Banebots/Cim motor programming question

[Japper]

On our robot,we are using 2 wheel drive with one CIM motor per side and mounted back to back (non-shaft ends towards each other).

In this configuration, the polarity of the victor outputs are positive to red lead and negative to black lead on one motor and positive to black lead and negative to red lead on the other motor. This seems elementary due to the orientation of the motor and its relative rotation to one another....

What causes some issues is that I am finding that these motors are not linear in regards to direction, where as for example, a PWM of 200 will spin the motor at a different speed while connected with the polarity for forward than it will with the polarity in reverse...

I understand how the PWM works where values greater than 127 is forward and less than 127 is reverse but I am talking about the motor speed relative to how the polarity of the motor is connected as these motors seem to move faster forward than backwards with the same relative PWM output and the polarity set for each case....

And thus the dilemna where one motor spins ever so slightly faster causing the robot to track towards one side.. Is anyone else experiencing this?

Can this be overcome with programming or can the motor speeds change in time and at different rates due to heat and friction?

The default code treats the PWM values the same per the 2 joystick control values with the assumption that one of the motors (at least in our configuration) must have its polarity set opposite of the other motor.

With the two joystick control, we can make the robot track straight with a little technique but I would like to take as much of the need for technique out of the equation as the students will most likely be in a high level of excitement during the competition. (yes we are rookies this year...)

Any suggesions on how to normalize the effects of the motors so that these track straight?

Thanks in advance.