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I have a simple two-bone rig. One bone has the constraint "child of" pointing to the other bone. Both bones are identical, in the same spot, with all axes pointing in the same direction. They are set to Euler ZYX.

I have the "child of" constraint's influence set to 0.5, so it follows with a lag. I rotate around the parent bone around the X axis a full 360deg.

The issue: when the parent reaches 180deg, the child bone suddenly flips to point towards its negative Y axis.

I've tried turning off everything except "rotation x" in the constraint settings, but the results are even worse - the location of the child bone suddenly drops along the negative Z.

Any ideas what could be going on? Can attach the Blend file if I find a way.

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You could try with drivers. In Pose mode, select the "parent" bone and right click on the X rotation value in the N panel > Copy as New Driver:

enter image description here

Then select the "child" bone and in the N panel right click on the X rotation value and Paste Driver. The value should now be pink:

enter image description here

Then right click again on the same value and Edit Driver, it will open a pop-up window, in this window choose Type > Scripted Expression and in Expression type Rotation Euler/2:

enter image description here

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  • $\begingroup$ Hi, thanks for this! It is less the rig I am trying to build and more that I am trying to understand the basics of how bones work - so I am wondering what's going on mathematically that would make the bones do this, for greater understanding when I build more complex rigs (if weird stuff is already happening with just two bones, then...) $\endgroup$
    – karlaton
    Commented Dec 1, 2022 at 0:54

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