RChung

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.

JanMM

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?

Jeff Wills

Coefficient of Drag times Area... or "real" drag. Bob's already asked this on the WISIL forum.

There may be something in the back issues of Human Power: https://www.ihpva.org/hparchive.htm

RChung

Thanks. Yeah, I've already gone through all of the back issues of that, and done searches through a handful of other technical design oriented archives, including for motorized vehicles of different sorts. Thus far I haven't found anything other than wind tunnels, coast downs (some of them quite clever, though) and the "classic" constant speed flat course trials with power measurement. As I said over on the recumbents.com forum, I thought the recumbent world would be a natural place to ask. (As a side-effect of my searches, I'm evolving a guess as to why my initial thought was wrong).

Steamer

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?

RChung

Hmmm. Does this mean you *have* measured your CdA with coast downs, or are you giving reasons why you *haven't*? For the record, I do this as an athletic and leisure time activity, too -- but even were that not so I'm not sure why it is relevant to this discussion.

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.

Square & Compas

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.

Jay D

Yeah, I measured, it's 42.

RChung

Perhaps your question is the better question but I'd expand it a bit: it doesn't appear that the hpv racing community does much testing, either. I asked at a hpv racing forum and (mostly) got the same response as here: "why would anyone want to know that?" plus a bunch of reasons why it's too hard, too expensive, or not possible at all.

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.

purplepeople

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.

RChung

Hmmm. I don't think I was then nor am I now sore. As I said, I wasn't there to promote or defend my approach to anyone else's; that's why I didn't (and, for the record, I don't think I ever said my method was great. I don't think I even mentioned it until pretty late, and I only did that parenthetically. Please go look if you doubt that). I was there to learn what others had been doing. When it became clear that I wasn't going to learn what I was looking for, I left. That, btw, should have been a clue: if I'd been looking for converts why would I leave? Besides, I'm glad you think it shows promise but I don't need to have anyone test it -- it's way past testing stage and it's being used now in other arenas (google it up if you doubt that). The results have been validated by wind tunnel tests and "classic" regression methods. And, as for theory vs. practice, guys who have won races have credited this approach with helping them hone their drag (both aero and rolling) in the absence of a wind tunnel. I did think it was kinda funny when Sean said estimating drag with a power meter was impossible both because power is too jumpy and that power meters have insufficient precision -- right after saying that he doesn't use a power meter. He's fast but he doesn't know power meters. As long as you're googling things up, google me and you'll see that I'm slow but I do know power meters.

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.

BlazingPedals

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:

RChung

Tailfairings lower drag but don't appreciably affect A. Disc wheels sometimes increase A but lower drag. And CdA (or CxA) can vary with yaw: many modern TT bikes have lower drag at positive yaw than at zero yaw even though "A" increases in those cases.

But that's all secondary to my original question: if you've measured CdA, how did you do it?

Steamer

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.

RChung

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?

purplepeople

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.

Steamer

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'.

BlazingPedals

I haven't, and I wouldn't know how. Sorry.

gcottay

I have not. Have you? If so, how did you do it?

RChung

Whew. (Though actually I'm more interested in the how than the whether)

It's more accurate to say that watts/total drag is well matched. Power varies quite a bit (for an obvious example, Cancellara has way more power than Contador) but since we race in all kinds of wind and weather we need to optimize as best we can and then run what you brung. But because the competition is categorized into age groups and there are far more races than in your world there are lots of opportunities to feel bad because you lost out a podium spot by the equivalent of a 1/20 of a second per km. Then you obsess because you chose one brand of tire over another, or didn't tape the vents of your helmet, or rode with gloves instead of bare-handed.

You know, the power meter isn't necessary. I use it because it's faster and more flexible, not because you need it. (That's also the reason why I never said anyone needed a power meter to measure drag. Go back and look around if you doubt me). You *can* get reasonably precise drag numbers from coast downs but you need high precision in speed measurement. If you do that you don't need to control speed or to control mass, and the trials go faster. Not as fast as having a power meter, and the power meter lets you test at higher speeds and over a wider range of venues, but the essential thing is precise speed recording, not power. Here's the thing: if anyone had asked me how to do coast downs on the cheap without a power meter, I would have told them -- but as you've made clear, you guys don't test for CdA at all so that never came up. And although we've been talking about CdA a similar thing goes for Crr. We seek out the things that minimize Crr and have figured out ways to test that, too.

No apologies necessary. However, you've completely misinterpreted my reactions. As I explained to Warren, I'm a professor at what some people consider a pretty major research university. In the bizarrely arcane little world I inhabit, when I go to experts and see what they're doing I don't get "disappointed" if I discover they haven't worked on a problem. If you go to experts in the field and they've worked on the problem then you're happy because you can learn what they did. If they haven't worked on the problem you're even happier because the decks are clear.

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?

purplepeople

No, but many people have the hope that they can capitalize on their work. Take my case, where I am actively working on the next prototype of my tilting trike with an eye to selling them in a cottage quantity production run. And that doesn't include the fact that the design has applications in motorsport and in the exploding field of high-mileage vehicles and electric vehicles. As a result, I just don't give out any details regarding the steering geometry, even to those who would like to build one for personal use. The world is littered with inventive people whose work and future income have been co-opted by others.

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.

Steamer

Here is my straight talk, hopefully you can deal with it: this is a recubment forum. Have you ever ridden one?

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.

The Smokester

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?

RChung

Nope. I was very clear on the recumbents.com forum: I'm not interested in recumbents; I'm interested in how people measure drag (both rolling and aero) in the field. I've asked this same kind of question not only on conventional cycling forums but also on some motor sports forums. I have to say that the sort of reception I've gotten in the recumbent cycling forums is unusual.

You have a higher opinion of professors than I. We tend to be selected for our depth, not our breadth, so when we can't figure something out that's no indication that someone else hasn't already done it, done it better, written a paper, and has it named after him. We do tend to be selected for stubbornness, a high boredom threshold, and a low regard for the money value of time so mining sand is pretty much what we do. Even so, I've already learned some things and gotten some ideas both here and in the recumbents.com forum and in my world that makes it a good day.

RChung

Thanks. There are several ways to measure drag (both aero and rolling) from field tests and the choice among them depends on how much precision you want or need. Coast downs of various protocols normally give something like one-and-a-half significant digits of precision (i.e., you can tell big changes in position or equipment or position but not small ones). I do something vaguely similar in spirit to Smokester's GPS runs but with a power meter under windless conditions. The precision is quite good but it requires some special hardware. I think I've developed another way to do it with coast downs that takes doesn't have onerous hardware requirements (but takes much more time -- can't get something for nuthin'). I'm trying to find out what other people have been doing so I can figure out whether my approach has already been done.

There are a few, though I've mostly been focusing on the ones that use power meters. Candau et al. did coast downs in hallways using electric timing lights. Would you be interested in that?

Yeah, they make guesses and occasionally they guess wrong. They know that in a flat run-in to the finish, the peleton can pull back a solo escapee at close to 10 sec/km and a small group at 8 sec/km. On a stage with a steep hill finish, the GC contenders can pull back close to a minute per km depending on who is up the road. Everyone knows what the finish of each stage looks like, and with radios they know who is up ahead and how much of a danger they represent.