Flywheel balance

The following study paper was written by Richard Taylor about the effects of flywheel imbalance on a Lambretta engine and is published on with his consent.

You may fancy some background tunes while you take this in :~)

Flywheel balance

For many years I’ve been merrily bolting on Indian flywheels without a care. Flywheel balance is something I never bothered to consider in all honesty as I’m sure many have done and will probably continue to do so, but I won’t be one of them. I’ll be dynamically balancing all flywheels going forward.
Please read through this information and perhaps replicate some of the bench tests. In addition I would recommend you find a local company who can perform dynamic balancing.
Some people may tell you that flywheel balance only affects certain RPM’s, this is NOT true. If the flywheel is out of balance then it is out of balance throughout the entire rev range. Importantly the loads get exponentially worse with higher revving engines.

What is dynamic balancing

Static balance is achieved by balancing the object between centres and detecting which edge drops under gravity. Dynamic balancing spins the object at a known RPM. This has the advantage of increasing the measured force and defining angular position by means of a rotary encoder. It’s much the same as wheel balancing but it is typically done vertically (to negate gravity) and much faster, typically 800-1000 rpm.

What is the balancing unit of measure

The typical unit of measure is g/mm or Kg/mm (conversion between the two is obviously 1000). In layman’s terms if a flywheel has an imbalance of 200g/mm then it would require 200grams of material to be removed from the object 1mm off the rotational axis, at the point of imbalance. The weight of material required to be removed gets less the further away from the rotational axis. For example 200g/mm would require the following weights removed at different radii.

200 grams @ 1mm
20 grams @ 10mm
2 grams @100mm

How to calculate out of balance load

Out of balance g/mm or Kg/mm can be converted into a load. You can look this up on the internet and create a graph on excel. The load is not a linear relationship to RPM, this is very significant. Below is a graph that shows the load imbalance for a number of flywheels imbalance values.
As an example you can see that the flywheel with an imbalance of 0.4Kg/mm (or 400g/mm) creates a load of 19Kg @ 6500 rpm. It is also important to understand that this is not a constant load; it oscillates at the same frequency as engine RPM (108Hz in this case) because the crankshaft is horizontal and therefore the load is influenced by gravity at the lowest part of the rotation. In this example the crank experiences a peak load with 19Kg, 108 times a second.
From the graph you can clearly appreciate that high imbalance and high RPM greatly increase the load. The load imparted by the flywheel also deflects the crankshaft which has further consequences that are significantly influenced by flywheel weight.

One Reply to “Flywheel balance”

  1. Brilliant understanding of your knowledge is most valuable to anyone that rides and enjoys their scooters. Thank you for highlighting if this the case then bearings and cranks must get a longer life if treated with respect of other components to suit.

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