12v LEDs and use on a motorbike
If the voltage from the battery is fluctuating - you could look at using a voltage regulator to keep it stable. You can get fixed value regulators (no resistors or other components required!) or variable ones (2 resistors are used to set them, but they're both quite easy to use). The one I've used, LM317T adjustable voltage regulator, requires the input voltage to be around 2v more than the output voltage (in order to stay stable) and they have a maximum input to output differential of 40v. In other words - you want to have LEDs that are rated at a lower voltage than your supply - then use the voltage regulator to reduce the supply and provide the correct voltage for the LEDs.