Rigging for Forearm Twist?

Anyone know a good way to model/rig/skin the forearm to get realistic forearm twisting (or at least without nasty deformation at the wrist or elbow)?

If you don’t know what I mean try this:
Stick your arm straight out, rotate your hand from palm up to palm down.
Note that the rotation doesn’t occur at the wrist - it occurs in the fore arm as your radius and ulna (the two bones in your forearm ) rotate around each other, allowing approximately 180° of rotation.

I’m not suggesting trying to rig a radius and an ulna. But in my current character rig (with 1 bone for the upper arm, 1 for the forearm and a main hand bone) I get some nasty deformation when I try to rotate my model’s hand up or down.

I’ve adjusted the weights of vertices between bones, and made sure that I pose the forearm bone as well as the hand bone for the rotation. This gives me some succes at distributing the deformation between the wrist and elbow.

But does anyone have any suggestions on better ways to handle this?
Thanks in advance.

What I do usually is add a bone between the forearm and the hand (a wrist bone more or less) and use it to control the rotation of the forearm on its axis. Then, you can tweak how the blending of the rotation goes with weight paint.


Expanding on theeths sugestion here is the setup I got going at the moment…


The 2 wrist bones sit in the arm bone and are parented to it. Use Wrist.R to control rotation and since Hand.R is parented to it the hand follows. Wrist_T.R also follows the twist due to the copy rotation constraint, but at a reduced rate since the influence is lowered.

You need enough loops in the forearm to divide up the mesh between 3 bones. I found it best to give ArmLo.R some low level influence ove most of the arm but mostly red just up near the elbow. Also you’ll probably want to really play with the weight painting tool to get the right amount of twist for each bone. You can get an idea by looking at the weight painting chart in my WIP(Warning:The influences of Wrist.R and ArmLo.R kinda look like their merging)


Work fine with IK have not tried it FK


Hi CrackerJack, I have a link for you but it’s in french and for Maya.

I haven’t tried it yet so I don’t know what it’s worth but maybe the pictures will allow you to understand what it’s about.

Hope this helps.


Thanks Everyone!
Useful advice. Now let’s see how I do.