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motors balance unbalanced

Unbalanced motor output

Ok, so I've got these cheap motors in the DFRobots 2WD chasis that are so unbalanced that the thing almost spins in a circle! I've been searching the forums and it seems like everybody's using PWM to adjust the motor output, so I've been trying to adjust mine that way. Problem is, I can get it just about balanced at a higher speed but then it's totally off at a lower speed. And it got me thinking about "why" the motors are unbalanced. Is there some kind of electrical imbalance within the motor that's causing it to run at different speeds with (presumably) the same input voltage?