Shimano's budget hub generators have been widely sold and used but the answer to the simple question 'what exactly is the difference' (DH-3N30 vs DH3N20) has escaped various people including me. Both produce the same amount of electricity, about the same amount of drag, weigh about the same, look about the same....
The parts even interchange for the most part;
Well, yesterday I had one in each hand and I think I know now; the DH-3N20 model appears to have a three-piece hubshell (where the left flange is pressed into the body; it is slightly lighter in weight too) but the DH-3N30 model has a two piece hubshell, where the left flange and the main body are made in one piece. The bearings, seals, axle etc are all the same parts, they weigh about the same and they come out of the same factory in Singapore. So to all intents and purposes they are going to work the same. If anything with the hubs I had, the DH-3N20 turned easier in my hand than the DH-3N30.
I've overhauled the bearings on a few of these now and I have a few tips;
1) You need a thin 17mm cone spanner and another 17mm spanner for the locknuts.
2) The seal is a push fit over the central part of the cone; you can pry/lift the seals up 2mm without disturbing the bearings at all, add some aerosol motorcycle chain grease to the bearings (using the skinny plastic tube), then push the seal back down. If all you do is do this once or twice a year, the hub should last a very long time.
3) If you loosen the LH cone and locknut you can service the LH bearing in the normal way. Once the LH cone is backed out 3mm or so there can be (with a lifted RH seal) a larger gap on the RH side. If you fit a squashy plastic tube (e.g. a short length cut from the pickup tube from the inside of an old paint aerosol) to a nozzle-type grease gun, more grease can be added to the RH bearing without disturbing the RH connector assy in any way.
4) The bearings use 3/16" balls. It looks as if there is one missing, i.e. that you might be able to add another ball to the bearing assy; having tried this, I can report that sometimes you can, sometimes you can't... so 'if in doubt, leave one out', i.e. use the same number as shimano fitted to start with.
5) If you disassemble the connector on the RH side, you can overhaul the RH bearing. You don't need to unsolder anything, but you do need to bend the copper wire some. Shimano don't tell you this but the RH cone is an identical part to the LH cone.
6) I recommend cutting a strip of thin plastic and inserting it in the axle slot between the copper wire and the RH cone; this will prevent the shellac insulation on the wire from being damaged as the cone is unscrewed and then replaced. It will also prevent a dead short from appearing there in service. Sometimes the wire is set in the axle groove using a rubber material, but sometimes there isn't much of it where you need it, up against the cone.
7) The usual comments apply to setting the bearings; if the hub (with the locknuts tight) has a little play in the bearings, which then disappears when the cone/locknut are reset 1/30th of a turn tighter, then that is it for a solid axle hub. [With a QR hub the same procedure applies, but the hub must be tested for play/no play with the QR tight down on some dropouts, or washers positioned to take the place of dropouts].
8 ) The hub bearing design is flawed in that, during reassembly, balls can sit on the upper edge of the cup (instead of in the cup) and you can't see this from the outside. If the hub is like this it won't run smoothly no matter what you do with it; I have seen hubs scrapped because of this. However the misplaced balls invariably push the seal assembly outwards slightly, to the point that there isn't enough room to easily fit a cone spanner on the RH side any more. If your seal assy doesn't sit down then this is a good sign that the balls are in the wrong place.
In general terms the hub bearings, seals etc are of pretty good quality on these hubs; they are also a fair bit stronger than on 'normal' front hubs because the cones are larger diameter and there are more balls in the bearing. If kept well lubricated and adjusted, I can't see them wearing out inside a decade of daily use. I have (as an experiment) reground a DH-3N30 cone to a different profile (for another hub) and the hard layer on the outside of the cone isn't thin like it is with some hub cones, and I've not yet seen one break up badly, either.
So, if you are after the absolute lightest hub generator with the lowest drag then 'yes' you can do better, at a price.... but for commuting, winter training etc I honestly can't see the point in spending more.
The updated DH-3N35 (and the rollerbrake/disc brake versions) are mechanically similar (but not identical, the cones have the same threading but are different parts), however instead of an octagonal spanner being required to remove the generator assy, there is a large splined fitting instead.
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