Cycling UK Forum

I realise that on the face of it this question has an obvious answer (which probably already appears on the forum, but the search function is no help here), but…

Will a 2.4W hub adequately power both front and rear LED lights?

I ask because these hubs (eg the 2N35) seem to be available at much lower prices than the 3W hubs I've always bought in the past.

Additional details for context: it'll probably be used with a Cyo Premium and some sort of Topline rear, on a touring tandem (so the additional drag and weight of an entry-level hub isn't really concern, but output at very low speeds when climbing perhaps is).

If not, no biggie… might just take the battery route anyway, given that it'll only see occasional use and the front wheel already has a perfectly good conventional hub in it.

Ah yes. Doh! Thanks.

my understanding is that the 2.4W hubs don't ever produce as much current as 3W ones manage (into any given load, but note that these generators are usually current-limited at speed anyway). If so, the front light won't quite reach full brightness at speed, which tallies well with reports from folk that have tried this.

Regarding current output at low speed, all things being equal this might be pro-rata with the power rating of the hub, i.e. at low speeds the brightness might be 4/5th that of a 3W hub. However this is far from certain; note that the 3W hubs are built to meet a German standard which mandates (amongst other things) that the power output is a given percentage of 3W at some speed (10-12kph I think) when driving into a resistive load. This doesn't always translate into the same fraction of power when driving LEDs (the generator doesn't see them as a purely resistive load) and in addition to that, the 2.4W hubs are built to satisfy a Dutch standard; they are not necessarily even tested in the same way.

FWIW the 2.4W hubs are built to drive a 2.4W front light only (cf the 3W hubs which are intended to drive a 0.6W tail light and a 2.4W headlight). So in theory you ought to do OK if you use a battery powered tail light, and dynamo front light. However I suspect that you won't do quite as well as that; modern LED dynamo tail lights often take a lower current (as little as 30mA instead of 100mA) which leaves more juice for the headlight; say 2.8W from 3W, rather than 2.4W.

cheers

Brucey wrote:... note that these generators are usually current-limited at speed anyway ...

Totally true, however I like to share again this table not covered at all by any copyright since i've been working it out by myself, showing that voltage will continue to increase with speed anyway.

which model is that you are testing? Its been a while but IIRC in a DH-3N30 I measured a peak current that was maybe ~750mA into a dead short.

A clue to the internal differences might be had by comparing the exact weights of equivalent 2.4 and 3W models

cheers

Have a look at this site for all sorts of useful info on dynamos and led lighting.
http://www.pilom.com/

Brucey wrote:...

In my tests I’ve been using a Shimano DH-3D32. The dead-short current indicated (0.6A approx) is not very significant IMO as it’s basically associated to a pure reactive power. To make significant comparisons I'd rather prefer to refer to a "definite" load, 12 Ohm resistive load would be a nice compromise.

I think the current into a dead short is quite a useful measure with this type of generator, in that in practice it is extremely unlikely that you will get any more current than this, driving into a 'normal' (for a light) load. One way of thinking about it (which isn't entirely accurate but Is still useful) is that the current is limited by the internal resistance of the windings.

cheers

Brucey wrote:...current is limited by the internal resistance of the windings

Actually current is limited by either resistance and inductive reactance of the coil, depending on the number of turns, cross section of the wire used and overall magnetic arrangement. Coil resistance doesn't vary with the speed whereas its reactance varies linearly with it. I might have the case of a generator "B" having double the number of turns compared to a generator "A". Both will likely to exhibit the same current on a dead-short but "B" will likely to perform better than "A" when connected on a certain load being in such a case the external "added impedance" less impacting on the overall circuit.

Brucey wrote:Regarding current output at low speed, all things being equal this might be pro-rata with the power rating of the hub, i.e. at low speeds the brightness might be 4/5th that of a 3W hub.

I would point out that 80% brightness is difficult to distinguish from 100% brightness, by eye, and without direct back & forth comparison.

It's like comparing a Cyo Premium (80 lux) with the older Cyo 60 - what's obvious is the wider spread of the beam, rather than any increase in brightness.

ConRAD wrote:

Brucey wrote:...current is limited by the internal resistance of the windings

Actually current is limited by either resistance and inductive reactance of the coil,..etc.

yes but

a) that isn't quite what I said and
b) AFAICT stvzo approved hub generators and the loads they usually drive don't seem to vary enough in inductance for this to be more than a theoretical concern, do they?

cheers

AFAIK StVzo just mentions "load", implicitly "resistive load" I guess, since related to a traditional or halogen filament lamp.

Hi all,
Apologies if this info is right there and I couldn’t find it (have searched a bit), feel free to point at any threads covering the topic.

Can anyone report on real-life experience of using a 2.4W dynamo with front and rear LED lights in daily commuting and utility riding?

I am looking at a replacement utility bike and the Gazelle Chamonix range seems good (I tested a C7 today and like it. Cycle reviewed it recently). However, it comes with a battery rear lamp, dynamo front and 2.4W Shimano dynamo. Obviously I could rebuild the wheel with a 3W dynamo but wondering if it is worthwhile (getting the shop to do it right away means I could get the tax relief on cycle to work though).

I would prefer to replace the lights with a B&M Topline Plus (the one that brightens when you brake sharply) on the back and a Cyo of some description on the front with the light sensor (I’ve got a couple of different ones in the cupboard).

It’s not a problem if I lose a little brightness, having it all automatic would be the goal (like my car!) I wonder if the “brake light” function might suffer though? Also, what about actual low-speed performance? I have a few points on my commute where I need to turn a sharp bend at slow speed and it is not well lit.

Peter H

I hope that I’ll be pardoned for asking what might seem a stupid question but do all of the front headlamps on the market take the same power or are some less greedy than others? IIRC, which is in doubt, writers here have commented about successfully using a SA Dyna Hub with LED lights so low power use must be possible?

I’m still in the dark ages with Halogen bulbs (a 1980’s solution which for my so very occasional use is still fine enough for me) but at some point I’ll throw caution to the wind and buy a replacement LED unit. Having said that such LED headlights (on eBay) seem to claim various light outputs but don’t say much about their actual power consumption ... which I find a frustration to reasoned choice.