A Well-Balanced Flywheel

A well-balanced flywheel, with all that, from the previous story, out of the way, let's plug the weight of some imaginary components into our formula and balance a hypothetical engine to 60 percent. Note that although some engine builders take pains to match piston weight exactly, others feel that a few grams 'difference is insignificant. To limit the story length, we'll ignore the controversy and work with combined weight rather than the weight of each component.

Rotating Weight: Rotating end of Rods: 865grams, Crankpin and keys: 565grams, Crankin nuts: 80grams, Bearing assemblies: 70grams, Lockplates and screws: 10grams, Total rotating weight: 1,590grams,

Reciprocating Weight: Reciprocating end of rods: 420grams, Pistons: 850grams, Wrist pins: 190grams, Retainers: 10grams, Rings: 80grams, Total reciprocating weight: 1,550grams,

Combining all of the these numbers and plugging them into the formula yield: (1550x60percent) + 1590/2 = 930 + 1590/s = 2520/2 = The magic number of 1,290grams. The magic number, which is in this case 1,290grams, will be the weight of the bobweight. This represents the ideal combined weights of the reciprocating and rotating components over a range of riding speeds, while taking into account other factors, such as combustion pressure, piston ring drag, and the engine builder's experience.

The flywheel/bobweight assembly is placed on a machined shaft, and the shaft placed on a leveled stand. Gravity then causes the out-of-balance flywheel to rotate. The flywheel will settle with the heavy side down, of course, indicating where it must be lightened. Because of the heavy counterweight opposite the crankpin hole, that side will usually be heaviest, requiring that weight be removed from the counterweight. If the reverse happens, weight is added to the flywheel's counterweight.

After removing the flywheel from the stand and drilling a hole or two in the counterweight, the engine builder will place the assembly back on the stand and let it rotate again. Balancing is complete when the flywheel stays put at any position on the stand.

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