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  1. #1
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    Wheel Weight - Flywheel Effect

    It's commonly accepted that lighter wheels (or less rotating mass) make climbing and accelerating easier.

    But say you were riding on a completely flat course; Would heavier wheels make maintaining speed easier? There must be some science behind this (conservation of momentum?).

    I looked up the land speed world record for human power and found that they are using fully faired devices. Weight must be less of an issue than aerodynamics.

    Opinions? Facts?

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    recent post to Randon about bicycle physics

    essential takeaway -- on flats, weight is definitely less of an issue than aerodynamics. when you climb, weight becomes more important.

    If you want to get faster overall, learn to climb. that's less about shaving weight on your bike and more about developing power as a rider.

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    just another gosling Carbonfiberboy's Avatar
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    Here's a total oddity that might contribute to the discussion. I mostly climb in the saddle. I'm not a hollow-boned waif. From time to time, I get up to rest or power over a little bump. Every time I do that, my HR will pop up about 5 beats, even though I hold the speed the same and shift up to a more appropriate resistance. However, riding a spin bike with that heavy flywheel at a climbing resistance and low cadence, if I get out of the saddle at the same resistance and cadence, my HR drops about 2 beats. That has to be something to do with the flywheel effect on what is effectively a fixie. So is that why some people enjoy climbing with their fixies?

    Further, on the OP's point, I don't notice any particular difference in climbing with lighter or heavier wheels if I'm sitting and spinning with a decent circular power stroke. I do notice it out of the saddle.

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    Quote Originally Posted by Carbonfiberboy
    Here's a total oddity that might contribute to the discussion. I mostly climb in the saddle. I'm not a hollow-boned waif. From time to time, I get up to rest or power over a little bump. Every time I do that, my HR will pop up about 5 beats, even though I hold the speed the same and shift up to a more appropriate resistance. However, riding a spin bike with that heavy flywheel at a climbing resistance and low cadence, if I get out of the saddle at the same resistance and cadence, my HR drops about 2 beats. That has to be something to do with the flywheel effect on what is effectively a fixie. So is that why some people enjoy climbing with their fixies?
    your legs can produce more torque when you're standing on your pedals, because you're now using gravity to assist in your downstroke; but your body also has to do more work on the upstroke (both to lift your leg against gravity and retain balance). So you can make yourself climb faster when out of the saddle, but you're forcing your body to do more work, and that work increases in relation to your weight.

    On a fixed wheel, the momentum from the wheel is also assisting on your upstroke, while gravity is still providing the same boost on the downstroke as on a freewheel bike, which is why your body would do less work and your HR drops.

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    Wheel weight is significant mostly in acceleration, heavier wheels
    will accelerate more slowly for a given power input. Since you
    can't climb without accelerating the same applies. Aerodynamics
    comes into play between 17 and 20mph and becomes rapidly more
    important as the speed increases above 20mph. Power required
    goes up approximately with the square of the velocity at low
    velocities, with the cube of the velocity at higher velocities. Low
    and high were not defined in the article I looked at but I suspect that
    upper 20s would be considered "high". Lance' and Tom Danielson
    doing 34+ average mph in a 50km TT on the Tour 2yrs ago requires steady
    state power outputs in the 400+ watt range.

  6. #6
    Senior Member Richard Cranium's Avatar
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    Would heavier wheels make maintaining speed easier? There must be some science behind this (conservation of momentum?)
    Wheels aren't a big deal, but the overall weight of a rider/bicycle combination is.

    A case in point, I rode a touring setup on a ride and was carrying substantially more weight than usual. I would get a really good "roll" going down long down-hills, and the guys drafting me were finding it harder to come around me because of the increased speed. Of course, I was wasted trying to climb........

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    littlecircles bmike's Avatar
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    Quote Originally Posted by spokenword

    On a fixed wheel, the momentum from the wheel is also assisting on your upstroke, while gravity is still providing the same boost on the downstroke as on a freewheel bike, which is why your body would do less work and your HR drops.

    interesting theory / take on fixed riding vs geared riding


    (warning, its a PDF)

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    littlecircles bmike's Avatar
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    Quote Originally Posted by Richard Cranium
    Wheels aren't a big deal, but the overall weight of a rider/bicycle combination is.

    A case in point, I rode a touring setup on a ride and was carrying substantially more weight than usual. I would get a really good "roll" going down long down-hills, and the guys drafting me were finding it harder to come around me because of the increased speed. Of course, I was wasted trying to climb........
    Agreed. But bicycle weight is usually a very small percentage of the engine weight. (unless you are touring)
    My goal is to find a lightweight engine that puts out lots of power.

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    When I was a junior -- what's getting to be a long time ago -- I was on a team that was loaned some first generation Gitane time trial bikes for the TTT of a stage race. These things looked like they'd been run over by a steamroller and weighed about 35 pounds apiece. The wheels were the Ambrosio "metacrillium" disks front and rear and they had to contribute half the weight of the bike. We were all shaking our heads, but the things were actually pretty damn fast, at least on the flat course. It took forever to get them up to speed, and I would have hated to try to lug the silly things up any real hills, but they also smoothed out the usual surges and gaps of the team event, as well as giving a feeling of momentum on small rises.

    Regardless, it seems to me that testing in the late '80s/early '90s -- might have been Moser and Conconi; remember his hour record bike with the gigantic rear wheel? -- showed that heavier wheels were slower than light ones in all conditions. I could be misremembering, and the odds of me finding a link to it are small, but that is my recollection.

    HTH!

  10. #10
    just another gosling Carbonfiberboy's Avatar
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    Aha! This is the piece of information I was hoping to elicit. This completely explains what I was experiencing on the spin bike. However enamoured the author is of fixies, it takes an exceptional athlete to keep up with a normal person on a geared freebie over any kind of distance. Also, he totally neglects the spreading of the pedaling load among more leg muscles, which freebies encourage. So I guess SS riders have the worst of both worlds, in some way.

    That said, I totally believe the physics of the fixie phenomenon. My question now is: why don't we have mechanisms to lock the freehub for climbing? That makes so much sense - gears and a locked freehub. I've climbed with enough superb fixie riders to see that they are in serious pain on long 10% + grades while I, a complete nobody, am spinning fairly comfortably.

    Back to the OP: No, heavier wheels wouldn't help most riders maintain speed on a flat course, for reasons explained in the PDF. They would if you were riding a fixie on rollers, but that's not what you're talking about. Any maybe they would if you had a really awful pedal stroke, but I doubt it. It would be interesting to experiment, but to what point? I don't think lighter wheels help much in climbing, either, unless you climb out of the saddle, in which case you are accelerating with every pedal stroke. Otherwise, contrary to ach, you are not accelerating when climbing. In fact, the object of that game is to not accelerate, but rather to keep your angular momentum unchanged. When I finally learned to pedal hard over the top of the stroke, it made a huge difference to my climbing.

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    My question now is: why don't we have mechanisms to lock the freehub for climbing? That makes so much sense - gears and a locked freehub.
    If I understand what you are saying, the trouble is that it's very easy to munch your rear derailleur on a fixed gear. Even momentarily trying to freewheel will stretch the daylights out of the return spring.

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    And after going through the article, I have a number of disagreements.

    The author writes that the human leg is not capable of turning cranks at constant velocity, and that the freewheel "decouples" to allow this at the bottom of the peddle stroke. This cannot be correct as it is possible to feel when this happens as the result of a crummy pedal stroke -- the rider notices that the crank has to turn against no resistance for a fraction of a degree as the cranks "catch up" to the freewheel.

    Moreover, even if this argument IS correct, there would still be no power or speed advantage from riding fixed. In fact there would likely even be power loss as power is robbed from the drivetrain in order to carry the leg throught the "dead spot" in each pedal stroke.

    The author then goes on to argue that the fixed gear allows a 1/3 longer power stroke and provides 50% more torque and acceleration. This is disproved not only by results, but also by hard numbers. Those familiar with Powermeter numbers from road and track racers know that they are essentially the same with individual athletes.

    The author, IMO, is looking for complex answers where simple ones suffice: Why are fixed riders sometimes faster on hills? Because they have to be. If you've got lots of gears you have a decision to make: do I grunt or do I spin? But if you've got one gear you're either going to turn it or you aren't.

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    It comes as no surprise that fixie riders are forced to train at higher intensities on hills so thier fitness levels and hill climbing power should increase over a rider who chooses more of a spinning gear ratio. Still, the geared rider can conserve more energy and over a longer distance this really gives the geared rider the advantage.

  14. #14
    just another gosling Carbonfiberboy's Avatar
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    Ah, I hadn't thought about the poor derailleur. You're saying that of course the bottom chainline will tighten when the wheel drives the cranks.

    Quote Originally Posted by Six jours
    And after going through the article, I have a number of disagreements.

    The author writes that the human leg is not capable of turning cranks at constant velocity, and that the freewheel "decouples" to allow this at the bottom of the peddle stroke. This cannot be correct as it is possible to feel when this happens as the result of a crummy pedal stroke -- the rider notices that the crank has to turn against no resistance for a fraction of a degree as the cranks "catch up" to the freewheel.

    Moreover, even if this argument IS correct, there would still be no power or speed advantage from riding fixed. In fact there would likely even be power loss as power is robbed from the drivetrain in order to carry the leg throught the "dead spot" in each pedal stroke.

    The author then goes on to argue that the fixed gear allows a 1/3 longer power stroke and provides 50% more torque and acceleration. This is disproved not only by results, but also by hard numbers. Those familiar with Powermeter numbers from road and track racers know that they are essentially the same with individual athletes.

    The author, IMO, is looking for complex answers where simple ones suffice: Why are fixed riders sometimes faster on hills? Because they have to be. If you've got lots of gears you have a decision to make: do I grunt or do I spin? But if you've got one gear you're either going to turn it or you aren't.
    Well, you make a good argument, also. But what about my HR dropping while out of the saddle on the spin bike fixie? It really is a totally different feeling than climbing out of the saddle on my freebie. You probably can remember that it was a trick learning how to pedal out of the saddle on the flat, at low cadence against small resistance. There's a tendency to have a slack chain at about 11:00, so one has to consciously push that top foot forward at the top of the stroke, which is quite hard to do out of the saddle as there's nothing to push against. The fixie removes that bit of difficulty and your foot is automatically presented with a pedal at the appropriate position for a power stroke.

    I think there's no difference while in the saddle. TT'ing on a spin bike feels exactly like TT'ing on my road bike. Well, a lot sweatier, but more fun with better scenery.

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    Ah, I hadn't thought about the poor derailleur. You're saying that of course the bottom chainline will tighten when the wheel drives the cranks.
    Yeah, I learned that the hard way, fooling around as a kid. We all put fixed gears on for the winter, leaving the derailleur in place but bypassing it with the chain. Being smarter than everyone else, I saw no reason to go to all that work so just threaded on a fixed cog and reinstalled the wheel. One less Super Record rear derailleur in the world.

    Well, you make a good argument, also. But what about my HR dropping while out of the saddle on the spin bike fixie?
    You mean in spin class? Dunno, for sure. My HR goes up when I'm out of the saddle. I think it's a very individual thing. Some folks love to climb out of the saddle, others hate it.

    You probably can remember that it was a trick learning how to pedal out of the saddle on the flat, at low cadence against small resistance.
    Well, sort of. I have to hold my hips a bit low and hold the bike upright. I also don't have any reason for every doing that, though.

    There's a tendency to have a slack chain at about 11:00, so one has to consciously push that top foot forward at the top of the stroke, which is quite hard to do out of the saddle as there's nothing to push against. The fixie removes that bit of difficulty and your foot is automatically presented with a pedal at the appropriate position for a power stroke.
    I can see that. I know that trying to spin out of the saddle under low resistance is fatiguing to muscles of my legs that normally aren't heavily stressed. But my critique still stands: if the fixed gear is helping the rider through some component of the pedal stroke, then the fixed gear must be robbing power from the drive train to do it!

    I noticed that when some folks tried to revive the "retro" technique of fixed gear training, they neglected the point that, back in the day, the fixed gear would be very small. Like 42x23. So these guys spent a winter on the 48x16, pedaled around at 80 rpm -- or slower, up the hills -- and then wondered why they were pedaling squares when they got back on the freewheel. What happened, of course, was that they got lazy and began allowing the fixed gear to carry their legs through the "dead spot" at the bottom of the pedal stroke. Once back on the freewheel, they had to retrain their legs to eliminate the dead spot through muscle power.

    Good stuff, this thread.
    Last edited by Six jours; 04-13-07 at 01:41 PM.

  16. #16
    just another gosling Carbonfiberboy's Avatar
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    Quote Originally Posted by Six jours
    I noticed that when some folks tried to revive the "retro" technique of fixed gear training, they neglected the point that, back in the day, the fixed gear would be very small. Like 42x23. So these guys spent a winter on the 48x16, pedaled around at 80 rpm -- or slower, up the hills -- and then wondered why they were pedaling squares when they got back on the freewheel. What happened, of course, was that they got lazy and began allowing the fixed gear to carry their legs through the "dead spot" at the bottom of the pedal stroke. Once back on the freewheel, they had to retrain their legs to eliminate the dead spot through muscle power.
    So if I understand you correctly, you're saying that the conventional wisdom of training fixie in the winter may not be wise after all.

    Sometimes on group rides in the winter, I'll run my road bike like a SS, in a 60" or 67" gear. I did that the first time when I broke a derailleur cable and found it such fun that I've done it from time to time, but with the B group. It probably does help that neuromuscular coordination thing. The gear you quote for fixed back in the day is 50". That seems really small. The 48X16 is 80" - way too high.

    A 2 used to come out with us in the winter on his 67" steel fixie. He'd peel off at 35 miles, while we nobodies would continue on for another 15-20 on our geared bikes. Still, an impressive performance.

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    littlecircles bmike's Avatar
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    Quote Originally Posted by Six jours

    I noticed that when some folks tried to revive the "retro" technique of fixed gear training, they neglected the point that, back in the day, the fixed gear would be very small. Like 42x23. So these guys spent a winter on the 48x16, pedaled around at 80 rpm -- or slower, up the hills -- and then wondered why they were pedaling squares when they got back on the freewheel. What happened, of course, was that they got lazy and began allowing the fixed gear to carry their legs through the "dead spot" at the bottom of the pedal stroke. Once back on the freewheel, they had to retrain their legs to eliminate the dead spot through muscle power.
    I can't comment on the physics or accuracy of the article. I read it and found it somewhat enlightening... but I think every bit of data or opinion has to be taken with a grain of salt.

    I've been riding my FG since February (when I built it up). I've been riding 42x19 on the road and a bit of trail (mild dirt path) and having a blast. (riding 35mm knobbies) Longest ride to date is 32 miles. Can't wait for the snow to stop and get some slicks mounted to see if they are faster on the pave. I'll drop to a 42x17 at this point and call it good for all around riding.

    I have to admit that my $1000 steel fixie with basic components and no bells and whistles is getting ridden far more than my custom Ti IF distance machine with Record and carbon goodies. Not sure what it is yet - either infatuation with the "new", or the fact that I really like the way FG feels.

    I've had the geared machine out for 3 centuries and the last 2 I was wishing I had the FG... at least for the first half.


    I think the important thing to remember about gearing and any "flywheel" effect is that most of our cycling is relative and constrained by limits. In a perfect world we'd have the ideal gear for every climb and headwind, we'd pedal at our ideal cadence for the whole ride, and if we wanted our power output would be a smooth curve taking us off into the sunset.

    This is not reality though, and at some point we run into the limits of the physical world and balance how many gears with how strong the engine and how steep the terrain - and how light our bikes and gear with how heavy we want our wallets to be. For some doing LD rides efficiency and durability will play a role - so even if those Rolf 2 spoke wheels will help with the climbing on BMB they might not survive the road surfaces of the North East!

    I'm finding this winter riding the FG to really challenge what I thought about bikes, gears, and riding. I'm hoping to do a brevet on my FG and see how the LD stuff starts to compare... and I'm sure that at some point I'll fall in love again with my geared machine. (but I'll admit I'm already eyeing an ENO hub and new wheels, or having a framebuilder modify my ride with horizontal dropouts)



    Per the last post I think spinning a lower gear in the winter has been helping me - even if I'm limited to just 1. On my training ride with some hill repeats I'm forced to grind it out or stand - and I raise my HR and work on my strength. On the flats I spin at higher RPM than I would normally - and I'm still going slightly slower... and on downhills I need to concentrate (or ride the brake) or my hips and knees will eject from my body.

    I'm also not sure that FG riding makes your pedal style lazy. I'd have to argue that clipless pedals could do the same thing, constantly bringing your foot around in time with the power stroke. I think the possibility of having the bike push your legs along exists, esp at mid range RPMs - but being mindful of this is no different than being mindful of trying to pedal circles on a FW machine.


    Good posts all around.


    (edit - on a side note - a randonneur did BMB fixed last year on 42x16, she also qualified for RAAM at the 508 on her fixie... same gear ratio)
    Last edited by bmike; 04-13-07 at 02:47 PM.

  18. #18
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    Look at disc wheels for the rear and think about it. They're heavier, the aero and lack of turbulance -"spoil", provide benefits over weight. The flywhell effct does come into play,not solely based on weight.The lack of wind turbulance created by the wheels ALLOWS the momentum to continue,rather than create more resistance as speed increases like an old Schwinn excersize bike.The weight MAY help,I never thought about the flywheel comparison as such,interesting notion.

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    So if I understand you correctly, you're saying that the conventional wisdom of training fixie in the winter may not be wise after all.
    Well, it depends. In the "old days" the point was to increase your ability to spin at high RPM, to increase your "suppleness", and to smooth out your pedal stroke. This was accomplished by prolonged efforts at very high RPM -- 120+. At the same time, it was thought that the tiny gear allowed your body to recover from the season's efforts. Back in the day, the common belief was that you had to take a month or two very, very easy in order to recuperate, and the tiny gear assisted with that. So if the modern goal is to improve pedalling and/or recover from a season of racing, then modern riders are going at it incorrectly, IMO.

    If, however, the goal with the fixed is to increase strength, force yourself to pedal the whole time, grind up hills, or whatever, then okay.

    As an almost irrelevance, Sean Kelly had a semi-famous quote to the effect that one should never use a gear bigger than 42x17 in the off season. He was talking about freewheels, but it does illustrate the attitudes commonly held at the time.

    FWIW!


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    I'm also not sure that FG riding makes your pedal style lazy.
    I'm absolutely positive of it. I saw a great deal of it firsthand during my coaching career, and I experienced it myself prior to that. The catch, though, is it doesn't affect all riders the same. Some experience utter disasters after a few months of fixed gear "grinding" while others seem unaffected by it.

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    Also FWIW, I think the fixed phenomenon is one of the greatest things to have happened to the sport in modern times. The race bikes have gotten so complex and expensive that the fixed stuff is, IMO, kind of an extension of some folks' dissatisfaction with that scene. I absolutely applaud the folks riding and racing on fixed and SS. I think there ought to be a new racing federation and a new style of racing, both returning to the roots of the sport, with long distances, unpaved climbs, random "controls", and simple rules. Only hard men (and women) need apply!

    </ highjack >

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    Quote Originally Posted by Six jours
    I absolutely applaud the folks riding and racing on fixed and SS. I think there ought to be a new racing federation and a new style of racing, both returning to the roots of the sport, with long distances, unpaved climbs, random "controls", and simple rules. Only hard men (and women) need apply!

    </ highjack >
    at the risk of hijacking this thread further -- have you heard of / checked out Japanese Keirin racing? pretty much sounds like fixed criteriums with a bit of a NASCAR flavor due to the added pace car / moped.

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    Senior Member filtersweep's Avatar
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    I will take an aero wheel any day over a lighter wheel.... wind resistance is always your worst enemy, unless you have some odd, all uphill TT.

    Quote Originally Posted by Barre
    It's commonly accepted that lighter wheels (or less rotating mass) make climbing and accelerating easier.

    But say you were riding on a completely flat course; Would heavier wheels make maintaining speed easier? There must be some science behind this (conservation of momentum?).

    I looked up the land speed world record for human power and found that they are using fully faired devices. Weight must be less of an issue than aerodynamics.

    Opinions? Facts?

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    at the risk of hijacking this thread further -- have you heard of / checked out Japanese Keirin racing?
    Absolutely. And to tell you the truth, I think American/Continental Keirins are a bit more exciting, what with the motorcycle and the shorter tracks. Japanese Keirin is run rain or shine though, which ought to shame the typical U.S. roadie.

    If American refs saw what Japanese Keirin riders do to each other they'd have heart attacks, though!

    The deal with fixed/SS road is a bit different, though. A friend pointed out a few "unofficial" (read: outlaw) races with a lot of folks showing up on fixxies. Very much like shorter versions of turn-of-the-century road races: few rules, simple machines, the worst and steepest roads available, complete self-sufficiency, and no-nonsense riders.

    As the road scene becomes ever more effete, this is exactly the antidote, IMO. (Donning Nomex suit...)

    < edit > After checking out the link, I see that they are pretty much describing American Keirin. Japanese is much different, with much longer tracks and no motorcycle. The pacer is usually another cyclist, not even a tandem. IMO, Japanese rules are much less exciting, but the level of bike handling is outstanding.

  25. #25
    Curmudgeon Wil Davis's Avatar
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    Quote Originally Posted by Barre
    It's commonly accepted that lighter wheels (or less rotating mass) make climbing and accelerating easier.

    But say you were riding on a completely flat course; Would heavier wheels make maintaining speed easier? There must be some science behind this (conservation of momentum?).

    I looked up the land speed world record for human power and found that they are using fully faired devices. Weight must be less of an issue than aerodynamics.

    Opinions? Facts?
    I remember reading somewhere that one single ounce off the rim of a wheel is equivalent to three ounces off the frame; I don't have the article or the science to back it up, but seem to remember it as one of those maxims learned at a very young age while helping my ol' Dad in his workshop.

    - Wil
    Last edited by Wil Davis; 04-13-07 at 07:43 PM.
    "" - Marcel Marceau

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