Select Tread1b as its morph target. We need to create a morph envelope for Tread1a. Leaving the default key at Frame 0 alone, create a key at Frame 20. Exit the Envelope and Objects panels, and position your Camera for a good view of Tread1a. Play this back in endless loop mode.
Treads are heavy and they usually sag on top.
While the initial curve in Modeler could be modified to include some sag, I don't recommend this. Try adding a couple to Tread1a. Give them a Limited Range of 1 and keep them in their default rotation, laying down on their sides. With a Limited Range, they will not affect the entire object and you won't need to mess with lock-down bones. Place them along the top of the object and position accordingly.