Adding Inertia

Future Exploration

I feel the motor could use some additional inertia. (I've always appreciated heavy flywheels on trials bikes.) There is no “carry through” when you shut the throttle – forward momentum just ceases.

Adjacent is a shot from the service manual.

The motor's rotor shaft extends through the housing on the right (non-drive) side. The motor shaft is 16mm.

There is some room for an external flywheel. The space available is about 4.5" by about 1". That space was originally intended for a cooling fan.

Right side of motor. showing area intended for a fan

Moment of Inertia

I did some calculations for a solid steel disk measuring 110mm OD x 16mm ID x 22mm thick. Such a disk weighs 1.6 kg and has an MoI of 2475 kg*mm².

For comparison, I estimated the S3 flywheel addition I used on my OSSA at 0.5 kg and 1483 kg*mm². This produced a noticeable change. Whereas, the OSSA flywheel itself I calculated at nearly 3kg and 11,440 kg*mm².

Keyless Shaft Bushing

I found a really cool attachment method by Googling “keyless shaft bushing.” A number of products exist. They all seem superior to a key in every way but cost – typically $50 - $120 each.

I bought the item shown via eBay for $27. It has a 16mm ID, 25mm OD and is 30mm long. It's Fenner part number TTQM1626. It has a 22mm hex nut for tightening.

The thing I like best is its simple on/off nature. One can ride a section with and without the additional inertia to see exactly how it changes the bike's behavior.

Unfortunately, I have not even gotten around to ordering steel for the flywheel yet.

Fenner TTQM1626

Inertia Ring

If it were possible to locate an inertia ring directly inside the 5.7's motor, it might have about the same inertia as the external disk with only one-fourth of the mass.

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