Originally Posted by
mulveyr
Yep, other than the atrocious drag you would get with the ancient bottle dynamos we had as kids, I suspect most people would not be able to tell if a dynamo was engaged or not in a blind test, especially a modern hub dynamo. The couple of percent efficiency lost is easily overwhelmed by other factors like, say, wearing a slightly baggy jacket on a chilly day. :-)
Bottle dynamos are a drag.
I was amazed when I got my 1963 Raleigh Superbe with a S-A hub dynamo. It was noticable whent he light was on, but just barely. My wife and I often had 'coasting races' on some of the longer gradual downhills around out neighborhood... with the light off we were eveny matched, witht he light on I lost every race by a bit.
I had a frustrating argument with the guy at the antique store when I bought the bike. He suggested swapping out the front wheel to get rid of the drag of the dynamo. I said I was looking forward to using it, and besiades, it will only make a difference when the light is on. He was convinced that the light being on or off would have no effect on the drag, but I knew different.