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I'm trying to rig a leg that has two thigh bones (one to move the leg around the hip and the other to rotate the thigh around the bone's main axis):

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The IK constraint on the Shin bone reaches up to ThighB; ThighR has X and Z locks enabled on the bone's IK tab so as not to bend the leg between ThighB and ThighR. This works fine.

I tried to lock the ThighB's Y axis on the bone's IK tab but this has no effect - ThighB still changes it's roll when I move the leg (I also tried using a limit and setting a high stiffness, this also didn't work):

enter image description here

So I have two questions:

  • What exactly is the "Limit Y" / "Lock Y" option good for, since it doesn't seem to work as I expect it to?
  • Is there a way to keep the IK Solver from rolling/rotating ThighB around the y axis?

I'm aware that I could set up a second chain that just used a single bone and then copy the x,z rotation to the ThighB and the roll or the y rotation to ThighR, but I'm looking for a simple solution first.

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2 Answers 2

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With no pole, locking the Y axis works, and can be seen by locking/unlocking it in pose mode. However, what exactly it means to lock the Y axis isn't necessarily intuitive; in reality, orientations aren't sets of 3 independent axes.

There is one situation where locking the Y axis (of the first bone in the chain) will do nothing: when you are using a pole target. In this case, the calculations used to track the pole will replace the limited Y axis rotation of the first (only) bone in the chain.

Note that pole targets aren't necessary for IK; they're an option, and there are alternative controls.

If I wanted to create an IK thigh twist bone with a pole target, here's what I'd do:

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The thigh bone of the IK chain is non-deforming. The deforming thigh twist bone is parented to it. Note the hierarchy: the deforming (upper, non-twist) thigh bone is not parented to it. Instead, it's parented to the hip (or whatever the IK thigh's parent is), and then acquires the proper orientation via a damped track constraint, in this case, targeting the head of the thigh twist bone.

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Set same angle(so no lock, but if you want set min=max) to IK + rotation limit, local space)

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