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Old 01-29-14, 03:31 PM   #1
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Apparent CdA reduction drafting at 12m

I was randomly wondering about the percent change in apparent CdA while drafting "normal" people at a "normal" drafting distance.


Instead, I found this dude's personal experiment with 2 friends riding a track. Idk if he was making this s*** up or not, because it's hard to believe.

He found a reduction of apparent CdA of ~0.033m[SUP]2[/SUP] (~30 watts) at 40 kph, 12 meters back. Lead rider was a small guy in TT position.
http://alex-cycle.blogspot.com/2013/...-me-draft.html


What do you think, possible or BS?
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Old 01-29-14, 03:41 PM   #2
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That's from Alex Simmons, whose opinions i would trust quite a bit. So 30w saving from a baseline of 240w, that's only 12% saving. In road racing situations, you save 35% as the second in line and 40% as the third rider and onwards. So not at all surprised.

PS. He'snot just some dude. One should read his seven sins blog post before training with a PM
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Old 01-29-14, 04:08 PM   #3
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That's from Alex Simmons, whose opinions i would trust quite a bit. So 30w saving from a baseline of 240w, that's only 12% saving. In road racing situations, you save 35% as the second in line and 40% as the third rider and onwards. So not at all surprised.

PS. He'snot just some dude. One should read his seven sins blog post before training with a PM
Yeah, 35%-40% in road racing sounds reasonable to me, but 12 meters just seems sooo far back to get that much (30 watts) savings.

I've stumbled across that blog post a few times, but seeing as I don't know him I just call him "dude".
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Old 01-29-14, 04:27 PM   #4
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Seems plausible to me, anybody who's tailgunned a pack can tell you the draft pocket goes way back behind the last rider.

Also a big +1 to Alex being a credible and reliable thinker on such topics. Not to mention a true badass.
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Old 01-29-14, 04:32 PM   #5
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Give it a shot at your next crit.. in the leadout, sit 36 feet behind a rider for "optimal draft" and let us know how it goes!

Actually I'll tell you how it'll go: about 8 guys will slot in front of you, and you'll finish 9th. =]

The factoid may be true, but I find it of little use for pack racing.
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Old 01-29-14, 04:33 PM   #6
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It's also not 12 meters of separation. So front hub to front hub is 12 meters, this include the wheelbase of the first bike, about a meter and the diameter of a wheel, which is about 75 cm. so 10.25 m of separation
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Old 01-29-14, 04:34 PM   #7
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Same difference.
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Old 01-29-14, 04:36 PM   #8
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Another thing I've noticed about drafts that nobody seems to talk about: imagine a crosswind situation, typical case is a few riders in echelon at the lead, then a long tail of single file riders in the gutter. Conventional wisdom seems to be that the single file riders are getting zero draft, e.g. they are catching full crosswind and having to make speed entirely by their own power.

But in my experience / perception, that's not always true. In some cases it seems as if the group mass of the leaders and trailers is actually creating a localized aerodynamic envelope, partially (sometimes completely) providing (some / imperfect) draft to the single file riders.

Perhaps that helps explain why that single file tail always seems able to hang on for so long? Because those riders are not actually giving full gas to stay in contact, but in fact are getting "just enough" draft to keep up with the echelon that's rotating just ahead of them (and providing indirect shelter as well).
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Old 01-29-14, 04:37 PM   #9
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Ever tried to bridge a gap? When you got 12 meters back from the rider ahead, did you naturally sort of slot into where the draft should be? Why?
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Old 01-29-14, 04:39 PM   #10
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Again I present to you some supporting evidence: Mr. Fabian Cancellara, bridging a gap to the breakaway... note how he adjusts his line as he approaches the back of the break group. (0:06 to 0:12)


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Old 01-29-14, 05:26 PM   #11
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There would be a difference between an indoor track and an outdoor race. Indoors, I can see the draft making a difference at 12 meters. I doubt it would help as much outside.

Apples and oranges, but if you're driving a car at Indy, then the draft is as important as if you're racing a bike. In that situation, if you can see a car in front of you, then the draft is affecting you. Straights at Indy are about 3/4 of a mile long. So you're talking about an effect that acts over a hell of long distance in this case.
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Old 01-29-14, 06:57 PM   #12
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Quote:
Originally Posted by Alex Simmons
A larger rider adhering to the 12-metre draft rule when following a single smaller rider at speeds of ~40km/h in calm conditions gained a benefit of ~27-30W reduction in power required
I believe that his experiment showed this, though that is a surprising result, >25W savings 5 bike lengths back. The experiment was done at an outdoor track. It would be less surprising at an indoor track, where riders generate a cyclone effect. There may be a similar effect of less magnitude at a relatively sheltered outdoor track.
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Old 01-29-14, 07:06 PM   #13
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the takeaway from this is to not give up just because you can't close a bike length or two. ****ing focus and pedal and know that even at that distance your rabbit is having to work 12% harder than you are just to remain equal. Match him and you're golden.
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Old 01-29-14, 07:36 PM   #14
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I believe that his experiment showed this, though that is a surprising result, >25W savings 5 bike lengths back. The experiment was done at an outdoor track. It would be less surprising at an indoor track, where riders generate a cyclone effect. There may be a similar effect of less magnitude at a relatively sheltered outdoor track.
I'm too lazy to read the methodology, but when they run flying 200 heats at indoor velodromes you can measure a significant wind alongside the track that increases for the first several riders if you hold up an anemometer.
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Old 01-29-14, 07:45 PM   #15
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What do you think, possible or BS?
http://xkcd.com/54/
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Old 01-29-14, 07:48 PM   #16
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Nice, that graph caused some troubles back in the day.
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Old 01-29-14, 09:50 PM   #17
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I think the thing you need to look for is the time it takes for the turbulence of the first rider to settle out. My guess is that it probably takes about 2 seconds for the air to 'settle' after a rider passes a certain point. If you're more than 2 seconds back, then you're probably seeing about the same air as the lead rider. Once you're inside that window, though, you're getting some sort of advantage due to the draft.

If we're considering a still air, then when the leading rider disturbs the air he adds energy to it and causes it to move around in a somewhat unsteady, chaotic manner. Moving air has less pressure than still air (said Mr. Bernoulli), so the high pressure area directly ahead of the trailing rider is somewhat less than if the air was completely still. The closer the trailing rider is the leading one, the more disturbed the air will be (and a bigger advantage to the trailing rider). If I'm correct about it taking 2 seconds for the air to settle out, then the effect might start to be measurable at a gap of 20-25 meters and further for a big group. Pretty amazing if you ask me. I've never really considered it in these terms.
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Old 01-29-14, 10:30 PM   #18
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I think the thing you need to look for is the time it takes for the turbulence of the first rider to settle out. My guess is that it probably takes about 2 seconds for the air to 'settle' after a rider passes a certain point. If you're more than 2 seconds back, then you're probably seeing about the same air as the lead rider. [...] If I'm correct about it taking 2 seconds for the air to settle out, then the effect might start to be measurable at a gap of 20-25 meters and further for a big group.
Depends on the size and speed of the thing displacing the air. A big truck will have a larger wake than a butterfly. When doing aero field testing on the open road a passing car will disturb the air in a detectable way (that is, you can see its effect on the apparent CdA) for more than 30 seconds. We've detected the effect of a single rider circling a 250m velodrome lowering his own apparent CdA at a speed of around 40 km/h or so (i.e., so that there's more than 20 seconds per lap).
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Old 01-29-14, 11:37 PM   #19
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We've detected the effect of a single rider circling a 250m velodrome lowering his own apparent CdA at a speed of around 40 km/h or so (i.e., so that there's more than 20 seconds per lap).
super interesting. i assume facilities like ERO account for this, but it seems to show that other riders on the track or comparing 1st/early laps to later ones to have potential for serious error.
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Old 01-29-14, 11:52 PM   #20
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once upon a time i used to know how to derive that distribution from basic principles of statistical mechanics. oh well...
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Old 01-30-14, 12:28 AM   #21
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super interesting. i assume facilities like ERO account for this, but it seems to show that other riders on the track or comparing 1st/early laps to later ones to have potential for serious error.
Yup. That's why you typically toss the first couple of laps and only have one rider on the track at a time. Having said that, there are ways to correct for the difference between air and ground speed if you know what you're doing.
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Old 01-30-14, 12:30 AM   #22
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I stared at that for a few minutes, and decided I have never seen it before.
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Old 01-30-14, 12:36 AM   #23
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Give it a shot at your next crit.. in the leadout, sit 36 feet behind a rider for "optimal draft" and let us know how it goes!

Actually I'll tell you how it'll go: about 8 guys will slot in front of you, and you'll finish 9th. =]

The factoid may be true, but I find it of little use for pack racing.
Yeah I guess it isn't useful (except for what Fudgy said about keep going a few bike lengths back), but sometimes I find things interesting even if they aren't very useful.

Isn't the rule for TTs 25 meters? I remember thinking it seemed long, but I guess its reasonable!
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Old 01-30-14, 07:44 AM   #24
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I stared at that for a few minutes, and decided I have never seen it before.
Google Planck's law.

i hope you've at least heard of Fermions and Bosons

Even if we aren't scientists and engineers, we should have at least a basic understanding of history of science

Last edited by echappist; 01-30-14 at 07:47 AM.
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Old 01-30-14, 08:09 AM   #25
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Wow-over 10% reduction at almost 40 feet-at just 25mph
Never would have guessed it was that much.

How?
The drafter is over 1 second back- those air molecules are bouncing around at 1100 fps(or so)-
Viewing it simply-I would have guessed(wrongly) the "defect" in the air column would have been "filled in" by the time the 2nd rider arrived-
I would have thought that tumbling air would have been pretty dirty-

Never would have guessed you get so much boost at such a long distance such a low speed.

Car hypermilers has disavowed drafting -safety-it probably was not uncommon in 2006-2007-but responsible hyper milers won't "intentionally" do it.
This does make me wonder if safe distances-maybe 180 feet at 60 mph- would save much fuel? Most cars can stop from 60 mph in about 150 feet+ reaction time of maybe 60 feet?
Yeah makes me think. Big truck-200 feet-65mph ??Hmmm
Great stuff!
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