To prevent flipping when a joint chain slides down its curve

1. In the Animation menu set (press F2), select Skeleton > IK Spline Handle Tool > .

   The IK Spline Handle Tool settings appears.

2. Turn off Root on Curve, Auto Parent Curve, Auto Create Curve, and Snap Curve to Root.
3. Turn on Auto Create Root Axis.
4. Click the start joint, the end joint, and then the NURBS curve.

   This creates the spline IK handle with a parent transform node above the start joint. In one of the following steps, you’ll put the node on a motion path that prevents the start joint from flipping.

5. Select the parent transform node, then -click the curve.

   You can select the parent transform node by dragging a selection box around the start joint.

6. Select Animate > Motion Paths > Attach to Motion Path > .

   The Attach to Motion Path Options window appears.

7. Turn on Start/End.
8. For the Start Time and End Time, enter the frame range for the joint chain’s motion.

   The parent transform node and its child joint chain move from the start of the curve to the end of the curve in the specified frame range.

9. Turn on Follow.

   If the curve has a 3D looping shape, you may also need to turn on Normal for the Up Direction to avoid unwanted flipping.

10. Leave all the other options at their default settings.
11. Click the Attach button.
12. Now when you play the animation, the parent transform node and joint chain move along the curve path. The movement should be free of unexpected flipping. However, flipping is unavoidable if the path is very complex.
    Note

    You can roll and twist the joint chain with the IK handle’s roll and twist manipulators for additional control. See Pose joints with IK handles.