Rotor / Flywheel


The engine rotates counterclockwise when viewed from the alternator side. The required flywheel puller has an M27 x 1.0 RH thread (same as used by GasGas with Kokusan Denki flywheel).

As you can see, there are only 5 flywheel “trigger teeth” (locations highlighted with a Sharpie pen), one of which is longer to allow determination of absolute position.

Even with the head off, the engine does not spin easily. There are 12 distinct “detents” in one complete crank revolution. This is due to 12 powerful permanent magnets (locations highlighted with “X”) in the flywheel. I've heard this called “magnetic drag.” I do not really care for that term, but can not offer anything better. It takes a fair amount of electrical energy to run an EFI system, and this alternator is much more powerful than one on a carburated trials bike. I also think the engine's primary compression ratio must be quite high, as I feel a lot of “pumping resistance” in addition to the magnetic drag. A high primary compression ratio makes sense to me as the simple exhaust system is not at all like a 2T expansion chamber.

Positions of magnets and trigger teeth annotated with Sharpie.

Flywheel trigger positions

Flywheel Trigger Events

From an oscilloscope capture of the pickup signal, I took the following measurements: 0.647" per division, 5.577" per revolution = 8.6 divisions @ 5ms per division = 43ms per revolution = 23.2 revolutions per second = 1392 RPM.

The diameter at the root of trigger teeth = 133.5 mm implies a 419.4 mm circumference.

  • Narrow tooth: 18mm long

  • Long tooth: 53.5mm long

  • Normal Tooth: 12mm long

  • Chordal gap: 57.25mm long

  • Long gap: 67.4mm long

Narrow tooth + Long tooth + 4 * (Chordal gap + Normal tooth) + Long gap = 415.9mm

18 + 53.5 + (57.25 + 12) + (57.25 + 12) + (57.25 + 12) + (57.25 + 12) + 67.4 = 415.9mm

The Image Carousel shows all trigger teeth in succession.