Hub dynamo with 2.4 watts vs. 3 watts
While visiting in Europe, I am borrowing a bicycle with a Shimano hub dynamo that is rated 6 volt 2.4 watts. It powers headlight only and seems to do a decent job for lower end hub. Bottle dynamos and some other hubs I have seen generate 3.0 watts. What determines the watt ratings? A local shop has a similar 6 volt 2.4 watt Shimano dynamo hub at a reasonable price. I was thinking that it might be useful on my utility bike. Is there a compelling reason to avoid a 2.4 watt generator? I would only power a head lamp.
A lot of people with 3W hubs split 2.4W to the head lamp and 0.6W to the tail lamp, so I guess that's the rationale behind the 2.4W head-lamp only version. Obviously, 3W would be brighter. Personally, I would like more than 3W, but no one makes one (on account of German standards, I think).
Originally Posted by folderfan550
As for the rating, it all has to do with the design: magnets, coils of wire, other things that I vaguely understand. There might be less inductance in the 2.4W version, so it might be marginally more efficient, but I think you're pretty much 'stuck' at 80% of the power at any given speed.
Incidentally, I have measured the inductance, resistance, and voltage production of my 3W hub: if anyone has the numbers for a 2.4W hub, let's compare notes.
Read through Peter White Cycles page. He's probably the US expert on hub dynamos.
Formerly Known as Newbie
German law states all bikes above certain weight limit must have non-battery powered (front?) lights. It also specifies maximum wattage for front (2,4W) and a bunch of other things (at which speed the dynamo must produce enough juice to run the 2,4W in full). That's the reason for specs.
I once specifically asked a web shop based in Germany to sell me a dynamo light but to replace the bulb with 3W as I'd be using a battery powered rear LED. They declined. The bulbs are readily available, so it's not a big issue to change it yourself.
As far as I know there's no reason why 2,4W dynamo should be automatically better (or worse) than a 3W dynamo. It all depends on how well it's been designed and built.
I ride with a SON dynohub, 6V 3W. I do notice when I have the additional 3W load when my head light is on. I'd love to have a 15W dynamo and headlamp but the load would be so great I'd probably end up going backwards.
I don't recall seeing a 2.4 watt dynamo, just 3W dynamos then 2.4W and 3.0W headlights and 0.6W taillights. Are you sure the dynamo is rated to 2.4W and not the head lamp?
The 2.4 watt hub on the bike I borrowed is a Shimano HB-C050-D. A local shop has a Shimano DH-2N30-E and the HB-C050-D for sale. Both are marked 6V 2.4W.