References to previous theoretical discussions of engine balance are cited prior to consideration of vibrations in four, six or eight-cylinder engines that may either be felt or heard in the car and result from lack of balance. Dynamic arrangement of the engine, unequal forces set up by the unequal weights of moving parts and vibration arising from elasticity or yielding of the parts themselves are the major causes of unbalance, of which the unequal weights of the parts are within the manufacturer's control.
Unbalance of the conventional four-cylinder engine is of considerable magnitude, due to the angularity of the connecting-rod that produces unequal piston motion at the upper and lower parts of the stroke, the unbalanced force reversing itself twice per revolution and acting in a vertical direction. The actual magnitude of this force varies directly with the weight of the reciprocating masses and as the square of the speed. Hence, a reduction in the weight of the reciprocating masses has a direct effect upon the magnitude of unbalance and the use of aluminum pistons and light weights has a tremendous influence toward vibration reduction.
From a production standpoint, the influence of mechanical unbalance of the parts and of variation in their weight that may create different unbalanced forces is of principal concern. Specific instances are illustrated and analyzed. The commercial balancing of crankshafts, flywheels, clutches and propeller-shaft assemblies is outlined, six and eight-cylinder-engine balance is considered and vibration-test results, in terms of actual measurement on the dynamometer and in the car, are presented. It appears that an engine in theoretical balance should have the reciprocating and the rotating parts balanced to close limits, but a more definite understanding of how this should be accomplished is highly desirable. Balancing of the engine components is essential, and reasonable tolerances can be established that can be adhered to commercially.
