HELP! How to rig arm rotation for ulna/radius in a skeleton rig

I’m super new to rigging and this was my first rig on a model that I’ve made. I’m trying to rig this skeletal arm to move and rotate just as it would irl. So far, I’ve got most of the rigging functioning properly but the ulna/radius rotation (bones in the forearm) is super tricky. I’ve spent hours already and can’t figure out how to get the wrist to rotate with those bones correctly. When the wrist rotates I don’t want the ulna (forearm bone closer to the pinkie) to be rotating with it, I only want the radius bone (forearm bone closer to the thumb) to be rotating. Here’s a reference video for what I’m trying to accomplish:

I originally had the ulna and radius parented to a single forearm bone but that’s not going to work for the rotation I’m trying to get. The problem is if I separate them into two separate bones, I’m not really sure how I’ll get the up/down rotation of the forearm to be synchronized for the ulna and radius while separating the movements of the two bones for pronation/supination of the hand. Any help would be appreciated, thank you!!!

so is the ulna influenced in any way by the wrist rotation? if not, parent the hand to the radius bone, parent the ulna bone the radius bone as well?

I tried doing that, but then my IK rig doesn’t work properly…I’m also new to IK so I’m not sure if I’m just not use the right targets or something. Right now, when I use the radius as the IK bone with a target at the hand, the IK rig only moves the hand, radius and humerus (arm bone), but not the ulna.

Here’s a schematic of what I want to happen and what’s actually happening when I have a single forearm bone setup vs. a separate bone setup for the radius and ulna. When I use a single bone for both the radius and ulna, the IK rig works fine, but I can’t individually rotate the ulna. When I use separate bones, the rotation of the radius works fine, but the IK isn’t working properly.

Is it something like that? (a bit complicated though)


Yeah!! That’s exactly how I wanted it to work :smiley: it’d be awesome if you could explain that or send me that blend file

The ulna is parented to the upper arm, it has an IK constraint with a Chain value of 1, its controller has a Copy Location with the other controller as Target. Also the controllers are unparented and the hand is parented to the radius controller. Here is the file.

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Thanks, that did the trick!
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great then! :wink: