In the above video we discuss in a fair bit of detail what the purpose and advantage of an inner rotor is, and when you might consider installing one – Really worth a watch, even if just to improve your understanding of how your scooter ignition works.
To summarize, the stator in your scooter (similar to an alternator in a car) uses the rotation of the engine to generate electricity which it uses for various purposes on the bike. A factory stator has a number of coils, each of which perform a specific function (e.g. Spark for the spark plug, charge for the battery, power for the headlights, ect).
The principle of charge generation revolves around a magnet, passing over a coil. Each time the coil and the magnet interact, they generate electricity – as the engine revs and the magnet and coil interact at a higher rate the voltage increases (this is why the charge rate increases as the engine revs).
An inner rotor ignition uses this same principle, however it functions in the reverse to the factory stator. The factory system is based around a flywheel with a magnetic ring around it, which rotates around the stator (coil) to generate power.
For the engine to rapidly rev, it needs to be able to do so with as little drag as possible. The weight of the factory flywheel creates significant inertial drag on the crank as it tries to rapidly increase in RPM. This drag is compounded by the build up of centrifugal force, as the weighted component of the flywheel is relatively far away from the centre of the crank.
An inner rotor solves this problem by lightening the flywheel significantly, and placing it inside the coil component. This drastically reduces the drag, and centrifugal force build by placing a smaller amount of weight, much closer to the centre of the crankshaft – meaning the engine can increase in RPM far more rapidly.
An accurate analogy would be the difference between trying to run while dragging a 20kg bag of sand tied to a rope around your waste, or running without.
Because centrifugal force builds up exponentially as the engine revs, high revving engines experience far worse affects compared to lower revving engines. For this reason, we generally recommend installing an inner rotor ignition on scooters with racing cylinder kits – where the most benefit will be experienced (as these are not a cheap part).