Dynohub with a drum brake - a hub of choice for all-weather no-maintenance utility bikes. It runs on cartridge bearings well hidden from dust and water. The hub is totally silent. It does have some little mechanical drag (freely spun wheel stops sooner than the same wheel with a similar hub without the generator), but I didn't measure it.
The brake drum diameter is 90 mm (smaller 70 mm X-FDD version is also available), it brakes very well in my 20" wheels and I wouldn't be afraid to use it for larger wheels too. Overheating at long descents is not as big a problem as with the 70 mm variant because there is an additional centimetre of aluminium between the brake drum and generator magnets (which must not exceed 80 °C or they cease to be magnets), but even a larger thermal capacity can get filled up. Wheel removal is no problem, the cable unhooks without any tools and the reaction arm pulls out of its bracket easily. You can see my special modification for a cargo trike on the photo: the arm is anchored by an M6 screw which goes through a hole drilled in it. Brake maintenance is trivial, just adjust the cable stop and you're done. I expect the brake pads to last almost forever, spare ones can be bought if needed.
Nominal power is 3 W (there is also a 2.4 watt version). I have the hub laced in 47-406 mm (20×1,75") rear left wheel of a trike and a speedometer in the front 50-406 (20×2") wheel with a circumference of 1543 mm, the difference between the two wheels is negligible. 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 45 V at 35 km/h. Premature end of some of the curves was caused by tired engine.
Absolute maximum current is 570 mA.
The nominal 3 watts would be generated with a 28" wheel at 20 km/h.
Pn is nominal power of 3 W. The zigzag shapes of some of the curves are caused by measurement errors.
Optimum load is something around 30 or 40 Ω. If you draw more current, you get more power at lower speeds, but less at high speeds. Compared to Shimano and SON generators, Sturmey gives a bit less power at higher speeds and with more scatter than SON.
Source data to download (XLS, Excel 97).
To get inside, unscrew the nut on wheel axle:
Nothing more to disassemble, the rest is held together by one-way spring washers, we can only clean and lubricate. When the pads are worn, probably this whole subassembly gets replaced.
By pulling the actuating lever, a rectangular block between the brake arms (at the front of third picture) rotates, pushing them apart. Direction doesn't matter, it works the same in both. The lever is fitted on a square end of the axle and the retaining nut is removable, so we can put the lever in any position we like and direct the cable in other direction than towards the reaction arm (I plan to take advantage of this on my next recumbent which will have this hub at the rear and the cable running forward instead of up).
The aluminium end plate has a draining notch at its lowest point to let out rainwater and wear particles. It didn't quite work in my nonstandard application: by lowering the reaction arm to horizontal position, the notch rised up and forward and let water in instead of out. And due to boxlike mudguards, the hub gets much more spray than usual. It resulted in weak braking until the insides dried out again. To fix this problem, I filed a new notch at the correct place:
Other part of the fix was mudguard modification to direct water away from the hub. The brakes work reliably since then.