So now the two shaking forces are becoming widely separated. What results is called a “rocking couple.” As the right-hand piston decelerates toward BDC, it pushes the right-hand cylinder downward. But at the same time, the left piston is decelerating toward its TDC, yanking the left-hand cylinder upward. The leverage for all this yanking is one-half of the separation between the two cylinder centerlines. The result is a whacking great side-to-side rocking motion from the engine. Because shaking force is proportional to the square of speed, vibration increases rapidly with rpm. Yamaha’s parallel twin streetbikes, making peak power at maybe 7,500 rpm, produced only half as much rocking couple as did its 10,000-rpm roadrace version. That was too much for the racer’s chassis, whose engine mounts were, one by one, broken by the fatigue stress (at an average 8,500 rpm, a 10-minute race applied 85,000 stress cycles, so it didn’t take long).