Theory And Practice In Motor Balancing

By Uncle Frank

From the December 1945 Issue of Motorcyclist magazine

Note: We are reprinting this article by Uncle Frank from our April, 1937, issue due to the demand we have had for back issues of this particular copy. Extra copies of this reprint are available by writing The Motorcyclist, 947 Bendix Building, Los Angeles 15, enclosing 15¢ for each copy. The April, 1937, issue is definitely a collector s’ item due to no extra copies remaining in our files.

This subject is so deep in mathematics and theory we would have to wade in with hip boots to understand it. We will try, however, to give you the elements of motor balancing as simply as possible. You need never worry about your stock motor balance as it was received from the factory because the motor was balanced for best all around service. However, if the pistons are replaced with lighter pistons, then rebalancing might be in order. We might add that pistons two or three ounces lighter than original pistons may be fitted to a motor without rebalancing the wheels and still the motor would run quite smoothly. If we do need to rebalance a motor, then we have formulae and set rules to follow for satisfactory results.

The service for which a motor is intended has much to do with the plans adopted for balancing flywheels. We would not balance a standard type of motor as we would a very high r.p.m. racing or hillclimbing motor. The standard type motor must run reasonably smooth at both extremes of speed, low and high. Whereas the racing motor functions more or less at full open throttle its low speed balance need not be considered. In fact, racing singles actually gallop at the lower speeds.

Inherent Forces Which Make Balancing Necessary

Due to the complex nature of the working members in an internal combustion motor we must consider four things when balancing such a motor.

1. The unbalanced rotating parts

2. The unbalanced reciprocating parts

3. Torque reaction

4. Torsional flexibility of flywheels (crankshaft)

The object in balancing a motor is to try and cancel out the forces which would make excessive and objectionable vertical and horizontal vibration. It is impossible to balance any motor perfectly! This is an impossible feat because of the complex forces which vary with the two extremes in a motor’s speed, setting up vibration harmonics. Vibration occurs when a mass in motion is interrupted or varied.

Engineers can, however, strike a happy medium in balance which will give a very satisfactory motor with out objectionable vibration.

Rotating Masses

The crankpin, nuts, washers, rollers, roller retainers and the lower half of the connecting rod make up the rotating mass in a motor. Naturally this revolving mass creates much centrifugal force. It would be rather easy to provide a counterweight to offset such a mass by adding weight on the opposite side of the crankpin. Thus, mechanical balance could be perfected. But we have more than rotating mass to deal with. The constantly varying force of the reciprocating mass sets up a harmonic that must be balanced to reduce vibration so that it is not objectionable. Since the lengths of the connecting rod, the stroke of the motor and the maximum r.p.m. all have to do with balance, these must be considered along with the weight of the reciprocating parts. Compression ratio is also a factor considered in balance formulae.

Reciprocating Masses

By Uncle Frank
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