Shimano dynomo hub question

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Si
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Shimano dynomo hub question

Post by Si »

I've got my hands on one of these DH-3N7 Shimano dynomo hubs, but never had experience of one before. I'm comparing it to the one on my folder (made by a different maker).

If I spin the wheel on the folder it just spins nice and smooth like any other wheel...you wouldn't know that it was a dynamo hub if you had your eyes closed.

However, if I spin the wheel with the shimano dynamo hub in it then it feels very notchy like there is something clicking inside. The wheel goes round OK, but feels a bit like a back wheel with a stiff freewheel ratchet.

My question is: are they meant to be like this and if not then what do i need to do to it?
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Si
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Re: Shimano dynomo hub question

Post by Si »

Tsk, think I've found the answer: http://www.ctc.org.uk/file/public/featu ... ynamos.pdf good thing the CTC has a technical specialist eh!
beardy
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Re: Shimano dynomo hub question

Post by beardy »

Though some would argue that everything you ever wanted to know about cycling is available on the CTC Forum.

viewtopic.php?f=5&t=92777&hilit=rose+dynamo
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Si
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Re: Shimano dynomo hub question

Post by Si »

cheers, looks like everyone is getting dynohubs for Christmas
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[XAP]Bob
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Re: Shimano dynomo hub question

Post by [XAP]Bob »

It's fairly normal to really feel it strongly if you hold the axle, and to feel it turning the wheel by hand - once there is an appreciable rate of turn the magnetic poles are coming so thick and fast that they blur into one another...
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pete75
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Re: Shimano dynomo hub question

Post by pete75 »

Presumably a small wheeled folder? Dynamos for small wheels spin faster so have weaker magnets etc than dynamos meant for 700c wheels. That's probably why you find a difference when spinning.
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Brucey
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Re: Shimano dynomo hub question

Post by Brucey »

whilst it is the case that nearly all hub generators feel 'notchy' when you turn the axle by hand, there are two things to note;

1) it doesn't have to be like this, and

2) this may be almost entirely unrelated to the losses incurred when riding, anyway.

Here is an explanation, based on what I know and what a chum of mine (who does this kind of thing for a living) tells me.

The first thing to understand is that there is always a magnetic flux going from the permanent magnets through the soft iron parts of the stator. [NB this could be eliminated if these machines were alternators instead, but they are not...]. The flux coupling is an attraction, just as with any strong permanent magnet near soft iron or steel. If you slacken the hub bearings in a hub generator, the stator is always pulled off-centre because this increases the total flux linkage. So similarly if the total flux linkage varies as you turn the hub, there will be a force that turns one way and then the other.

Now it is possible to design a generator that has the same output but that also has the same total flux linkage at all times; this requires that the pole pieces overlap the magnets in the right way (which might be more expensive to make/design or package into a small space). If you get such a generator, turning the axle slowly produces very little notchyness. How is this possible? Well it is actually the flux reversal in the soft iron which does the generation, and this can be made almost independent of the variations in the total flux linkage.

The second thing to understand is that an alternating force (if exactly equal and opposite) isn't going to slow you down. An analogy is that if the road you ride along has small bumps and dips with a gentle profile, these won't in any way affect your average speed; their positive and negative effects will exactly cancel out.

The third thing to understand is that the losses in a generator are largely because of the magnetic hysteresis in the soft iron material, and the eddy currents that are produced in them as the flux reverses. Bad generators suffer from both issues, where good generators suffer neither in the same way. A notchy generator may suffer near-identical losses to a non-notchy generator in fact, because all each chunk of soft iron ever sees is a reversing field and the amount of reversal it sees can be (to a first approximation) similar regardless of variations in the total flux linkage.

Arguably a badly notchy generator is badly designed or badly made, but you don't really know if or how much this is really the case; they may just have traded some weight for some static notchyness. If you look at generator efficiency tests, you can tell if there are bad hysteresis/eddy current losses because the generator will drag as badly when the lights are off as on. Whenever I've opened up such generators, I've found single lamination, cold-formed pole pieces, which are probably made in mild steel rather than soft iron. This is done for cheapness, no other reason. Some of these generators are actually quite good in terms of static (i.e. total flux linkage) notchyness, so you just can't tell.

The fourth thing to understand is that you will get a 'notchy resistance' as soon as you connect a load (that draws any current) to the generator. So a non-notchy generator is only likely to be that way when there is no load on it. The way the mechanical resistance on the generator changes as it rotates is intimately liked to the characteristics of the generator and the load. If you get an unfortunate combination, you can get the total flux linkage forces in phase with the current generation forces, and both of these in phase with the resonant frequency of (say) the forks. The result of this is that the hub and the bike will 'hum' or even vibrate through the handlebars. If I ever get a really bad example of this, I intend to experiment with varying the reactance of the load circuit so as to reduce any phase linkage between the total flux linkage forces and the generation forces.

hth

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
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Re: Shimano dynomo hub question

Post by Vantage »

[XAP]Bob wrote:It's fairly normal to really feel it strongly if you hold the axle, and to feel it turning the wheel by hand - once there is an appreciable rate of turn the magnetic poles are coming so thick and fast that they blur into one another...


I had the same concerns as Si when I got my 3N72 but figured that as the LBS built it into the wheel, the shop owner would have noticed any issues with the hub when new and reordered if it was faulty.
One thing I notice when riding, is that at low speeds (2-6mph) the front light gradually 'strobes' faster until I reach high enough speeds that the little light blips merge and it becomes a steady beam.
What other people think when they see me crawling up a hill I've no idea.
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Re: Shimano dynomo hub question

Post by mjr »

Vantage wrote:One thing I notice when riding, is that at low speeds (2-6mph) the front light gradually 'strobes' faster until I reach high enough speeds that the little light blips merge and it becomes a steady beam.
What other people think when they see me crawling up a hill I've no idea.

The 3N30(or maybe another thirties number) here does the same as your 3N72. I think it's quite useful to strobe at other road users as one starts and stops... but I can see that it might be less desirable if it's happening while climbing.
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Re: Shimano dynomo hub question

Post by MikeF »

Si wrote:However, if I spin the wheel with the shimano dynamo hub in it then it feels very notchy like there is something clicking inside.
The "notchiness" makes adjusting the cones harder. The only one I've found (in my very limited experience) that had about the correct adjustment was my 3N80, but that might have been chance.
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Re: Shimano dynomo hub question

Post by MikeF »

mjr wrote:
Vantage wrote:One thing I notice when riding, is that at low speeds (2-6mph) the front light gradually 'strobes' faster until I reach high enough speeds that the little light blips merge and it becomes a steady beam.
What other people think when they see me crawling up a hill I've no idea.

The 3N30(or maybe another thirties number) here does the same as your 3N72. I think it's quite useful to strobe at other road users as one starts and stops... but I can see that it might be less desirable if it's happening while climbing.
The strobing is much more noticeable until the standby capacitors have charged eg when you first start riding. This is as would be expected as there is a greater current being taken at the same time as there are slow pulses.
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Re: Shimano dynomo hub question

Post by thidwick »

I've no experience of dynamo hubs either. But I bought - a bit of an impulse buy - a Shimano DN-2N30-E dynamo hub on eBay when I was looking at prices of some (non-dynamo) hubs to have a go at wheel building.
The hub is brand new, with a QR skewer, 36 hole.....and cost me £9.99 plus £4.50 postage. Came from Merlin Cycles. A bargain surely?
Turning the hub by hand feels very notchy, but answers and the link reference here are reassuring.
Next stage is to buy a 36h rim. and 36 spokes, and build the wheel.
The interesting question will be if I can just connect the output to any LED cycle lights.... but they are all 3 volt, so need to connect two lights in series. (I'm guessing). Does this work with LED?
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Re: Shimano dynomo hub question

Post by edocaster »

thidwick wrote:I've no experience of dynamo hubs either. But I bought - a bit of an impulse buy - a Shimano DN-2N30-E dynamo hub on eBay when I was looking at prices of some (non-dynamo) hubs to have a go at wheel building.
The hub is brand new, with a QR skewer, 36 hole.....and cost me £9.99 plus £4.50 postage. Came from Merlin Cycles. A bargain surely?
Turning the hub by hand feels very notchy, but answers and the link reference here are reassuring.
Next stage is to buy a 36h rim. and 36 spokes, and build the wheel.
The interesting question will be if I can just connect the output to any LED cycle lights.... but they are all 3 volt, so need to connect two lights in series. (I'm guessing). Does this work with LED?


When you say 'any LED cycle lights', if you mean just connecting them up to the dynamo... no. They will need a circuit to convert the AC to DC: http://www.pilom.com/BicycleElectronics ... rcuits.htm - moreover, if you mean converting a self-contained battery light to dynamo, the only part you will need from the original light's circuit would be the LEDs themselves - battery lights have all manner of drivers which are designed to work with a fairly steady voltage source. A dynamo would probably zap them.

Just follow the circuits on the link, and you will get a working light. The voltage of the dynamo (i.e. the '6V' rating) isn't critical - you can get over 12V from a dynamo at perfectly rideable speeds.
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Re: Shimano dynomo hub question

Post by iandriver »

mjr wrote:
Vantage wrote:One thing I notice when riding, is that at low speeds (2-6mph) the front light gradually 'strobes' faster until I reach high enough speeds that the little light blips merge and it becomes a steady beam.
What other people think when they see me crawling up a hill I've no idea.

The 3N30(or maybe another thirties number) here does the same as your 3N72. I think it's quite useful to strobe at other road users as one starts and stops... but I can see that it might be less desirable if it's happening while climbing.


I've never noticed this effect apart from pulling away for the first time on a ride. I've assumed it's the stand light capacitor making up for any difference that's good for about 10 minutes stationary. Maybe in need to find some longer hills :?
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Re: Shimano dynomo hub question

Post by peterh11 »

edocaster wrote:
thidwick wrote:I've no experience of dynamo hubs either. But I bought - a bit of an impulse buy - a Shimano DN-2N30-E dynamo hub on eBay when I was looking at prices of some (non-dynamo) hubs to have a go at wheel building.
The hub is brand new, with a QR skewer, 36 hole.....and cost me £9.99 plus £4.50 postage. Came from Merlin Cycles. A bargain surely?
Turning the hub by hand feels very notchy, but answers and the link reference here are reassuring.
Next stage is to buy a 36h rim. and 36 spokes, and build the wheel.
The interesting question will be if I can just connect the output to any LED cycle lights.... but they are all 3 volt, so need to connect two lights in series. (I'm guessing). Does this work with LED?


When you say 'any LED cycle lights', if you mean just connecting them up to the dynamo... no. They will need a circuit to convert the AC to DC: http://www.pilom.com/BicycleElectronics ... rcuits.htm - moreover, if you mean converting a self-contained battery light to dynamo, the only part you will need from the original light's circuit would be the LEDs themselves - battery lights have all manner of drivers which are designed to work with a fairly steady voltage source. A dynamo would probably zap them.

Just follow the circuits on the link, and you will get a working light. The voltage of the dynamo (i.e. the '6V' rating) isn't critical - you can get over 12V from a dynamo at perfectly rideable speeds.


The other option is to buy an LED headlight designed for a hub dynamo. These are now available at very reasonable prices and will save you time and hassle.

Check out the range on www.bike.de (click union jack flag to get English version), and search for "lumotec lyt" to see that you can get a basic but decent headlight with built in standlight (for when you stop) for under 20 Euro now. There is plenty of choice in a range of price brackets and the IQ Cyo range are good headlights available with standlight and light sensors to come on automatically when it is dark, available for around 50 Euro - search for model "175QRSNDI" to find this one. When set up right, they also direct the light nicely on the road on front of you and don't dazzle oncoming road users - a problem with many cheap battery LED lights.

Peter H
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