[BN]

There are two separate types of vibration in regard to a rotating crankshaft/flywheel/drive line. Lateral vibrations and torsional vibrations. Lateral vibrations are caused by mismatch of weights (pistons, rods, unbalanced flywheel, etc). Lateral vibrations (at 2 x RPM) are also caused by the vertical acceleration at BDC being different than that at TDC, which are inherent for a 4 cylinder in line engine. Torsional vibration is oscillatory twisting of the crankshaft and drive line. For example (when looking at the front of the crankshaft) #1 rod journal will have a different oscillatory “clocking” than #4 rod journal at a given load and RPM. You cannot usually “feel” torsional vibration, although you can sometimes hear it at a given RPM if the clutch pressure plate springs resonate with the torsional vibration. However, the crankshaft journal fillets DO “feel” the vibration in the form of oscillatory stresses (fatigue stresses) that, over time, can cause fracture. The magnitude of these oscillatory stresses is related to the firing impulses and to the natural frequency of the rotating assembly. The natural frequency (there will be several of them) will change if the flywheel weight (or more specifically the moment of inertia) is changed. It will also change if the diameter of the crankshaft journals is changed (which changes the torsional stiffness). The maximum magnitude of the oscillatory stresses will occur at a resonant condition where the firing impulse frequency is at or near a natural frequency of the rotating assembly. This is called a “Critical Speed”. On many engines, a harmonic damper is installed at the front of the crankshaft (as a part of the pulley) to reduce the oscillatory amplitudes and associated fatigue stresses at critical speeds. So, changing the flywheel weight will change the critical speed at which the maximum fatigue stresses occur and may increase or decrease their value, as well as the location they occur (#4 main journal, #3 rod journal, etc). Also, the torsional vibration characteristics for a Model A engine vs a Model B engine are different – since the Model B has larger crankshaft journals and therefore more torsional stiffness. This may be the reason Ford went to a lighter flywheel on the Model B.