Continuing the discussion from Throwing my hat in the sled modification ring:
I’m, splitting this idea from @awlinc out of the massive ring sled discussion, because it’s my new favorite and I think it’s very promising. A lot of the other triangular kinematics designs require custom hardware like those offered by @bar (laser cut steel ring & v-groove bearings upcoming), @dlang (steel top ‘pantograph’), and @pillageTHENburn (laser cut birch 45 degree ‘pantograph’). All of these solutions look awesome, but the parts may be hard to come by in remote locations.
I’ll summarize what was discussed so far, as I interpreted it. Most of these ideas are someone else’s, my apologies if I didn’t give appropriate credit, please feel free to mention if these are some of your ideas. The basic design would look like this, but with the grooved ring elevated off of the base sled for proper COG offset.
The ring could be made with a maslow, or with a router, a board, and a nail, to a high degree of accuracy. To build with a router & board, the nail would be hammered through the board into plywood, and attached to the router on the opposite end, with the nail moving positions in the board (but at the same center point in the plywood) for successive cuts.
Construction would involve one 1cm deep dado indentation on the outside of a ring, followed by a full depth cut a little further out, leaving a circle of ply with an indent on the outer edge. A succesive cut would remove the center ~2" in from the circle, leaving a ring. This would be done twice, then a 3rd thinner ring with no dado indent would be stacked on top. This would create a circle 3 layers of ply thick when stacked, with 2 grooves to hold a #25 chain. The groves could be sanded to give them a slight taper to help with chain alignment.
Once the circle is built, it would be installed a few inches off of the base of the sled, near CG, with standoffs. Then, approx 1.5 meters of a chain loop would be rapped around each indent in the circle. An idler sprocket (these can come with built in bearings!) would be placed inside the loop of chain on each, then an intermediate linkage, similar to the one @bar uses on his ring, would attach the sprocket to the chains.
This approach shares a part with the standard Maslow, #25 chain, and the idler sprockets are available just about anywhere the chain is sold. The overall price could be as low as $30, and no special tools or skills would be required to build. Here are some parts I’ve found online that would work for a total of 22.78 + some shipping:
Long enough chain for both loops - $16.28 incl shipping
Idler sprocket with 8mm / 5/16" opening - $3.25 each + shipping
https://www.phidgets.com/?tier=3&catid=43&pcid=36&prodid=809
One thought that was mentioned was that the chain wouldn’t need to go all of the way around, and less chain could be used if screwed in to a section of chain that wouldn’t have to lift from the ring, but there’s a chance this could introduce some error and inconsistency.
I hope this is helpful, and I’d love to get everyone’s thoughts 
(free!!)
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