Flywheel balance

Collated data from testing to date.

Importantly, all of these flywheels can be re-balanced to less than 10g/mm. This will resolve reliability issues such as crank twisting, splaying and spurious ignition timing problems such as miss-fires at high revs, unexpected contact between stator/pickup and magnets, curious timing issues.

Comparative notes

During this process I’ve revisited several other crank and flywheel combinations with regard to the ratio between flywheel and crank weight. CR250/YZ250 flywheels are approximately 18-20% of crankshaft weight. RS125 flywheels are approximately 25% of the crankshaft weight and many of the BSSO racers are using much lighter flywheels and or internal rotors. One particular champion has always been lightening dynamically balancing his flywheel. The general guidance from alignment with other manufacturer’s product is that performance engines must use lighter, balanced flywheels in order to increase reliability and prevent crankshaft damage.
It’s been recognised for many years that the quality of Indian parts has got exponentially worse. Indian flywheels seem to have escaped scrutiny until now, but I’d put this down to the general practice of assumption and the ability/process for checking not being widely known or practiced.

It is also worth mentioning that the calculation for imbalance and deflection would end up as a circular argument, forever one process increasing the other to the point of failure. Clearly this does not happen which is why I only applied one level of crank deflection as a best case. The reasoning behind using one level of crankshaft deflection is that there must be some level of specific frequency in the crank web and pin assembly which prevent the effects from being immediate. For example, if specific frequency is achieved there may be some rapid damage which is not evident from reving the engine through. To try and investigate and substantiate a piece of work like that would be hugely expensive and ultimately require a lot of high speed data acquisition and measurement from many running engines of exactly the same build, something that just does not exist in our market.


I used the company Andrews Precision Balancing in Bromsgrove. Suitable testing fixtures for GP tapers had to be made for their equipment (relatively low cost fixtures £100 or so). They charge £30 set up and £25 to balance. Money well spent IMO but please look for a local supplier so perhaps you can form a relationship with them so you can develop and confirm your own opinions.


The learnings I’ve taken out of these tests are quite simple.
1. Personally, I’ll be dynamically balancing every flywheel, regardless of make. The cost is quite small, typically a £30 set up and £25 to balance an individual flywheel.

2. I’ll always be using the lightest flywheel available because the comparative loads are much smaller should any imbalance occur

3. Tick-over with a light flywheel is achieved with an ignition that retards at start-up and tickover. My preference has always been RS125 conversions for the last 8 years or so but there are others, Kheper, LTH over-rev and the soon to arrive SIP.

4. Ignitions timing boxes that only retard are not good as they require inertia mass to throw the crank over TDC with increased advance at low RPM.

5. Even modern production flywheels had quite large imbalance (50 – 140 g/mm). IMO everyone is worth checking, the presence of balancing holes means nothing especially on Indian products.

6. Indian flywheels, especially those purchased on Ebay or other various routes from India have been massively out of balance 240 – 690 g/mm. The tapers have also been distorted by welding to the boss. It is essential they are balanced.

7. These findings are not unique to GT cranks; they apply to all 40.5mm wide cranks. I tested GT, PM, Indian and MEC, the effects are the same.

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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