The solution was fabricate a nifty swing-out rack to cover the RotopaX and provide a solid base on which to re-mount the tail bag using the original clips I made. The piggy-back rack (piggy-rack?) is made from leftover stainless steel from our old dishwasher fascia (I've gotten a lot of mileage--literally and figuratively--out of this scrap!) and the aluminum bottom plate from what was once the fastest optical networking switch in the world--a $250,000 circuit board that I can't bring myself to throw away. Poetic afterlife for both abandoned appliances.
An important design goal was to keep things really simple: no weird parts; simple construction that can be fixed anywhere with any available hardware, metal and tools; quickly openable to access the RotopaX; and quickly removable altogether without any special tools. After a few minutes of doodling I had a concept, took some measurements, and got busy.
First I made the top plate from the 1/16" circuit board aluminum. I wasn't really thinking this part through: I just traced the original rack and located where the RotopaX mount would potentially stick through. In retrospect, a plain rectangle would work better and the hole was unnecessary.
Now I needed some standoff hinges and this is where the stainless steel came in. From strips sheared by hand, I used a 3/16" steel rod as a mandrel (and future hinge pin) to bend a tab and form the main hinge body. While it looks tricky (the first one was educational), I was able to make three more in about 15 minutes each. All the bending was done in a bench vise by hand and using a ball peen hammer to tap clean edges. Note that the hinges lack cut-outs for the other half; these will be cut later.
Dry-fitting the four standoff hinges around the bottom rack and RotopaX revealed a fitment error. I'd aimed for a 1/4" clearance between the top of the pack and the underside of the top plate to allow for manufacturing variations of the packs and distortion from fuel expansion. However, I'd bent all four standoffs precisely at the wrong location. The top plate was too tight on the pack as a result. Oops. Rather than make new hinges, I was able to heat the bends red hot in a propane torch and then gently forge them flat on my anvil. Other than some minor oxidation, this removed most evidence of my blunder while relieving the metal of any work hardening stresses that would likely lead to cracking under road vibration.
It was then easy to re-bend the hinges in the correct location to get the right standoff height. Fortunately I'd left long enough tails to make this possible! The lesson is to not trim until you're sure about the fit.
Now to make the other halves of the hinges. This was done in much the same manner as the standoffs, using the same 3/16" rod as a mandrel and trimming the final size only once the bending was complete. By this time I was getting the hang of mandrel bending and it only took a few minutes to make each piece. It's important to file the edges square or the next step won't be precise.
A vague memory of traditional blacksmithing techniques guided the step of scribing and cutting the standoffs to accept the centre hinge pieces. First I cut slots through the barrel with a hacksaw, then I used a Dremel cut-off wheel to slice along the hinge axis and pop out the centre section. Some filework cleaned up the cut and gave a precise fit with just enough play to allow the hinge rod to slide easily.
Next was dry fitting and drilling hole locations. Unfortunately, my intended locations for the rear hinges wouldn't work because of the large holes cut into the 6mm bottom rack (that's something to fix in v2!). I had to move them to an inelegant location. I also debated whether or not the hinges should mount to the tops or undersides of the bottom and top rack plates for mechanical durability. For the bottom rack, I decided that having the hinge hidden away as much as possible would reduce the risk of catching it on something. This is also why I decided to mount the centre portion of each hinge permanently on the rack: it was the least intrusive component. They were attached to the 6mm plate with 6-32 machine screws tapped right into the aluminum. The tops of the standoffs were bolted to the 1/16" aluminum top plate with M4 bolts and nyloc nuts.
Eventually I'll replace all the hardware with one standard metric equivalent (probably M4), and rather than use tapped holes for the bottom hinge, opt for a countersunk bolt from above. Again, keeping it simple and standard makes it easier to fix on the trail with simple tools and common parts.
The hinge pins were cut from the 3/16" steel rod. The ends were rounded into smooth bullet shapes on the grinder to facilitate insertion through the hinges. A small hole was carefully drilled through each end to allow for a safety-pin style retaining clip (which I may wire to the hinge so it can't get lost). This would prevent the pins from vibrating out of the hinges. Both side assemblies are identical and the rack can swing either way by simply sliding out the appropriate hinge pin.
Here's the piggy-rack mounted on the WR. It's really fast and easy to open and close--no tools required! The whole thing weighs practically nothing. The hinge pins are the heaviest components.
I was initially skeptical that using such light gauge material would result in a durable design. However, it's surprisingly sturdy and immovable when you consider that the RotopaX itself bears most of the load via the blue pads I made from some old closed-cell sleeping pad material. I intend to make these pads bigger to improve load distribution. Keep in mind that the rear subframe is only rated to about 15 lbs, so it's not like you need to strap a goat on there (unless it's a small one).
Here's the finished assembly. I reused the original stainless steel clips I made for the tail bag straps, bending them higher and slipping a small length of split vinyl tubing over the sharp inside edge to reduce the risk of slicing through the Velcro. This is OK for now, but a sewn buckle system would be more secure and faster to remove.
Overall I'm pretty pleased with how this turned out although getting on and off my bike now requires some acrobatics. As a test mule, the rack really needs to be flogged in the field to see how well it works in practice. Already I have a list of improvements to the base rack, standoffs, and piggy-rack that I may incorporate into a commercial version that can be adapted to different motorbikes. Would you buy such a product? How much would you pay? I'd appreciate your thoughts on that.
Update (April 4, 2014)
Update October 5, 2014
This crude top rack has proven surprisingly durable despite overloading and trail abuse. However, the sideways flip is awkward to manage when the bag is full, so it's time to think about a redesign. Next version may flip forward, so the bag is supported by the seat when accessing the RotoPax. I may also widen the rear rack to provide better access to the side slots for bungie hooks (currently covered by the RotoPax) and room to rig a mount for soft side bags. There's a TIG welder in my near future which should also help me progress my jet engine project.