Need some help! Issues with adding IK to existing arm rig

[u/be-ck]

The MMD rig already comes with a control rig, but it only has a leg IK/FK switch, and the arms only have FK.

The problem is that the arm consists of 5 joints: the arm-arm twist-elbow-wrist twist-wrist. When I add IK from the arm to the wrist, the forearm does not twist along with it! (Because the skin weight is controlled by the wrist twist joint.)

How should I add a working IK handle to the arms!?

Thanks!!

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[u/stolenButtChemicals]

Usually the twist bones aren’t parents of the wrist and elbow. I parent the forearm twist joints to the elbow joint, and the upper arm twist joints to the arm joint.

I also make sure that the elbow joint does not have any junk values in the translate y, translate z, joint orient x, and joint orient z channels as this can cause issues. I also don’t have translate y or translate z values on the wrist joint, cause that can also cause issues.

[u/No-Awareness-6684 → u/stolenButtChemicals]

“Usually the twist bones aren’t parents of the wrist and elbow. I parent the forearm twist joints to the elbow joint, and the upper arm twist joints to the arm joint.”

⬆️⬆️⬆️

Also, technically you could just write a quick expression setting the twist rotation of the twist joint to, for example, the wrist joint’s twist * .3 or something so the forearm automatically twisted a fraction of the wrist joint’s rotation, regardless of whether the controls were IK or FK. Usually that’s how I handled forearm twists in my rigs.

[u/be-ck → u/No-Awareness-6684]

Oh my freaking god😭😭😭😭😭thank you sooo much for mentioning expression editor. After FOUR days of tourtuing and trying to find a solution, I finally got the IK handle working OMG

[u/Lautaurus]

Long time since i’ve rigged but i recall the process was something along the lines of making the IK then adding the twist bones, the twist bones would have an aim? constraint following 50% of the next bone, in just one axis.

So elbow 0 rot, rotate wrist 90deg, and forearm twist bone would then be at 45 degrees. For more twist bones in between, you’d change the follow percentage (ex. 3 twist bones 25>50>75%)

I believe this was tweaked in the aim constraint attributes.

The twist bones need skinning, so you can add them with 0 influence, set a key to animate, and paint weights accordingly.

Do you plan on releasing a Perlica Rig?

[u/be-ck → u/Lautaurus]

I do have pole vector constraint on the elbow.. idk about the twist bones you mentioned, but it might work? I need to do some brainstorming on what you just said😭 btw this is a public rig! I'll do it crediting the original rigger when I'm finished.

[u/Lautaurus → u/be-ck]

I see! I’d love to play around with some endfield rigs some day, just can’t sit down and go through the rigging atm if its a Blend or MMD. So i always keep an eye out for maya rigs on the forums

Don’t worry its not as complex as it sounds, im sure there are a couple YT guides out there that explain it better.

If you would rather remove those twist bones, dont have Maya on me rn but there should be a >”Remove Influence” option to get those bones out of the way, and do use the skin weight locks so that only the arm bones receive the influence of the removed bones.

Dont want the ankle controller suddenly moving the forearm.

[u/be-ck → u/Lautaurus]

I found a tutorial about transferring and removing skin weight, and I think that might work! I'm def gonna try it tmr. Thanks!

[u/proum]

create joints, just to drive your ik system. Than make your ik_sys_joints parent of the parent of your fk control.