The ideal balance would be theoretically obtained with a horizontally opposed (180° cylinders) motor with rods and cylinders in perfect alignment, in the same plane. Here outward and inward masses would be cancelled by the constant reverse motion of the pistons.
Balancing in Practice
In practice, we find it possible to balance a twin motor, or a single for that matter, by offsetting two-thirds of the reciprocating parts’ weight. Harley-Davidson motors have been balanced very satisfactorily with this method. This means that the pistons, rings, pins, locking rings, rods (with bushings), crankpin, rollers, retainers, nuts, locking washers and screws were weighed. One-third of this weight is taken and used to balance EACH of the flywheels. Thus, two-thirds of the reciprocating mass is balanced. The wheel with the determined weight, located in crankpin hole, is placed on true and level rails and the rim drilled at the proper point to allow the wheel to remain stationary in any position.
For very high speed motors, it is advisable to allow the counterweight side to be slightly heavier (2 or 3 ounces) than for stock motors. In other words, the reciprocating mass is lighter and the revolving mass is heavier.
Indian balancing may be done by using one rod with piston and rings, and one rod with piston removed, the flywheels then drilled to make this mass balance at any position on level test rails.
For a complete formula and illustration on flywheel balancing, we would refer you to the Question and Answer Book, pages 52 to 57 inclusive under Question No. 73. If you are interested in the mathematics of motor balancing, then refer to the work of Dalby’s “Balancing of Engines”, A.W. Judge’s “Automobile and Aircraft Engines”, or Ricardo’s “Internal Combustion Engine”. These are all English publications and may be obtained at larger libraries. End.