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Old 10-23-08, 03:06 PM   #1
folderfan550
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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.
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Old 10-24-08, 02:03 AM   #2
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Originally Posted by folderfan550 View Post
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).

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.
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Old 10-24-08, 07:23 AM   #3
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Read through Peter White Cycles page. He's probably the US expert on hub dynamos.

http://www.peterwhitecycles.com/
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Old 10-24-08, 07:30 AM   #4
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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.

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Old 10-24-08, 01:18 PM   #5
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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?
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Old 10-24-08, 03:56 PM   #6
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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.
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