Hub dynamos for 20" wheel

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hercule
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Hub dynamos for 20" wheel

Postby hercule » 25 Aug 2014, 5:59pm

Last year I bought a cheap factory built 20" wheel for my recumbent with a Shimano 3N20 dynamo hub - the price was very good and it had to be better than the howl of the bottle dynamo. I have duly been impressed, having lighting on tap whenever wanted (without the howling bottle dynamo) is a great advance! I have however thought that the hub is pretty draggy even with the lights off and by way of experiment swapped the wheel for a non-dynamo version and found the trike ran much more freely. I've got an idea to swap the current dynamo hub (with a wheel rebuild) into my hack bike and get a "nicer" hub dynamo for the recumbent, which tends to be used for much longer rides. The B+M IQ Fly I use has a light sensor so it was handy in the winter to automatically switch lights on and off according to conditions, including under trees, etc. I've not got any plans to power anything via USB… yet.

Apart from a SON which costs the earth, are there any suitable low rolling resistance hubs (especially with lights off) out there which would work in a 20" wheel? Or am I as well sticking with my Shimano hub and simply swapping wheels as needs dictate? The trike did seem to be a fair bit quicker without it, I don't think that's wholly in my imagination!

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[XAP]Bob
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Re: Hub dynamos for 20" wheel

Postby [XAP]Bob » 25 Aug 2014, 6:18pm

20" wheels rotate faster than larger wheels, so hub dynamos need to take that into account. I have a SON, but noone else did (does) a stub axle dynamo.

Does anyone else do a "small wheel" dynamo?
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andrew_s
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Re: Hub dynamos for 20" wheel

Postby andrew_s » 25 Aug 2014, 6:33pm

The 3N20 is bottom of the range Shimano, and whilst it's good for town bikes, it's draggy enough to put most people off using one for longer distances. It's also designed for 26"/700c, and will spin faster in a small wheel so you'll get 25kph drag when only doing 18 or 19.

A cheaper-than-SON alternative is the SP SV-8, £85 (or the SD-8 for £5 extra if you think you may use disc brakes in the future).
It's functionally pretty much the same as the SONdelux 20" hub. The difference is that if it fails out of warranty (2 years, I believe), it's currently either return to Taiwan or just buy a new one, whilst SON are 5 years, return to Germany via SJSC for £50 service/return charge afterwards. I've not heard of lots of failures, though they have only been available for 2 or 3 years.

The lower-end hubs can have quite significant lights-off drag. Shimano's original dynohub (NX10) famously had more drag with the lights off than with them on. When I found out, I got rid of the switch on my commute bike, and plugged and unplugged the lights with the clock change.

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Si
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Re: Hub dynamos for 20" wheel

Postby Si » 26 Aug 2014, 8:36am

I believe that mine has a "BioLogic Joule Dynamo Hub" which seems to work very well so far....can't feel any drag. Of course that may be because there is a load of drag when it's turned on AND when it's turned off so there is no perceptive change when flicking the switch, or it may be that there is little drag either way....who knows?

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squeaker
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Re: Hub dynamos for 20" wheel

Postby squeaker » 26 Aug 2014, 10:27am

andrew_s wrote:A cheaper-than-SON alternative is the SP SV-8, £85 (or the SD-8 for £5 extra if you think you may use disc brakes in the future).
Been very pleased with the SD-8 I fitted to my Grasshopper (with Trelock LS8885 light), but only 2 years ago so can't comment on durability. No noticeable drag, and the auto light on is priceless on a 'bent ;)
"42"

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simonineaston
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Re: Hub dynamos for 20" wheel

Postby simonineaston » 26 Aug 2014, 10:51am

I've just been trying out my latest tourer - a Moulton 'Safari', which has the Ultegra-level dyno-hub on a 16" wheel and I can't say I've noticed it's drag. By comparison I have a cooking Shimano product on the Brompton and by virtue of the cheap switch on my front lamp being broken and permanently on, the dynamo is always working and noticably a bit draggy... The top end Shimano dynos crop up on various German on-line stores at 'keen prices' quite regularly, 'specially as autumn looms i.e. now!
My posh Shimano is the DH-3N72 model ( I get very confused over their various model numbers...)
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Brucey
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Re: Hub dynamos for 20" wheel

Postby Brucey » 26 Aug 2014, 12:44pm

if you get a Dutch-spec 2.4W generator (or even a new style 1.5W German spec generator perhaps) it will have noticeably less drag than the equivalent 3W model, and will make for a better choice in a 20" wheel (barring some generators which are meant for small wheel use). You will almost certainly get a lot more than 2.4W (or 1.5W respectively) out of them if you run them in a small wheel too.

cheers
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squeaker
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Re: Hub dynamos for 20" wheel

Postby squeaker » 26 Aug 2014, 1:06pm

hercule wrote:Apart from a SON which costs the earth....
There might be other reasons for avoiding SON (see section 3.2) :?
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RonK
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Re: Hub dynamos for 20" wheel

Postby RonK » 26 Aug 2014, 2:04pm

hercule wrote:Apart from a SON which costs the earth, are there any suitable low rolling resistance hubs (especially with lights off) out there which would work in a 20" wheel?

Yes, these Shutter Precision hubs are the go. The Audax boys love 'em. I'm using a PV-8
If you think you may at some time want to charge USB devices, then get the standard hub, not the 20" model. The hub will produce more output at lower speeds. Specialist 20" hubs are really only needed in Germany where there are strict regulations about the brightness and alignment of bicycle lights.
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sore thumb
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Re: Hub dynamos for 20" wheel

Postby sore thumb » 26 Aug 2014, 2:31pm

I was going to recommend a SP dynamo as I have one onmy MTB. They also do one for 20" wheels.
But it looks like everyone has beaten me too it.

Great hubs!!!!!

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andrew_s
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Re: Hub dynamos for 20" wheel

Postby andrew_s » 26 Aug 2014, 2:36pm

Si wrote:I believe that mine has a "BioLogic Joule Dynamo Hub" which seems to work very well so far....can't feel any drag.

The Biologic Joule hub is a rebranded SP hub. The same goes for the Joule III. The Joule II is less obviously an SP hub, and may be someone else's.


Wouter has a bit of a thing about vibrations. Maybe it's his bike, maybe it's that he's taught himself to look out for them automatically. Most people either don't notice, or notice only occasionally and then forget about them.

hercule
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Re: Hub dynamos for 20" wheel

Postby hercule » 26 Aug 2014, 8:20pm

I have been leaning in the general direction of the SP hubs. They also do one similar to the Supernova switchable hub, but that would cancel out the desired 'lights on' trickery of the front light. Slight concern about the narrowness of the hub and the fact it's going to go on a trike (so significantly greater side loadings in corners) but in a smaller wheel maybe this isn't so significant. My Kettwiesel trike has no brake on the front wheel (it's very lightly loaded) but I'm considering the disk rotor model so I can swap it between the trike and my Nazca Fuego recumbent bike.

Are the Biologic hubs not 74mm OLN, ie for narrow forks on Dahon folders? When I looked at them before they also came with weird spoke counts - 22 ISTR - which severely limited access to suitable rims.

Brucey wrote:if you get a Dutch-spec 2.4W generator (or even a new style 1.5W German spec generator perhaps) it will have noticeably less drag than the equivalent 3W model, and will make for a better choice in a 20" wheel (barring some generators which are meant for small wheel use). You will almost certainly get a lot more than 2.4W (or 1.5W respectively) out of them if you run them in a small wheel too.

cheers


Would 2.4W be enough to power front and rear lights (both LED - IQ Fly front, Topline Flat rear)? Average speeds are in the realm of 12-16mph (the latter on a really good day!) so I'm no road rocket!

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Re: Hub dynamos for 20" wheel

Postby Brucey » 26 Aug 2014, 8:36pm

if you use a hub meant for 27" wheels on a 20" wheel it will be going ~33% faster than in a larger wheel at any given road speed, i.e. it will have the same output as if you were actually going x27/20 faster. The test for these generators is that they should produce some large fraction of the rated output at a speed of ~12kph or something. At 12-16mph on a 20" wheel you should be achieving the rated output very closely, because so far as a hub (meant for 27" wheels) is concerned, you are doing about 20mph.

As to whether any given light achieves full brightness on any given generator, it depends on the specific light and generator.

Some 2.4W generators will produce the same current as a 3W generator, ( they just need to be going a bit faster to manage it) and this extra current is usually what makes an LED light brighter.

However if the generator produces a fixed current value at an ever increasing voltage, this is less useful for more commonly available lights despite the fact that it is also 'more power'.

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simonineaston
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Re: Hub dynamos for 20" wheel

Postby simonineaston » 29 Aug 2014, 10:27am

simonineaston wrote:My posh Shimano is the DH-3N72 model ( I get very confused over their various model numbers...)

A self-fulfilling prophesy if ever there was one - it turns out the dyno-hub I built into the 16" wheel on my 'Safari' is the DH 3N80 model. Sorry if I misled anyone... here's a decent review of said product: http://road.cc/content/review/33808-shi ... hub-dynamo
(rides: Brompton nano & ever-changing Moultons)

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Mick F
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Re: Hub dynamos for 20" wheel

Postby Mick F » 29 Sep 2017, 1:26pm

Thinking of fitting a dyno hub to my Moulton.
20" wheels and 28h.

The Son 28 can be got in that spec, but at £180 it's a bit steep. :shock:
https://www.sjscycles.co.uk/dynamos/20- ... ub-silver/

Any other recommendations?
Any links?

Must be 100mm OLN, silver, 28h, and for rim brakes.
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