Attaching Shoulder Flange to A&A Frame

Attaching JAG-compatible shoulder flanges to the A&A frame wasn't as straightforward as I had hoped. The A&A guys made the bolt circle on their JAG stops considerably smaller than that of the Worr shoulder flange I was using.

You can see both the A&A PVC JAG stop and the aluminum shoulder flange in the photo below.

It wasn't immediately obvious to me how I was going to mount the flange, and thus the legs, to the frame. I had a couple of missteps in solving this. I'll detail the first and biggest misstep below because it was the one with the most time and money invested in it. I did get there in the end, though.

Once Step Forward, Two Steps Back

In my travels, I stumbled across another builder with the same frame and flange as me. His solution was to machine a well in the flange for a second set of bolts that would connect the flange to the A&A JAG stop. The legs would mount to the flange using the original holes. It was certainly an interesting approach. But I didn't have the resources or the expertise to accomplish the same thing. I'd need to do something different.

I saw that I basically had to choose one of two paths - 1) Replace the JAG stops with something of my own design, or 2) fabricate some sort of adapter. Both seemed like a bit of work. I (WRONGLY) opted for an adapter.

I ordered two 6" diameter 1/8" thick aluminum disks from MetalRemnants.com ($14.00 + $12.75 shipping (ouch)). I probably could have gone a bit thinner, but I wanted something that could handle the stress the legs would be placing on the entire shoulder area. 6" is actually a smaller diameter than the shoulder flange, but it was sufficient for the bolt locations and it should be hidden anyway.

I next created a drill template in Adobe Illustrator, attached it with some spray adhesive, and then drilled everything out. All holes accommodated 1/2" bolts. The center hole is for the electrical wiring. The inner holes are for the JAG stops. The outer holes are for the flange.

I was pretty proud of this solution. For months I scooted R2 around like this, letting friends and family drive him around the basement (where I work). All the while, I was unaware of a problem that was about to knock the builder-hubris right out of me.

The day came to test mount the skins. All I really wanted to do was to make sure the frame and the skins were up to spec. So I wrapped the front skins around the frame, carefully aligning them so that the front cut outs were perfectly centered, and then I taped the skins into place. I took one step back and my jaw hit for the floor. There were awful gaps between the skins and the shoulder flanges.

My first thought was that my 1/8" disks were too thick. But I took some measurements and found the gap was greater than 1/8" in places. Thinner adapters, or even no adapters, wouldn't have mattered. A gap of some size would still exist.

I needed to take another pass at this problem. Side Note: To this day, I still haven't learned how the A&A folks make this work. I know a number of builders have used this frame successfully. So it's a mystery to me. I'd be grateful if any of those folks could reach out to me and tell me why this wasn't an issue for them. :-P

The Solution

My final solution was to design a custom aluminum mounting plate for the flange. I considered designing a drop-in replacement for the frame's A&A JAG stops. But after some measuring and experimentation, I didn't feel confident that I could create a workable solution that could accommodate the bolt circle of the shoulder flange, be thin enough to eliminate the gaps, and not actually get in the way of the skins.

I decided to throw out the JAG stops and put my mounting plate right against the frame. There would still be a small gap between the aluminum plate and the frame's shoulder plate, which would accomodate the heads of the bolts for the flange and the legs.

I iterated on the design a few times before I found one that would work well. Each time, I prototyped it using a piece of plywood. The drawing for the final design can be downloaded HERE.

The plates were waterjet cut out of 5/16" aluminum by Big Blue Saw. This is what they looked like fresh out of the box.