How to move the head/tail along closed loop spline chain control rig

Does anyone know how to set up a Control Rig controller for a closed-loop system?

The plan is to create a circular chain (like a motorcycle chain), but it will be used as a pullable chain (people can pull it).

So I need:

  • a controller to adjust the spline shape, and

  • another controller to drive movement along the spline path.

I’ve already found the variable that controls the head/tail mesh position along the spline, but the variable is a static value.

Is there a way to make this variable non-static / driven dynamically?

(post deleted by author)

Trying to understand you want to change the min and max dynamically right?

Yes. I want the animator can change the min/max value in sequencer