So I've been using tutorials to set up an FK/IK switch for my rig. After completing it and looking at the deform bones, they are working as expected when switching between FK/IK, but the mesh itself doesn't fully deform when FK has influence. Mesh has no issues when IK has influence so I don't think it's a weight painting problem. I copied the driver directly from the same set up I did for the legs (and retargeted to the proper bones) which have no issues so not sure what is different about the arms. Any ideas? I've attached the blender file at the bottom.

When IK has influence of 1 (IK rig = red | FK rig = Orange): enter image description here

When FK has influence of 1: enter image description here

Showing the deform bones by themselves:

IK influence enter image description here

FK influence enter image description here

Blend file:

A guide to my mess of bone layers for anyone brave enough to take a look: enter image description here


1 Answer 1


Let's enter weight paint on the badly deformed mesh, select a bad vert, and check out its weights on the sidebar:

enter image description here

We see a wide variety of weights, many of which I doubt you want. When an armature has bones labelled "DEF", I expect all deforming weights to be to "DEF" bones. But in this case, this vertex is being deformed, in addition to DEF bones, by 3 other bones. One of these is labelled "IK", suggesting that it's weighted to a bone involved in IK, so even when you switch the DEF bones to transform on the basis of FK instead, the IK bones still deform the mesh.

You can disable the "deform" property on a bone while in edit mode, in properties/bone/deform, which will prevent it from deforming the mesh. I would recommend doing this on any bones which should never deform the mesh. Doing this at this point, you may need to reweight afterwards (or, you may not.)

  • $\begingroup$ Yep! That fixed it. I had originally rigged and parented the mesh with the IK bones before I found out about IK/FK switching so I guess I missed that when I unparented the mesh from the IK bones. I knew it was something dumb. Thanks! $\endgroup$
    – ikillpxls
    Commented Dec 3, 2023 at 16:59

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