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Pete · 07-04-2019, 04:03 PM

The degree of balance in any rotating part is directly proportional to the rigidity of the mounting.
A 4 cylinder engine with a single plane crank cannot be be so called "completely balanced".
It has the same problem as a single cylinder engine.
You can only balance it for a narrow rpm range.
This is why you still feel a model A engine shake at certain rpm ranges even after being balanced.
You can change the rpm it shakes at but you can't eliminate the shake without engine
re-design.
Certain types of industrial and racing 4 cylinder engines have achieved something close to ideal balance
by using "balance" shafts. This is a counterweighted shaft that is mounted on the oposite side of the
engine from the camshaft with the weights synchronized to counteract the vibration of the crank.

In a model A/B engine, the ideal situation would be to have the harmomic damper the same weight as the
clutch/flywheel assembly to reduce harmonic vibration. This however will not eliminate the felt vibration.
It will prolong the life of the engine though. In the real world this is difficult to achieve but can be
approached by using a damper from a big block Chev.

Reducing flywheel weight has an added advantage in making shifting easier and faster.
All of this modification will all but eliminate TORSIONAL vibration but will not eliminate total engine vibration.
You do not normally feel torsional vibration in the seat of your pants.
Crankshaft counterweights help reduce overall vibration by adding rotating mass and hence damping of felt vibration.
There is not enough room in the "A" crankcase to add enough counterweight to achieve a comfortable "seat of the pants"
vibration level. 60% is about all the weight that can be squeezed in without redesigning the whole engine.

A couple of definitions are offered here for a better understanding about balancing.
Static balancing is balancing in a single plane.
Dynamic balancing means the object is balanced over it's whole length.
Both are done while the part is rotating in the balancer.
Bob weights are not used to balance single plane cranks except for extreme rpm racing engines.

07-04-2019, 04:03 PM	#2
Pete Senior Member Join Date: May 2010 Location: Wa. Posts: 5,408	Re: Balancing The degree of balance in any rotating part is directly proportional to the rigidity of the mounting. A 4 cylinder engine with a single plane crank cannot be be so called "completely balanced". It has the same problem as a single cylinder engine. You can only balance it for a narrow rpm range. This is why you still feel a model A engine shake at certain rpm ranges even after being balanced. You can change the rpm it shakes at but you can't eliminate the shake without engine re-design. Certain types of industrial and racing 4 cylinder engines have achieved something close to ideal balance by using "balance" shafts. This is a counterweighted shaft that is mounted on the oposite side of the engine from the camshaft with the weights synchronized to counteract the vibration of the crank. In a model A/B engine, the ideal situation would be to have the harmomic damper the same weight as the clutch/flywheel assembly to reduce harmonic vibration. This however will not eliminate the felt vibration. It will prolong the life of the engine though. In the real world this is difficult to achieve but can be approached by using a damper from a big block Chev. Reducing flywheel weight has an added advantage in making shifting easier and faster. All of this modification will all but eliminate TORSIONAL vibration but will not eliminate total engine vibration. You do not normally feel torsional vibration in the seat of your pants. Crankshaft counterweights help reduce overall vibration by adding rotating mass and hence damping of felt vibration. There is not enough room in the "A" crankcase to add enough counterweight to achieve a comfortable "seat of the pants" vibration level. 60% is about all the weight that can be squeezed in without redesigning the whole engine. A couple of definitions are offered here for a better understanding about balancing. Static balancing is balancing in a single plane. Dynamic balancing means the object is balanced over it's whole length. Both are done while the part is rotating in the balancer. Bob weights are not used to balance single plane cranks except for extreme rpm racing engines.