Flywheel balance

Effects of heavy flywheel
If a flywheel has a very low imbalance measurement then the effect of its weight is greatly reduced (see the Imbalance load graph example 0.01Kg/mm). For example, during this work I measured an original Ducatti 3.2Kg electronic flywheel. Its imbalance was 9g/mm, staggeringly low. Clearly Innocenti were precisely balancing flywheels over half a century ago.
Should crankshaft deflection occur, an additional imbalance is created and this increases the overall imbalance load. Below is graph to calculate additional imbalance if a flywheel is running off centre (either through deflection or bearing runout). For example If a 3Kg flywheel is running off centre by 0.025mm (1 thou “) the additional imbalance is 0.08Kg/mm (or 80g/mm).
You can now appreciate the relationship between Load imbalance, crankshaft deflection and flywheel weight.

Bearing run out
Worn or second hand bearing have a big influence of crankshaft load. Even if the flywheel is perfectly balanced the use of a worn out bearing immediately adds runout which can be considered as flywheel offset. The geometric relationship between drive bearing, mag bearing and the CoG of the flywheel is approximately 1:1. Example, a 0.02mm drive bearing runout would add 0.02mm offset to the flywheel. If both the mag and drive bearings had 0.02mm run out the two values are added to give an overall flywheel run out of 0.04mm. We could then calculate the additional load and deflection of the crankshaft through the RPM range plus the influence of bearing runout.

Compound effects on balance, deflection and bearing run out

Below is a graph where the initial flywheel imbalance of 0.048 g/mm is plotted against RPM (red). The green plot is the initial flywheel load plus the additional load created from the crankshaft deflection.

The purple line is the additional bearing runout load based on 0.05mm runout of a 2.67Kg flywheel. What started out as a comparatively low 4Kg load at 6,500 rpm ends up at 12Kg. The worn bearing is tripling the crank load!
The blue line at the bottom it the original Ducatti electronic flywheel with 9g/mm imbalance. If I run the additional curves for crank load deflection they virtually superimpose each other because the values are so low.
The typical Indian mid weight flywheel is between 0.248 – 0.277Kg/mm imbalance. Between 7 and 8k rpm they have between 20-30Kg load before crank deflection is added. It is also worth noting that the balance holes drilled in the Indian flywheels bore no resemblance to where the imbalance was.

Collected data and notes

Andrews Precision advised that the flywheel weight should be 25% of the crank weight or less. In the majority of cases the Lambretta flywheels are considerably heavier than the entire crank and it begs the question how this could have come about (after all Innocenti were no fools).
There are examples of Lambretta flywheels being much lighter, for example LD’s. LD’s also have an advancing cam which allows the ignition to retard for starting and tick-over. It is possible that with the introduction of the Li engine came with an aggressive cost down exercise where the engine design was optimised for fast production and low cost. Perhaps in this process the cams were ditched because of cost and replaced with a fixed ignition which relied on an increased inertial mass for tick-over. Innocenti may have mitigated reliability concerns by precisely balancing the flywheels. I’m not aware of any documentation to confirm this; it’s a hunch.