The rotor on the XR/XL is used to create a variable magnetic field and produce current through the stator coils. It is also used as a flywheel with a significant moment of inertia to store and release rotational energy and smooth out engine pulses. The more mass rotating at a given speed and radius, the more moment of inertia, the more stored rotational energy and the more “resistance” to acceleration or deceleration. In other words, an engine’s smoothness at low RPM for instance (resistance to stalling) can be helped by a flywheel with more inertial mass. Acceleration is also better controlled (less abrupt). This thinking is reflected in the choice of rotor/flywheel installed on the XR versus the XL. The XL model is mainly intended for street use with occasional off highway escapades — it thus has a rotor/flywheel with more inertial mass. The XR on the other hand is intended for sportier use where engine acceleration is favored over low RPM smoothness and resistance to stalling — its rotor/flywheel thus packs measurably less inertial mass (see picture below).
The XR rotor weights 2.31 kg (5 lbs and 1.75 oz) whereas the XL rotor weights 2.96 kg (6 lbs and 8.75 oz). The difference of 0.65 kg (22.9 oz) with a rotational radius of about 70 mm (2 3/4 in) is quite significant. Swapping one rotor for the other will result in an absolutely perceivable change in engine character. For this project I’m more interested in a very responsive motor than anti-stalling capabilities or smooth delivery of power and thus I’ll install the lighter XR rotor.
On the stator side, the XR and XL differ as well. The XR is delivered with minimal accessories – no blinkers, no battery, no real world headlight, just an enduro interpretation of a headlight. Its stator thus is a reflection of this minimal electrical power needs (see picture below).
The XL electrical system on the other hand includes blinkers, a real headlight and a battery which needs charging. Its stator thus incorporates beefier coils which provide the necessary current to the bike’s electrical needs (see picture below).
I intend to keep the project bike as simple and light and possible and thus don’t plan on mounting blinkers or a battery — I’ll use the XR stator. And if I change my mind, I’ll just swap the rotors and re-wire!