Using IK with two forearm bones

Hi all,

I am trying to setup IK for my character’s arms to be able to move the hands to any position.
Using the Two Bone IK node, this works well using Unreal’s standard human rig but not our character’s rig.

The rig we are using has two bones in each forearm, to be able to properly animate twist.

With this setup, the Two Bone IK node does not work anymore, since there are three bones in the arm:

FABRIK does not work either since it resolves the bone chain by ‘breaking’ the forearm:

Is there any way to prevent this, for example by ignoring or constraining the second forearm bone for the IK calculations?

Thanks in advance

Hi, do you have to do the twist setup like that?
I am currently using the Unreal ART tool to generate rigs and they do it differently, with an extra duplicate bone under the elbow that can be manipulated to twist the mesh.
I haven’t tried your method so can’t compare pro’s and con’s of each so you would have to try.
But with that method you can do basic Two Bone IK. Although I am struggling with the IK/bones not being able to lock the IK solve to a plane for good elbow bend. But that is a different story.


i know this is a pretty old post, but i found a workaround for rigs with multiple forearm bones. You actually can create a secound arm, that has only one forearm bone and isnt skinned to the character, but is animated the same way as the deforming arm. You then can apply an IK solver (FABRIK for example) on your secound arm bones and then copy over the rotations and transforms from these bones onto you deforming arm bones (there is a new experimental constraint node for the animGraph). For the upper arm and the forearm you want to copy the rotations and apply them to the first of the multiple bones in the upper/forearm and for the hand.