.
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Old 11-27-2021, 07:23 PM   #13
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I’d agree that some flex is potentially good, but too much can lead to lots of issues. See all the threads on frame separation of 25’ and 27’ units in recent years. In this case, the frame is deforming WAY too much, thereby causing the shell to dimple in the front storage compartment.
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Old 11-27-2021, 08:13 PM   #14
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Yes..if the frame did not flex it would break and acquire many cracks…
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Old 11-28-2021, 12:43 PM   #15
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Quote:
Originally Posted by tjdonahoe View Post
Yes..if the frame did not flex it would break and acquire many cracks…
As would the panels & rivets of your AS, being that they are a part of the 'frame'.


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