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Shimano Nexus DH-3N20

Low-cost 3 watt dynohub designed primarily for city bikes. Its bearings are classic cups and cones (12 loose balls each), sized like for an average rear wheel. Seals are pretty tight with rubber rings on both sides of each bearing, so the interior should be watertight. The wheel should only rotate in one prescribed direction since the right-side flange is threaded; there is no effect on the generated alternating current, of course.

At certain speeds, engaged generator resonates with my bike's flexible fork and I feel soft humming in the handlebars. Other than that, it behaves nicely. At the beginning when I remembered having no dynohub on this bike, I felt a slight increase in resistance after engaging the generator, but now I don't notice it anymore. Resistance when disengaged is negligible and my average travel speed (part day, part night) went up.


Measured with 28×1,25" (32-622 mm) wheel, circumference 2093 mm was set in the speedometer. For wheels of different size, divide the speeds below by circumference on my wheel and multiply by circumference of yours.

Open circuit voltage rises more or less linearly up to 49 V at 35 km/h, I didn't measure it any further.

Short circuit current doesn't exceed 520 mA regardless of speed.

Smaller resistances work better for lower speeds, larger ones for higher. Nominal 3 W are generated at about 20 km/h, at 35 it's more than double.

Best power output occurs with about 50 Ω of load. Wide power variations complicate the required circuitry a little. I'd be worried about standard 2.4 and 0.6 W light bulbs, those 7 watts at higher speeds (when they are needed most) could burn them. Maybe I'd add a "long range" headlight that could be switched on for downhills. If you use a high-power LED that can safely eat the whole current output of the dynamo (520 mA), it's easy - no protection needed, just add a rectifier. Serial/parallel combinations of smaller LEDs running on higher voltage and lower current require some overvoltage protection.

Source data to download (XLS, Excel 97).


In normal use the hub is maintenance-free; I know a fellow who has been using it for over 15000 km and unless his bike got stolen, he uses it till today. I was unlucky and got a faulty one: it started to whirr after first 30 km of lighting. The cause was a loose pole yoke being shaken by the magnets. A warranty claim would take too long and I would have to relace the wheel, so an emergency repair took place:

First step: loosen a locknut, unscrew left cone and remove the balls.

This is the cone. The widest thing at the bottom is a labyrinth dustcap, the narrow top end is the cone itself and the conical object between them is a rubber seal. Another rubber ring at the inner side of the cup prevents the grease from getting in the electric stuff.

Second step: loosen right side flange by the octagonal segment. The photo is just an illustration, you actually need to clamp the octagon in a vise and turn the whole wheel, a wrench is not strong enough. Try to avoid damaging the aluminium. The thread is normal right-handed.

Third step: remove right flange together with the stator. Don't touch right bearing unless absolutely necessary (for regreasing etc.) - wires go through there and the threads are secured by some putty.

This is the rotor. Magnets are black, probably standard ferrite.

There is just one coil in the stator, magnetic circuit closes by a lump of pole yokes. They are rather massive and made of thin iron sheets, similar to transformer cores. They are held in place by a black plastic cage. I reshaped a bit of the cage by a moderately hot soldering iron, that fixed the loose fit problem. To be sure, I dripped some superglue in the seams. The hub is silent and reliable since that.