Alright, it’s late and I’m tired and this was another throw-together test but I learned something about the linkage designs!
To test a possible bar construction technique:
I cut 1" wide strips of ~1/2" plywood into approximately 9" lengths (using a hand saw to keep it real). I carefully measured and marked one with three points, each exactly 4" apart (starting 1/2" in from an end). I stacked all six bars and clamped them together with two C-clamps. I then carefully drilled through all pieces at the marked spots. I used a drill press but it could be done with a hand drill. Hand drilling will make accuracy more difficult here. I think this can be a sufficiently accurate way to make the bars but laser or plasma or water jet would be WAY better.
I used a screw pin shackle to give the chain rotational freedom while still using the hole in the center of the bar. This way you don’t need specially shaped bars. May still need refinement but this is much better than direct attachment.
I tried the 3-Bar Top Mount in two configurations and the 3-Bar Balanced. I wanted to try them both with the same materials and hardware at the same time so I could really see the differences in performance.
The biggest thing I noticed was that with the Top Mount design any error or slop in the joints added due to the compression-tension trade off. In the Balanced design any error or slop in the joints effectively cancel each other (they probably still lead to minor inaccuracy but it’s a consistent offset no matter where the sled is, with the Top Mounted one the compounded error changes based on arm angles and that’s not fun at all).
Of the two Top Mounted set ups I tried the one with closer arms felt slightly more stable.
I think you can see in the pictures of the Top Mounted one that the vertical arms are slightly splayed, this is the compounding error I was noticing.
Even with my weights directly centered on the sled it feels quite “bottom heavy” using the Top Mount, [EDIT: after sleeping and re-assessing, I no longer feel this to be true. It was a perception when held in my hands but on the chains it is very well balanced) but with bricks on your sleds I imagine you’re already used to that. I just added 15lbs of weight in the center of the sled.
Another possible problem with either design (if stacking the bars like I did) is that the chain attachment points are not on the same Z-plane. This can probably be fixed and my not affect too much but it probably matters enough to think about it.
Here’s a far too small version of the Balanced design, there’s clearly not enough clearance with 8" spacing like this.
I just wanted to feel it compared to the Top Mount using the same weight and materials.
While I like the idea of the Top Mounted one because it can be made smaller and more compact, if I were to build myself one right now I’d probably go with either the Balanced or the 45° design. I just don’t like compounding errors!