This is the part that makes a passive system like this difficult.
My opinion is also that a closed 4 chain system like the first one you proposed also requires too much tension to be safe or achievable on a wooden frame. It seems to me like this would only work if you drastically decreased the weight of the entire system including the chains.
An idea that I have bantered around for 3+ years, but never achieved, is a 3 chain system with a passive counterweight. In theory with three chains, nothing is ever over constrained. I have always thought of this design as a “vectored gravity approach” in a way moving the angle of gravity on the sled to keep both chains in tension. My hunch is that an angle in between the two chain angles is ideal, but I am not smart enough to know how to calculate this.
Anyways, here are two drawings of the idea. I am happy to discuss it if anyone wants. The vector of the passive counterweight is controlled by a 4th motor on a slide.
