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  1. #26
    Climbing: Ropes or Wheels PiLigand's Avatar
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    Quote Originally Posted by asgelle View Post
    See the Broker paper. Their data is the source of the 30%. I know if it were me, I wouldn't publish any numerical results unless I had validation against their data.
    Will do, thanks. Though worth mentioning, in research, you have the obligation to publish your results regardless of how they compare to accepted values. But that's beside the point for now.

    I appreciate the help!

  2. #27
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    Quote Originally Posted by RChung View Post
    Um, yeah, lots of people have looked into this as a possibility. I looked at some data last year in the run-up to the Olympics measuring drag for a pursuit team.
    So let me ask this question, without divulging specific information, would you say the there have been significant improvement in how the problem is addressed since Broker, or is it more a matter of applying these methods to the specifics of a particular team of riders?

  3. #28
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    Well, I wasn't involved with the 1996 Atlanta Olympics so I don't know what they did but I believe that the precision of measurement is probably better now than it was then, and that precision means we were able to do certain things that I think would be hard to do with less precision.

  4. #29
    rugged individualist wphamilton's Avatar
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    Regarding the drafting giving a boost the the front rider, a while back I saw an examination of this that looked reliable. Sorry, no link, but while there could be a boost in theory it's tiny, and to even get that tiny boost the drafter would have to be practically glued to the cyclist's back. Millimeters. So for all practical purposes, as far as I'm concerned it's busted.

  5. #30
    pan y agua merlinextraligh's Avatar
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    It seems petty simple to me; there's some theoretical reason to believe that the front rider gets a very small benefit, but at normal bike speeds there may be no, or virtually no effect.

    There's no robust data to support the existence of an advantage to the front rider. There is some limited data from computer simulations, and less than rigorous observations to support that there may be a tiny effect. So it's still open for further study.

    And the fact that it only matters to determine who wins a pedantic debate on a bike forum may explain the lack of conclusive, rigorous testing to resolve the issue.
    You could fall off a cliff and die.
    You could get lost and die.
    You could hit a tree and die.
    OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.

  6. #31
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    Quote Originally Posted by merlinextraligh View Post
    And the fact that it only matters to determine who wins a pedantic debate on a bike forum may explain the lack of conclusive, rigorous testing to resolve the issue.
    Maybe we can some how combine this with the frame stiffness thread. We could debate whether a flexible frame would slightly bend to become more streamlined while under speed, sort of like an F1 car with flexible rear wing mounts.

  7. #32
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  8. #33
    Senior Member floridamtb's Avatar
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    From what I've heard the slipstream only projects back about 6 feet. This means that the third rider is not seeing a benefit from the slipstream from the first rider.
    Kevin S

  9. #34
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    Quote Originally Posted by asgelle View Post
    See the Broker paper. Their data is the source of the 30%. I know if it were me, I wouldn't publish any numerical results unless I had validation against their data.
    The Blocken paper I linked did compare the isolated model to wind-tunnel data of a single rider on a bicycle. The error ranged from 10%-0.7%.
    Here's the link to the published article (which few can read because you need a subscription, hence the preprint):
    http://www.sciencedirect.com/science...45793012004446

    Remember that you're looking for changes in the overall drag, so a 10% error in the model really means that 3% change is drag would be 3%+-0.3%.

    The Blocken paper seems to be independent authorship from the web link, so you've got two groups basically in agreement, compared with no credible disagreement.

  10. #35
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    Quote Originally Posted by floridamtb View Post
    From what I've heard the slipstream only projects back about 6 feet. This means that the third rider is not seeing a benefit from the slipstream from the first rider.
    In calm conditions we've measured a "drafting effect" way, way, way farther out than 6 feet for "typical" TT speeds.

  11. #36
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    Quote Originally Posted by gsa103 View Post
    The Blocken paper seems to be independent authorship from the web link, so you've got two groups basically in agreement, compared with no credible disagreement.
    Broker et al. disagree. They were able to model the data without decreasing the drag coefficient on the lead rider.

  12. #37
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    Quote Originally Posted by floridamtb View Post
    From what I've heard the slipstream only projects back about 6 feet. This means that the third rider is not seeing a benefit from the slipstream from the first rider.
    ??? The third rider is behind the second. The third rider could still get a bit of extra benefit from the first. The second rider could cause the slipstream to extend farther than 6 feet by the mere fact that it has to go around him.

    A link I ran across months ago (can't find it now) indicated that the third rider benefits more than the second.

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    Quote Originally Posted by asgelle View Post
    Broker et al. disagree. They were able to model the data without decreasing the drag coefficient on the lead rider.
    You seem rather vested in this. Any particular reason?

    Just throwing some numbers into analyticcycling using myself as a model I get that a 1% decrease in Cd (with in the range of the Blocken and Ferguson findings) would reduce the watt requirement by about 2.5 from 300 to maintain 27mph.

    Personally I consider that to be most likely not noticeable or meaningful in a rotating pace line. But since multiple sources indicate that a very small gain is probably realized under the right conditions while cycling, why so interested in fighting about it?

  14. #39
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    Quote Originally Posted by canam73 View Post
    You seem rather vested in this. Any particular reason?
    Not vested, not fighting. Just looking to separate the signal from the noise.

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    At something around 1% that is going to be tough. Figures from the Blocken study show the gap length and riding position of the riders having many times the effect of a possible 'push'.

  16. #41
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by njkayaker View Post
    ??? The third rider is behind the second. The third rider could still get a bit of extra benefit from the first. The second rider could cause the slipstream to extend farther than 6 feet by the mere fact that it has to go around him.

    A link I ran across months ago (can't find it now) indicated that the third rider benefits more than the second.
    I think you're right, and it's fairly intuitive - up to a point, lengthening fills in the turbulent areas reducing induced drag, while increasing skin friction drag. Thinking of the several cyclists as one complex object, the third cyclist has the first effect but from his perspective no increase in surface friction.

  17. #42
    Stand and Deliver FLvector's Avatar
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    I always hate getting behind the 110 lb, 5 ft tall little guy that rides in his drops and barely breaks the wind at all. Its almost like riding up front. Now getting behind the big behemoth is nice bonus and will make a big difference in effort and can add up on a long ride. There's so many variable to consider and the size of the rider and how close you tuck in are just a few.

  18. #43
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    Quote Originally Posted by FLvector View Post
    I always hate getting behind the 110 lb, 5 ft tall little guy that rides in his drops and barely breaks the wind at all. Its almost like riding up front. Now getting behind the big behemoth is nice bonus and will make a big difference in effort and can add up on a long ride. There's so many variable to consider and the size of the rider and how close you tuck in are just a few.
    I tried to draft a recumbent once.



    I've hated recumbents ever since.

  19. #44
    Blast from the Past Voodoo76's Avatar
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    Quote Originally Posted by RChung View Post
    In calm conditions we've measured a "drafting effect" way, way, way farther out than 6 feet for "typical" TT speeds.
    You really notice this on a track, you feel it a lot more easily in a fixed gear. Running up on another rider, or better yet a motor, easily 5 or 6 bikes bikes back you start accelerating at the wheel in front.

    So what would happen if we got enough riders together to circle a track, we are all in the draft, how fast?

  20. #45
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by Voodoo76 View Post
    You really notice this on a track, you feel it a lot more easily in a fixed gear. Running up on another rider, or better yet a motor, easily 5 or 6 bikes bikes back you start accelerating at the wheel in front.

    So what would happen if we got enough riders together to circle a track, we are all in the draft, how fast?
    That's a pretty awesome idea! Maybe even motor-pace them up to some ridiculous speed, pull off letting the lead cyclist close up the circle. I'd like to see that.

  21. #46
    Senior Member rebel1916's Avatar
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    Quote Originally Posted by canam73 View Post
    You seem rather vested in this. Any particular reason?
    since multiple sources indicate that a very small gain is probably realized under the right conditions while cycling, why so interested in fighting about it?
    Because 41

  22. #47
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    Quote Originally Posted by canam73 View Post
    Maybe we can some how combine this with the frame stiffness thread. We could debate whether a flexible frame would slightly bend to become more streamlined while under speed, sort of like an F1 car with flexible rear wing mounts.
    Or the correlation between bar tape color and power output.

  23. #48
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    Quote Originally Posted by kenji666 View Post
    Or the correlation between bar tape color and power output.
    I don't use bar tape. It's too heavy.

  24. #49
    Senior Member Homebrew01's Avatar
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    Quote Originally Posted by Voodoo76 View Post
    You really notice this on a track, you feel it a lot more easily in a fixed gear. Running up on another rider, or better yet a motor, easily 5 or 6 bikes bikes back you start accelerating at the wheel in front.

    So what would happen if we got enough riders together to circle a track, we are all in the draft, how fast?
    They would go faster & faster until they melted ...... or their carbon frames assploded.
    Bikes: Old steel race bikes, old Cannondale race bikes, less old Cannondale race bike, crappy old mtn bike

  25. #50
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    Quote Originally Posted by Voodoo76 View Post
    You really notice this on a track, you feel it a lot more easily in a fixed gear. Running up on another rider, or better yet a motor, easily 5 or 6 bikes bikes back you start accelerating at the wheel in front.

    So what would happen if we got enough riders together to circle a track, we are all in the draft, how fast?
    It is well-known that riders on indoor velodromes create their own "draft" by circling the track. This effect is large when the number of riders is large (for example, in a points race) but is even measurable when there is only one rider on the track (for example, for the hour record), which shows that the effect can extend out at least 250m. Even a straw can stir up a bath tub, and even a single rider can set up a circulating current in a velodrome. In a typical velodrome the draft varies between the straights and the curves, and from the infield up to the top of the banked curve.

    The amount of benefit varies with the number of riders but also their speed. For a single rider on a 250m indoor velodrome at individual pursuit-like speeds the reduction in "effective" drag area due to this effect can be close to .005 m^2.

    Field measurements of aerodynamic drag based on velodrome tests need to be corrected for this effect. That is, real-time measurements of drag will decrease for the first few minutes while the rider "stirs the bath tub" until it reaches an equilibrium value. You can also see this in the power-to-speed relationship for hour record attempts and in team pursuit events. If you do field tests outdoors in calm conditions, you can see the effect on aero drag if another rider is even 15 seconds or so up the road (if a car passes, you can see the effect for at least 30 seconds).

    So, I don't know where the "6 feet of slipstream" came from but that sure ain't what I've seen.

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