Dynamo system choices

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markjohnobrien
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Re: Dynamo system choices

Postby markjohnobrien » 16 Feb 2018, 5:57pm

mercalia wrote:
Brucey wrote:
Giles Pargiter wrote:This is where we disagree then Brucey.

Due to the extra around 500grams (approx, depending on exact dynamo, IRC the B+M weighs 120grams,) you always carry with a hub dynamo...


for £65 you can buy a hub generator that weighs between 360 and 390g depending on who you believe. This is only ~225g heavier than a fairly light conventional hub, not +500g. And only ~100g heavier than a bottle dynamo. Most folk have a £ per gramme saved when choosing parts that strongly indicates they don't give a monkeys about weight.


and the slight eddy drag of even the best, taking into account the fractional extra drag of a bottle dyno in operation you will find that you will need to be using lights at least 90% of the time for the best hub dyno's to be more efficient.

Your maths is broken. The best hub generators drag less than 1W extra (lights off) at touring speeds, but are ~5W less draggy when the lights are on vs the best bottle dynamo that you can presently buy (just). You would need to use the lights about 20% of the time to break even, not 90%.

Note also that this assumes that having yet another warty thing hanging off the side of your bike creates no aero drag, which is obviously wrong. I don't know how much it takes to push a bottle dynamo through the air but it could easily be 1 W.

I don't think the point about which tyre you choose is very relevant really as this is in no part dependant on the dynamo. You do not need a tyre with a dyno track whatever type of dyno you use.


I disagree and have witnessed the consequences first hand.

Not to mention that with all things considered hub dynos are enormously more expensive.


you can buy a bottle dynamo for £1.25 if you like. It will probably drag the thick end of 20W when you are using it, and have a very short life expectancy. Your favoured dynamo presently costs £35 (where you can find it NOS), which would enable you to have a choice of pretty reasonable hub generators.

The World has moved on; if bottle generators were so great we'd all still be using them and B&M would still be making them. My abiding memory of using them is getting to the end of any given journey with a sense of profound relief; relief that the blinkin' thing didn't just break (again) and relief that I wouldn't have to listen to a sound like a kitten being strangled any more, shattering the silence of the night and fraying my nerves to shreds.

cheers


well mine b&m has never broken down in nearly 18 years or so the sound is a low whine of an electic motor. the only thing that would concern me is the extra effort that is in short supply thse days, why i prefer a battery system- changeing the tyres not really possible as i spend too much time on bad surfaces. by the way batteries rule in london, rarely see a dyno of any kind. the way ahead is a lamp with a separate powerbank


Well, I must be the odd one out as I've commuted, and used bikes, with led dynamo lights (and front dynamo hub) for years in London.

Practical; safe; cheap to buy dynamo wheels from Rosebikes; etc.

StephenW
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Re: Dynamo system choices

Postby StephenW » 20 Feb 2018, 9:25pm

What about a 1.5W bottle dynamo? If 3W gives an adequate amount of light for town riding using incandescent bulbs, surely 1.5W ought to be more than enough for town riding with LEDs. I know that 1.5W hub dynamos exist, but I haven't heard of a 1.5W bottle dynamo.

I agree that the noise of a bottle dynamo can be a bit annoying, but I think for shorter trips around town it is OK. A low-powered bottle dynamo would give zero lights-off drag, without requiring an especially high-quality and expensive design, and acceptable lights-on drag. If either the dynamo or the wheel develops a problem, one can be replaced without affecting the other (on a cheaper bike perhaps it is not worth the labour cost of rebuilding the front wheel?). There are surely lots of bikes out there which would benefit from dynamo lighting, but have perfectly serviceable front wheels which it would be a shame to throw away.

Another thing which I would like to investigate is running a bottle dynamo on the rim, having first slightly roughened the rim surface (this bike doesn't have rim brakes).

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andrew_s
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Re: Dynamo system choices

Postby andrew_s » 21 Feb 2018, 12:33am


mercalia
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Re: Dynamo system choices

Postby mercalia » 21 Feb 2018, 1:23am

andrew_s wrote:Here you are: a 1.5W rim dynamo
https://www.bikeradar.com/road/gear/art ... amo-51783/


wow but at 150 euros....... too much too late? I wonder what Brucey thinks of it? I read it has between 60-75% efficiency!

StephenW
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Re: Dynamo system choices

Postby StephenW » 21 Feb 2018, 11:29am

I was thinking of something costing about £15, not £150!!

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andrew_s
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Re: Dynamo system choices

Postby andrew_s » 21 Feb 2018, 1:38pm

I think the thing with rim drive is that as traction is limited, there's only so much power that you can take out before there's slip in the wet.

1.5W output at the 67% efficiency of a Velogical is only 2.25W at the roller. Use a standard £15 bottle dynamo, and it would be 1.5W at 33% efficiency (4.5W at the roller).
That's always assuming that you can get a light that will only use 1.5W. LED lights are more efficient at low power, so you'd generally drive a standard 3W nominal LED light at 1.5W using a lower input power from a smaller dynamo. If you used a standard bottle dynamo and a normal 3W light, you'd need 9W at the roller.

Maybe roughening the rim would be enough, but you may have to be a bit brutal.

Brucey
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Re: Dynamo system choices

Postby Brucey » 21 Feb 2018, 4:23pm

I think the basic premise is correct, for a set of 'be seen' LED lights for urban use it ought to be possible to create a bolt-on system that uses a small, low drag generator of some kind. Reelights for example.

Maybe 1.5W is enough to 'see by' in undemanding circumstances too; with LEDs it ought to be no worse than a lot of older 3W systems with tungsten bulbs, but maybe people's expectations have moved on.

However the appeal of lower drag than a full fat 3W bottle dynamo with a 1.5W system should be offset against the fact that a lower powered system could be just as complex (expensive) and will likely have just as many possible points of failure as a normal bottle generator system. In fact there could be more places where a failure is likely, because there will be a temptation to make the generator parts even smaller and more flimsy than normal. In other words perhaps only about half the objections to a normal bottle dynamo are likely to be addressed this way.

An easy way of lowering both drag and output is simply to fit a larger wheel onto a bottle generator. This is how the Basta (Union) 'turbo' and AXA HR models worked, although they still made 3W. If you fit a larger wheel to a standard bottle generator, you win twice over; there will be less drag because the generator is turning more slowly, and in addition the drag is likely to be less anyway whenever you make the drive wheel bigger.

Various attempts have been made to create a bolt-on generator system that makes the system simpler or takes the drive away from the wheel periphery (where 'exciting things' tend to happen) but to date only a few systems have been very convincing. Even the Reelight system's inherent simplicity has been diluted in order to make lights that are 'legal' in more territories.

BTW I don't consider hub generators as being a 'perfect solution' by any means, but they are the best we have at present.

cheers
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Giles Pargiter
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Re: Dynamo system choices

Postby Giles Pargiter » 1 Mar 2018, 6:28pm

Again I have to disagree with Brucey, who I think is very well informed and his posts are mostly well mostly well considered.

Plainly ;it is a matter of opinion only whether hub dyno's are " the best solution we have at present".

Many of the objections to bottle dynamos are spurious or else rarely happen. Today I took out my "Mule" fitted with road tyres - because a freind could'nt get to feed her horse due to road conditions, stuck 4x4's already blocking her route. . .

Due to the fashion police on this thread I ran my bottle dynamo and found that through the snow drifts I encountered that in this dry powdery snow it coped quite happily. So very rare that they won't work due to snow - I did change to my mountain bike for a further commute for neighbours supplys though due to grippy tyres being a definite advantage ( Subject of "todays commute thread).

I wonder - I bet a marathon or similar tyre may well have the highest [u]average[u] rolling speed on mucky or glass infested roads.

Brucey
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Re: Dynamo system choices

Postby Brucey » 2 Mar 2018, 2:07am

I suppose ultimately it is a matter of opinion but in my case it both an opinion that has been hard-won and that is shared with many others.

I rode my hack bike (which lives out of doors) today and I guess it must have thawed slightly (dunno how... :roll: ) and then frozen again since it was last used. All kinds of things on it were not working quite properly, including the gears, the chain, the brakes etc but the hub dynamo lights worked fine and this is exactly what I had anticipated.

With a bottle (or other tyre driven) dynamo I'd have been less sure; I have been let down by these things enough times that I just don't trust them; after a long period of no use they often don't turn freely and otherwise they have just packed in when I've been trying to use them. It is invariably dark when this happens, and usually the weather is bad. Several times I have been left by the roadside in a fury, trying to resist the temptation to kick the living wotsits out of the bike or just chuck it over the nearest hedge before now.

By contrast I have done many tens of thousands of miles in the darkness with hub dynamo lighting (over about a 40 year period) and have yet to run out of electricity....

BTW there is one good use for a broken/redundant bottle generator; if you take it to bits there is an excellent magnet inside.

cheers
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fatboy
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Re: Dynamo system choices

Postby fatboy » 2 Mar 2018, 6:42am

As an avid dynamo user (I have 4 bikes and they all have dynamos, 3 hubs and 1 bottle) I thought that I'd add my two pennies.

Hubs are superior in terms of efficiency etc but there is theoretical drag when off and they cost more.

I've had both hub and bottle dynamos fail (maybe I'm unlucky but a few months back my hub dynamo went open circuit and I have had a bottle seize up) and when a hub fails it's more work even if you can build wheels.

Despite the experience above my view is that hub dynamos are not only the best dynamo system but the best lighting system.

BTW it's my old Brompton that has the bottle dynamo and it's fine for the sort of use that it gets (and I'm quite fond of the dynamo hum if I'm honest!), but if I upgraded my Brommie it would have a hub dynamo.
"Marriage is a wonderful invention; but then again so is the bicycle puncture repair kit." - Billy Connolly

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Sweep
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Re: Dynamo system choices

Postby Sweep » 2 Mar 2018, 9:49am

Since brommies have come up, I'll pop in.

My 5 speed brommie (traveller?,) Came with a rack and a bottle dynamo as its added features, for which a premium was charged. Have never used the rack, but I like it being there and may use sometime. The bottle dynamo was a right waste of time. Positively dangerous since I never knew if the back light was on. I chopped the entire system off and just went to battery lights, which for the use a brompton tends to get is just fine and dandy.
Sweep

mercalia
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Re: Dynamo system choices

Postby mercalia » 2 Mar 2018, 1:07pm

Brucey wrote:I suppose ultimately it is a matter of opinion but in my case it both an opinion that has been hard-won and that is shared with many others.

I rode my hack bike (which lives out of doors) today and I guess it must have thawed slightly (dunno how... :roll: ) and then frozen again since it was last used. All kinds of things on it were not working quite properly, including the gears, the chain, the brakes etc but the hub dynamo lights worked fine and this is exactly what I had anticipated.

With a bottle (or other tyre driven) dynamo I'd have been less sure; I have been let down by these things enough times that I just don't trust them; after a long period of no use they often don't turn freely and otherwise they have just packed in when I've been trying to use them. It is invariably dark when this happens, and usually the weather is bad. Several times I have been left by the roadside in a fury, trying to resist the temptation to kick the living wotsits out of the bike or just chuck it over the nearest hedge before now.

By contrast I have done many tens of thousands of miles in the darkness with hub dynamo lighting (over about a 40 year period) and have yet to run out of electricity....

BTW there is one good use for a broken/redundant bottle generator; if you take it to bits there is an excellent magnet inside.

cheers


did you ever have the B&M one ?

cycle tramp
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Re: Dynamo system choices

Postby cycle tramp » 2 Mar 2018, 3:05pm

Sweep wrote:Since brommies have come up, I'll pop in.

My 5 speed brommie (traveller?,) Came with a rack and a bottle dynamo as its added features, for which a premium was charged. The bottle dynamo was a right waste of time. Positively dangerous since I never knew if the back light was on. I chopped the entire system off and just went to battery lights.


If you're using both front & rear lamps (fitted with a bulb) with any dynamo system, its worth considering not using the lamps in parallel (where both lamps get an earth & positive feed) but to isolate them (from the earth of the bike frame, perhaps by using nylon bicycle reflector mounts and then use the lamps in series (where the positive feed goes to the front lamp, then an earth wire takes it from the front lamp to the positive contection of the rear lamp and an second wire runs from the earth connection of the rear lamp back to the earth connection of the dynamo. That way you know when there's a problem with your rear lamp because the front lamp fails as well. Not sure about the modern LED stuff...

Although just because you've started your journey with a working battery powered rear lamp doesn't give 100% guarantee that you'll end the journey with a working rear lamp.
(Which is why i use battery & dynamo lamps when i go out at night as well as my reflective jacket as well as lots of refectors :-) )

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Sweep
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Re: Dynamo system choices

Postby Sweep » 2 Mar 2018, 3:10pm

Hi cycletramp.

Only really use one rear on the brommie due to positioning issues, but I check it frequently and it's only usually short night-time trips.

Totally agree about using more than one rear - I often have two or even three - in fact a few months ago in after dark London I was complimented on my lighting by a bobby on the beat. Not sure whether he was more amazed by my lighting or me my his presence afoot - though it was a posh area where maybe (silk) strings can be pulled.
Sweep

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ConRAD
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Re: Dynamo system choices

Postby ConRAD » 2 Mar 2018, 4:10pm

cycle tramp wrote:... with any dynamo system, its worth considering not using the lamps in parallel ... use the lamps in series ...

... it works of course ... but you're loosing almost 1W, this conclusion being based on real tests with a dynohub at 20 km/h, from 4.3W in parallel to 3.5W in series !!
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