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What I have:

Current rig

The four bones inside the arm (DEF_Arm_01 to DEF_Arm_04) are being controlled by an IK constraint. The IK targets Hand (selected) and goes up to the DEF_Shoulder (not including it). The pole angle is controlled by Elbow, floating behind the arm. The IK works as desired.

What I need: I need DEF_Arm_01 to DEF_Arm_04 to gradually follow/copy/otherwise propagate the roll of DEF_Shoulder to Hand.

How far did I get: I added a Copy Rotation Constraint, Y only, from local space to local space on DEF_Arm_04, having Hand as target. It worked like a charm! I realized I should do something similar to the other bones, adding two of these constraints to them (with Hand and DEF_Shoulder as targets) and changing the influence of the second constraint accordingly, for each bone.

I tried this. I added a Copy Rotation to DEF_Arm_01 having DEF_Shoulder as target. No go.

It seems that, as the DEF_Arm_04 has an IK constraint, it works as a stack and solves things in order, and the other ones are controlled by the IK and simply ignore other controls over their rotation and who knows what else. Also, I've tried to use an avulse bone as target, still no go.

So... What is one supposed to do to solve this?

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How did I end up solving this, in case anyone else wants to do something similar:

Four bone chains.

The first one is the IK, and it pretty much follows the rotation of the entire chain pole angle, but you can add a second bone chain to the arm (four bones) then apply a constraint to copy the rotation of the shoulder bone and then a damped track to the first, IK chain bone target onto it. In this very order. This way, you'll have a chain that follows the IK chain, but has the same up of the shoulder bone. Let's call this the "shoulder chain".

Then, add a third chain, and on each bone, add a constraint copying the rotation of the hand bone (the one that acts as a target for the IK chain), then the same damped track constraint to the IK chain bones' targets. This way, you'll have a chain that follows the IK controls, but each individual bone up direction follows the hand. Let's call this the "hand chain".

Then, the fourth, deforming chain: each bone will have two copy rotation constraints, each pointing to the correspondent bone of the shoulder chain and hand chain. But the later constraint will have proportionally less influence over the bone as you go down on the chain. As I have a four-bones long chain, I put it's influence in 0.125, 0.375, 0.625 and 0.875.

This way I have a chain that both follows the IK control and I can control it's twist along the chain using the shoulder roll and the hand roll as well.

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