Dynamo system choices

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mercalia
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Re: Dynamo system choices

Postby mercalia » 14 Feb 2018, 6:16pm

Just had a look at Rose Cycles and they dont seem to have any so stocks must be low? The only list Axa

SA_SA_SA
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Re: Dynamo system choices

Postby SA_SA_SA » 14 Feb 2018, 6:17pm

mercalia wrote:.....When I was up Scotland I met a Brompton owner who had a hub dyno and regretted getting it due to the drag when off

I read someone else saying that:
I wonder if it is to do with Shimano supplying QR hubs with bearings tight, and as the Brompton Hub is a QR with allen-key fasteners, the bearings are presumably overtightened as standard
unless
a) Brompton themselves take the time to adjust the bearings to be correct with wheel installed
b) the user does so themselves
?
Also, surely a small wheel dynamo should be more efficient as spins faster ?

If the basic low cost Shimano hubs 3N31 etc are acceptably efficient what else would make the capreo noticeably draggy when off?
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Giles Pargiter
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Re: Dynamo system choices

Postby Giles Pargiter » 14 Feb 2018, 6:29pm

"Also, surely a small wheel dynamo should be more efficient as spins faster ?"

That old chestnut, :shock: for a bottle dynamo - think on a little - you need the same number of meters of tyre/ mile whatever tyre size you have, therefore at the same speed, er, they turn at the same speed. :wink:

I may have misunderstood you though - a hub dyno would spin faster- more/mile of course.

Brucey
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Re: Dynamo system choices

Postby Brucey » 14 Feb 2018, 7:25pm

Giles Pargiter wrote: ....if you do your calculations even though they do drag slightly (a fraction of a W) more than hub dyno's....


IIRC I am pretty sure this is not the case. The very best bottle dynamos currently available (like B&M dymotec 6 model) still drag several watts more than a hub dynamo in manufacturer's tests, which will (unlike reality) be under absolutely optimum conditions. Typical bottle dynamos drag at least double vs a hub dynamo, for a similar output.

The basic Dymotec 6 model is still available here

https://www.sjscycles.co.uk/dynamos/left-busch-muller-dymotec-6-sidewall-bottle-dynamo/
and the instructions
http://www.sjscycles.com/Instructions/Busch_&_Muller/Busch_&_Muller_Dymotec_6_Instructions.pdf

show on p16 B&M's drag data for various dynamos. The now-extinct S6 and S12 models were noticeably more efficient, and assuming that the setup is good you could get comparable on-load drag to a medium quality hub generator. The basic Dymotec 6 model is not this good though, needing about 8W in for 3W out, even under optimal conditions.

Regarding off-load drag in hub dynamos, this varies with the model. A few are designed in such a way that they are electrically inept, e.g. because of eddy current losses etc they drag more when the lights are off vs when they are on... :shock: . However most are not like this and drag for reasonably good generators has been measured at between 1W (for the best SON) and 3W (for a basic shimano DH-3N30 or similar).

However because shimano hubs will have been tested 'as received' they may have had draggy, inadequately lubricated seals, and hopelessly badly adjusted bearings. In trials I carried out, the off-load drag in a new shimano DH-3N72 hub dynamo roughly halved once the hub had been set up correctly. It did indeed seem to be between 2W and 3W beforehand (and could indeed have been 3-4W in a small-wheeler...). However do bear in mind that Dynamo or not, hubs with seals have some drag rather than no drag anyway.

If you suppose that the extra drag of a half-reasonable hub generator, off load, is about 1.5W then to put it into perspective this is 1/10th the drag of a single tyre, if that tyre is one of the best racing tyres you can buy. If you are trundling around on Marathons or Marathon pluses then your tyre drag (at about 20mph) is probably going to be at least 50W anyway. You would almost certainly gain more than 1.5W by simply sticking about 2 or 3 psi more in your tyres....

So yes, the off-load drag in a hub generator is not zero, but is a price worth paying if you do much riding at night.

I have personally broken/worn out at least half a dozen bottle dynamos, which when you think that those have only seen about 1/20th of my total night-time mileage, does not speak well of their reliability. I'm sure that they suit some folk, but I shall not mourn their passing in the slightest.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Giles Pargiter
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Re: Dynamo system choices

Postby Giles Pargiter » 14 Feb 2018, 8:09pm

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 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.
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.

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

mercalia
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Re: Dynamo system choices

Postby mercalia » 14 Feb 2018, 8:15pm

Brucey wrote:
Giles Pargiter wrote: ....if you do your calculations even though they do drag slightly (a fraction of a W) more than hub dyno's....


IIRC I am pretty sure this is not the case. The very best bottle dynamos currently available (like B&M dymotec 6 model) still drag several watts more than a hub dynamo in manufacturer's tests, which will (unlike reality) be under absolutely optimum conditions. Typical bottle dynamos drag at least double vs a hub dynamo, for a similar output.

The basic Dymotec 6 model is still available here

https://www.sjscycles.co.uk/dynamos/left-busch-muller-dymotec-6-sidewall-bottle-dynamo/
and the instructions
http://www.sjscycles.com/Instructions/Busch_&_Muller/Busch_&_Muller_Dymotec_6_Instructions.pdf

show on p16 B&M's drag data for various dynamos. The now-extinct S6 and S12 models were noticeably more efficient, and assuming that the setup is good you could get comparable on-load drag to a medium quality hub generator. The basic Dymotec 6 model is not this good though, needing about 8W in for 3W out, even under optimal conditions.

Regarding off-load drag in hub dynamos, this varies with the model. A few are designed in such a way that they are electrically inept, e.g. because of eddy current losses etc they drag more when the lights are off vs when they are on... :shock: . However most are not like this and drag for reasonably good generators has been measured at between 1W (for the best SON) and 3W (for a basic shimano DH-3N30 or similar).

However because shimano hubs will have been tested 'as received' they may have had draggy, inadequately lubricated seals, and hopelessly badly adjusted bearings. In trials I carried out, the off-load drag in a new shimano DH-3N72 hub dynamo roughly halved once the hub had been set up correctly. It did indeed seem to be between 2W and 3W beforehand (and could indeed have been 3-4W in a small-wheeler...). However do bear in mind that Dynamo or not, hubs with seals have some drag rather than no drag anyway.

If you suppose that the extra drag of a half-reasonable hub generator, off load, is about 1.5W then to put it into perspective this is 1/10th the drag of a single tyre, if that tyre is one of the best racing tyres you can buy. If you are trundling around on Marathons or Marathon pluses then your tyre drag (at about 20mph) is probably going to be at least 50W anyway. You would almost certainly gain more than 1.5W by simply sticking about 2 or 3 psi more in your tyres....

So yes, the off-load drag in a hub generator is not zero, but is a price worth paying if you do much riding at night.

I have personally broken/worn out at least half a dozen bottle dynamos, which when you think that those have only seen about 1/20th of my total night-time mileage, does not speak well of their reliability. I'm sure that they suit some folk, but I shall not mourn their passing in the slightest.



cheers



well on Marathons for me more like 11mph so what then? do the figures get better for bottle dynos?

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andrew_s
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Re: Dynamo system choices

Postby andrew_s » 14 Feb 2018, 8:58pm

Giles Pargiter wrote:Not to mention that with all things considered hub dynos are enormously more expensive.

Axa HR bottle dynamo £14
dynamo mount £2
Deore T610 Front Hub £10
Total £26

3N31 Hub Dynamo £25

Giles Pargiter
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Re: Dynamo system choices

Postby Giles Pargiter » 14 Feb 2018, 9:17pm

Slight fly in the ointment their Andrew is that you don't need to have a wheel built to fit a bottle dyno.

mercalia
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Re: Dynamo system choices

Postby mercalia » 14 Feb 2018, 9:38pm

I personally dont see the need anymore for dynos of any kind for urban riding as cheap lights like the lidl ones at less than a tenner are bright enough even on low power with 11 hrs run time the front one: so that leaves touring, I dont aim to do much cycling in the dark, get it over with asap during the light of the day: so a bottle is good enough for those exceptional occasions.

Brucey
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Re: Dynamo system choices

Postby Brucey » 14 Feb 2018, 9:40pm

Giles Pargiter wrote:Slight fly in the ointment their Andrew is that you don't need to have a wheel built to fit a bottle dyno.


you can buy a cheap wheel with a hub generator in of course, eg from taylors wheels. Often this works out cheaper than you can buy the bits for.

cheers
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andrew_s
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Re: Dynamo system choices

Postby andrew_s » 14 Feb 2018, 9:45pm

Schwalbe Marathon Greenguard 37-622 - rolling resistance 19.6 watts at 75 psi, per tyre
Conti GP4000 700x28 - rolling resistance 12.7 watts at 80 psi, per tyre

So there's a saving of about 14 - 1.5W dynamo drag =12.5W using a hub dynamo with lights off, over a non-running bottle dynamo. :twisted:

The Greenguard is the fastest tyre on the bicyclerollingresistance.com site that's got a dynamo track, and I figure that you can't run a sidewall dynamo on a lightweight, skinwall tyre without damaging it.

I have been around long enough to have used sidewall dynamos, but the "can't run on lightweight tyres" problem saw me switch to using the even more draggy and short-lived roller (bottom bracket) dynamos until modern hub dynamos became available.
Last edited by andrew_s on 14 Feb 2018, 10:02pm, edited 2 times in total.

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andrew_s
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Re: Dynamo system choices

Postby andrew_s » 14 Feb 2018, 9:48pm

Giles Pargiter wrote:Slight fly in the ointment their Andrew is that you don't need to have a wheel built to fit a bottle dyno.

a) I build my own wheels, so there's no cost.
b) A non-dynamo front wheel also has to be built, and building costs are the same whether the hub is dynamo or not.

Brucey
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Re: Dynamo system choices

Postby Brucey » 14 Feb 2018, 9:50pm

mercalia wrote:

well on Marathons for me more like 11mph so what then? do the figures get better for bottle dynos?


not really. Both hub and bottle generators don't create drag in proportion to speed, when the lights are on, but tyres do and so (more closely than not) do hub generators when the lights are off. With your tyres at 11mph you are probably going to be using about 27W (or more) just to overcome rolling resistance and maybe there is another 0.75W to push a hub generator round, something like that.

The very fastest tyres are thought to be about half the drag and weight of the ones you are using. So you have already traded quite a lot of speed/weight for toughness/freedom from punctures etc. The tiny bit more to have lights that just work and don't need to be remembered, batteries charged etc or make a noise like a kitten being strangled is a good trade...

cheers
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Giles Pargiter
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Re: Dynamo system choices

Postby Giles Pargiter » 14 Feb 2018, 9:52pm

As I said earlier I could'nt care less what legal lights others use - so long as you are happy with it, but many of the imagined problems with bottle dynos either rarely happen or don't exist that's all. Of course they have developed (as have hub dynos) a great deal from those we remember using years ago.

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andrew_s
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Re: Dynamo system choices

Postby andrew_s » 14 Feb 2018, 10:00pm

Giles Pargiter wrote:Of course they have developed (as have hub dynos) a great deal from those we remember using years ago.

Not so much.
The Axa HR is pretty much the same as it was 15 or 20 years ago.

More efficient sidewall dynamos were developed (Lightspin, B+M S6), but they were expensive enough that they failed to compete with SON dynohubs, and were killed off when Shimano brought out their better hubs (3N70 etc).