Be sure you add a diode in series with a + supply (a 1N4000-whatever). And it would be a good idea to put a Zener to ground (anything in the 20-30V range will do) after the 100 Ohm resistor. That'll help get rid of any noise spikes (which can be both positive and negative) from the bike's supply. Bikes are probably not as bad as cars, because they don't have lots of motors and solenoids for things like wipers and power windows/locks, but it's still not a "clean" electrical environment like your benchtop supply is.
Ran