The coupling rod's off-center attachment to the crank pin of the driving wheel inevitably creates an eccentric movement and vibration when in motion. To compensate for this, the driving wheels of an inside-frame locomotive always had built-in counterweights to offset the angular momentum of the coupling rods, as shown in the figures above. On outside-frame locomotives, the counterweight could be on the driving wheel itself, or it could be on the crank outside the frame, as shown in the adjacent figure.