generator hubs

[mig]

do all hubs fully power all the (high light output) front lights (such as those 100 lux jobs IQ-X from B&M?)

[the snail]

The 'standard' output is 3W which is probably best (shimano 3W hubs have a 3 in the model name e.g. dh3n80) there are also 2.5W and 1.5W hubs, probably best avoided if you want to run bright lights, rear lights or recharge gizmos.

[andrew_s]

The following hub types are commonly available:

Standard 3W for full size wheel
--> Most generator hubs are these - Shimano 3N72, 3N80, T780 ; Schmidt SON 28; Shutter Precision (SP) PD8 and PV8 etc
There are also "own brand" versions of the SP hubs from Alpkit, Supernova, and maybe others.

Standard 3W for a 20" wheel
--> Schmidt SonDelux; SP SD8 and SV8
When used with a 26" or 700c wheel, these are fine with lights, give a bit less light at very slow speed, a little less drag at normal speed, but may be a bit short on power if you want to use them to power gadgets or charge things.

2.4W
--> Shimano 2Nxx
1.5W
--> Shimano 1Nxx
These are mostly for German/Dutch commuter bikes, and aren't readily available in the UK.

[Brucey]

do all hubs fully power all the (high light output) front lights (such as those 100 lux jobs IQ-X from B&M?)

there will be some differences for sure, but the answer is basically 'yes' if the generator is a 3W rated one (for the wheel size in use). The extra output from the lamp comes from a mixture of clever electronics in the lamp (which will work slightly differently with different generators), better optics, as well as newer, better LED devices that are more efficient.

I'm due to be fitting an IQ-X to someone's bike in the coming weeks, so I'll know more about the volts and amps (on load) then. A lot of older generation lamps 'waste surplus power' at speed, because the LEDs in them can't handle the full output of the generator.

cheers

[DaveP]

As the subject has come up, I'd like to add a special plea for guidance in choosing a hub dynamo for a 26" wheel.
I only seem to be able to find details for 700c and 20" wheels
Would the PV8 be ok?

FWIW I would just like to be able to run respectably bright lights for commuting.
Any "device" charging would be a bonus which may or may not come in handy one day.

[freiston]

As the subject has come up, I'd like to add a special plea for guidance in choosing a hub dynamo for a 26" wheel.
I only seem to be able to find details for 700c and 20" wheels
Would the PV8 be ok?

FWIW I would just like to be able to run respectably bright lights for commuting.
Any "device" charging would be a bonus which may or may not come in handy one day.

If my sums are correct, then for the same wheel rpm, the 700c wheel would only give you 1mph more speed than the 26", and to match speed, the smaller wheel would need to be spinning at 7% more rpm (approximately) so in my estimations, the difference between 700c and 26" wheel rpm is not enough to worry about (I would expect more variation between different riders using identical bikes). Maths is something that I'm a bit hit and miss with though and I could be completely wrong. I used gear-calculator.com to get my wheel circumferences - 26" 40-559 @ 2030mm and 28" 32-622 @ 2170mm and I derived the mph difference for same wheel rpm by changing the wheel size in the web page chart and noting the different speeds for the sprocket/chainring combinations.

[meic]

The Shimano hubs have the range of diameters which they are recommended for printed on the side of the hub. That is they outside of the tyre, rather than the rim, the same as you would enter into your speedo.

Here is one saying from 400 to 716mm, which covers 20" and 700 with quite fat tyres on.
http://www.dutchbikebits.com/image/cach ... 00x500.JPG

[freiston]

You mean I did my sums for nothing!

[Phil_Chadwick]

It makes no real difference (wheel size)

Indeed, many people use the 700C dynamos in small wheels (20" and less) so that full brightness is achieved at lower speeds, whilst some use the 20" version in 700 C wheels as it has slightly lower drag and if you're a fast rider you won't notice the slight trade-off in low-speed performance.

Electrically, all 3 W dynamos are sufficiently similar thta you won't notice the difference. I have 2 SONs and 4 Shimanos of various models, and they're all much of a muchness.

[james01]

It makes no real difference (wheel size)

Indeed, many people use the 700C dynamos in small wheels (20" and less) so that full brightness is achieved at lower speeds, whilst some use the 20" version in 700 C wheels as it has slightly lower drag and if you're a fast rider you won't notice the slight trade-off in low-speed performance.

Electrically, all 3 W dynamos are sufficiently similar thta you won't notice the difference. I have 2 SONs and 4 Shimanos of various models, and they're all much of a muchness.

Agreed. Especially now that most lights are LED, with good low-speed performance. As a matter of interest I have a modern state of the art Bush & Mueller headlamp powered by an ancient 20+ year-old AXA HR 3 watt bottle dynamo.

[Brucey]

the original German standard test stipulates that the output of the generator must be a certain percentage of the nominal 3W at a certain speed, as measured when driving an essentially resistive load (to simulate tungsten bulbs). [IIRC it is something like 80% at 12kph or something like that].

This was intended to give a certain light output at that speed, again with tungsten bulbs in use, so that 'ordinary cyclists' would be seen OK. In terms of light output vs power, such bulbs are very non-linear, and (to a first approximation) give near-enough no light at all at low speeds, so that (say) 60% of the power output would be more or less useless to the cyclist, but 80% might make you visible.

This test isn't very relevant to the performance of LED lamps; the light output is more likely to be almost Iinear with power, and are anyway far brighter than tungsten bulbs, so at low speeds, even 50% of the rated 3W is still a pretty bright light, and in many cases easily able to outperform tungsten lights at full output.

Thus even a hub generator meant for a small wheeled machine will still give plenty of output if you use a decent LED lamp. It mightn't reach full output until a slightly higher speed, but most keen cyclists will usually be riding fast enough that this won't be of any practical concern, and output at low speeds is more than enough for most folk too.

cheers

[DaveP]

I never really managed to get my head around the maths and physics of electricity generation, so Thanks For That - and cue the next stupid question: Am I likely to do any damage by "over revving"?

As you probably realise, I don't normally ride that fast, but there are one or two good downhills around

[Brucey]

most modern lights have a voltage limiting circuit of some kind and all hub generators that I have examined will self-limit in current output (because of the internal resistance of the windings) even though the 'no load' voltage of the generator increases with speed.

Together these things mean that most systems are resistant to excess speed (within reason) but if you have a badly insulated setup, you might get shocks from it as you ride down the road (esp when the lights are off!).

cheers

[Phil_Chadwick]

Bike dynamos give (once you're up to any sort of speed) pretty-much constant current of about 500 mA. At 6V that's 3W.

The output voltage depends on how fast you go, and with no light connected it can get very large.

A light switched on will clamp it down to 6-15 V typically (depending on the light), which is how a modern LED light can get 6W or so from a nominal 3 W hub.

In he old days, i used to have two halogen lamps wired in series, with a bypass switch on one. Once I got about 15 mph or so I'd switch in the second light to take advantage f the extra volts available.

[Abradable Chin]

all hub generators that I have examined will self-limit in current output (because of the internal resistance of the windings)

Is it not saturation of the soft-iron core that is the major part of the current limiting?