There is some strange twisting on a bone in my armature that I can’t figure out. This twisting only happens on the right arm and not the left. It also happens only when the right arm is rotated by the X axis by between -13 and -30 degrees. In the attached blend file, the bones are rotated by -25 degrees. I divided bbones for the arm into five segments and the ease in/out are set to 0. I set the ease in/out to 0 so that the bones remain straight on the X and Z axes but allow different amounts of twist around the Y axis for each segment.
In this rig I actually have two armatures: Armature.BC which is set to display octohedrons and Armature.BA which is set to display sticks. Armature.BC is meant to be controlled by me directly (BC stands for body controller) and Armature BA (BA stands for body adjustment) is meant to adjust the mesh. The mesh has an armature modifier for Armature.BA. The bones of Armature.BA have constraints to copy the location or rotation of a bone in Armature.BC.
I can’t find anything different about the left and right upper arms. Both bones are divided into 5 segments with ease in/out values of 0, and all bones connected to them have the same number of segments and ease in/out values as the corresponding bone on the other side of the body. What could be causing this twisting of the right arm and how could I fix it?
bbone_twist_20160908.blend (1.23 MB)