Re: [RBW] question re hub-dynamo drag

2016-09-28 Thread Philip Kim 
Ahh thanks! Super helpful illustrations.

On Wednesday, September 28, 2016 at 3:08:14 PM UTC-4, Peter White wrote:
>
> Actually, shorting the terminals will do no damage at all, at least not to 
> any Schmidt hub. And I doubt it would damage any other dynamo hub. It's 
> nothing to worry about.
>
> Also, the drag the OP noticed with the bike upside-down is real, but the 
> effect is exaggerated if there's little or no mass being moved, as is the 
> case with the bike upside-down. With the rider on the bike, the rider's 
> mass acts as a buffer to the effect of the drag. Inside a SON 28 there are 
> 26 magnets attached to the inside of the outer hub shell, between the spoke 
> flanges. They are, in a sense, trying to keep the hub from rotating. As the 
> hub does rotate, there are 26 points during each rotation of the wheel 
> where the magnets are trying to keep the hub from rotating, and they 
> alternate with another 26 points where the hub wants to rotate. As the hub 
> rotates a bit in one direction or the other, the magnets are trying to 
> bring the hub shell back to its earlier position. It's like riding down a 
> road that goes up and down, up and down. Gravity tries to keep you in one 
> of the low points on the road. So imagine that the road is essentially 
> level over a long distance, but undulates up and down every 20 feet or so. 
> From the top of each rise, you speed up just a bit for 20 feet until you 
> get to the bottom, and then you slow down for 20 feet. This repeats over 
> and over. The rider's mass is momentum, and the heavier the rider, the less 
> he will speed up on those 20 foot down sections, and the less he will slow 
> down on the 20 foot up sections. Over a long distance, there's hardly any 
> difference in your average speed.
>
> So, when you're riding a bike with a dynamo hub, with each revolution of 
> the hub, you slow down and speed up 26 times, but since you have a great 
> deal more mass at the rim rotating with you riding than with the bike 
> upside-down, the effect, the amount that the wheel's speed actually changes 
> 26 times per revolution, is tiny. Flip the bike upside-down and spin the 
> wheel, you can easily see the speed changing.
>
> In both scenarios, rider on the bike and rider off, the actual drag is the 
> same. And with the lights off, it's equivalent to the added work you do 
> climbing 1 foot of elevation every mile you ride. And it's about 5 times 
> that with the lights on. Can you tell the difference if you're riding a 
> road that's perfectly level, or if it's gaining 5 feet in elevation every 
> mile you ride?
>
> I didn't think so. ;-)
>
> On Wed, Sep 28, 2016 at 12:32 PM, Eric Norris  > wrote:
>
>> I would check with Peter White, but my guess is that shorting the 
>> terminals on hub would be very, very bad for the hub.
>>
>> I ride with dyno hubs on several bikes. With modern hubs and LED lights, 
>> the drag is negligible. Turning the lights off reduces the drag even more, 
>> but I like to have them on as running lights for better visibility.
>>
>> --Eric N
>
>
>
> -- 
> Peter White
>

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Re: [RBW] question re hub-dynamo drag

2016-09-28 Thread Peter White 
Actually, shorting the terminals will do no damage at all, at least not to
any Schmidt hub. And I doubt it would damage any other dynamo hub. It's
nothing to worry about.

Also, the drag the OP noticed with the bike upside-down is real, but the
effect is exaggerated if there's little or no mass being moved, as is the
case with the bike upside-down. With the rider on the bike, the rider's
mass acts as a buffer to the effect of the drag. Inside a SON 28 there are
26 magnets attached to the inside of the outer hub shell, between the spoke
flanges. They are, in a sense, trying to keep the hub from rotating. As the
hub does rotate, there are 26 points during each rotation of the wheel
where the magnets are trying to keep the hub from rotating, and they
alternate with another 26 points where the hub wants to rotate. As the hub
rotates a bit in one direction or the other, the magnets are trying to
bring the hub shell back to its earlier position. It's like riding down a
road that goes up and down, up and down. Gravity tries to keep you in one
of the low points on the road. So imagine that the road is essentially
level over a long distance, but undulates up and down every 20 feet or so.
>From the top of each rise, you speed up just a bit for 20 feet until you
get to the bottom, and then you slow down for 20 feet. This repeats over
and over. The rider's mass is momentum, and the heavier the rider, the less
he will speed up on those 20 foot down sections, and the less he will slow
down on the 20 foot up sections. Over a long distance, there's hardly any
difference in your average speed.

So, when you're riding a bike with a dynamo hub, with each revolution of
the hub, you slow down and speed up 26 times, but since you have a great
deal more mass at the rim rotating with you riding than with the bike
upside-down, the effect, the amount that the wheel's speed actually changes
26 times per revolution, is tiny. Flip the bike upside-down and spin the
wheel, you can easily see the speed changing.

In both scenarios, rider on the bike and rider off, the actual drag is the
same. And with the lights off, it's equivalent to the added work you do
climbing 1 foot of elevation every mile you ride. And it's about 5 times
that with the lights on. Can you tell the difference if you're riding a
road that's perfectly level, or if it's gaining 5 feet in elevation every
mile you ride?

I didn't think so. ;-)

On Wed, Sep 28, 2016 at 12:32 PM, Eric Norris  wrote:

> I would check with Peter White, but my guess is that shorting the
> terminals on hub would be very, very bad for the hub.
>
> I ride with dyno hubs on several bikes. With modern hubs and LED lights,
> the drag is negligible. Turning the lights off reduces the drag even more,
> but I like to have them on as running lights for better visibility.
>
> --Eric N



-- 
Peter White

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Re: [RBW] question re hub-dynamo drag

2016-09-28 Thread Lungimsam 
See this thread and what was said
https://groups.google.com/forum/m/#!searchin/rbw-owners-bunch/Drag/rbw-owners-bunch/XtLm5IU2xpE

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Re: [RBW] question re hub-dynamo drag

2016-09-28 Thread Eric Norris 
I would check with Peter White, but my guess is that shorting the terminals on 
hub would be very, very bad for the hub. 

I ride with dyno hubs on several bikes. With modern hubs and LED lights, the 
drag is negligible. Turning the lights off reduces the drag even more, but I 
like to have them on as running lights for better visibility.

--Eric N
www.CampyOnly.com
CampyOnlyGuy.blogspot.com
Twitter: @CampyOnlyGuy

> On Sep 28, 2016, at 9:07 AM, Bill Lindsay  wrote:
> 
> To the question posed in the original post:
> 
> The drag will be the same. Unplugged or Off, the load on the hub is the same. 
> It's an open circuit. 
> 
> The thing I don't know is what the drag would be if you shorted the hub 
> terminals together 
> 
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[RBW] question re hub-dynamo drag

2016-09-28 Thread Bill Lindsay 
To the question posed in the original post:

The drag will be the same. Unplugged or Off, the load on the hub is the same. 
It's an open circuit. 

The thing I don't know is what the drag would be if you shorted the hub 
terminals together 

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[RBW] question re hub-dynamo drag

2016-09-28 Thread Jim S. 
Hi all,

I was curious about the effect on drag of turning my dynamo headlight on or 
off. 

So I flipped my bicycle over, spun the wheel, and compared "light on" to 
"light off." The answer, unremarkably, is that the dynamo does cause some 
drag. Maybe more than I expected.

But I'm curious - can I reduce drag even more by unplugging the dynamo 
wires? The answer is probably clear and obvious to those who understand how 
these things work, but I couldn't really tell from my clumsy attempts to 
spin the wheel and compare. Hard to know if I'm spinning with exactly the 
same force, etc. 

Does anybody know the answer?


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