Do you know your recumbent's CdA?
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Do you know your recumbent's CdA?
Has anyone here measured the CdA of their recumbent?
I'm actually not interested in what the values are, I'm more interested in how you did it: wind tunnel, field tests, or something else.
I'm actually not interested in what the values are, I'm more interested in how you did it: wind tunnel, field tests, or something else.
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Combined deficit Algorithm. Computated duck Analogy. Coaxial deductive Amortization.
Have you tried Bentrideronline? If you get no helpful responses here. Someone probably does know what you are talking about. Coefficient of drag something-or-other?
Have you tried Bentrideronline? If you get no helpful responses here. Someone probably does know what you are talking about. Coefficient of drag something-or-other?
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There may be something in the back issues of Human Power: https://www.ihpva.org/hparchive.htm
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There may be something in the back issues of Human Power: https://www.ihpva.org/hparchive.htm
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For most of us this is an athletic and leisure time endeavor. I take off my engineering hat at approximately 5:00 pm every weeknight.
But seriously, I think the simplest way to do it without the benefit of any sophisticated meters (that most of us don't have and simply aren't going to waste money on) would involve the classic coast down test (recording terminal velocity) on a known (constant) gradient. If you have a steady grade that is long enough, and bearing drag can be assumed to be negligable for the sakes of the computation, then what would be better?
But seriously, I think the simplest way to do it without the benefit of any sophisticated meters (that most of us don't have and simply aren't going to waste money on) would involve the classic coast down test (recording terminal velocity) on a known (constant) gradient. If you have a steady grade that is long enough, and bearing drag can be assumed to be negligable for the sakes of the computation, then what would be better?
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For most of us this is an athletic and leisure time endeavor. I take off my engineering hat at approximately 5:00 pm every weeknight.
But seriously, I think the simplest way to do it without the benefit of any sophisticated meters (that most of us don't have and simply aren't going to waste money on) would involve the classic coast down test (recording terminal velocity) on a known (constant) gradient. If you have a steady grade that is long enough, and bearing drag can be assumed to be negligable for the sakes of the computation, then what would be better?
But seriously, I think the simplest way to do it without the benefit of any sophisticated meters (that most of us don't have and simply aren't going to waste money on) would involve the classic coast down test (recording terminal velocity) on a known (constant) gradient. If you have a steady grade that is long enough, and bearing drag can be assumed to be negligable for the sakes of the computation, then what would be better?
Anyway, I agree that coast downs may be the simplest way. But since you ask what would be better, I'd answer that whether something is better depends on how small of a change you want to measure with reliability. I got lousy precision with coast downs until I got a way to track speed accurately and precisely -- lousy in the sense of, with coast downs I could reliably detect changes in CdA of something around 0.02 m^2. With my current setup precision is now quite a bit better. In one test I added a 5cm cube to my bike and estimated the change in CdA at just under .0030 m^2. I leave it to you to figure out what the expected change in CdA should have been.
BTW, as long as you brought up wasting money on sophisticated meters, in any early experiment I scrounged up a cheap data logger to capture speed with stuff around my house (I did happen to have an ancient and obsolete microcassette recorder that I used to use for dictation). Total cost was zero. Not the most convenient solution but the speed precision was quite good, and with the coast down method that's the key.
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I think the better question is does any recumbent rider not competing and just riding really give a **** what it is? I know I don't and could really care less. If I was going for some record or competing at Battle Mountain or Bonneville Salt Flats I might remotley care, other wise I could care less. When I ride my Vision my cda is the last thing on my mind.
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I think the better question is does any recumbent rider not competing and just riding really give a **** what it is? I know I don't and could really care less. If I was going for some record or competing at Battle Mountain or Bonneville Salt Flats I might remotley care, other wise I could care less. When I ride my Vision my cda is the last thing on my mind.
There are a lot of dedicated recumbent riders and it appears that there is a lot of engineering knowledge in this community so I was casting my net over a larger area. So far, considering that one of the three most often expressed advantages of a recumbent is that it has lower drag, I'm sort of surprised that so few have actually measured it. Only about half a dozen riders have said that they have: one who has been in a wind tunnel and the rest who have done field tests of various sorts (coast downs on hills, coast downs on flat ground, power runs on flat ground). As I said, I'm not that interested in what the actual drag numbers are: I'm interested in how people who have done it, did it.
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BTW, since you left that forum at your self-imposed 100 post limit, somebody has tried your method and has reported that it has promise. They will probably now show that your technique works, then tweak it to make it better. Sadly, your impatience with the process of tell and show will only serve to keep you from learning more about it from the people that actually push the envelope.
:)ensen.
PS: Did it ever occurr to you that those who do know their drag values don't want to reveal it to their competitors on a public forum. Even the knowledge that the value is known can reveal very important techniques used in the background. For instance, I think a lot more people are using CFD, but just don't want to admit it. If I raced, I certainly wouldn't want to give away any edge I might have.
Last edited by purplepeople; 10-31-09 at 12:52 PM.
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I saw that thread over at WISIL and I think you're just sore because we asked you to provide more concrete proof that your theory works vs. convention coast-down methods, tuft tests and more expensive wind tunnels. In racing, theory only goes so far to the finish line, especially in HPV racing where multi-buck engineering and computerized theory have so far yielded only one record that has since been proven to have benefited from the bow-wave drag reduction of a large chase vehicle. When you showed up saying your methods were great, they asked you to prove it. Presentation of an accredited academic paper means nothing to someone whose life is at risk going 70 mph down the road. IMO, it's not your theory that is in question, it's your method to gain its acceptance.
BTW, since you left that forum at your self-imposed 100 post limit, somebody has tried your method and has reported that it has promise. They will probably now show that your technique works, then tweak it to make it better. Sadly, your impatience with the process of tell and show will only serve to keep you from learning more about it from the people that actually push the envelope.
ensen.
PS: Did it ever occurr to you that those who do know their drag values don't want to reveal it to their competitors on a public forum. Even the knowledge that the value is known can reveal very important techniques used in the background. For instance, I think a lot more people are using CFD, but just don't want to admit it. If I raced, I certainly wouldn't want to give away any edge I might have.
BTW, since you left that forum at your self-imposed 100 post limit, somebody has tried your method and has reported that it has promise. They will probably now show that your technique works, then tweak it to make it better. Sadly, your impatience with the process of tell and show will only serve to keep you from learning more about it from the people that actually push the envelope.
ensen.
PS: Did it ever occurr to you that those who do know their drag values don't want to reveal it to their competitors on a public forum. Even the knowledge that the value is known can reveal very important techniques used in the background. For instance, I think a lot more people are using CFD, but just don't want to admit it. If I raced, I certainly wouldn't want to give away any edge I might have.
Anyway, I specifically said I wasn't interested in the drag numbers themselves. That Stumpf guy thought I was fishing for his drag but I wasn't interested in that at all -- in fact, once I found out that his drag numbers came from a wind tunnel, I was even less interested. As for CFD, it certainly has strengths but it also has weaknesses, and if you've done much CFD you know that it takes some skill to do it right. It really not "dump in the shape and turn the crank." Knowing that someone has been working on a CFD model but not knowing anything else about the model may be interesting but, really, there's very little actionable information there.
I was sincere when I said that I was interested in seeing what people had been doing and I had presumed that dedicated racing enthusiasts with technical know-how would have been testing their drag so it surprised me that you all weren't. I'm still interested in how people who have done it have been estimating their drag.
As an aside, I looked on BROL, too. There are several threads where guys said they'd like to know but there appeared to be very little actual measurement going on.
And, if you guys push the envelope, that's great. I'm sure I'll eventually hear about it. Good luck.
#12
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I'm sure that knowing your CdA would be endlessly useful, but most of the fast crowd is more interested in decreasing their A. Presumably, the bikes with the smallest frontal area will have the lowest drag, regardless of the CdA. Which do you think will be the slipperiest bike:
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But that's all secondary to my original question: if you've measured CdA, how did you do it?
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It is relavent (in my mind at least) because you gave no explaination as to why you need / want this information. I hope it is more than intellectual curiosity. I have not bothered to do such a test because I don't care what my CdA is one little bit. From plenty of riding experience I know my hiracer bent has a signficantly lower CdA than any of my DFs do. Exactly how much better is a detail that doesn't concern me. For most of us with limited time and money to devote to our leisure activities, the time it would take to determine CdA would be better spent doing a hard training ride (assuming the ultimate (though indirect) goal of such a coast down test was to go faster). Once I have my CdA determined with painstaking accuracy, what do I do with that info exactly? If I am trying to understand the effects of a particualr aerodynamic enhancement (fairing, disc wheel, whatever), a reasonably well done coast down test with and without the enhancement will reveal the relative improvement of such an enhancement, and that is ususally more than enough info needed to make a decision about whether to use that particular enhancement on a given ride / event.
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It is relavent (in my mind at least) because you gave no explaination as to why you need / want this information. I hope it is more than intellectual curiosity. I have not bothered to do such a test because I don't care what my CdA is one little bit. From plenty of riding experience I know my hiracer bent has a signficantly lower CdA than any of my DFs do. Exactly how much better is a detail that doesn't concern me. For most of us with limited time and money to devote to our leisure activities, the time it would take to determine CdA would be better spent doing a hard training ride (assuming the ultimate (though indirect) goal of such a coast down test was to go faster). Once I have my CdA determined with painstaking accuracy, what do I do with that info exactly? If I am trying to understand the effects of a particualr aerodynamic enhancement (fairing, disc wheel, whatever), a reasonably well done coast down test with and without the enhancement will reveal the relative improvement of such an enhancement, and that is ususally more than enough info needed to make a decision about whether to use that particular enhancement on a given ride / event.
But this has nothing to do with my original question: if anyone here has measured his CdA, how did you do it?
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Okay.... I'll accept that you haven't had any other agenda than to find out whether people are measuring their CdA.
I haven't needed to in the past as most of my designs have been for un-faired recumbents. But recent interest in my leaning trikes, especially WRT to fairings and my own weather related needs where I now reside have me working on a fairing design. While initially this may be executed in Coroplast without compound curved surfaces, at some point I'll be looking at vacuum formed polyethylene sheets.
At that point, I will probably get into one of those on-line CFD services to fine-tune the 3D model, then run tuft tests on the final shape out of the mold. The thing is that I wouldn't be looking to tweak the design as I plan for it to be relatively optimised and compromised for both aerodynamics and commuter functionality. So would I need to measure CdA on a regular basis? Probably not, since I'd already be looking at the minimal sectional area while still being functional as a daily velomobile, while the shape itself will be as laminar as I can get it within those same dual constraints. I could imagine testing to see if accessory features were impacting drag, but the only external I can think of at this point would be mirrors. Everything else, including lights would be part of the overall surface and not impact the drag.
Now, seeing as how you are from a sport in which 6 seconds can cover the top three spots, it seems to me that the power sources in your game are relatively well matched, something that hasn't happened yet in recumbent racing. For the moment let's assume that since you've been on a number of cycling forums, that this would in fact be the typical UCI-type 1-hour time trial. Unlike recumbent racing, the vehicle form is highly restricted there, so in fact, CdA is very, very important to the designer, especially since rider power at the elite level is going to be the same within say +/- 2%. The smallest details can play large when both input power and overall shape are nearly uniform across the field....witness the new sequential shifters from Zipp in which the levers return to a center (and level) position after a shift.
As I said this kind of uniformity does not apply to recumbent racing... at least not yet. Measuring CdA before and after a change is going to be rare on vehicles where tweaks are almost all internal and external changes are considered to be a core re-design of the entire vehicle.
So no, I don't expect to measuring CdA anytime soon. If I did, I'd probably not bother with a power meter as that same expense can be put towards either wind tunnel rental or the building a small backyard tunnel both of which will provide consistent and accurate results that can also be viewed with smoke, tufts or other aids.
I hope this clarifies things. Finally, please understand that the tone of your posts so far has seemed to show a general disappointment over the non-high-tech-ness of recumbent design methods, and that may often be interpreted as disapproval. My apologies if I interpreted your contributions in that way.
:)ensen.
I haven't needed to in the past as most of my designs have been for un-faired recumbents. But recent interest in my leaning trikes, especially WRT to fairings and my own weather related needs where I now reside have me working on a fairing design. While initially this may be executed in Coroplast without compound curved surfaces, at some point I'll be looking at vacuum formed polyethylene sheets.
At that point, I will probably get into one of those on-line CFD services to fine-tune the 3D model, then run tuft tests on the final shape out of the mold. The thing is that I wouldn't be looking to tweak the design as I plan for it to be relatively optimised and compromised for both aerodynamics and commuter functionality. So would I need to measure CdA on a regular basis? Probably not, since I'd already be looking at the minimal sectional area while still being functional as a daily velomobile, while the shape itself will be as laminar as I can get it within those same dual constraints. I could imagine testing to see if accessory features were impacting drag, but the only external I can think of at this point would be mirrors. Everything else, including lights would be part of the overall surface and not impact the drag.
Now, seeing as how you are from a sport in which 6 seconds can cover the top three spots, it seems to me that the power sources in your game are relatively well matched, something that hasn't happened yet in recumbent racing. For the moment let's assume that since you've been on a number of cycling forums, that this would in fact be the typical UCI-type 1-hour time trial. Unlike recumbent racing, the vehicle form is highly restricted there, so in fact, CdA is very, very important to the designer, especially since rider power at the elite level is going to be the same within say +/- 2%. The smallest details can play large when both input power and overall shape are nearly uniform across the field....witness the new sequential shifters from Zipp in which the levers return to a center (and level) position after a shift.
As I said this kind of uniformity does not apply to recumbent racing... at least not yet. Measuring CdA before and after a change is going to be rare on vehicles where tweaks are almost all internal and external changes are considered to be a core re-design of the entire vehicle.
So no, I don't expect to measuring CdA anytime soon. If I did, I'd probably not bother with a power meter as that same expense can be put towards either wind tunnel rental or the building a small backyard tunnel both of which will provide consistent and accurate results that can also be viewed with smoke, tufts or other aids.
I hope this clarifies things. Finally, please understand that the tone of your posts so far has seemed to show a general disappointment over the non-high-tech-ness of recumbent design methods, and that may often be interpreted as disapproval. My apologies if I interpreted your contributions in that way.
:)ensen.
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Man, you guys sure are suspicious. I feel like the honest traveling salesman who came through River City right after Harold Hill. I've been upfront about my interests, both here and on the recumbents.com forum: I'm interested in figuring out how to improve the current methods of estimating cycling drag (both aero and rolling) from field tests. I have no ulterior motives. I'm not selling anything, I'm not trying to get anyone to test any method I've developed, I'm not writing a book, I'm not developing a fee-for-information web page, I'm not trying to screw up your recumbents by feeding you bad information, I'm not trying to steal anyone's recumbent or streamliner designs, I don't have a financial interest in any power meter or software company, and I'm not trying to convince anyone to do CdA estimation (except maybe that one guy who was wondering whether to fair his steerer tube). I was somewhat surprised that so few of you measure your drag (as I've said several times, my a priori expectation was that a group of dedicated enthusiasts with a lot of technical knowledge would have figured out clever ways to do this) but I'm coming to the realization that you don't. That's okay. To that list above add this: I'm not trying to make anyone feel bad because they don't measure drag. If you're happy with what you've been doing then I'm happy for you, too. I happen to operate in a world where after an hour of racing six seconds can cover the top three spots but if you don't there's no need for you to estimate drag with the kind of precision that we do. (But if you don't need to and you haven't even tried it, don't tell me it can't be done).
But this has nothing to do with my original question: if anyone here has measured his CdA, how did you do it?
But this has nothing to do with my original question: if anyone here has measured his CdA, how did you do it?
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[...]Now, seeing as how you are from a sport in which 6 seconds can cover the top three spots, it seems to me that the power sources in your game are relatively well matched, something that hasn't happened yet in recumbent racing.
[...]So no, I don't expect to measuring CdA anytime soon. If I did, I'd probably not bother with a power meter as that same expense can be put towards either wind tunnel rental or the building a small backyard tunnel both of which will provide consistent and accurate results that can also be viewed with smoke, tufts or other aids.
I hope this clarifies things. Finally, please understand that the tone of your posts so far has seemed to show a general disappointment over the non-high-tech-ness of recumbent design methods, and that may often be interpreted as disapproval. My apologies if I interpreted your contributions in that way.
For the record, it is not suspicion, it's confusion. You still haven't said why you are interested. As best as I can gleen, the reasons have nothing to do with recumbent riding or racing - yet this IS a recumbent forum, not a forum about a world where after an hour of racing six seconds can cover the top three spots (whatever that is). You need to consider that this makes your inquiry is essentially 'off topic'.
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And, notwithstanding the trade secrets, it is still racing and giving away your edge is like giving away the race, even if the guy you are losing to is your friend.
:)ensen.
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Holy cow. I've said exactly what I'm after. Do you guys operate in such a duplicitous world that you don't know how to deal with straight talk? As for being OT, I started separate threads to collect responses to a very narrow question. If you read back in this thread and the other thread on recumbents.com you'll see that you guys are the one's who went OT. I still haven't received a single reply to my question: if anyone here has measured their CdA, how'd you do it?
A forum is only remotely interesting when there is a backdrop of common interest. On this forum the common interest is riding recumbent bicycles and tricycles. Supporting engineering research projects that have very litte or nothing to do with recumbents isn't the cup of tea recumbent forum folk find appealing. I find it strange that you thought you'd get an answer to your question that was better than the ones you already received.
I don't regularly follow the recumbents.com forum so I missed those threads first time around (I just went and looked at them actually - boy folks got a tad bit fired up...), but I gotta say, if nobody there has a a satisfactory answer for you, there is no way in heck this forum can help you (or BROL for that matter).
Please understand that I was actually trying to help. Often times when working with other engineers they come to me with a question, oftentimes the best reponse is for me to dive into their situation with them and find out the motivations and purposes behind their question. Sometimes I find they aren't asking the right question and we decide to attack the problem a different way. Since I still have absolutely no understanding of the context since you won't share that with us, then I personally have nothing to offer.
Consider that if you are a college professor who ponders questions like this for a living, then it is likely you are much more suited to answering your own question than anyone here.
Please understand, I have none of the evasive or paranoid feelings you ascribe to me, I am simply trying to explain to you why you are only mining sand here.
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Well, I AM interested in measuring CdA and the other parameters that go into a model describing(understanding) bicycle dynamics with a view to understanding the differences between DF and 'Bents in real-world use. This is just an amusement for me, but I am taking the equations of motion for the various bikes I ride and fitting GPS data to time, position and elevation data for runs over the same courses. To determine CdA I fit many unpowered downhill runs to a model using these gps data and a derived estimate of the varying slope (velocity vs slope). This allows me to combine pleasant rides (i.e I don't need to obsess over the measurements during the ride) with the accumulation of interesting (to me) data. I preselect data to maintain uniformity in weather conditions, bicycle and rider. It is too early to tell whether this approach will be meaningful since I haven't had the 'bent very long.
I would be interested in knowing what other technical references on such empirical modeling there are on the web.
As a side note, I notice from watching various Tours de ... that breakaway groups are allowed to get quite close to the finish line before the main peleton is unleashed to catch them and the peleton almost always wins. I have wondered whether these sophisticated teams have real-time models of the course and bike performance that allow them an edge in judging just when to start the chase. Anybody know?
I would be interested in knowing what other technical references on such empirical modeling there are on the web.
As a side note, I notice from watching various Tours de ... that breakaway groups are allowed to get quite close to the finish line before the main peleton is unleashed to catch them and the peleton almost always wins. I have wondered whether these sophisticated teams have real-time models of the course and bike performance that allow them an edge in judging just when to start the chase. Anybody know?
Last edited by The Smokester; 11-02-09 at 10:17 PM.
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Consider that if you are a college professor who ponders questions like this for a living, then it is likely you are much more suited to answering your own question than anyone here. [...] I am simply trying to explain to you why you are only mining sand here.
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Well, I AM interested in measuring CdA and the other parameters that go into a model describing(understanding) bicycle dynamics with a view to understanding the differences between DF and 'Bents in real-world use. This is just an amusement for me, but I am taking the equations of motion for the various bikes I ride and fitting GPS data to time, position and elevation data for runs over the same courses. To determine CdA I fit many unpowered downhill runs to a model using these gps data and a derived estimate of the varying slope (velocity vs slope). This allows me to combine pleasant rides (i.e I don't need to obsess over the measurements during the ride) with the accumulation of interesting (to me) data. I preselect data to maintain uniformity in weather conditions, bicycle and rider. It is too early to tell whether this approach will be meaningful since I haven't had the 'bent very long.
I would be interested in knowing what other technical references on such empirical modeling there are on the web.
As a side note, I notice from watching various Tours de ... that breakaway groups are allowed to get quite close to the finish line before the main peleton is unleashed to catch them and the peleton almost always wins. I have wondered whether these sophisticated teams have real-time models of the course and bike performance that allow them an edge in judging just when to start the chase. Anybody know?