I'm planning to replace the original "Pipe Frame" with a more traditional "Ladder Frame". This is a major commitment and undertaking and probably the most serious challenge I'll face with this renovation project.
There are a few things driving me to entirely replace the frame:
- fresh and grey water storage
- space available to route the plumbing
- the original pipe frame is not structurally robust and prone to failure
Lots of people have renovated their Clippers with large water tanks placed under benches, etc. I've tried diligently but I can't figure out how to do this and maintain a safe weight distribution while also keeping the trailer tongue weight under 500lb (a typical max for midsized SUVs). As the water drains from the freshwater tank to the grey tank the weight distribution shifts and tongue weight shifts.
Placing the tanks close to the axle in the center of the trailer with a low center of gravity is the optimal location. The pipe frame has some space for this but it would limit fresh and grey water capacity to about 15 gallons. The pipe running down the center of the frame really limits the options for underfloor tanks of any size.
Another consideration is routing of the plumbing. The central pipe has limited clearance above and below it so plumbing can easily run from one side to the other. This is particularly true of the 1.5" diameter grey waste dump valve and the large water fill line. Both need to be accessible on the "street" side but one of them would need to run to the "curb" side past the center pipe frame.
It makes my head hurt. I really wanted to make this as simple as possible.
Another consideration is the soundness of the original design. Pipe frames were phased out of Silver Streaks and Airstream in the early 1950s for good reason. While they are exceptionally light, they are rotationally flexy when compared to ladder frames. Additionally the single central tube limits the weight carrying capability when compared to a dual frame rail design.
As a side note I found it interesting to see that the advertising brochure of the day featured a "4 inch 11 Gage Steel Seamless Shelby Tube Design". A little online research revealed that "Seamless Shelby Tube" is a reference to a particular manufacturer and the type of tube was used in the aircraft industry (as well as bicycles). Yet another tie-in to the post-WWII aircraft ancestry of many of these trailer designs and fabrication techniques.
I plan to keep the weight down as much as possible but a large water tank immediately adds 200+ pounds. Add a few batteries for boondocking and it starts to add up.
The other interesting design element is the axle attachment. There are two steel C-channels running parallel to the central pipe and the axle attaches to these as it would any other ladder-type frame. But these C-channels are attached to the aluminum cross-members which provides another failure-prone connection. Decatur had several of these fasteners missing or rusted out when I purchased it and they needed to be replaced before I could tow it home.
Most Clipper and pipe-frame Airstream renovations I've seen have had to address frame failures. Often the aluminum cross members are replaced with steel ones that are welded to the central pipe. This seems to be a reasonable fix although it can rapidly increase the weight as steel is 3x more dense than aluminum. It also doesn't address the plumbing issues I mentioned above.
In the end most of the more thorough renovations I've seen have gone the route of a frame replacement. Most of these replacements utilize a ladder frame design with external cues that nod to the original pipe frame, for example a pipe extending from the back even though it isn't the central spine of the frame.
I plan to take this same approach.
At a top level the frame will feature rectangular steel tubing as the main ladder elements. My analysis indicates that 0.120" 4"x2" rectangular tubes will be more than sufficient.
A simple comparative analysis of the frame rails can be done by comparing estimated beading of rectangular tubing to the round tube used originally. For a given size of tube, say 4" diameter like the original, a rectangular tube 4" tall with the same wall thickness will bend less than a round tube of 4" under the same load. Analytically this is because beam deflection is proportional to 1/(moment of inertia) and the moment of inertia of a rectangular tube is higher than a round tube hence the beam deflection is lower for the rectangular tube. Given that I'll be using 2 beams, each of which resist bending better than the single round tube, I'm confident this approach is sound. I'll also note that I did a simple finite element model of a single rectangular beam loaded at 4x the rated axle weight (3500lb) and it indicated negligible beam deflection.
The cross members will primarily be fabricated from 12 gauge mild steel. I say fabricated because I intend to use a "tab and slot" design approach to build up the crossmembers. This approach gives me complete control of the crossmember webbing design meaning I can remove material to cut weight but also run computer analysis to make sure I'm not impacting necessary strength. I'll do a post with more detail on this as I've just completed the outrigger design and ordered the initial set of laser-cut parts that I'll be welding up.
The other feature of the crossmembers will be different frame depth sizing. I've found some 30 gallon water/grey tanks that should work but they are 7" tall. So Decatur is going to have a 2 inch "baby bump" around the axle area. The belly pan is going to drop down a little to accommodate the tanks. Of course the frame crossmembers in that area will also need to be deeper to accommodate the bump out. I'm planning on 8 inch deep crossmember in that area vs the normal 6 inch belly pan depth.
A few crossmembers will be made from angle iron (probably 1.5"). The ones in the very front and rear where they really only support the subfloor are strong candidates.
The tongue will use a 32 degree couple vs the traditional 50 degree coupler. This fits the design better and will lessen the apparent deviation from the pipe-frame underpinnings. It will also provide an additional foot of tongue that will allow me to mount the spare tire between the propane tank and the trailer body front end.
Finally I do plan to maintain the protruding pipe from the rear of the trailer. I have in mind that I'll weld a 2" hitch receiver inside the pipe to allow for bicycle carriage.
As I mentioned I've ordered the first set of outrigger parts so I'm off to the races.
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