From the moment internal combustion engines were invented, balancing proved to be a top concern. Part breaks signalled important troubles for manufacturers, not to mention vibrations produced by inertia pressures and gas combustion.
The mobile parts of an engine are submitted to acceleration and deceleration phases that are particularly violent considering their mass and inertia. That's why engineers have to factor in static balancing (idling body) and dynamic balancing (moving body). Cyclical regularity also comes into play, whether it's a single- or multi-cylinder engine.
On a two-stroke mill, the crankshaft needs to rotate a full 360 degrees to ensure complete combustion, whereas a four-stroke unit will require 720 degrees -- hence the frequency and amplitude of vibrations. This phenomenon is bad for any engine, especially a four-stroke single-cylinder.
Consequently, many engineers will choose to incorporate a balancing weight to counter vibrations and make the riding experience smoother, while ultimately reducing friction and wear. The engine will also operate more smoothly as it revs up. Of course, the addition of such a balancing device increases the overall weight and inertia. It can also affect performance substantially due to the effort required from the engine on the move.
A balancing weight is capable of neutralizing inertia entirely during the combustion cycle when the piston is at the highest point of its stroke. You should know that a general rule calls for the crankshaft to weigh the same as the connecting rod, pin and piston combined. At mid-stroke, however, a centrifugal force disrupts the balance and vibrations are peaking.
To solve this problem, the engine is fitted with one or several counterweights for the reciprocating mass of the crankshaft. These weights are actually driven by the crankshaft through a pinion-and-chain system or a gearset. Vibrations are most often felt on single-cylinder engines; in other words, cyclical regularity improves as the number of cylinders increases. That being said, the layout of the cylinders also impacts the engine operation.