Originally Posted by
JohnDThompson
I'm not sure I'd describe it as a "severe" disadvantage, but Dynohubs do have more drag than ordinary hubs, even when they're not powering the lights. But the advantage of being able to have light at the flip of a switch is very nice.
Any mechanism that extracts power from the movement of the bicycle to power the light will add drag. Whether the extraction mechanism is a dynamo hub, a side wheel dynamo, or a contactless magnetic light dynamo. Right now the hub dynamo is the most efficient (think 3.5 - 5 watts of drag for 3 watts of output -- approximately)...