Many of the robots so far have been powered by 2 independent motors, each driving one side of the robot. Whether treads (Tankbot) or wheels (Tribot) are used, the principle remains the same: drive both sides at the same speed to go straight, and drive one side faster than the other to turn. Since driving straight requires both motors to be turning at the exact same speed, slight variations in the motors can cause the robot to drift quite a bit to the left or right.
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