Originally Posted by
noglider
Now they have higher output, so the big disadvantage with bottle dynamos is that they present more drag than hubs. Still, the higher drag is sometimes justified by the greater convenience.
The greater convenience is that you can disengage them when you don't need the light. (I'm sure that's what you meant.) Disengaged = no drag at all, of course. Some people say then can feel the drag of a hub generator, but measurements also show that it is quite small.
LEDs require so much less power than incandescent bulb, so a lower-output generator of any sort should work. Also the drag difference between the light switched on or off is much smaller.