Aero

[Calnago]

Kind of getting a bit off topic now (Hooray!), but I just never warmed up to the Madones, particularly due to the under and behind BB brake placement and just its general aesthetics. But I've always known their geometry would work for me. I love the new Emondas however. Really well designed bikes. Looks like Trek is kind of leaving their Madones out to dry in the wind (pardon the pun) in favor of the new Emonda with its decidedly massive and oh so un-aero down tube. Rides really nice.

[ergott]

I'd love to snatch up a M7 cheap if they are on their way out;-)

[Calnago]

No more donuts? Now you are talking crazy.

ha! Yes, you're right. All this technical talk made me momentarily forget why I like biking so much. Donuts for everyone!!

[Butcher]

'Mmmmmmmmmmmm Donuts'. Homer Simpson

[Calnago]

Glad to see at least a few folks here haven't lost total perspective on things.

[davidalone]

@53x12: the chung method isn't simplistic science. the application of the chung method is important. how many variables did you control for? time of day? humidity? temperature? wind conditions? clothing? shoes? Groupset ? helmets? shades? exact position? tyre, tyre pressure? helmet? what power meter did you use? was it calibrated? did you zero it? type of speed sensor? head unit? was your drivetrain clean? level of fitness? type of tube? wheel? position? type of handlebar? cabling? heck, even the type of bar tape? unless you control for ALL these variables, and possibly more, you cannot say the 30 watts you see in the chung method is ENTIRELY due to the frame. So, scientifically speaking, it is not ethical to publish findings saying our frame saves 30 watts!

If it weren't simple science I wouldn't have been able to figure it out. (Besides, the "30 watts" isn't how we do it. We measure in terms of CdA, which gets translated to watts at a particular speed. I've been trying to get people to stop quoting in differences in terms of watts and start quoting differences in terms of CdA because of the confusion you're demonstrating). Anyway, you don't have to control for time of day -- that's silly. We do measure humidity and temperature and baro pressure and elevation -- you need that to get the right air density. When we're measuring frame differences, yes, we do control for tires and tire pressure; we've just swapped the wheels with the tires and tubes at the same pressure over to another bike (though since we also get an estimate of Crr, even if the tires and tubes weren't the same we can separate the CdA estimate from the Crr estimate). The power meter is the same because we've been using Power Taps, so when we swap the wheels the PT is the same. If you use a PT you don't need to worry about dirty chains since the power is being measured at the rear hub. We either measure the wind or else we do the tests under calm conditions (but as an aside, one of the virtues of my method is that you can tell when a test is spoilt by wind or a passing car or the use of the brakes -- or a change in position). Yes, the rider should wear the same shoes, clothing, and helmet (interestingly, in the Martin et al. paper that validated field tests with the wind tunnel, the one outlier was for a guy who didn't wear the same clothing -- which, in a backwards way, validates that field tests and wind tunnel tests can detect small differences in test conditions).

There are only a few wind tunnels large enough (and slow enough) for team pursuit but results from team pursuit testing on the track properly predict the power for speed team members see as they rotate through and off; that is, we measured the drag for each rider in each position, when the rider is in front or sheltered. The results were validated in the 2012 Olympics. Also, the drag measured at sea level properly predicted drag at Aguascalientes Mexico (which is at 2000 meters, so obviously on a different track) for a couple of recent hour record attempts.

The results from careful field tests are the same as the results from careful wind tunnel tests. The results from either produce estimates of CdA that predict the proper speed for power, or power for speed when on the road or the track, for a single rider or riders in a group.

thanks for chiming in rchung. Im not an aero guy, so I don't claim to be an expert by any means ( I'm a biomaterials guy). I'm not saying you can't control for all these variables- I'm sure when you were testing your formulas you tried to control as much as possible- my point is that many people don't, then claim that their testing is accurate. I live in a tropical area and it is hellishly difficult to get the air density right, unless you do the test in the middle of the day, where traffic is the heaviest.

I do know the difference between CdA and watts- I was just using the example raised by 53x12, who raised the '30 watts saved' - which to me, is unlikely. I would prefer if people reported in CdA as well!

[rchung]

Actually, sometimes you learn the most when things *aren't* controlled, like the example where the guy didn't wear the same clothing. Recently I got some data from a friend where one of the runs didn't line up with the previous or following runs. My friend said "oh, I forgot to tell you to skip that lap; a car passed, and after that I figured the run was ruined so I sat up." So the model had properly predicted that something was different and a post-analysis investigation uncovered it. That's a very strong result, and a good test of any model: the model made a prediction that was subsequently verified.

Psychology and feel and perception are important (I would never say they aren't), but they don't always make predictions that you can check and verify afterward. What we've been seeing is that wind tunnels and field tests give the same estimates of drag (when done carefully -- you're right that when one or the other isn't doesn't carefully then they're not always comparable). When you insert these estimates into models that calculate speed for power (or power for speed) you get testable predictions, which are subsequently borne out by real riders on real bikes in real races. That's pretty cool.

[53x12]

I was just using the example raised by 53x12, who raised the '30 watts saved' - which to me, is unlikely. I would prefer if people reported in CdA as well!

It isn't silly or unlikely. Just look at the equation yourself and solve for Cd if you would like rather than grams force (watts).




Here is an obvious visual to show the large differences that tube shapes can have on Cd.

[ergott]

Don't forget that even a round down tube isn't round to the wind since it's on an angle.

[Calnago]

Omg... Will someone please pass me another donut.

[Ahillock]

Omg... Will someone please pass me another donut.

Grown ups are discussing in this thread. Maybe time to leave this one since you don't like it and have stated you are "done with it" several times already. Maybe keep your word and be done with it and let the adults discuss?

[Calnago]

I came back when I saw something that actually was worthwhile, the chart that @Tapeworm posted. But now it's gone back to entrenched people trying to convince other entrenched people why they are right and the other is wrong. It's just repetitive at this point. And quite frankly, hilarious and sad at the same time to see such childish behavior from "grownups". Sometimes free donuts are needed, even for grownups.

[Rick]

It isn't silly or unlikely. Just look at the equation yourself and solve for Cd if you would like rather than grams force (watts).

Just a note of caution; the Cd isn't really a constant, but varies with conditions and even the velocity at which it is measured.
The Cd is really just "the number that makes that simplified approximation work under the exact conditions tested".
Also, the Cd of those shapes are valid approximations, but the Cd of a tube in a flow that is already disrupted by stuff in front of it is going to be vastly different.

Maybe you already know all this stuff. Just cautioning against oversimplifying just because the equation will still look "valid".

Example:

[davidalone]

@ 53 x12- thats an oversimplification. for a single airfoil, at laminar flow. As Rick pointed out- the Cd of your tubes are going to be different because of turbulent flow. at typiucal bicycle speeds we can assume that Cd is constant with Reynolds number, though.

For reference, ERO sports sees a difference of 0.08-0.1 change of CdA between a change of road to aerobars ( note, I'm taking the average here anyone seeing 0.2 CdA changes in their Cda from changing from a road to TT position needs to fire their previous fitter) . see here: http://www.ero-sports.com/index.php/what-is-cda
ERO sports test LOTS of atheletes. in a velodrome, with real time riding conditions. so I would say they have possibly the most 'real world' data there. I do believe they use the chung method.

at 11m/s ( 40 kph- a realistic racing speed for most of us) - that works out to anything between 50-80 watts saved from changing from a road to a TT position. Now, I'm assuming you did your chung method test at similar conditions ( unless you're telling me you did the test faster, by which any means, you have strong legs, sir.) a 30 watts saving that your claiming is between 40-70% of the benefit we see from changing from a road to a TT position. we know from other various researchers that position is the main player in cyclist drag- most researchers claim 80%. clothing, wheels and frames make up the remaining 20 percent, with clothing and helmets making up the majority of that. ( see here: http://cyclingtips.com.au/2010/04/bigge ... equipment/) 30 watts saved from the frame ALONE is simply, by virtue of previously collected data, not likely. you are either very lucky, your Felt AR has some weird shit going on, your OLD frame has some weird shit going on, or something is wrong with your test.

@rchung : as before, I already said I never said the test isn't capable. my caution is against claiming unrealistic results ( see above) without proper scientific rigor

[Rick]

Reynolds Number for 40 kph in 20°C Air for a characteristic length of .5 meters (the bicycle overall ?) is about 3.7 X 10^5

So it would seem that "bicycle speeds" span a big portion of the chart, including some areas of pretty whacky jumps in value as vortices are breaking off, etc.

Again, i am not claiming any particular value. I am just pointing out that theory is probably little more than a "ballpark guess" for something like this. I would trust emprical data, but even then it can't be extrapolated too far from the exact condition under which it is measured.