I build control panels, but I'm no expert on vehicles. Based on experimenting with some LED tail lights I built, I would say DC gets the nod. At low rpm you will see flickering when using AC. At high rpm the LEDs were bright, but not quite as bright as on DC.
I'm happy with AC for lighting as it keeps everything simple. The type of light you want to use will be the deciding factor. For incandescent including halogen AC is fine. If you want HID then you need DC.
For total light output, the wattage of your system is more important than the type of power. On a typical four stroke dirt bike, the windings are tapped such that probably 85% goes for lighting, with the remaining 15% rectified for battery charging. This keeps the regulator small and lightweight. When you float the ground, you now have full output going to the regulator. The regulator must be replaced with a higher capacity unit.
I played around with the 12 - 13 volts AC going to a headlight. I connected a full wave rectifier and got 17 volts DC out of it. I had intened to use this for a cooling fan, but figured the 17 volts would be too high. Measuring AC can be confusing as most meters will give you an RMS value which is less than the AC peak voltage.
These projects are a lot of fun as long as you don't burn out any expensive stuff.
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