Hub dynamo output

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denmancentral
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Joined: 20 Jun 2009, 10:28pm

Hub dynamo output

Postby denmancentral » 19 Jun 2013, 10:09am

I want to build a hub dynamo wheel, with a disc brake. My fork has the disc on the right side (unusually), so the hub dynamo will be rotating 'backwards' compared to a normal setup.

My question is, does this matter? The hub dynamo is likely to be a Shimano Alfine.

Will
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Joined: 16 Jan 2007, 6:39pm

Re: Hub dynamo output

Postby Will » 19 Jun 2013, 12:51pm

I don't know about Shimano dynamos, but Peter White has details on using different models of Schmidt hubs with the connectors on the left hand side: http://www.peterwhitecycles.com/schmidt.asp

See the section titled 'Hub Alignment' about 3/4 down. It looks like some models cannot be used this way.

I don't think there will be any issue regarding the output from the dynamo as it is in reality an alternator rather than a dynamo so it will produce AC whichever way the wheel is rotated.

Will

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[XAP]Bob
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Re: Hub dynamo output

Postby [XAP]Bob » 19 Jun 2013, 2:10pm

The dyanmo OUTPUT will be fine.

However the threads will all be "backwards" and so internals may unscrew themselves, or overtorque themselves.

I know that some of the Shcmidt dynamos can run backwards (the stub axle one for instance) but don't know about the shimano...
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tatanab
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Re: Hub dynamo output

Postby tatanab » 19 Jun 2013, 2:21pm

Shimano say that the dynamo connector should be on the right. http://techdocs.shimano.com/media/techd ... 605466.pdf

denmancentral
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Re: Hub dynamo output

Postby denmancentral » 19 Jun 2013, 4:44pm

yes, it seems that Shimano aren't keen on the idea. They say, slightly mysteriously:

"If the side with the connection terminal is facing toward the left, the hub dynamo may not turn properly while riding"

Looks like I'll need a new fork

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andrew_s
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Re: Hub dynamo output

Postby andrew_s » 19 Jun 2013, 5:51pm

If a Shimano dynamo, or one of the earlier cylindrical SON dynamos is run backwards, the section in the end of the hub shell that the magenets & windings are put in through may start to unscrew, possibly breaking the wires through to the external connectors and rendering the hub useless.
To be run with the connectors on the left the hub shell threading would have to be left-hand, for the same reason that LH pedal and RH BB threads are left hand.

Some people have used Shimano hubs backwards for extended periods with no problem, but it remains a possibility.

The ball shaped SON and SP dynohubs separate in the middle to allow the magnets & windings in, and may be run either way round.

It may well be cheaper to get an SP dynohub rather than a Shimano hub and a pair of forks
http://www.sjscycles.co.uk/sp-dynamo-pd ... prod29695/
http://www.sjscycles.co.uk/sp-dynamo-sd ... prod29699/

Having the disc brake on the front of the right-hand fork blade is better in principle, as there is no chance of braking torque causing your front wheel to fall out.

denmancentral
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Joined: 20 Jun 2009, 10:28pm

Re: Hub dynamo output

Postby denmancentral » 19 Jun 2013, 11:10pm

thanks for the advice. I will consider one of those SP hubs.

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7_lives_left
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Re: Hub dynamo output

Postby 7_lives_left » 22 Jun 2013, 1:06am

andrew_s wrote:If a Shimano dynamo, or one of the earlier cylindrical SON dynamos is run backwards, the section in the end of the hub shell that the magenets & windings are put in through may start to unscrew, possibly breaking the wires through to the external connectors and rendering the hub useless.

It is not just that the dynamo can be damaged. CJ has described in posts elsewhere on the forum that if the windings unscrew they can jam the front wheel solid, throwing the rider over the handle bars without warning. So it is really a safety critical problem not an reliability problem.

Need to find a link to CJ's post.

Brucey
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Re: Hub dynamo output

Postby Brucey » 22 Jun 2013, 9:32am

CJ's post?

http://forum.ctc.org.uk/viewtopic.php?f=5&t=29928&p=234760&hilit=jam+unscrew#p234760

My take on this is that a shimano generator will likely be fine the wrong way round in normal running, provided the internals are refitted carefully and are loctited well...; the precession forces are not huge in normal use. (This situation is arguably less bad than, say, an Italian BB, even if the potential consequences are worse.)

So this could be fine, until you put the brakes on, at which point the guts of the hub will see a far larger precession force than normal because of the brake loading. To a first approximation, the loads on the hub bearings during braking are increased in proportion to the wheel size vs the disc size, so (say) a 160mm disc on a 700C wheel with a 100kg load will make for a ~450kg precession load during a hard stop (admittedly not all on the one side of the hub). Loctite or not, there is a fair chance that the hub guts will unscrew themselves eventually. So my advice would be 'don't do it!'

cheers
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