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  1. #1
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    I received my DH 3N72 yesterday

    And that thing is HEAVY! When I picked up the padded envelope I figured there must have been a catalog inside, but no, it was just the hub in the box. The spec says 558 grams, and the spec on my front road wheel says 640 grams for the complete wheel. So figure 558, plus 440 for an XM317 rim, and about 160 for spokes, that comes to 1158 grams for a wheel, over 2.5 pounds.

    Now the lightest dynamo hub is the SP series at about 400g, lighter but not that much. A generic front weight for a decent hub is generally in the 125g range.

    And let's add up the cost: $78 for the hub, 40 for a rim, 35 for spokes, for $158. And dynamo lights are generally not so cheap. The B&M lights start at $75 or so and go up. Shaped beams yes, but limited output. And no flash mode either, which is almost a requirement in NYC. It is what distinguishes a bike from everyone else. (keep the argument in the other thread).

    Maybe the self contained single LED lights are a better idea. (Ducks for cover from the dyno light crowd.)

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    400g sp dynamo + 400g road rim + 160g spokes + 40g nipples + 150g dynamo light = 1150g for all night illumination.

    100g regular front hub + 400g road rim + 160g spokes + 40g nipples + 250g self contained battery light = 950g for a few hours of illumination.

    I can handle 200g of weight penalty. Cost otoh is a no brainer, you just can't beat $50 dual xml china lights.

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    I didn't think the weight would matter either until I lifted the hub. And the other difference is that I don't ride with the light on the bike if I'm not going out at night.

    I'm going ahead with this anyway, but I'm actually still skeptical about whether I'll like it or not. The dyno light advocates are all gung-ho, but I'm going to try it in spite of thinking that it isn't really the right thing for me. I don't ride through the night, I actually like the light spill of a circular beam without a cutoff, I want a blinking light.

    In NYC, you want to be seen, and not only by cars, but also by the clueless pedestrians walking with their heads down at their phones, and also the wrong way delivery guys coming at you from every direction on their bikes. Flashing bright lights work for that. What I want to know is whether a controlled beam will be effective or will it deflect all the light onto the road, but leave little to be seen. I don't really need the street illuminated.

    I do now understand one particular reason for not using dynamos. The cost of this is adding up. That number I quote above is the low end. I'm going to want at least the $100 light, which is a lot more than I'd spend on a battery light, and then there's the cost of the wheel. At least I found a good deal on the rims, 2 for $50 for XM317s. I needed to rebuild the back wheel too.

  4. #4
    Senior Member Road Fan's Avatar
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    Quote Originally Posted by zacster View Post
    I didn't think the weight would matter either until I lifted the hub. And the other difference is that I don't ride with the light on the bike if I'm not going out at night.

    I'm going ahead with this anyway, but I'm actually still skeptical about whether I'll like it or not. The dyno light advocates are all gung-ho, but I'm going to try it in spite of thinking that it isn't really the right thing for me. I don't ride through the night, I actually like the light spill of a circular beam without a cutoff, I want a blinking light.

    In NYC, you want to be seen, and not only by cars, but also by the clueless pedestrians walking with their heads down at their phones, and also the wrong way delivery guys coming at you from every direction on their bikes. Flashing bright lights work for that. What I want to know is whether a controlled beam will be effective or will it deflect all the light onto the road, but leave little to be seen. I don't really need the street illuminated.

    I do now understand one particular reason for not using dynamos. The cost of this is adding up. That number I quote above is the low end. I'm going to want at least the $100 light, which is a lot more than I'd spend on a battery light, and then there's the cost of the wheel. At least I found a good deal on the rims, 2 for $50 for XM317s. I needed to rebuild the back wheel too.
    I have a 650B wheel with an older SON (I'll just guess 450 g), a Velocity Synergy (about 450 g), 160g for spokes, and 330? g for a Hetre with tube, for an estimated total of 1390 g for the front wheel. But remember the effect of wheel mass at the hub is the same as added mass on the frame or the motor (added pork or selecting a Brooks/Berthoud/SA versus a Toupe or 1.5# more stuff in your jacket pockets. These types of mass increase affect riding in terms of acceleration, climbing, and to a much smaller degree, rolling resistance due to increased tire loading.

    The concentration of mass at the rim is the most important thing while riding, because wheels more dominantly have moment of inertia - the inertia of the wheel to rotating forces and motion. The effect of moment of inertia is a lot greater than frame or hub mass, because of the circular geometry. If you double the mass at the rim, the effect on rotation is multiplied by the square of the radius. For mass increase at the edge of the hub, the radius is about 1.5 inches, for a factor of 2.25. For the same mass increase spread along the rim, the radius (for a 650b) is about 12.2 inches which gives a factor of 149. The addition of mass at the rim has 66 times more significance than the same mass at the outside of the hub.

    Bottom line is, I would not worry about weight, if you do not weight-weenie out at the addition of a Brooks saddle or if you eschew carbon seat posts, stems, bars, and cranks. After you get the hub and lights installed, see if a lighter rim exists that you can use, or lighter tires. Lots more bang for the buck!

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    I'm not a weight weenie anyway, but the reason I have dynohubs on all my bikes is because I always have working lights with me. I didn't have a light ready when I put together a new bike this summer. We went for a ride that was supposed to end in daylight. Fortunately, my riding companion had extra lights, because we rode for over a half hour in full darkness on a mountain road. I don't even like to have bikes without rear dyno lights any more. It's very freeing, I can decide to go on a longer ride at any time or start a ride late in the day without worry. I can't count the number of times I went out for a ride late in the day and had to worry about riding home in the dark, those days are over.

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    Senior Member Looigi's Avatar
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    Quote Originally Posted by Road Fan View Post
    The concentration of mass at the rim is the most important thing while riding, because wheels more dominantly have moment of inertia - the inertia of the wheel to rotating forces and motion. The effect of moment of inertia is a lot greater than frame or hub mass, because of the circular geometry. If you double the mass at the rim, the effect on rotation is multiplied by the square of the radius. For mass increase at the edge of the hub, the radius is about 1.5 inches, for a factor of 2.25. For the same mass increase spread along the rim, the radius (for a 650b) is about 12.2 inches which gives a factor of 149. The addition of mass at the rim has 66 times more significance than the same mass at the outside of the hub.
    All sounds good, but: Moment of inertia is only relevant to acceleration and deceleration and its contribution to the force needed to accelerate the entire bike and rider is small. The static weight is still what contributes to the work need to go uphill and to a very small extent, the rolling drag of the tires.
    Last edited by Looigi; 12-15-13 at 11:22 AM.

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    Quote Originally Posted by Looigi View Post
    All sounds good, but: Moment of inertia is only relevant to acceleration and deceleration and its contribution to the force needed to accelerate the entire bike and rider is small. The static weight is still what contributes to the work need to go uphill and to a very small extent, the rolling drag of the tires.
    Weight is only important to pro racers who need every advantage. Even then the low weight is limited by rules.
    For the rest of us it is not a real factor.

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    at some point weight becomes an issue. I know people that ride long distance (not touring) on bikes that weigh 35-40 pounds, and I think that's crazy. My bike when fully loaded weighs well into the 20's, but there is not a lot that I would take off of it. You can definitely feel a pound when you start sprinting. I don't think you really feel it climbing. My rule of thumb is you can feel 10 pounds climbing.

  9. #9
    Senior Member Road Fan's Avatar
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    Please be specific about where you think I am spinning bull**** or attempting to mislead. I don't have any reason to do so, this site is for recreation, and I don't see misleading fellow cyclists as fun.

    Regarding your claim regarding inertia and accel/decel, yes, you are correct that it only affects those aspects of riding. I never said anything different, though I neglected to make that point. My comment about "the most important thing while riding" was badly chosen, I was trying to say it was a lot more important than hub or frame mass, since Zacster was so concerned about the extra pound added to his bike due to the addition of a dyno hub. My basic point is that the added hub weight does not make a significant difference.

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    The B&M lumotech fly N can be had for $30 from Peter White Cycles, I have one and it's a good light. I run that with a cheap handlebar light (~$10) in blink mode and it's fine. The extra light is a backup and can be used as a flashlight if any repairs need to be done after dark.

  11. #11
    coprolite fietsbob's Avatar
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    generating electricity requires steel , magnets and Copper wire . Aluminum wire may save a few grams..


    Fwiw, another cheap dynamo headlight: Planet Bike ,Its like their 1W superflash, with a plug in the hookup wire ,
    so you can put it in your pocket when you park your bike. (just have to remember to not leave it behind)
    [a bolted on lights will be there . on the bike..

    adding a second one, powered with a battery , is a good plan.

    In addition to the Shimano for Brompton Hub, my Brompton Upgrade:

    B&M Eyc,.. + a Ixon IQ , that... I can share with a couple other bikes ..
    Last edited by fietsbob; 12-17-13 at 03:47 PM.

  12. #12
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    Quote Originally Posted by unterhausen View Post
    at some point weight becomes an issue. I know people that ride long distance (not touring) on bikes that weigh 35-40 pounds, and I think that's crazy. My bike when fully loaded weighs well into the 20's, but there is not a lot that I would take off of it. You can definitely feel a pound when you start sprinting. I don't think you really feel it climbing. My rule of thumb is you can feel 10 pounds climbing.
    I like to know what they're putting on their bikes to make it 35#-40#. My long distance bike is 20#, that's with standard stuff like open pro rims, 32 hole 3x, fenders, and sp dynamo and handlebar bag. Only carbon parts are the fork and the seatpost.

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    I have no idea what my MTB/Commuter actually weighs. But it is heavy once I put a laptop and the other stuff I carry in my bag. So, will a 558g hub make that much of a difference? Maybe, maybe not. What I do know is that there is no way my commuter bike rides like my road bike, 16 lbs CF, with 1450g wheels. I don't expect it to. I understand the concept of the mass at the rim vs. at the hub. I could also stand to lose 25 pounds myself, all dead weight.

    As for lights, the $30 Fly N does not have a standlight, and is already 5 years old. The standlight models cost more. LED tech has changed a lot in the last few years. I was looking at either the EYC N or T models $73 or $80, or the IQ Premium Cyo T Senso Plus for $110.

    As I said already, I'm going ahead with this with a good deal of skepticism.

  14. #14
    Senior Member Road Fan's Avatar
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    My feeling with my first dyno hub, is that I like it despite the weight. I too have a very light roadie, and no, the rando/commuter does not ride the same. It's quite good, but very different from an 18# steel ELOS frame with 21 mm tubulars.

    But that's no reason not to have good lights, is it?

  15. #15
    Senior Member dougmc's Avatar
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    Quote Originally Posted by Road Fan View Post
    The concentration of mass at the rim is the most important thing while riding, because wheels more dominantly have moment of inertia - the inertia of the wheel to rotating forces and motion. The effect of moment of inertia is a lot greater than frame or hub mass, because of the circular geometry. If you double the mass at the rim, the effect on rotation is multiplied by the square of the radius. For mass increase at the edge of the hub, the radius is about 1.5 inches, for a factor of 2.25. For the same mass increase spread along the rim, the radius (for a 650b) is about 12.2 inches which gives a factor of 149. The addition of mass at the rim has 66 times more significance than the same mass at the outside of the hub.
    Qualitatively, you are correct -- mass on your wheel has a larger impact on your total inertia than mass on your frame (or your body.) But qualitatively, you're wrong -- mass at the outer edge of your wheel, no matter what its radius, only counts twice as much as mass on your frame or at the hub.

    Mass at the hub is close enough to the center that it counts approximately as much as mass on your frame, but mass at your rim and wheel is close enough to the outer edge of your wheel that it counts approximately twice as much as mass on the frame.

    And note that this is only about inertia -- accelerating and deaccelerating -- not climbing. For climbing purposes, it doesn't matter where the mass is on your bike, it has the same effect.

    And yes, in general the only time that such things are really important is during a race. A few lbs extra on the bike really only matters when 1) every second counts (i.e. a race) or 2) if you're carrying the bike. But if it is a race, it's better to have the weight on the frame or hub than at the outer part of the wheel.
    Last edited by dougmc; 12-15-13 at 08:21 PM.

  16. #16
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    Quote Originally Posted by unterhausen View Post
    I'm not a weight weenie anyway, but the reason I have dynohubs on all my bikes is because I always have working lights with me. .
    I heartily agree, when you turn them on-they work, everytime!
    R

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    If all you want is a flashing light to be seen, then I'm not sure you'll be happy with your proposed setup. When the B&M lights are aimed properly, they are a lot like a low beam on a car, illuminating the road without blinding other road users. If you aim it like a high beam, maybe you'll get what you want. As for the standlight feature, my IQ Fly Plus standlight is really not that bright and I wouldn't count on it to be seen when stopped.

    What I really wonder is, if you are so down on the whole dynamo lighting thing, why wouldn't you try a cheaper setup from the start? I got an Axa tire driven dynamo, canti mount, canti light bracket, and IQ Fly Plus for $75 total with shipping. There are downsides to tire driven dynos, but weight and price aren't among them. As for the IQ Fly Plus being old, for $20 ($52 on Peter White's site is a misprint) it's a hell of a deal/performer and will not need to be upgraded any time soon.

  18. #18
    Senior Member Road Fan's Avatar
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    Quote Originally Posted by dougmc View Post
    Qualitatively, you are correct -- mass on your wheel has a larger impact on your total inertia than mass on your frame (or your body.) But qualitatively, you're wrong -- mass at the outer edge of your wheel, no matter what its radius, only counts twice as much as mass on your frame or at the hub.

    Mass at the hub is close enough to the center that it counts approximately as much as mass on your frame, but mass at your rim and wheel is close enough to the outer edge of your wheel that it counts approximately twice as much as mass on the frame.

    And note that this is only about inertia -- accelerating and deaccelerating -- not climbing. For climbing purposes, it doesn't matter where the mass is on your bike, it has the same effect.

    And yes, in general the only time that such things are really important is during a race. A few lbs extra on the bike really only matters when 1) every second counts (i.e. a race) or 2) if you're carrying the bike. But if it is a race, it's better to have the weight on the frame or hub than at the outer part of the wheel.
    I don't see where you get that the force required to overcome inertia due to mass at the rim is twice that needed to overcome inertia due to the same mass on the frame. Can you explain that? I have reviewed my math and concluded that my factor of 66 was correct for moment of inertia, but not for the force required to accelerate the wheel, driving it from the hub. I think the force required is actually half:

    F_applied = (total_mass_at_rim x bike_acceleration)/2.
    Last edited by Road Fan; 12-16-13 at 09:11 AM. Reason: Better clarity.

  19. #19
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    Quote Originally Posted by likebike23 View Post
    If all you want is a flashing light to be seen, then I'm not sure you'll be happy with your proposed setup. When the B&M lights are aimed properly, they are a lot like a low beam on a car, illuminating the road without blinding other road users. If you aim it like a high beam, maybe you'll get what you want. As for the standlight feature, my IQ Fly Plus standlight is really not that bright and I wouldn't count on it to be seen when stopped.

    What I really wonder is, if you are so down on the whole dynamo lighting thing, why wouldn't you try a cheaper setup from the start? I got an Axa tire driven dynamo, canti mount, canti light bracket, and IQ Fly Plus for $75 total with shipping. There are downsides to tire driven dynos, but weight and price aren't among them. As for the IQ Fly Plus being old, for $20 ($52 on Peter White's site is a misprint) it's a hell of a deal/performer and will not need to be upgraded any time soon.
    I'm not really down on the dynamo lighting, just skeptical that it will be what I need. As for the cost, I can always resell it on ebay if I really don't like it. I'm buying everything at prices that are below the average price, so it's more likely to be able to recoup most of my investment. I had a tire driven dynamo many years ago and they just seem like toys, plus it had a lot of drag. But this is a long time ago.

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    And one other thing, the naming convention on these B&M lights is really confusing. It must be the translation from the German.

  21. #21
    Zoom zoom zoom zoom bonk znomit's Avatar
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    Quote Originally Posted by zacster View Post
    And one other thing, the naming convention on these B&M lights is really confusing. It must be the translation from the German.
    +1
    Took me a week to find the one I wanted (standlight yes, running lights no).

    Weight wise I use a 3n80 on my carbon roadie, between touring wheels I don't really notice the hub for the rides I do where I need the dyno.

    On the MTB getting rid of the dynohub (the bearing do not like swimming in 6ft deep rivers) made a huge improvement to the handling.

  22. #22
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    Quote Originally Posted by zacster View Post
    I'm not really down on the dynamo lighting, just skeptical that it will be what I need. As for the cost, I can always resell it on ebay if I really don't like it. I'm buying everything at prices that are below the average price, so it's more likely to be able to recoup most of my investment. I had a tire driven dynamo many years ago and they just seem like toys, plus it had a lot of drag. But this is a long time ago.
    The tire dynamo I've got does indeed have some noticeable drag when in use, but It's not too bad riding at night when speed is lower anyway. The upside is that I can totally eliminate drag when not in use by disengaging the dyno.

    I chose that route because I was like you, unsure of how I'd like dyno lighting. For me, the downsides (less reliable wet weather performance and increased drag) were acceptable because the investment was low. With the tire dyno I'm getting all of the light, and none of the weight penalty also. In addition, any future upgrade to a hub dyno will only result in miniscule loss of my initial investment when you factor in the use I get out of the tire dyno in the mean time.

    As for tire dynos being toys, untrue. The nicer tire dynos are being used extensively in Europe and giving good service. Are they the coolest, most prestigious way to power your light? Absolutely not, but they are a viable alternative.

    I'm not trying to be argumentative, your setup is going to be really nice when you are done, and I hope you really like it. I'm only posting to benefit any other readers who are trying to plan their first dyno setup. There are many different lighting setups and each have their up/downsides. Finding that out late in the game could be costly, and nobody likes losing $$.

  23. #23
    Senior Member Road Fan's Avatar
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    Quote Originally Posted by zacster View Post
    And one other thing, the naming convention on these B&M lights is really confusing. It must be the translation from the German.
    Peter white has a really good secret decoder ring in his section on lights, but there are just a lot of choices to deal with. I don't think it has to do with German.

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    Quote Originally Posted by likebike23 View Post
    The tire dynamo I've got does indeed have some noticeable drag when in use, but It's not too bad riding at night when speed is lower anyway. The upside is that I can totally eliminate drag when not in use by disengaging the dyno.

    I chose that route because I was like you, unsure of how I'd like dyno lighting. For me, the downsides (less reliable wet weather performance and increased drag) were acceptable because the investment was low. With the tire dyno I'm getting all of the light, and none of the weight penalty also. In addition, any future upgrade to a hub dyno will only result in miniscule loss of my initial investment when you factor in the use I get out of the tire dyno in the mean time.

    As for tire dynos being toys, untrue. The nicer tire dynos are being used extensively in Europe and giving good service. Are they the coolest, most prestigious way to power your light? Absolutely not, but they are a viable alternative.

    I'm not trying to be argumentative, your setup is going to be really nice when you are done, and I hope you really like it. I'm only posting to benefit any other readers who are trying to plan their first dyno setup. There are many different lighting setups and each have their up/downsides. Finding that out late in the game could be costly, and nobody likes losing $$.
    I really didn't take it as argumentative.

    That old tire dynamo I had was made for an old 3 speed "English Racer", so it probably was close to being a toy. I can't even remember why I had that bike, it wasn't mine. I just remember riding it home after working a shift driving a taxi when I was in college. Maybe it was after I had crashed my Atala and before I bought my Austro-Daimler Inter10.

    Maybe I'll have a nice 26" rim brake dynamo wheel to sell you! As I found out though, 26" rim brakes are not much of a standard anymore between disc brakes and 29ers as serious MTBs.

  25. #25
    Senior Member dougmc's Avatar
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    Quote Originally Posted by Road Fan View Post
    I don't see where you get that the force required to overcome inertia due to mass at the rim is twice that needed to overcome inertia due to the same mass on the frame. Can you explain that? I have reviewed my math and concluded that my factor of 66 was correct for moment of inertia, but not for the force required to accelerate the wheel, driving it from the hub. I think the force required is actually half:

    F_applied = (total_mass_at_rim x bike_acceleration)/2.
    To simplify things, let's assume that we need to worry about three places with mass on your bike -- on the frame, at the hub (at radius = 0) and right at the edge of the wheel.

    We should all agree on how the mass on the frame affects the bike -- it creates inertia that's proportional to the mass, it makes the bike harder to get up hills, yet it makes the bike go down hills faster (but this doesn't make up for the slowness going up the hill.)

    Now, as for weight at the hub -- it rotates, but since r=0 in our approximation, it doesn't "move" at all relative to the frame. So mass at the hub has the same effect as mass on the frame.

    Now, the mass at the edge of the wheel is what's interesting. Relative to the ground, the mass at the bottom of the wheel doesn't move at all, the mass at the top moves twice as fast as the bike, and the mass on the front and back moves sqrt(1^2+1^2) as fast, or 41% faster than the bike. If you consider that kinetic energy is proportional to the speed squared and integrate over the entire rim, you find that in total, it takes twice as much energy to bring a rolling wheel to speed (assuming all mass is at the rim) as it would if it were not rolling, hence the double contribution to inertia.Or,

    Or, to look at it in another way, look at the motion of the wheel relative to the frame. If the bike is moving at a given speed, the inertia of the wheel has two components -- one, its mass, moving with the rest of the bike, and its velocity -- so its momentum is mass * velocity.

    But there's another component, where the wheel is rotating independent of the bike. The outer rim of the tire is moving, also at the same velocity as the bike (but in a circle, not a straight line) and so the inertia from that is also mass * velocity, and so the total inertia from the wheel is 2 * mass * velocity.

    Now, this all assumes that all weight is at the outer point of the wheel, which is not true. But it's a good approximation for a standard bicycle wheel -- most of the weight is at the rim and tire. However, if you have a heavy hub, most of that weight is almost at the center of the wheel, so it doesn't hurt you more than weight on the frame, inertia wise.

    Also, I should point out again that this difference in inertia is only important if you're speeding up or slowing down. If you keep going at the same speed, it doesn't matter. And it's usually only a few pounds, so the overall effect is even less significant. If you worry about it because you're racing in a crit -- it'll probably matter. But if you're racing in race where you come up to speed and stay there ... probably not.
    Last edited by dougmc; 12-16-13 at 12:40 PM.

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