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Primary balance is when the crankshaft counterweights offset the weight of the piston and rod. Secondary balance is when the movement of one piston balances the movement of another. The 180-degree dual-pin flat-twin engine has secondary balance. The red/yellow dots represent the centers of gravity for the pistons. Their combined center of gravity is always right in the middle, at the green/black dot. Even the connecting rods are balanced.

Not represented in this diagram is an imbalance caused by the crankpins being off-center form each other. The engine has a slight rotational imbalance on an axis that is at a right angle to both the crankshaft and the piston bores. In the BMW boxer engine, the left piston is about two inches forward of the right one.

The crankshaft does not need counterweights, but the engine does need a heavier flywheel than a 90-degree V-twin engine does. In fact, since both pistons start and stop their movements at the same itme, they are together constantly exchanging kinetic energy with the flywheel. The result is that the crankshaft actually accelerates and decelerates through its rotation. As the pistons speed up through the center of their bores, the flywheel slows down. Then, as the pistons slow down near either end of their travel, the flywheel speeds up again. In addition to the vibration inherent from the pitsons accelerating and decelerating, there is the usual vibration that comes from the power strokes. These rotational vibrations are damped by a special spring-equipped shaft in the transmission.


Flat-Twin
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