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Sturmey-Archer X-FDD (2.4 W)

Dynohub with a drum brake - a hub of choice for all-weather no-maintenance utility bikes. It runs on cartridge bearings and is advertised as perfectly sealed against weather, but I can't confirm it because I haven't seen the internals myself (yet). The hub is totally silent and its resistance so small that I can't tell if the lights are on or off without looking at them (I have no direct comparison with a standard hub on this bike).

The brake drum has 70 mm in diameter (larger 90 mm XL-FDD version is also available). Stopping power with a 28" wheel is slightly better than that of old Favorit calipers, but as I'm now used to disks, I think it is not enough - certainly not enough for a quick emergency stop. Weak spot is long descents: the drum overheats and you have to let it cool down before the magnets in the generator get fried (80 degrees Celsius is their limit). Wheel removal is no problem, the cable unhooks without any tools and the reaction arm pulls out of its bracket easily. Maintenance is trivial, just adjust the cable stop and you're done. My brake pads don't show any signs of wear after 7 years and 6000 km, I haven't even have to adjust the cable yet (spare pads can be bought if needed).

Generator characteristics

Nominal power is 2.4 W (there is also a 3 watt version). Measured with 28×1,75" (47-622 mm) wheel, circumference 2231 mm was set in the speedometer. For wheels of different size, divide the speeds below by circumference of my wheel and multiply by circumference of yours. Output current was rectified by a bridge of four diodes 1N4004 and smoothed by a capacitor.

Open circuit voltage rises to about 30 V at 30 km/h.

Pn is nominal power of 2.4 W, the numbers are speeds in km/h.

Voltage and current consistently rise through the whole range of test speeds, I didn't reach any limit. From the shape of the plots it looks like current limit is around 460 mA at over 40 km/h, but it's just a guess, I couldn't ride that fast.

Standard 2.4 W light bulb can be connected directly, even though I would be afraid of it at higher speeds. Lights consisting of smaller LEDs need some overvoltage protection (Zener diode or the like) because the power at lower currents is not clearly limited. High-power LEDs can draw full output directly without any protection as long as the estimated current limit really exists, just add a rectifier. Optimal load yielding the most power from the generator is about 45 Ω.

Source data to download (XLS, Excel 97).