Question about position
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But more than external conditions is rider input. Even in the tunnel with the most disciplined rider, there's noise. You get a swing and give it the old WTF? Look at the camera overlays and you see what happened. Moved the hands/fingers/head a little. I'm doubtful a person pedaling out on the road or even coasting maintains near the position consistency they would in the tunnel, and the logistics to even verify this are another matter.
Then sometimes you come across someone like Racer Ex, or this kid Sergio that is rock solid in their position, and can nail it every time. Every time I measured this kid he was spot on. I was pretty impressed.
And like proximity to a tunnel, proximity to a location that affords favorable conditions is a bug in the rug. I had to wait over a month once in Texas for a day where the wind wasn't blowing and the temps were stable.
I like field testing BTW. It's a heck of a lot better than nothing.
I like field testing BTW. It's a heck of a lot better than nothing.
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165. No way could that kid do that position with 172.5. There's a thread on the triathlon site about him (Sergio). Super nice (and super fast kid). He was quite disappointed that he couldn't beat my drag numbers, but I laughed and told him I would gladly trade his power numbers for my drag numbers.
#29
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165. No way could that kid do that position with 172.5. There's a thread on the triathlon site about him (Sergio). Super nice (and super fast kid). He was quite disappointed that he couldn't beat my drag numbers, but I laughed and told him I would gladly trade his power numbers for my drag numbers.
also, what's the consensus on the tradeoff between lower initial profile (like Sergio Jr.) vs a flat back (like most time trialists)?
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1) - lower bars = less drag
2) - lower more = even less drag
3) - lower even more = Eh, kind of the same drag so lets stop lowering and work on helmets.
A few years ago Cobb decided to keep going and eventually, you kind of "break though" a wall and the drag starts dropping again. Sergio is an example of this. Of course, that position is not for everyone, and as always, what works for one person.... Cobb published a mini-paper on this that he passes out at the F.I.S.T. camps.
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I'm sure we could develop field test equipment that is as precise as a wind tunnel, given that most wind tunnel equipment was generated off of field technologies. And I know there are days when field testing yields very tight results. The problem with field testing (especially beyond zero yaw) is consistent external conditions. And by time you've invested the time and money into the equipment you'd need to offset this problem, you could likely go to the tunnel a bunch.
But more than external conditions is rider input. Even in the tunnel with the most disciplined rider, there's noise. You get a swing and give it the old WTF? Look at the camera overlays and you see what happened. Moved the hands/fingers/head a little. I'm doubtful a person pedaling out on the road or even coasting maintains near the position consistency they would in the tunnel, and the logistics to even verify this are another matter.
Well, above when I mentioned a sd of .0006 m^2 or a CV of 0.3%? That was done with an old PT Pro, the one that records at 1.26 seconds. It had been checked with a static calibration sometime shortly before the data collection, I don't remember exactly when, but it had also been checked several times over the preceding months and was also checked after the test runs, and over all static checks, both before and after, had been spot-on. [Edit:] BTW, what's the sd on the drag you measure at A&M?
Last edited by RChung; 07-04-11 at 05:46 PM.
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Well, above when I mentioned a sd of .0006 m^2 or a CV of 0.3%? That was done with an old PT Pro, the one that records at 1.26 seconds. It had been checked with a static calibration sometime shortly before the data collection, I don't remember exactly when, but it had also been checked several times over the preceding months and was also checked after the test runs, and over all static checks, both before and after, had been spot-on. [Edit:] BTW, what's the sd on the drag you measure at A&M?
My question was in regards to how accurate the power meter was. If a powertap is only accurate to +/- 1.5% how do you account for that in your precision testing? Also, what if the testing was done with a SRM? Should you adjust your findings because the power is derived at the crank and not the hub? There could be a 2% variation due to that. How do you account for these potential variations?
As for the tunnel sd, I'll need to check with them, but I'm curious now.
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I see where you are going with this, but noise can also be a detriment.
So you're saying it was tested a lot, but you don't know specifically when, with regards to your tests. What was the temperature difference between your static tests and your field testing?
My question was in regards to how accurate the power meter was. If a powertap is only accurate to +/- 1.5% how do you account for that in your precision testing?
Also, what if the testing was done with a SRM? Should you adjust your findings because the power is derived at the crank and not the hub? There could be a 2% variation due to that. How do you account for these potential variations?
Last edited by RChung; 07-04-11 at 07:26 PM.
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Having seen the variability of results from 3 different tunnels, and having run 100+ field test runs with a couple of riders, it's my experience that it's pretty rare for the field to hit the repeatability of the tunnel (and this is coming from someone who doesn't pee or drink during tests to keep the rider weight as static as possible). Can it be as precise, or at least close? Sure. Every time on any given day? Absolutely not.
The kid is good. I'm old and stiff.
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There's seldom high precision in field testing (see external conditions)..."real world context" by your definition makes noise a good thing, which it's not. It's dog poop on the rug. "Real world" is, to paraphrase Chung, a red herring unless you apply it to yaw. Which you can't field test.
Having seen the variability of results from 3 different tunnels, and having run 100+ field test runs with a couple of riders, it's my experience that it's pretty rare for the field to hit the repeatability of the tunnel (and this is coming from someone who doesn't pee or drink during tests to keep the rider weight as static as possible). Can it be as precise, or at least close? Sure. Every time on any given day? Absolutely not.
Having seen the variability of results from 3 different tunnels, and having run 100+ field test runs with a couple of riders, it's my experience that it's pretty rare for the field to hit the repeatability of the tunnel (and this is coming from someone who doesn't pee or drink during tests to keep the rider weight as static as possible). Can it be as precise, or at least close? Sure. Every time on any given day? Absolutely not.
2. I'm also saying that equipment exists that allows one to test at variable yaw, although that equipment is not generally available.
3. As I've said elsewhere, a wind tunnel works when the conditions outside are not conducive to field testing; a wind tunnel is faster than field testing; a wind tunnel can (often) be more convenient than field testing; if you don't have the right equipment, a wind tunnel lets you test at non-zero yaw; and a wind tunnel is way more expensive than field testing. I've also said that the key to a good testing protocol isn't just that you can get an answer out of it -- it's that you can tell how good (or poor) that answer is. If you've done 100+ field tests and got dog poop results out of them then I'm thinking maybe those methods you're using aren't the best use of your time.
4. BTW, I can produce the same level of field testing precision even without a power meter -- the testing is slower and more tedious but the precision is the same.
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I'm guessing that somewhere in the deep dark recesses of LabChung's supercomputer that there's a deleted spreadsheet or three. The people that stand head and shoulders above the crowd are usually standing on a pile of failed experiments (paraphrasing Erv)
I'm pretty OK with my protocol BTW. If you'd like to peer review it I'd be happy to forward it.
I've got zero concern with the absolute accuracy of the derived number which is probably where and why we're parting company on this discussion. The fish I'm frying is winning bike races.
Depends on what your time is worth.
As for the CV numbers from the tunnel, I'll leave that to Nate.
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Are you talking about force measurements? The Oran Nicks Tunnel lists the specs of the tunnel on their website. I don't know about the San Diego or A2 specs.
https://lswt.tamu.edu/info.htm
I'm having a tough time with the "thumb nail" CdA. A CdA diffrence of .001 m^2 works out to be, what, a watt? So your powertap, is reliable down to the 1 watt? Impressive. Have you published this test? I'd like to see your testing protocol, as it must have been really good.
https://lswt.tamu.edu/info.htm
I'm having a tough time with the "thumb nail" CdA. A CdA diffrence of .001 m^2 works out to be, what, a watt? So your powertap, is reliable down to the 1 watt? Impressive. Have you published this test? I'd like to see your testing protocol, as it must have been really good.
Last edited by Nate552; 07-05-11 at 11:13 AM. Reason: added link
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This. When it comes down to it, we're trying to figure out what is fastest at various wind conditions. A wind tunnel is superior than field testing for this purpose. I'm not saying field testing is bad, or worthless just that it has it's limitations.
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And I didn't dispute that. I've said many times in many places that field testing is not a substitute wind tunnel testing; I've said it's a complement. The part I was disagreeing with was the claim that field testing *always* has poorer precision than wind tunnel testing, and that field testing *can't* handle non-zero yaw. I've shown that field testing *can* have very good precision, and that it *is* possible to measure non-zero yaw.
Evidently we both don't know that. In my day job I look at a lot of "noisy" data. The history of this field is that sometimes the data that we think of as "noisy" can reveal systematic variation in addition to the random component. Being able to tell one from the other can often lead to better estimates. That's why my field tests turn out to have much better precision than traditional field tests -- traditional field tests couldn't differentiate between the two.
Sure. Maybe start a new thread.
Actually, I don't think this is where we disagree. If the tare on a wind tunnel balance is off then the absolute drag is off but most of us are knowledgeable enough that we don't care about that. Most of us care about comparing equipment or positions so even if the tare is off the relative difference is still good. That's what happens with well-performed field tests. The absolute CdA can be off but the relative comparison between equipment or positions depends on precision.
Um, what makes you think that? I haven't made any specific comment on what I do or don't have available to me.
Nope, not exactly. I've known for a while what the claimed accuracy and resolution of the force measurements are. I'm interested in the measured precision, not the accuracy.
Of course I've written this up. As I pointed out above, you don't even need to use a power meter -- it's just faster and easier (much easier) that way. If you don't have a power meter then you have to do the tests at zero power -- reliably at zero power. You think that's impressive? But by your questions I'm guessing you're not trained as a statistician. For example, we can talk about average global warming as measured in hundredths of a degree even though any individual thermometer may not have precision of 0.01 degrees. The Central Limit Theorem is a pretty neat thing. As for the protocols, they depend on the type of venue and situation you have available. Even if you have the right venue using the wrong protocol can give you poor results -- just as using the right protocol but in the wrong situation. I've never claimed that a good protocol magically makes everything work (I've claimed that a good protocol makes things better than a poor protocol).
Of course field testing has limitations. Who has said it doesn't? Y'all have been making blanket statements. I've been making nuanced ones: I've been saying that dismissing field testing because "it's not precise enough" or "you can't do non-zero yaw" is incorrect.
You're at nature's whims and while you'd now like to describe noise as a good thing, we both know it's not.
I'm pretty OK with my protocol BTW. If you'd like to peer review it I'd be happy to forward it.
Don't need [an estimate of the confidence bound on estimates]. My confidence comes from repeatability on a controlled set up during the tests. But I'm using the tunnel for comparative analysis, not for the aero equivalent of the Ewang FTP chart.
I've got zero concern with the absolute accuracy of the derived number which is probably where and why we're parting company on this discussion. The fish I'm frying is winning bike races.
I've got zero concern with the absolute accuracy of the derived number which is probably where and why we're parting company on this discussion. The fish I'm frying is winning bike races.
BTW, this is what I was referring to when I wrote about "by time you build it, you could have gone to the tunnel a bunch", not the zero yaw tests. Given that you're probably the most noted advocate and resource for field testing and you don't have the equipment or even an controlled estimate of time/cost/effectiveness of non zero yaw equipment...
I'm having a tough time with the "thumb nail" CdA. A CdA diffrence of .001 m^2 works out to be, what, a watt? So your powertap, is reliable down to the 1 watt? Impressive. Have you published this test? I'd like to see your testing protocol, as it must have been really good.
Of course field testing has limitations. Who has said it doesn't? Y'all have been making blanket statements. I've been making nuanced ones: I've been saying that dismissing field testing because "it's not precise enough" or "you can't do non-zero yaw" is incorrect.
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I have a State TT championship coming up in 3 weeks, and I want to know if the new helmet I have is faster between 10 and 15 degrees than my old one since that's the predicted yaw angles for 2 of the legs. How would I accomplish this using field testing with tools currently available to me?
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I have a State TT championship coming up in 3 weeks, and I want to know if the new helmet I have is faster between 10 and 15 degrees than my old one since that's the predicted yaw angles for 2 of the legs. How would I accomplish this using field testing with tools currently available to me?
I want to test in a wind tunnel tomorrow but my checking account balance is watched carefully by my wife. How would I accomplish this with the bank balance and schedule currently available to me?
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I have a State TT championship coming up in 3 weeks, and I want to know if the new helmet I have is faster between 10 and 15 degrees than my old one since that's the predicted yaw angles for 2 of the legs. How would I accomplish this using field testing with tools currently available to me?
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If you wear pantyhose over your head when you go into the bank and speak in a loud voice, they will often give you more than you have in your account. During the cooler months you could substitute a ski mask.
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But all of that requires a recording wind sensor.
[Edit:] I should add that although his estimates seemed reasonable, he couldn't afford to go to a wind tunnel so we didn't have any way to validate them. However, since Nate has been in the wind tunnel, if he was able to do something like this then he'd have an external validity check.
[Edit 2:] BTW, that same guy affixed a flat 5cm x 5cm plate to his bike and tried to estimate the difference in estimated drag. He estimated the difference in CdA at .003 m^2. The area was 5cm x 5cm = .0025 m^2, but the Cd of a flat plate isn't 1.0. Go look it up and figure out what the CdA of a 5cm flat plate ought to be.
Last edited by RChung; 07-05-11 at 03:30 PM.
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Because you haven't made any specific comment on what you do or don't have available to you.
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I thought you were referencing something that occurred with regularity say, every third run or at 3:30 into the test.
I would say that using prevailing wind and a gauge to do yaw testing is gong to be very iffy to get a good control. Wind seldom travels in a straight path outdoors so you'd want a bike mounted velocity and directional gauge with a black box recorder, and even then you're subject to the whims of nature, the road builder, and terrain to "decide" on the "test" angle. You'd have to layer the data to determine the average effective yaw as wind and bike speed changes...which really gives you an average drag number at an average yaw number, and not an actual result.
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Lost me here...sytemic or systematic? In any case the car would seem to fall under the heading of random noise, unless it was a train that ran on the hour, or something of that sort, in which case I would consider it systematic. But I get where you're going...much the same as I'll abort or note runs when I TV test if there's something that I know is going to skew the results you clip out the noise.
I thought you were referencing something that occurred with regularity say, every third run or at 3:30 into the test.
I thought you were referencing something that occurred with regularity say, every third run or at 3:30 into the test.
I would say that using prevailing wind and a gauge to do yaw testing is gong to be very iffy to get a good control. Wind seldom travels in a straight path outdoors
so you'd want a bike mounted velocity and directional gauge with a black box recorder, and even then you're subject to the whims of nature, the road builder, and terrain to "decide" on the "test" angle.
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