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I am 100% new to the dynamo world, and after unpacking it I was surprised how it behaves. I already googled and found out that some of my worries are "normal", but for some of the issues observed I didn't find explanation (or "it is normal" confirmation).

I am talking about just hub, I didn't build a wheel yet. I am holding/squeezing axle nuts at one hand, and try to turn hub body with the other.

When I hold hub at hand and try to turn it there is a lot of friction, there are distinct points (some like stop points) where hub feels click-in (there is not audible click). Far, far analogy but it resembles the rear hub somewhat -- there is also some friction and there are also some special points.

Friction in dynamo hub I have is much greater. Gathering from all "it is normal" I take my hub is not broken. Yet.

But there is more -- the rear hub movement is predictable, I turn it by hand, the hub has no inertia so it stays exactly when I stop turning it, in other words any point/angle is valid point/angle. With dynamo hub if I am less than midway between points it bounces back by itself. So there are only X designated points/angles at which it can rest. And as for friction -- forward turning is hard, turning it backward is even harder (at first I thought it is not possible at all).

So my question is -- is this "more" section above normal as well, or those are odd even in dynamo-hubs world and I probably have broken hub (or terrible quality one)?

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Remember a dynamo hub has a small electric generator inside of it. What you are feeling when you turn the shaft is the interaction between the magnets and wires in the the generator creating peaks of torque on the shaft.

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    Yep, speaking as an electrical engineer, the sort of "clicking" feeling you get is due to the permanent magnets moving past steel "fingers" inside. They are not actually rubbing (or at least they shouldn't be, in a properly functioning unit), and, since the magnets pull in in one direction for a few degrees and then in the opposite direction, there is very little energy lost due to this effect. (But when you put a light on the unit obviously that drains some power.)
    – Daniel R Hicks
    Commented Dec 14, 2020 at 15:01
  • Thank you, this explains the bouncing back. But out of curiosity, why there is such difference in direction of turning? If I remember correctly my high school lessons I could spin magnets both ways.
    – greenoldman
    Commented Dec 14, 2020 at 15:03
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    The feel of the magnets is too smoothly bouncy to be rubbing or mechanical notching. I don't recall a difference in direction on either of mine when the hub was loose. With a built wheel fitted to a bike, there's quite a flywheel, so the notching is much less noticeable, but I have spun the wheel quite a bit recently fitting some new lights on one bike, and the occasional backwards spin didn't feel different
    – Chris H
    Commented Dec 15, 2020 at 8:44
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The apparent turning resistance, and the presence of discrete resting positions is normal for dynamo hubs. When I first got my Shutter Precision PD-8 dynamo hub, I was shocked that I could barely turn the axle with my fingers. However, in operation the effective resistance of the hub is negligible as you would expect. The "cogging" between electrical poles doesn't slow the bicycle down much, because although it does take force to turn the axle toward one of the pole locations, you will notice that once the hub has rotated past the pole it actually "snaps" back to the next rest position, giving back most of the energy. In operation the repeated motions past the electrical poles evens out. There is still some added resistance from the electrical generation, but only a tiny fraction of what you would expect from turning the axle by hand.

I do not have an explanation for why the resistance would be different when turning one direction vs. the other. As far as I know, it should be the same turning either direction.

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  • Does the felt resistance vary whether the load is connected or disconnected ? I've never owned one.
    – Criggie ♦
    Commented Dec 14, 2020 at 21:18
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    Yes it does. Not sure whether you can feel it on a dyno hub alone without a wheel, but a free-spinning dyno-hub-wheel stops significantly quicker when loaded even with just a small LED light. When turning the wheel slowly by hand, you can also feel the individual poles more distinctly with the load connected.
    – Erlkoenig
    Commented Dec 15, 2020 at 7:09

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