Help! Elbow IK flips when bending the upper body

Hi everyone,

I’m having a strange IK issue with my arm rig.

When I enter Pose Mode and rotate ROOT.upper with R, the elbow suddenly bends in the opposite direction at a certain angle. It doesn’t flip 180° anymore, but there is still a clear point where the elbow abruptly changes direction.

I tested the same motion on several other character rigs, and their elbows remain very stable. So I believe there is something wrong with my rig rather than Blender itself.

I’ve already spent a long time checking, but I still can’t find the cause.

I’ve attached a simplified .blend file containing only the armature.
Since I’m a new user, I can’t upload attachments yet.I’ve uploaded the .blend file here:

If anyone could take a look and point out what I’m doing wrong, I would really appreciate it.

To reproduce the issue:

  1. Open the file.
  2. Enter Pose Mode.
  3. Select ROOT.upper.
  4. Press R and rotate it.
  5. Watch the elbow suddenly change direction at a certain angle.

Thank you very much!

That’s what the pole target is for, to fix these issues. Move the pole target until stuff looks good