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Old 02-12-09, 08:15 PM   #1
quester
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Generator hub vibration

I'm the guy with the last thread on the SRAM i-light. My problems were that there was a squeak, and vibration. Luckily the squeak went away, as the SRAM technical people didn't want to talk to me other than to explicitly warn that opening the hub was courting disastor.

The vibration is another story. It seems to max out at around 14 mph, slowly waning at higher speeds. It totally cuts out when I take the charger out of the circuit. This is the Busch and Muller "Ride & Charge".

Anyone have experience with this on the i-light or other (shimano? schmidt?) hubs? It's not that the vibration is all that bad (I'm usually going slower than that on tour anyway), but it's a constant reminder of energy being sucked away. Plus, it does interfere with the nice smooth ride of my stell LHT.

Thanks,
pete
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Old 02-13-09, 06:50 AM   #2
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All the dynamos do this. I guess I'd describe it as a pulsing rather than a vibration, but let's not argue semantics. I know Sheldon Brown has explained it somewhere, but I didn't see it in a quick glance at his dynohubs page. At any rate, what you're feeling is increased resistance as two magnets get closer together, and then decreased resistance as they come apart again. Yes, energy is used as you push them closer together, like when you pedal your bike up a hill; but it is released again when the magnets come apart again, like when you coast back down. So don't worry, not that much energy is actually being sucked away.

I'm so accustomed to the vibration from the dynamo that I hardly even notice it any more.
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Old 02-13-09, 05:36 PM   #3
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Hmm... my Shimano hub doesn't have any vibration at any meaningful speed. I can feel it when holding up the bike and spinning the wheel by hand, though.

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It seems to max out at around 14 mph, slowly waning at higher speeds.
Are you saying that it has the most vibration at 14 mph?

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It totally cuts out when I take the charger out of the circuit. This is the Busch and Muller "Ride & Charge".
I'll guess that the charger draws a bunch more current, which manifests itself as more force against the magnets in the hub.

I don't have a charger like that, however, and don't notice any ill effects from the dynohub that aren't overwhelmed by various gusts and breezes blowing against me.
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Old 02-16-09, 10:11 AM   #4
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Hmm... my Shimano hub doesn't have any vibration at any meaningful speed. I can feel it when holding up the bike and spinning the wheel by hand, though.



Are you saying that it has the most vibration at 14 mph?
Seems to be. Probably the "pulses" just get more and more strong as speed increases, but at higher speeds they just start to merge.

All of this makes sense, it's just that I was surprise that the vibration/pulses were so pronounced, and especially surprised that it disappears when I stop drawing a current. Again, this totally makes sense, it's the magnitude of the difference that I'm surprised at.

I need to go find a long downhill and coast down it a few times in succession, both with and without the charger in the circuit. If, as I suspect, that any downhill time difference w/ and w/o the charger is within the usual variation from one run to the next, even with everything the same, then I'll be a bit more comfortable with the whole thing.

The whole combination is both for commuting (light is important, don't really care about drag), and a long tour next summer, where the light is irrelevent (I almost always arrive where I want to be early), but where charging of batteries is important and any extra drag would give me a psychological disadvantage going up hills with my buddies.

On the other hand, I don't go up many hills at 14mph, especially with a full touring rig....

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I don't have a charger like that, however, and don't notice any ill effects from the dynohub that aren't overwhelmed by various gusts and breezes blowing against me.
Which hub do you have?

pete
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Old 02-16-09, 05:07 PM   #5
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I've got a Shimano DH-3N71 (or whatever it is; I'm out of town now). All it's powering is a B&M LED headlight and taillight.

I'll bet that what you're feeling is the additional current draw from the battery charger (and even then, I'm surprised it's making that much difference). I'd try finding someone with a Shimano hub and see if it reacts the same way.

Or, just "rough it" and don't bother charging batteries while riding.
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Old 02-16-09, 09:32 PM   #6
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Charging the batteries shouldn't induce more drag than turning on the lights.
What type of cells are you charging? How many cells do you charge and what is your charging setup? I suspect your setup isn't very efficient.
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Old 02-17-09, 08:51 AM   #7
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Ditto, with my limited electrical knowledge (and less present knowledge about Dynohubs) it sure sounds like you're attempting to demand more power out of the dynohub than it's designed to deliver. This could be using batteries with a difference resistance (demand) than the charger is expecting (or was engineered for). That, or your charger may be broken.

If the problem 'goes away' when you turn off the charger, the issue is the charge or something the charger is hooked to, not the hub.
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Old 02-17-09, 10:20 AM   #8
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Charging the batteries shouldn't induce more drag than turning on the lights.
What type of cells are you charging? How many cells do you charge and what is your charging setup? I suspect your setup isn't very efficient.
It's possible that the charging circuit uses some clever electronics to improve the dynohub output at low speed.

How much drag is 'induced' will depend on the relative loads, there is no reason why the charging circuit shouldn't 'take out' more from the hub than the lighting circuit at low speeds.

Why do you suspect the setup isn't very efficient ?
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Old 02-17-09, 10:59 PM   #9
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Quester: My SON hub dyno vibrates too (well the front end of my bike vibrates the handlebars), but at about 19 mph on a 406 size wheel. As others have said, I think its just the nature of the beast.--george
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Old 02-17-09, 11:19 PM   #10
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My shimano hub never used to vibrate until I heard people *****ing about it on the internet.
Now it does.

Seriously I only notice now and then on very very smooth roads.
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Old 02-18-09, 08:13 PM   #11
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Quester: My SON hub dyno vibrates too (well the front end of my bike vibrates the handlebars), but at about 19 mph on a 406 size wheel. As others have said, I think its just the nature of the beast.--george
The SON's are usually assumed to be the class of the crowd, so if you have vibration, then I'm not surprised I do with a SRAM hub.

For those that asked about the charging setup, it's a Busch and Muller "Ride & Charge" hooked to an Ixon IQ light containing four AA rechargeables.

Thanks all,

pete
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Old 02-18-09, 10:42 PM   #12
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A slight "humming" vibration is normal on the SON hub at higher speeds.
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Old 01-24-10, 06:14 PM   #13
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Quick update, OP here. I finally got around to replacing the i-light with a SON 28: no vibration, no noticeble effect at all for my five minute test ride (while charging batteries through the ride&charge and Ixon IQ). I might feel something on a longer ride tomorrow, but this is clearly much, much better than my sram hub.

Note that I'm not claiming that all sram i-lights behave the way mine does; it's possible I just had a bum hub (note the squeak that I had in the beginning).
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Old 01-25-10, 11:49 AM   #14
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Quick update, OP here. I finally got around to replacing the i-light with a SON 28: no vibration, no noticeble effect at all for my five minute test ride (while charging batteries through the ride&charge and Ixon IQ). I might feel something on a longer ride tomorrow, but this is clearly much, much better than my sram hub.

Note that I'm not claiming that all sram i-lights behave the way mine does; it's possible I just had a bum hub (note the squeak that I had in the beginning).
I'm pretty late to the thread but this vibration is a normal characteristic of any dyno hub. In motors this is called 'torque ripple' and is one of the design trade-offs made in designing a motor/dyno. Increasing the number of magnetic poles in the hub will generally reduce the amount of ripple/vibration, but there are trade-offs with this approach, too. The first hub may well have been a bad part. Unbalanced parts and/or asymmetrical magnetization of the poles can cause vibration.
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