i created a simple bone structure -> 3 bones with IK.
so the structure is like this : chain_root -> A -> B -> C
(A,B,C are bones.)

and by default the ‘goal’ will be placed at the end of chain (which is the end of bone C).
when moving the goal, the bone A,B,C will be adjusted , the IK solver will calculated the angle that will be used to rotate each of the bones.

the problem : when moving the goal , i want the solver to rotate JUST bone A and B,
so bone B and C are rigid.
in my case , Bone B and C will be used to control forearm. so the forearm will have 2 bones ( B and C) instead of just 1 bone , then i will use the C to twist the forearm. And the goal is still located at the end of C.
anyone know how to solve this kind of problem?

thanx

Here's something I just cooked up. Beware, I'm not a rigger your mileage will vary with the attached file. It uses a “shadow” forearm object to solve for the IK and then two “real” forearm bones for the RZ twisting. At the same time, I've tied the twisting to the arm controller's RZ value and orient constrained the palm bone to the arm controller. To play with it, you just bone the purple controller around and rotate it in RZ to perform twisting.

thanx a lot Edward,
that trick is very cool