My motorcycle (Keeway Superlight 125) has been burning headlight bulbs (S2, BA20d bayonet) at an impressive rate (one every a few months). By inspecting the bulbs I noticed that the little "reflector" inside (the piece of metal that shields the low beam filament) is noticeably warped like it's been overheated.
Voltage at the bulb with the engine running at about 6000 RPM is 12.70 V (+- 0.05 V) and seems pretty stable at varying engine speed.
I thought it could be caused by a poorly designed headlight reflector focusing light back on the bulb but the little "reflector" inside the bulb becomes red hot also when it's not placed inside the headlight.
Here the reflector does not seem warped but it fell off breaking both filaments
