[Al Skierkiewicz]

There are a few reasons that robots don't run straight or differ in speed side to side..
1. Significantly more wire feeding the motor(s) on one side of the robot due to asymmetrical placement of electrical components.
2. Speed controllers not calibrated by procedure or software.
3. Mechanical issues such as bent shafts, dragging chains, excessive loading on shafts or excessive grease in one transmission.
4. Although CIM motors have a small directional bias most motors naturally run faster in one direction than the other due to the design.

The mechanical issues can usually be deduced by rotating the wheels by hand and looking for a difference from side to side. Your robot is trying to talk to you, all you have to do is listen.