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? |
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 |
Originally Posted by echappist
(Post 16450993)
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 I've stumbled across that blog post a few times, but seeing as I don't know him I just call him "dude". |
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. |
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. |
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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Same difference.
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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). |
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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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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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. |
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
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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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Originally Posted by globecanvas
(Post 16451547)
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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Originally Posted by aaronmcd
(Post 16450972)
What do you think, possible or BS?
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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. |
Originally Posted by Fat Boy
(Post 16452063)
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.
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Originally Posted by RChung
(Post 16452166)
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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Originally Posted by tetonrider
(Post 16452315)
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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Originally Posted by mattm
(Post 16451126)
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. Isn't the rule for TTs 25 meters? I remember thinking it seemed long, but I guess its reasonable! |
Originally Posted by aaronmcd
(Post 16452374)
I stared at that for a few minutes, and decided I have never seen it before.
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 |
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! Charlie |
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