[Greg McKaskle]

Since you have encoders, can you plot them over time? I'd be curious how much difference there is under different conditions. In particular, how much difference is there when the robot is up on blocks? Does the half joystick and full joystick bias persist or does the behavior change?

Looking at the videos, in the half-speed one, the robot's acceleration is minor and the robot stays level. In the full-speed one, the robot accelerates hard. The weight rocks the robot on its back wheels, and that seems to be when it veers right. It doesn't look like a constant arc.

So another test would be to ramp the motor values more slowly. See if that sheds some light.

Greg McKaskle