Interesting. I see a P3C in your future.

Regarding the Chung testing, are your laps only about 500-600m long? I thought they would have to be longer, but this is good news as I might be able to find somewhere like this close to where I live.

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Searching for the bliss of ultimate exertion.

http://forum.slowtwitch.com/...rain%20bike;#1701847

Check out the info half way down that page. Cervelo measured the drag difference as 15g.

What was your error margin?

P2K grasshopper :-)

Oooops!

I never realised that the P2 came in a carbon and ali version!

this is the old discontinued P2K shown in the top pic. in any case, 0.023 is a huge margin for bike/frame changeout alone. Maybe 25% or more of the bike-only CdA/drag ...

I like the results! My 2007 bike was that very same P2K and my 2008 bike just happens to be a P3C !

Now the pressure to produce a 2min improvement in my 40k time ...... oh my!

---Francis

Nice work (and nice thread title <g>).

I'll ride my P2K for a few more years as there's other (cheaper) things to fix/change, but I'll start saving my pennies. Thanks Tom!

Great info. Now you just need to repeat for a P2C and a P3SL and the Cervelo Mafia will have all the info they need.

Steve

No. They're ~1100-1200m long. I'm using an "out and back" course that's in the shape of a "drawn-out half-pipe". In other words, my turns are done at low speed on the upslope side of a hill at either end.

In fact, the venue I did this test on is not the one I usually test at. I used it for logistical reasons (only a couple blocks from home, shorter lap length). The downside is that the elevation difference isn't as great and so the maximum and average speeds are lower. This means (as was seen) that it's more sensitive to cross winds affecting the apparent wind angle.

If I plan on testing more at that course, I may need to consider investing in a portable weather station with data logging :-)

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No pressure for you Francis for the simple fact that you go so fast that you see much less yaw component than us mere mortals, so you only get the 2 to 2.5 seconds per km gain. Plus, the ROT that gives us those time savings is based on something like 24 or 25 mph average speed. Again, you go a lot faster...so, less gain for you!

I'm thinking that all things equal, you should see at least 1.5 to 2s per km.

BTW folks...when Francis says "that very same P2K", he means THAT VERY SAME P2K. Guess who I bought the frame from :-)

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rmur asked me these questions in a PM and I hope he doesn't mind me answering them here so that anyone else with the same questions can see the answers



Well, the simple answer is, they're not my tires/tubes I was wondering if someone was going to notice that.

Seriously, I'm running a VF Record front and a Bonty RXLPro23 on the rear on my wheels (latex tubes, of course)...and last month at the Piru TT I had the perfect opportunity on my outbound leg to nail down a Crr of .0038 to go along with a CdA of .228 m^2. My friend's 404s had Vittoria Open Corsa CX 20s with what I had assumed to be latex tubes inside. I hadn't had time to switch out tires yet so I just went with what he had for the testing. When I got done, and started looking at the data, things weren't matching up with the true elevation difference with the slightly higher Crr I had originally assumed...so I quick popped off the front tire and discovered the -gasp!- butyl tubes inside.

Check this out though...so I took a look at AFM's chart. I looked at the percent difference between my tires and the Vittorias and adjusted the .0038 Crr. I then looked at the percent change from latex to butyl and adjusted it again. That gave me an assumed Crr of .0056. When I put that into the spreadsheet, the calculated CdA for the zero yaw condition worked out to be...ta da! .228 m^2, just like the Piru data. Pretty cool, huh? Shows pretty clearly to me the validity of Al's testing.



What I learned is that with the "mini-halfpipe" course I used this time for logistical reasons (closer and shorter, i.e. more laps) is that with the lesser elevation difference as with my original "halfpipe", I don't see as high of max and average speeds...so the testing is much more sensitive to crosswinds since they can have a bigger affect on the apparent wind angle.



Everything as "same" as I could get it. The funny thing is...if I had to choose which one "felt" faster just from perception alone...I would swear the P2K "felt" faster...go figure.

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Thanks...I thought the title was somewhat appropriate, especially considering your thread from last week.

The good news (for me) is that the whole reason I'm borrowing the P3C is for my district TT on Saturday :-)

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30 mph, actually, since that is the standard for wind tunnel testing. However, I also rounded every number to a one or five*, so the effect may be the same (I haven't checked, but take your word for it).

*I did some calculations in my head while riding this weekend, and realized that, at least for me, the same approach also works for Crr. Specifically, at my speed/mass, a change in Crr of 0.0005 equates to a difference in power of ~5 W, and hence also a difference in CdA of ~0.005 m^2, a difference in drag of ~0.1 lbs, and/or a difference in time of ~0.5 s/km. Neat how that works out, huh? :-)

Naah...these numbers match up pretty well with what Andy has posted before and what's been "leaked" out from stuff like the "brain bike" sessions...I think I'll believe the other numbers on the P2C and P3.

I'm more interested in testing out and finally getting some numbers on something like a Transition...or maybe the new Plasma 2...

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Knowing true elevation difference can be pretty useful for getting an estimate of Crr.

Absolutely :-)

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Yeah, I didn't know true elevation difference in my Bois de Vincennes runs so I couldn't pry Crr apart from CdA. Sometimes it's possible to pry them apart in other ways.

I'm agog that there's that much difference between a P2K and a P3C.

The P2K runs seem a little more stable than the P3C runs.

It gogs me up pretty good too ;-)

Based on Andy's reports, I was expecting (hoping?) to see something on the order of a .015 to .020 m^2 difference...they way the drag drops with just an ever-so-slight yaw component was the most amazing thing to me.

I guess Gerard wasn't BS'ing when he says that the amount of coverage of the rear wheel and "shape" of the seat tube really "works"...

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I agree with that assessment. I believe that's the downside of my venue selection for this test and it's lower speeds. I'm sure if I had some "real time" weather data that could be improved.

I'm thinking any future frame upgrades for me might have to be accompanied by a portable weather station purchase :-)

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Skeptic. ;-) I said in this post that my CdA was 0.020-0.030 m^2 lower on my Hooker than on a track version of the P2k:

http://forum.slowtwitch.com/...post=1344294#1344294

As a package, the Hooker was almost as good as the P3C (at 0 deg of yaw, anyway), but a good part of that was due to the Hooker "aero-or-die" handlebars. Take those away, and there's no comparison...

Not skeptical of you...but, skeptical of my ability to realize the same gains as you. There's a difference ;-)

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okay seems like you had the ducks nicely aligned. Sometime it'd be interesting to go out on a plain jane road bike and do this sort of thing! I can see you riding a P3C and drop bars in your crits soon!

Have losses due to frame differences other than aerodynamics been factored in? e.g. frame stiffness.

The tests "just" show that the P3C is a better bike than the P2K, but does not attribute the difference directly to aerodynamics...

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Slowtwitch Aeroweenie since '06

That's one of the beauties of measuring power at the hub! That said, and as mentioned in that other thread, while I see the potential for frame stiffness to affect efficiency, I don't think it's ever been demonstrated.

I think a better question may be 'does frame material/design affect Crr'?

edit: so Tom, do you assume a constant Crr, or does the Chung method determine it for you?

I disagree. I don't see how a stiffer frame would account for the differences. Math is math after all.

I think (he can correct me if I'm wrong) that chewgl was suggesting that one frame may have less drivetrain loss than another. If Tom were measuring power at the crank, this could be an issue in determining the aerodynamic differences between frames. I think these differences though fall into the 'insignificant' category though, and it's moot since he's measuring at the hub.

Except...my "plain Jane" road bike is a Soloist...which by my measure is only slightly slower than the P2K.

BTW, I've commented to some friends that the P2K would probably make a pretty decent crit bike. It's got a steep seat tube and a relatively high BB. Perhaps that's what duty this particular P2K may end up performing in the future ;-)

I've seen Arnie Baker riding a P3C with drop bars in road races before...with a disk and a Troxel helmet too. Do you think he likes to ride off the front? :-)

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The power measurement was "to the road", or more specifically "to the hub" since it was using the same PT SL 404 wheel. Everything between the hub and the road was identical.

IMHO, the evidence is pretty compelling that the differences measured were aerodynamically based.

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No doubt that most of it was probably aerodynamic, but I'm wondering if any can be attributed to frame stiffness (yeah... something along the lines of drivetrain losses...).

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Chewie
Slowtwitch Aeroweenie since '06

The method requires you to assume a Crr and solve for CdA, or vice versa.

That said, if you KNOW the actual elevation differences AND you don't have any appreciable wind effects, it's possible to narrow in pretty quickly on the correct combination of Crr and CdA when using an "out and back" type course. Although there's virtually an infinite number of combinations that will "level" the calculated elevation plot, only one combination of Crr and CdA will "level" the plot, have the proper elevation change, AND be symmetric about the turnaround point. That's why I said above I was lucky to be able to nail down a Crr for my tire combo at a previous TT. The outbound leg and a portion of the inbound leg of the TT was under near zero wind conditions and I was travelling between 25 and 35mph, along with the leg having a known elevation change of ~50 meters.

However, if all you're interested in is comparing relative aerodynamic drag effects in a "head to head" type comparison like the one above, any reasonable Crr assumption will still give you the relative aero differences. You just may not be able to compare it to absolutely to testing on different days, or with different tires/wheels/pressures. Make sense?

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My bad... power measured at the hub ==> losses measured probably come solely from aerodynamics.

I'd be curious to see how this would compare to measurements at the BB or crank: by comparing both, you could get some estimate of drivetrain losses due to changes in frame too.

That said: nice job Tom A!

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Chewie
Slowtwitch Aeroweenie since '06

I rode a P3SL in a road race last year. I pulled the 38 cm bars off my wife's road bike and used a 140 mm stem. When I was out on the hoods I was basically in my normal TT position as far as drop and extension, but of course my hands were much farther apart then normal, and I had only minimal forearm support from the bat-top.

It was a fast setup (I wore a skinsuit and silly pointy hat, as well). However, I have to say that a P3SL with narrow bars and long stem really does not handle particularly well and I don't think I would ride that setup in a crit.

Since Tom's riding on a recycled P2K, it's fun for me to note that this P3SL has also been recycled, and is now being ridden by someone who has reported (in another thread) large changes in CdA as compared to his former ride. It's a small world! :-)

Tom A: I've always wondered: how does the accuracy of the powertap (+/- 2%) figure into the calculation of errors for these field tests? Compared to an SRM Science (with +/- 0.5%)...

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Chewie
Slowtwitch Aeroweenie since '06

Ain't it cool?

That's not quite so easy. Part of the issue is that the PT and SRM measure in different ways (e.g., the PT is time-based), and we think that drivetrain losses may vary with chainspeed, chain tension, and cog size. In order to determine the effect of a frame, you'd need to be careful to partial out the other effects. Not impossible, but it is kinda tedious.

Yep, the evidence is fairly compelling. However, couldn't changes in frame stiffness cause a change in effective Crr? It's probably a second or third-order effect, but frame stiffness affects tire path, which affects effective Crr. I'd bet that for the normal range of frame stiffnesses the effect is insignificant, especially for the typical power laid down in field testing.

If you do a standard error propogation you'll see that speed is the factor that needs to be measured most precisely.

If the PT has had a static torque check, you can pretty much tell. The 1.5% quoted by Cycleops refers to accuracy, not precision. Because the PT is time-based while the SRM collects data by completed crank revolutions (and thus, is slightly less dependent on cadence), there's more record-by-record variability in the PT. However, when averaged over a few seconds, they appear to be quite close.

Yeah, that reminds me: you have to measure your rollout pretty carefully, and it helps to record in km/h. Speed gets recorded to the nearest tenth of a km or mile. I've toyed with figuring out how much of an error is introduced by that, but only in a half-assed kind of way.

Mr. Chung is probably best to answer this one, but my understanding is that the "integration and constrain" nature of the method tends to dramatically minimize the associated sampling errors (such as those reported accuracy levels you mention above). You're then basically dealing with systematic errors, which can be minimized and/or accounted for by other methods and/or experimental technique. See page 53 of Robert's .pdf presentation that was linked to in my original post.

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With one magnet, anyway. However, if you put more than one magnet on the wheel (or axle, if using a PT), you run the risk of the reed switch miscounting wheel revolutions. You can almost always correct such errors after-the-fact, but it's a PITA.

So does signal averaging.

Thanks Guo-Liang. Coming from a self-professed "ST Aeroweenie", that means a lot ;-)

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Very. In fact, I'm quite agog at how cool it is :-)

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True 'dat.

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Tom, haven't you said elsewhere your apparent precision -- well I prefer repeatability myself -- is about 0.001 m2 for your culvert? Ain't that down around 1W average power? Or maybe less at these test speeds?

IOW, I think that's getting down there!

Well...that's on my "preferred" half-pipe course. I seem to get pretty good repeatability there when testing identical configurations in the same session. Typically within +/- .001 to .002 m^2

Then again, maybe I'm just lucky ;-)

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uhm I was really wondering if you'd thought about the what the CdA results implied about the repeatability of the PM (not accuracy). Just a sense check you know eh? I'm not good at dis English stuff.

Damn you.

It appears that on occasion the results are sensitive enough that one can diagnose problems with the PM. Alex mentioned that his buddy had a problem with the cover screws in a track hub. I've noticed a difference in a PT Pro hub that varied a little with clamping force -- it turned out my cone needed adjustment.

Right. It's not the math so much as the constraints.

I knew I could do that for an SRM but I didn't know it was possible to put another magnet inside the PT. Where? How? I'm intrigued.

Yes. In fact, there was a second P3C run in this session (after the P2K run) that I did that didn't make any sense elevation-wise until I increased the Crr to .006 from .0056. As I related to Andy and Robert though, I had possibly "over-tightened" the skewer when swapping the wheel back into the P3C for this final run. Also, since it was getting later in the morning, I was starting to experience occasional automotive interference on the venue, and later after realizing how sensitive the runs had been to ambient wind conditions, I didn't think that run was worth presenting. The calculated zero yaw (again it was under dead calm conditions...except for the cars, of course) was on the order of .215 to .218 for that run. I was hoping it would be a good "confirmation" run as I've experienced at my other venue, but I really don't have much confidence in the validity of that particular run for the various reasons noted above.

This is my first experience with a PT SL hub (mine is an older Pro model) and I have to say I'm not very impressed by the lack of adjustability/locking on the bearing preload. When I first borrowed the wheel, I noticed that the NDS axle nut was loose. Expecting there to be some sort of locknut, I was surprised when I researched the bearing adjustment and found that the factory recommendation is to just torque each side to 12 N-m, and to use some loctite to prevent loosening. Even though they're cartridge bearings, you still need to be able to adjust the preload correctly, right?...especially if you want to account for skewer tension when it's installed.

On the question of repeatability...yeah, I think the PT hubs are a lot more "repeatable" than the quoted precision value. I also made sure to zero the torque before each run. I'm sure that helps.

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The magnet is embedded in the large nut/spacer on the left end of the axle, and the reed switch rotates around it. It therefore seems to me that it would be a fairly simple proposition to just bore another hole in that spacer and insert another magnet. I haven't tried this myself, though, so don't know if the reed switch would be happy with such an arrangement.

Cars and aero field tests simply don't mix. Indeed, I suspect that some of the residual variability in our testing out in LA is the result of having two cyclists on the track at once (although we were still able to detect a difference between a LG Rocket and a Bell Meteor II helmet).

okay just keeping you guys honest eh? :-)

One thing I'd thought about was changing the wheel rollout value in the PT head. I can double it -- but then I'd have to make sure not to exceed 50 km/h. I haven't done this, though.

According to this article http://www.tritopics.com/...ycling&Itemid=53 there could well be different rolling resistances between the p3c versus the p2k based on differences in vertical compliance of the frames themselves ( In spite of testing with the same wheel, tires and PT.) This would suggest that some of the differences measured are not purely related to aerodynamics.

what are your thoughts on this?

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www.true-motion.com Triathlete Casual Wear since 2007
(Twitter/FB)

True. But to be honest, in the past on my original venue I didn't seem to be bothered by an occasional "car on course". Of course, that could well be because of that course's longer lap length, greater elevation difference, and higher overall speeds. So, like the wind sensitivity issues that appear to be magnified on the "mini half-pipe" course, I was a bit surprised at how the car interference apparently affected the data quality in the second P3C run. Robert thought the first P3C run looked "less consistent" than the P2K run...the second P3C run is even less consistent.

It's a good thing I started so early and got in the first 2 runs before the neighbors woke up and started going places :-)

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"Check this out though...so I took a look at AFM's chart. I looked at the percent difference between my tires and the Vittorias and adjusted the .0038 Crr. I then looked at the percent change from latex to butyl and adjusted it again. That gave me an assumed Crr of .0056. When I put that into the spreadsheet, the calculated CdA for the zero yaw condition worked out to be...ta da! .228 m^2, just like the Piru data. Pretty cool, huh? Shows pretty clearly to me the validity of Al's testing."

And the validity of Tom's method for converting roller to flat data.

Thoughts?

My initial thought after reading just the first sentence, i.e. "Rolling resistance is the amount of energy required to overcome the friction between the road and tire.", is that whoever wrote that needs to read the article I wrote for Slowman to become better informed :-)

http://www.slowtwitch.com/...ling_events_226.html


My second thought is that the main "suspension" of the bicycle is the tires themselves. As long as they aren't overinflated and defeating the purpose of being a pneumatic tire, the compliance they have is much higher than any other part of the structure. When you put springs in parallel, the combined spring constant is going to be closer to the spring constant of the weaker spring element. The vertical compliance in this case (with rigid frames) is nearly completely driven by the tires.

I'm not seeing how any differences in the vertical compliance of either of those two frames in that test can make a measurable difference in the total "resistance to forward" motion.

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That's part of the problem that I have in trying to find a good place to do such testing: anything rural enough to have limited traffic is too rural to have any short loops. (The other problem is terrain: almost every possible loop is too hilly to avoid braking for corners, or at least deviating from a perfect aero position.)

"My second thought is that the main "suspension" of the bicycle is the tires themselves."

you have a great point there - the article was, after all using a softride as an example which is of course a different kettle of fish...

now I'll go read your article and edu-ma-cate myself...thanks.

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Instead of loops...what about "out and backs" like I've been using? Just find a road between 2 hills in a rural location...

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That would require driving my bike, etc., somewhere...in which case, I might as well stick to my "natural wind tunnel".

Well...depending on what all you want to test out, that might be an advantage in that you can carry more tools, equipment, etc. with you.

Just trying to help...

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The only real advantage of Robert's method is that you don't need to know the exact elevation profile of a course. It would therefore be quite handy if I could do as you did, i.e., make the measurements within riding distance of my house. Unfortunately, I haven't been able to locate a suitable location to do so...which means that I might as well keep on using the same stretch of flat road that I have been using for the past few years.

Who are you and why are you posting under Andy Coggan's name?

Yep, with most cyclecomputers you get speed with accuracy of about 1 part in 400 (0.1km/hr resolution divided by 40 km/hr speed). This has always struck me as unfortunate, because the computer is just counting revolutions and measuring the time delay, and it's easy to measure time to a very high level of accuracy.

Now V = dx/dt, and of course dx is the tire circumference, which I think you can measure accurately to say +- 0.5 mm. So that gives an achievable accuracy of roughly 1 part in about 2000, about five times better resolution than the computers report.

I've tried the multiple magnet approach and never had very good luck.

Another approach I've tried that works quite well is to use a small digital voice recorder. Just cut off the external microphone and splice a reed switch in. The resulting sound file is just "tick, tick, tick", and can be run through a simple script that pulls off the time interval between ticks. The hassle then comes in synchronizing the sound file to the Powertap file. Basically, the sound file gives you speed at fixed intervals in space (wheel circumference), while the PT file gives you data at fixed intervals in time (1.26s). It's doable and I've played with it quite a bit, but I haven't yet tried it on a real field testing file.

BTW - I suspect that the ibike uses high resolution speed (for power calculations) but the data you can download from the device (and view) are of limited resolution.

-Eric

I think Al's cat might've hacked into Andy's account...damned cat! :-)

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Excellent work Tom!

Here's a related quiz to test out everyone's "eyeball wind tunnel" capabilities in regards to "Chunging it" with two different framesets:

Off the top of your head: if the "Chung method" estimate for the CdA of this rider/bike combo is around 0.270-0.275 (based on multiple TTs)...
http://www.hcphoto.smugmug.com/...2858355_aZDzn/Medium

...what would you estimate as the CdA for this rider/bike combo? http://www.hcphoto.smugmug.com/...4318610_i69ri/Medium

In the interest of full disclosure, here are the only equipment/kit differences between the two:

1) Bento box behind the stem in photo #1 (according to the MIT interview, should provide some significant aero savings)
2) Shoe covers in photo #1 (according to most sources: small, but measurable aero savings)
3) Rider weight around 2-3 lbs. less in photo #2 (yeah, I know the riders calf looks weirdly large in photo #1, but I think that is an optical illusion - it hasn't changed in size)
4) Different aerobar extensions (same S-bend shape though)
6) Different saddle
6) Different sunglasses
7) Different frameset (2005/6 Motobecane Nemesis (same frameset as the Fuji Aloha 1.0) vs. 2005 Cervelo P3SL

Everything else equipment-wise is the same (including components) down to the tubes/tires and tape on the valve holes. Total bike weights are within 0.5 lbs of each other.

As you can see, the rider's body position is VERY similar in the two photos. I could give you the measurements, but there are only very minor differences. Based on testing, I don't believe any of these to be aerodynamically significant.

I'll wait until I get some guesses about the CdA difference, and then reveal the answer...

Thanks for playing!

Rik

I have never denied that it provide such an advantage. I just don't think that it is is a very significant one, at least in the big scheme of things. Wind, after all, is the true enemy, not lack of routes for which the elevation is known (note that I did not say "flat", since that is not a requirement when using either the signal averaging or regression approaches).

 
.250 - .255 m^2

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Your English isn't bad for a non-native speaker.

.249.

Candy@sses ... ;^>

I think I'm barred from entering this contest Gee that second frame looks familiar...

last month at the Piru

Damn... now that I know that... I shoulda gone. :)

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36 kona qualifiers 2006-'23 - 3 Kona Podiums - 4 OA IM AG wins - 5 IM AG wins - 18 70.3 AG wins
I ka nana no a 'ike -- by observing, one learns | Kulia i ka nu'u -- strive for excellence
Garmin Glycogen Use App | Garmin Fat Use App

1. Sweet lard: you can pedal in that position? Ouch!
2. How much wind was present for both datasets and from what angle? ;-)

I confess to not reading every single post in this thread, but Tom..your time savings look to be at least double what Andy has posted in the past, and also roughly double what the Trek whitepaper would suggest. Am I missing something, are you or your methods somehow doubling the savings?

Thanks for posting, BTW, very nice work overall!

Well then, you should've read every post ;-)

Here's what Andy had to say in post #23: http://forum.slowtwitch.com/...rch_string=;#1802599



Maybe I'm missing something, because I don't recall there being a P2K test in the Trek white paper.



Thanks. Considering that the TTX is recognized to be "neck and neck" with a P3C, I bet you're glad you upgraded from the P2K, huh? ;-)

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I must say I am finding this thread mildly amusing. Here we have the same people who kick anyone who tries to do a similar test to gauge power improvements seen with PowerCranks around the block, describing the method as being essentially worthless, stating the PM is the only acceptable method of measuring power, yet we are using the same technique (just putting in different knowns and unknowns) to assess aerodynamic drag and everyone thinks it is wonderful and amazingly accurate.

Hey, you work with what you have, and if you don't have $thousands to get into a wind tunnel to assess drag or don't own a PM to assess power, you do the next best thing. While this is an interesting result and probably indicates an improvement I would be extremely surprised if the real improvement is as great as is calculated here. It seems way too large (as someone stated, 25% of the total bike drag) when there are so many uncontrollable variables involved in the technique.

Anyhow, it will be interesting to see how much you actually improve your TT performance based upon your expected improvement based upon these numbers.

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An original Ironman and the Inventor of PowerCranks

I've been watching this thread all day wondering what would happen first...

1) Frank jumping on trying to bring PC's into the discussion.

or

2) Someone telling you that while you're wasting your time arguing endlessly about bullshit seconds, they're just gonna go train and get faster.


I guess now that I have an answer I can on about my business.

Oh, and thanks for posting your results - very interesting reading!

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Strava

I assumed the P2K was no worse than a Madone...which I didn't think was unreasonable.

I upgraded based on the data from the whitepaper and Andy...when I lose TTs now, I have only my own execution, preparation (and parents) to blame...just how I like it!

He probably made the same mistake I made. Using the degrees of separation (P2C and P3 are similar in drag, and the P3 and P2K look similar), I certainly wasn't expecting this much of a difference--though with Andy's results I shouldn't be surprised. I figured that once you put a disc on the bikes, the covered seat tube and shaped seat stays weren't a big deal. Obviously, I was way wrong on that one.

I've either underestimated the P3 or overestimated the P2K, but either way, it looks like my 'over-under' guess is way off. To echo what the others have said, nice work!

Very simple. You're attempting to test the effect of training with powercranks, and attributing the result (or at least part of it) to powercranks.

Difficulty in testing: to show that powercranks is effective, you have to measure:
Before: prior to any training
After: 1. Training with powercranks
2. Training an equivalent amount, without powercranks
Technically, to determine the effect of powercranks, you'd have to use the same test subject, which is impossible. You can't have someone train with powercranks, then "detrain" him back to his original state, then train back up without powercranks and measure the difference.

Hence, the statistical approximation: take a *large* sample of people
Before: measure everyone's fitness prior to training
*Randomly* assign people to train either with powercranks, or without powercranks.
After: 1. Measure "fitness" (power?) of people who trained with powercranks
2. Measure "fitness" of people who trained without powercranks
Both groups should have trained and equal amount, and powercranks should not have caused either group to train more or less, if you really want to measure the effect of powercranks. Of course you can consider this variable as endogenous and ignore it (consider it as part of the overall effect of powercranks).

Only then do you have a scientific study with powercranks.


It's much easier to do with a wind tunnel / aerodynamics test. Individual variables are easily isolateable. Tests are repeatable, using identical equipment, same wind tunnel, same testing conditions (or at least well defined corrections can be made based on the testing conditions). Which results in a more rigorous scientific study overall.

Anecdotal evidence is a first approximation. Powercranks *possibly* help, but until you (or someone else) does a full scale independent scientific test, people will always have reason to doubt you. Nobody doubts that aero helmets, aero frames, aero wheels do work: the evidence is out there.

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Chewie
Slowtwitch Aeroweenie since '06

Ugh, all we say is that our typical user is gaining 40% in 6-9 months after starting exclusive training with PC's. That claim does not say that every watt of increase is due to PC's. Some of it, of course, may be due to training effect and, in fact, some probably is. That is why one does independent studies to try to isolate the variables. However, 40% is much more than most people expect to see in that period of time so some must be due to the PC's and the independent studies that have been done seem to suggest a substantial benefit in both efficiency and VO2 max can be attributed to use of PC's.

So, if someone comes here and says I have increased my speed x amount in 6 months of training with PC's and that calculates to X watts increase. You may disagree as to how much of the increase is due to PC's themselves and how much they might have seen if they had trained traditionally but the result remains yet people come here and call them liars and shills and whatever. Yet, when similar "unbelievable" improvements are seen in aerodynamic drag by using one of current "favorite" bikes of this site everyone thinks it is just wonderful and justifies their last or future purchase despite the fact that no good control of all the variables existed in the test. If someone really wanted to know what the difference was they would go to a wind tunnel and put a mannequin on the bike(s) so everything would be known to be exactly the same between the tests, except the bike. Then we would know.

This result is an estimate yet it seems people are taking it as gold. And the number is so large it is hard to believe it to be true but people want it to be true which probably accounts for why everyone is taking it as gold. We will see how the racing goes. (of course, we will need a lot of numbers to assess whether race conditions affected the results one way or another.)

I simply pointed out the seeming hypocrisy.

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Frank,
An original Ironman and the Inventor of PowerCranks

thanks for trying... please, PLEASE, let's just ignore him and maybe he'll go away.

I was really enjoying this thread up 'til now.

Oh...thanks for that! For a second, I just thought you were just being an ass...

I'll be curious to see how many others view the 'hypocrisy'.



Phooey...

You're pointing out a double standard which I don't believe exists.

Mannequin? LOL... Cervelo did just that. They're using it in their wind tunnel tests.

I don't understand what you mean by "no good control of all the variables existed in the test". Could you elaborate? Or have you never been to or conducted a wind tunnel tests? We (people who do these tests) usually make an effort to ensure that proper controls are taken, and it shows: many of these independent tests arrive at the same conclusion, with very similar numbers. These numbers that we come up with are hardly "estimates": they're calculated, based on well established models. You're using the word "estimate" in the same way that creationists call evolution just a "theory".

Sometimes the numbers we come up with are large. And they are, cos that much of an improvement was made. You're insulting the people who test in the wind tunnel, as well as the people who conduct the test, by saying that people want to believe the results are true.

Oh, and if you don't mind, could you email / PM me the "independent studies that suggest a substantial benefit in both efficiency and VO2 max can be attributed to use of PC's"? Finals are gonna be over, and I think I'll have time to pour over them. I'm honestly curious.

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___________________________
Chewie
Slowtwitch Aeroweenie since '06

Weren't the tests involved that started this thread done on the road? As I understood it these were calculated improvements, not measured improvements. That is what I meant by "no good controls". It is not possible. 1-2 mph winds could substantially affect the results yet it would seem to be virtually calm to the rider. Is the rider riding the same straight line each trial, hitting the potholes the same, etc.?

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Frank,
An original Ironman and the Inventor of PowerCranks

Andy, quick question for you regarding your Rule of Thumb above (based on 28-29MPH?)

I will be testing using the iBike iAero, which gives me a static CdA readout. What formula should I use to calculate out the relative differences in CdA as applied over 40KM ... will your formula CdA improvement of 0.005 m^2 = 0.5 s/km faster work for me at 29MPH/76kg?

Thanks,

- Gary

"Weren't the tests involved that... blahblahblah..."

Frank, read the protocol.


We, as an "online community" should really begin to institute a consensual standard that requires a poster to demonstrate at least a rudimentary understanding of the topic under discussion before taking seriously their attempts at stone-throwing.


...of course, such a standard would effectively eliminate 90% of your "contributions" to this board...



Could everyone please just ignore the FDBS, and continue on with the excellent thread?


(back to lurking...)
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Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

No no, this thread is just getting interesting. No point in putting forward these claims and data without some critical reviews!

Full credit to the OP for doing this.
Full credit to Frank for laying down a critical eye, I just hope this thread stays clean so we see some real ideas surface.

Can I ask how many of the players in this thread are honestly neutral in their views? and not in the 'pro P3' or 'anti P3' clubs?
Its eaiser to find data to support what you want rather than what you dont want...

(This post was made with genuine interest, none of which was intended to bait)

The only problem is that Frank really didn't make a 'critical review' or even ask a question about the protocol (well he did--but the question was so silly that is shows he's never bothered to read the information on the Chung protocol).

Remember, this is the same guy who spent 8 pages arguing that Joaquin's file wasn't manipulated, only to then admit that he didn't even have a rudimentary understanding of power files?? I'm all for a critical eye....but this is just silliness...

You can always go to analyticcycling.com to double-check, but the short answer is: yup. That ROT pretty much applies.

So speaking of questions about the results....

Back to the rolling resistance issue: I'm still wondering if there couldn't be some differences in rolling resistance between the two setups. Forget about friction for a second--what about the potential for reduced hysteresis losses resulting from vibration damping of the frame? Something else I'm not considering?? I'll say in advance that as soon as you start talking about secondary springs, my head starts hurting.

I realize this is a stretch, but I figure it might be worth considering. Thoughts?

Many thanks.

"...as soon as you start talking about secondary springs, my head starts hurting."


This is one of those concepts that's really simple when you see it in action.

Link two springs in series, one with a lower resistance to elongation than the other. The spring with the higher level of resistance will not begin to elongate until the spring with the lower level has elongated fully, or to the point that it's resistance to elongation surpasses that of the other spring.


Vibration damping in regard to hysteresis losses strikes me as a not terribly promising line of enquiry. Wouldn't you need to see some type of significant system-level resonant frequency issues in order for damping to be much of a factor?


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Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

As long as we're talking about ROTs, at the speeds we're talking about a change in CdA of .01 has roughly the same effect as a change in Crr of .001. Tom is estimating a change in CdA of a bit more than .02 so, ballpark, you'd need a change in Crr of .002. That's huge, especially since he used the same wheels and tires. Unmeasured frame losses aren't the explanation for the difference.

If the frame geometries are not identical when comparing two frames, does it matter?


P2K


P3C

Well, my question was if changes in Crr could be responsible for a portion of the change--not all of it. That said, I'll be the first to admit it's unlikely. I still think it's worth asking though since Crr was assumed.

Gary, I'm not sure if I'm following the question...isn't the point that the frames aren't the same? In other words, the rider and his position (relative to the BB) along with the wheels and bars are duplicated. So the only thing that IS different is the frame, right?

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http://bikeblather.blogspot.com/

Well, Crr wasn't exactly pulled out of a hat. Knowing the true elevation delta over the laps puts pretty tight bounds on it.

I think he means different tubing dimensions (namely length) may give different damping characteristics. For the frame to make a difference to rolling resistance you would have to be claiming that it acted like full suspension - keeping the tyres in contact with the road more securely.

Well, I did read the protocol. Here are three excerpts I think revealing to my point.

1. the title: "Indirect estimation of CdA using a power meter" Key word, indirect.
2. from page 3. "how good of an estimate of CdA is it possible to get?
using usual approach, not very good at all
using indirect approach, not bad at all" key word, not bad. He did not seem to choose, good, very good, or excellent. So, it is a not bad estimate.
3. from page 7. "wind, if present, should be consistent in direction and speed". A condition that hardly ever occurs.

Anyhow, it looks to be a useful technique. But, there are enough "problems" that it could hardly be used as a substitute for wind tunnel testing. Hence, the difference in the drag numbers between these two conditions, which seem exceptionally large, must be somewhat suspect as representing reality.

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Frank,
An original Ironman and the Inventor of PowerCranks

"I get that part (kinda), but the question I have is does the second spring elongate less (I'm guessing so) or not at all??"


Not at all, at least until a threshold level is reached.

In practice, with imperfect springs, at the transition point, you can actually see a short period where there is alternation in elongation between the two springs, but this is really, really minor...


"...a long time ago I had seen a paper which discussed frame materials and efficiency. I don't remember much about it (I think it was a manufacturer sponsored, non-peer reviewed deal), and it made some suggestions that frame material affected efficiency to the ground. However, I don't remember the specific mechanisms by which they suggested those changes occurred."

I would like to see this if you ever dig it up. My understanding of these issues is informed almost entirely by my academic background in musical instrument design and construction, and when I see terms like "efficiency" and "damping" in the same sentence, I'm seeing antagonist forces...

I would like to refer this line of thought to your steel-tube bike discussion on another thread, and harken back to a conversation I once had with a frame builder of some repute. He believed that more (in his words "excessively") rigid frames were less efficient to ride, as they were less effective as a mechanism to absorb road vibrations, and were not demonstrably superior in efficiently transferring rider energy to the drive train. In his words, "any energy lost as a result of frame material or design stiffness would manifest as heat, and would be easy to measure if it occurred at a significant level."

Earlier in the thread I seem to remember that the frame that "felt" fastest was actually slower... Hmmmm...



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---

Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

Namely the head tube geometries, and the shape of the P3C head tube was my question. In this case it is taller than the P2K, yet although taller, possibly more aerodynamic?

"Anyhow, blahblahblah..."



...of course, since you are completely unfamiliar with the literature on the subject, you haven't read the field-test validation paper (which I think Andy actually contributed to) which demonstrated (rather conclusively) the ability to field test at a level of confidence approximating that of the wind tunnel...


...and, as has already been mentioned on this thread, some of the people actually posting on this thread are demonstrating reproducible test results with a higher level of resolution than at least one of the academic wind tunnels can, and they are using this protocol.


So, please... go away.


.

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Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

Hey, if you believe it to be true, so be it. I look forward to this result being independently reproduced with the same result. When that is done I would have more confidence in this representing reality. Otherwise, I still have my skepticism there is this much difference from this simple change.

I look forward to seeing how he does in his time-trial to see if he sees it in his race. If he doesn't see the improvements that he should see based upon this result I look forward to hearing the explanations as to why.

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Frank,
An original Ironman and the Inventor of PowerCranks

Actually, I can't. I'm just letting the tailwind push me along ;)

And by the way, that's not lard - I have my patch kit and CO2 stuffed in a ziplock down the front of my skinsuit. ;)


The CdA calculations are from several TTs on different nights with different wind conditions. I am using the Chung method as modified by Ron Ruff for "instantaneous apparent headwind". I get very similar results using the standard Chung method calculating for "virtual elevation change" The TT course is a very flat out and back, but with a changing road bearing with several turns, so it gets a variety of wind angles.

I calculate air density based on the readings from a nearby weather station. I also temperature-correct an assumed base Crr for that course and the tire/tube combination from AFM's dataset.

Any more guesses before I reveal the results?

Rik

Aah...I see. Yes, I'm sure the headtube area is one of the "difference makers", especially since it's a "leading edge". Let's not forget also that the forks are different on the 2 bikes as well. That may play a part in the differences MEASURED (that's for Frank <grin>) between these 2 setups.

In any case, faster is faster, right? :-)

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http://bikeblather.blogspot.com/

Well, as Robert pointed out, having a known elevation helps to tightly bound the Crr. If the Crr is "off" then when choosing a CdA that "levels" the elevation plot, the elevation difference will be either too big or too small. Don't forget that the 2 main drag sources vary differently with velocity...this allows one to "pry" the two values apart by having adequate speed variations throughout the laps. The "halfpipe" configuration helps in getting that speed variation and I also make a point of varying power (and thus speed) across each lap.

In any case, even if a change in the Crr component was measured and all the difference wasn't attributable to drag reductions, it still would have been measured to be faster, right? Who's to complain about that? :-)

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http://bikeblather.blogspot.com/

You are forgetting that this IS an independent verification of the differences Andy measured on virtually identical frames (P2T vs. P3C Track).

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http://bikeblather.blogspot.com/

OK, that's for explaining that in a way the this music degree equipped bike racer can understand! And yeah, whether it's Crr or drag reductions, faster is faster <tm>, they'd just scale a little differently.

Either way, I guess I should probably be happy I joined the arms race already....

Unless, of course, wind, or other uncontrolled variables, make the "faster" an illusion.

I would feel much better about the reliablity and accuracy of such a result if there were say 4 different people who said, lets do this experiment. We will each do this independently using this protocol and then compare results. If they all get the same result then I think we can feel very comfortable the protocol is pretty good and reliable. If they each get substantially different results, despite having their own internal consistency, then who is correct? The fact that one person has an internal consistency is not particularly good evidence the result is accurate.

I think the protocol is very interesting and a lot of work went into coming up with it. However, I haven't seen the work that proves to me that a result such as this can be trusted to represent reality, especially in view of the magnitude of the difference.

People here agonize over the accuracy of the various power meters yet I haven't seen one person here (except perhaps me) ask what the accuracy of this technique for determining aerodynamic drag supposedly would be. Why not?

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Frank,
An original Ironman and the Inventor of PowerCranks

That is you Eric. Thanks for digging the frameset out of your garage and selling it to me!

The answer is 0.0228 m^2.

We only have two entries in my little contest and RChung's "Price is Right"-style guess is closest. So I guess he has "smart guy" bragging rights over Tom A for the time being! Though he gets some style-points deducted.

OK, now that I have the data in front of me:

Photo #1. I have data for three 10-mile TTs on three separate nights from July-August 2007 for which I did a Chung method CdA analysis. Equipment the same as I described earlier and as shown in the photo, except for the 3rd TT where I had a water bottle cage mounted on the downtube (no bottle). CdA calculated at 0.2716, 0.2733, and 0.2735 . Wind speeds and direction, temperatures, and air density varied greatly on the three nights (I corrected for air density as well as the temperature effect on Crr), so I'm pretty happy that the results varied as little as they did.

Photo #2. I have data so far this this year for three 10-mile TTs from March to April on the same course as 2007. Equipment the same as I described earlier and as shown in the photo, except for the 3rd one where I had to use a 23 tire because I broke my 20. Wind speeds and direction, temperatures, and air density varied greatly on the three nights. CdA calculated at 0.2497, 0.2504, and 0.2499.

Average of the CdAs in the 1st set: 0.2728 m^2
Average of the CdAs in the 2nd set: 0.2500 m^2
Difference: 0.0228 m^2

That's the difference I found between an "aero" frameset and frameset that is actually aero. And based on the tube widths and aspect ratios, the "aero" Fuji Aloha 1.0/Motobecane Nemesis is more aero than a number of other "aero" framesets

What does this mean in terms of results? You know, that actually-going-faster part? Well, there just happen to be two TTs I did with almost identical air density and power output. Makes for an easy comparison:

TT#2 from the 1st set: June 20, 2007. Calculated air density = 1.167 kg/m^3; averaged 329 watts; time: 22:33
TT#3 from the 2nd set: May 7, 2008. Calculated air density = 1.168 kg/m^3; averaged 328 watts; time: 22:02

31 seconds over 10 miles = 1.93 seconds/km = 0.42 seconds/km per 0.005 m^2 change in CdA. Hey, somebody ought to write a rule of thumb ;)

Rik

Very cool.

so what do I win for guessing .205 correctly?

I really am sorry about the collarbone or I could have posted my data...

shame having these two identically equipped tt bikes sitting here not being ridden...


-g

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greg
www.wattagetraining.com

Frank, this is not one of those: teach physics to Frank threads so I'm not going to respond to any questions here. This method has been tried and criticized mostly elsewhere but it's a tad more complicated than explaining why a bike speeds up going round the corners in a velodrome.

It's not a training method, nor is anyone selling anything here ... so ...

Anyhow you missed Marco Pinotti's 4th place in the Giro TT. How come?

I'd be interested in seeing some tests like that. I could see a bad chainline being measureable, but would be shocked to see losses to to flexibility. A simple way to test would be temp of the frame. If power is being lost then its going somewhere and it should be heat.

An good test would be a full supension MTB set squisy then locked rigid. Ride them on the road like Tom did and measure the loss. That should be an absolute worse case scenario for frame stiffness losses.

Styrrell

I know I'm going to hate myself for this but I'm going to respond very briefly.

1. Indirect doesn't mean inexact. Neptune was discovered by perturbations in the orbit of the planet named for you, or more precisely, Uranus (thanks folks, I'm here all week. Tell your friends). Its position was subsequently verified through direct observation. All field-based methods are indirect methods since they use measurements on other variables combined with known relationships between the variables to estimate CdA and Crr. Regression-based methods are often classified as indirect methods. There's nothing unusual about indirect methods. You just haven't spent any time thinking about the distinction before.

2. As you noted, the "not bad" came from page 3. What was page 3 about? Oh yeah, a particular data set proposed as a challenge. That challenge data set contained only speed and power. There was no info on weight, weather, or route, except that it wasn't flat. The "not bad" was in reference to this particular data set, which was quite extreme in its parsimony.

3. "Wind": Wind is always a problem with field tests. A good thing about this approach is that --> you can tell when the wind was strong enough to mess up the results <--.

Lots of people have asked about the accuracy of this method, including some of the people participating in this thread. It's just that they asked earlier, in other threads or in other places. The reason why you're the only one asking now is because you're late to the party.

iow, there's a door charge to the Chung Comedy Club and, Frank, you've not paid it :-)

The next time you're in town, I'm buying you a beer! (or, the beverage of your choice, if you don't like beer).



Yeah...that's a real shitty deal. Have you found out if it's going to be surgery or not yet?




Hmmm...we're about the same size, right? How about letting me borrow them for awhile? I could do another "Something borrowed..." test :-)

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http://bikeblather.blogspot.com/

No, I'm quite familiar with the various Cervelo models...I went back & checked my records. I've got a private email from a reputable friend who was in a tunnel with a P2K and P3C, he quoted .5 - .8s/km @ 30mph. Maybe he was being conservative since he knew I was deciding whether to swap my P2K for a TTX?

Trek Whitepaper had P3C as a little more than .5s/km faster than a Scott Plasma, closer to .8s/km faster than a Madone (all @ 0' yaw). I assumed the P2K was no worse than a Madone...

Tom's work appears to double the difference, at least, which is why I wonder if something has been missed.

ROTFLMAO!!!

Say what? You compare anecdotal reports of performance improvements due to use of your product with measurements made using a powermeter, then state that they are the same technique??

Is that with or without a rider, and at what yaw angle(s)?

You tell me!

The Trek whitepaper data was, as you know, with a rider, and I said I was only quoting the 0' numbers . Looking back at it, it is roughly doubled at even just 10' yaw (if I assume the P2K is as bad at yaw as a Plasma).

Did I miss Tom's estimation of the apparent wind during his testing? If so, I apologize for bringing up the point, as it appears even minor wind could easily result in these sorts of differences once we make a couple assumptions.

I'll take you up on the beer soon... talk to j and maybe we'll add ya to the interbike list if you can swing a hallpass...

I'll loan you the bikes if you pay shipping and ride them in my position (you can move saddle/posts) and have the back in 4 weeks...

:P

-g

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greg
www.wattagetraining.com

Sorry, I missed where you said that it was at 0 deg of yaw.

Anyway, differences in how CdA changes as a function of yaw could certainly explain the difference. Indeed, I recall John Cobb once commenting that the Hooker (which used narrow NACA-profile tubes, similar to the P2k) didn't perform as well in crosswinds as it did in a pure headwind. I still "sailed" pretty well upon it, but that doesn't mean that a better design (e.g., P3C) wouldn't be even faster under such conditions.

Indirect doesn't have to mean inexact, execpt the pertebations of Neptune told astronomers there should be something in this area. It did not tell them exactly where to look. Now that it is found, people know exactly where to point there telescopes each time. It is hard to imagine an indirect measurement being more accurate than a direct one (except when direct measurement is not possible). Hence the comment on accuracy.

Second, the measurement starts by using a device with a known accuracy and then introduces additional steps on top of that. The method, it seems to me, cannot be any more accurate than the power meter one is using and, because of the additional steps involved should be substantially less accurate. While others may have questioned the accuracy of the method in other threads I haven't seen anyone question the accuracy of this result, people seem to accept it as being true.

Even when methodology is potentially very accurate, errors can be introduced by those performing the methodology, despite their best efforts. I remember analytical chemistry very well. I could never get a very accurate result for whatever reason, despite what I considered my best efforts, while others could reliably do so. In that instance, the methodology is proven yet, subtle errors can be introduced throughout.

As I said, the methodology seems very inventive and I am impressed. But, until it is compared to known standards and its reliability when done by others less skilled is known we cannot necessarily trust this particular result as being anywhere close to the truth. That is what I am trying to say. If he saves 2 minutes off his next 40 k TT using that bike (and everyone else rides pretty much what they have done before, on average except, of course, those who have changed to this bike who shuld all be 2 minutes faster also) then I would take that as an independent confirmation the result is pretty accurate, at least in relative terms. If he doesn't, where are the inaccuracies in the result going to be explained?

As near as I can tell from watching the big people ride TT's, the bikes they are on make little difference in performance. This result says otherwise so is a little hard to believe. It is even better, as I understand, than the manufacturers claims. It just makes no sense to me that this much change can supposedly be due to the bike alone. It might be true, but I would like a little more rigor in the process before it is taken as gospel.

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Frank,
An original Ironman and the Inventor of PowerCranks

Not only has this been done, the results of such studies have been published in the peer-reviewed scientific literature. The bottom line is as alluded to by others: at least under optimal conditions/in the right hands, field testing using a powermeter can be just as accurate as testing in a wind tunnel, and is nearly as precise. It does, however, have some disadvantages: 1) it's generally more time-consuming/less convenient, and 2) more importantly, you can't test at controlled yaw angles. The latter is critical, because cyclists are NOT bluff bodies.*

*A while back I was surfing the web for information on building a small wind tunnel, and came across an aerodynamic primer for model rocket hobbyists. It gave the Cd value of a long, multi-stage (fuselage of two diameters) rocket as being 0.7...IOW, the same as a cyclist in the aero position on an aerodynamic bicycle! Remember this the next time somebody starting spouting off about how non-aerodynamic a "floppy" human pedaling a bicycle is...

Maybe they're on the wrong bikes, or maybe you're just not a very good observer...

Anyway, here's what I know: when my wife was riding a P2T, she had to produce 410 W to ride 3 km in 3:51.X. Five years (and one DVT) later, she goes nearly as fast on a P3C while producing only 330 W. Conversely, when I was riding a Hooker my 40 km TT times were routinely in the 53:00-53:30 range, but when I retired that machine and started using a Javelin Arcole or P2T (my wife's bike) instead, the fastest I went was 54:12, despite producing just as much power as previously.

John, I hear what you're sayin'...for example, assuming that the P3C is slightly (let's say ~.05lbs @ 30mph) lower drag than a Hooker Elite at zero yaw (as Andy has reported) and that the P2K isn't appreciably different from the original P2 at that angle, the data Andy presented in the post below indicates the difference to be around 0.2 lbs of drag, or ~.010 m^2 of CdA.

http://forum.slowtwitch.com/...oker%20drag;#1109749

So yeah...my "zero yaw" number looks to be at least double than what was reported there.

All I can say is the data "is what it is"... :-)

In regards to the apparent wind conditions, the testing was done in the early morning hours with the intent of minimizing crosswind affects. However, as I discussed earlier in the thread, even though in the past I haven't found much problem with dealing with slight winds on my original course, on this course there seems to be more sensitivity to winds, however so slight, due to the lower average and max speeds encountered. I have to emphasize however, that the amount of wind encountered during this test was extremely light...in fact, I'm not even sure if it would have registered on a wind gauge.

Here's some more data that may help to serve as a "sanity check". Below is a plot from the first lap of a 40K TT I did a few weeks back on the P2K. As I mentioned earlier, the outbound leg of this lap provided an ideal opportunity to nail down a CdA and Crr since it was done in near zero wind conditions and the outbound speeds are high due to it being a generally downhill run. Knowing the actual elevation difference of the course was 50meters, and the fact that the wind didn't start picking up until ~1/4 to 1/3 of the way into the inbound leg, by choosing a Crr and CdA combo that matched the elevation difference and made the elevation profile symmetric about the turnaround point (up until the part where the wind started) I came up with a Crr = .0038 and a CdA = .228 m^2, which basically matches the "zero yaw" measurement from the field test.




Now, at the same race was another rider who is nearly the same size as myself and has a similar position on his bike. The main differences are that he rides a P3C and his wheels are slightly different (404 front vs. my TriSpoke, and tubulars vs. my clinchers) although that shouldn't have a huge affect on zero yaw performance. This other rider was, in fact, my 30s man so it's safe to say that we were both racing under VERY similar conditions. Using the same technique above, 2 things stand out. First, as expected, his Crr is higher due to the use of tubulars, and his CdA turns out to be .207 m^2.




So...knowing that, I don't find the difference I measured to be unreasonable, especially when considering Andy's data in the mix.

Oh yeah...the second lap of that TT was done with a steady wind. Using the Crr determined from the first lap, my calculated CdA dropped to .220 m^2 (just like in my testing when I thought there might be some wind present) and IIRC (I don't have THAT data in front of me right now) the other rider's CdA dropped to ~.190 - .195 m^2. I know that's not super compelling data since it's 2 different riders, but it WAS under near identical conditions for both of us. The fact that the general magnitudes of the changes were similar lends some confidence (in my mind at least) to the results in the P2K vs. P3C testing.

But, I'll admit...there's some question about how truly zero yaw my "zero yaw" measurement were. Then again, the data also points out how very little yaw it takes for the CdA to drop significantly on frames like these and further emphasizes how testing in a wind tunnel at only zero yaw can be misleading as to the differences between setups.

I'm thinking I might have to take the P3C over to my "preferred" course and do some runs prior to returning it to it's rightful owner. That's not going to happen this weekend though...since I'll be racing it :-)

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http://bikeblather.blogspot.com/

Tom,

1. Small density difference between those files 1.178 vs. 1.19 (66 degF vs. 61F ??)
2. Which point on the course is at 613ft? I know very minor but as there's 50m delta E ... there's some variation in rho as well.
3. How does the pavement on this TT course compare to that of your test culvert?
4. For your original post (and I know this is obvious in hindsight) I would have swapped back to the P2k and repeated the baseline.

Thanks, good stuff to mull over!

Aah...yes. Thanks for pointing that out. I had forgotten that the rider who sent me that data had looked at it first using Alex Simmon's spreadsheet posted over on wattage. He had used weather data from a different weather station than I did, and the numbers in that file reflect that. I was trying to see how closely our spreadsheet calculations matched using HIS data.

Check this out...I just "copied and pasted" the weather data from my file into his. Guess what the CdA for him works out to be now? .205 m^2...can you believe it? :-)



Actually, that's the elevation of the weather station that's across the river valley from the course. You're right though, to be more exact I really should use the average course elevation, huh?



Pretty similar. They're both asphalt pavement. I'd say the TT course might be slightly rougher.




Well...I only had so much time...and as I related before, the second P3C run was beginning to suffer from "automotive interference" anyway. It only would've gotten worse as it got later....

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http://bikeblather.blogspot.com/

I wold love to see a link to the study, if available. I do love the caveat, "under optimal conditions/in the right hands".

And, exactly what do you mean when you say it can be "just as accurate" and "nearly as precise". What is the difference?

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Frank,
An original Ironman and the Inventor of PowerCranks

Hi guys. I only just came across this thread this morning. Very cool reading, well most of it, but Robert gets the joke of the month award.

Just on the air density calculation - I did reload the files for both the Chung and regression methods onto the Wattage forum files section on 5 May - the latest version has the altitude correction factor included.

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_________________________________________________________________________________
Training Plans -- Power Meter Hire -- SRM Sales Australia -- cyclecoach.com -- My Blog -- Sydney Turbo Studio

Um, Frank? You know just a bit above where you wrote: "I remember analytical chemistry very well"? I'm thinkin' you don't remember it quite as well as you think you remember it.

Can you persuade him to ride the P2...

How do you know that your position on the P3C is exactly the same as on the P2K?

Um...

A ruler, a level and a little bit of time???


G

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greg
www.wattagetraining.com

I hardly remember anything about analytical chemistry other than I wasn't very good at it even though I took as much care to be "accurate" as I knew how. Is there a problem with asking him how he is using these words? One statement implies an equivalence and the other doesn't.

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Frank,
An original Ironman and the Inventor of PowerCranks

He's using them the way anyone who remembered chemistry, or engineering, or statistics, or any of a large number of other fields that require measurement and evaluation would use them. How about this: go to google.com, type in "accuracy" and "precision" and click on "I'm feeling lucky."

Oh, btw, I almost forgot: you know how you were pointing at little bits from the title page and page 3 of that thing? One of the studies that says that field testing can produce results comparable to a wind tunnel is listed on the page between those two.

Coincidentally, on the first day of my 10th grade chemistry class the teacher showed us a couple pictures of bullseyes not unlike the two found on a certain wikipedia page. We discussed accuracy and precision for a couple minutes and somehow it stuck with me since way back then.

One quick search with the googles turned up the wikipedia page... now that wasn't too difficult to find.

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Erik
Strava

Very simply. Measurements.

I have diagrams of "stack and reach" to all the touchpoints for both my road and TT positions. This case was easy because I had the P2K to measure off of right next to the P3C.

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http://bikeblather.blogspot.com/

sounds solid once the density was equalized. Hopefully conditions for the upcoming TT are amenable to getting another solid CdA reference. Oh yeah, hope you go fast too ;-)

Sure, as long as you realize that it is just a rule-of-thumb arrived at via some rounding, whose purpose is to simplify doing calculations in your head.

How is the bottle cage attached to the aero bar? Do you really keep water in it?

Thanks,
Scott

which bike had powercranks,insert smilley face,been tting for twenty years saw a lot of wheel covers in the beginning,now see a lot of discs deep dish etc, see a lot of Crevelos, never see a round tubed steel bike,at one time got a disc that would make me faster,,,,the one thing i do not see is people going faster,the times for everyone should have dropped over the years with all of this equipent to make you go faster,everyone should be doing 48 minute 40ks if this stuff worked

Zip-tied to one extension with bar tape under it for better "grip".


Absolutely. Perhaps not completely full during a race, but at least enough to "wet my whistle" midway through a 40K TT.

It also works out good for the event that pic was taken from, which is a local monthly 10 mile TT that I ride to from my house. Since the start and finish are at 2 different locations, this allows me to take along some water for my warmup without having to worry about retrieving a bottle at the end.

Note the seatbag with tire repair supplies as well for the same reason ;-)

BTW, I haven't formally "Chung'd" this bottle location yet. In fact, it was left off during the testing vs. the P3C and I'm not planning on putting it on the P3C for the race. That said, I have no indication that this bottle location is any slower (or faster) for me than no bottle.

edit: If my arm position was flatter, I most likely would have the bottle mounted above the extensions instead of below...just thought I'd add that.

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http://bikeblather.blogspot.com/

If I could make as much power as I could ~20 y ago, I'd be under 50 min even at sea level. As it is, I'm still several minutes faster now than I was, say, in the mid 1980s (when aero wheels, aero bars, skinsuits, etc., first became popular), despite a ~20% reduction in my power output.

I know your background,i know the aero stuff can work,but with every product saying you go 2 minutes faster in a 40k,i do not see the whole as a group at states tt going faster then twenty years ago.Better bikes better training,but the times about the same, Has anyone beat John Frey's tt time from around the late 80's?.
I know your Old bike (Hooker) to your new bike is not as a aero difference as some of us has had,(round steel to Aero), I just can not wrap my head around a lot of the aero claims

1. You don't think that somebody like Dave Z could take that record if they wanted to?

2. People don't take TTing nearly as seriously as they used to. For example, back in the mid 1970s it wasn't uncommon for Wayne and Dale Stetina and Tom Doughty (all Olympians) to contest our state (district) TT. Nowadays, most cat. 1 riders don't even bother to show up, preferring instead to find a mass start race or just train instead.

1 really not sure
2 been seeing the same guys for years,after a 10 year absence of not doing our state i was supprised to see a lot of the same people,better bikes, about the same times (plus new people) I do think your right about not taking TTing seriously we would have twenty people at a Thursday night TT then nobody now 4 or 5 of us get together once a month

Well duh: like me, they're apparently attempting to delude themselves that they aren't getting older by continuously upgrading their equipment. ;-)

You know those ROTs? You need to add another sorta like: $X is roughly equivalent to 0.5 s/km.

So if you don't have a "loop" course, can you get the same results by "repeating" a particular stretch of road (in the same direction, cruising back to the "start" each time)? Post test you would delete all of the "cruising" data and be left with only the "laps". Granted, they're not starting/ending at the same elevation (nothing here in Colorado does), but if you have accurate altitude data over the stretch of road, will this method still work well?

Sorry if the answer is clear when you study the paper...I thought asking would be quicker than reading/studying. :-)

Well i sure have done the upgrade thing ,i guess my brain can not understand,i got slower and older,so i should sell all of my new go fast stuff,get back on the first TT bike i ever built(not assembled,built brazed tubed kind of built)and just enjoy myself

because outside of a stage race, they don't count for upgrade points...

:/

g

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greg
www.wattagetraining.com

better read the paper, then re-read the paper, ....

If you don't fully understand the basics - any shortcuts are likely to lead to some ugly dead-ends !

I think the presentation suggests a half-pipe like the one Tom uses but there's actually a fair amount of flexibility. You can use loops or laps or an out-and-back or, in fact, any segment where you happen to know the true net elevation gain: I've actually used repeats on a stretch of road exactly in the way you're asking about. The loops and laps are because we know the true elevation gain nets to zero. If you do decide to use an out-and-back course it's probably a good idea to do at least a couple of laps at different speeds. Winds are always an issue so this approach lets you focus on finding a venue that helps you minimize wind effects without also having to find one with constant slope. It's actually good not to have a course that's too flat.

The bottom line is, this approach isn't magic -- it just means you can expand the places where you could test and puts the onus on you to take good measurements.

robert,
speaking of non-ideal venues, what is your take on the "smart guys" approach of neutralizing the VE of an unavoidable (flat) braking zone by setting the VE for each record in the entire zone to that of the entry point. IOW, flattening that portion of the loop.

Better brake relatively hard and correct for it vs. raising your head, sticking out the knee ... bleeding off speed in an less obvious way?

Uhm, I'm assuming the braking zone will be short in relation to the length of the lap - otherwise - 'tis obviously not going to work too well ;-)

:-)

There's a physician/popular author/radio personality out in California (whose name I can't remember), and in one of his books he provides advice for aging men who still want to remain competitive in sports (I was sent a courtesy copy after answering a few questions for one his editorial assistants...don't have it any more, though). One thing he suggested was getting into new sports before they became really popular (his example was inline skating), and started attracting athletes better than yourself. Another piece of advice, however, was to use your wallet, i.e., to outspend younger men on coaching, equipment, etc.

Thanks. Since someone has said that the technique is "just as accurate" and "nearly as precise" this implies these terms have been quantified for the approach. What is the standard deviation of the technique that would define "precision"? Would the standard deviation depend upon the accuracy or the precision of the power meter? wikipedia also stated this as regards precision.

What is the repeatability an reproducibility of the technique? I am mostly interested in reproducibility as that seems to be the major flaw in the technique for general use, since good results require great attention to detail.
This looks like it might be a great technique for individuals to assess how changes might affect them overall. Are they better or worse off for the change and by approximately how much. However, I can't see it as being particularly useful in comparing the aerodynamics of two bicycles, as was seemingly done here. Was the person doing the test blinded to the bicycles? Was the person evaluating the test blinded to the bicycles? How do we know the head was held the same the entirety of the two runs? Or, a myriad of other potential "problems"? However, if they feel what they are doing is aerodynamically advantageous (whether it actually is or not), this could help them psychologically. So, where is the racing benefit coming from?

Further, was a mistake made in the original calculations, in that the air density was not properly accounted for between the two runs? It would appear that a great deal of attention to detail must be applied here for this to be the least bit accurate, even for the overall number. I haven't quite figured out if this "error" refers to the original post or to a post on another site. Either way, it is one of those "detail" things that can greatly affect the results, which goes go my point.

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Frank,
An original Ironman and the Inventor of PowerCranks

Attorney: "Objection, Your Honor! Asked and answered."

Judge: "Agreed. Move on, Mr. Day."

Hmmm. I've thought a tiny bit about this but not a lot. If this is an out-and-back and you're talking about a turnaround at mid-point, I'd tend to think there isn't much of a problem: the braking and re-acceleration out of the turn gets picked up and you just have to match up the start/finish. If this is a loop course with a lot of turns, and one of the turns is one where you have to brake, then it's slightly harder: basically, I've split the file into segments ending on the turn and matched up the segments. That might be equivalent to what you're suggesting and it's a pain. BTW, that's one of the reasons why I try not to fit on VE peaks. I always begin by doing the VE plots but I don't always end there.

it's a loop course that I'm looking at here - quite good except for one fastish, sharp, blind 90 ...

Tom. A and another person have suggested that approach ... sounded good to me vs. chopping up the file - which would have been my 1st thought. Now that zone must be truly flat of course or another error will be introduced. The loop is 4km and I figure maybe 25-50m to brake ...

edit: but I will admit I've not thought about it as much as I should have!! Setting the VE flat during braking would be fine if braking were the only source of slowing --- but we are very likely to be coasting as well -- so the drop in speed due to drop in power which should be captured will not via this method. Or am I thinking too much?

Darn Tom and his half-pipe!

Exactly. Isn't that the point? Faster is faster, right? :-)



That just indicates that you don't fully understand the approach and the underlying physics of what is being measured. No big deal, a lot of people don't...



What would the purpose of the blinding be (besides perhaps causing the rider to crash)? Remember, the calculation is based on the recorded values of speed and power; Nothing else. Are you thinking that there would be some sort of "placebo effect" in the rider being able to telekinetically change the speed vs. power recording? You might want to think that one through again...




In this case, you'll just have to trust me ;-)

Seriously, what you point out is valid, but is just a part of using good experimental technique. To give you an idea of how well I can apparently "hold a position", in the past I've been able to get repeatability of ~.001 to .002 m^2 between separate runs of the same configuration during a session. Another thing to remember is that the "visual" nature of the technique allows one to see when things "aren't quite right" in that the laps won't be as consistent as they should be (i.e. the virtual elevation calculation "peaks and valleys" will vary inordinately). If you've read the whole thread you'll see that it was the excess variability in the second P3C run that I did which caused me to basically "toss" that run.



Such as? I believe we've covered all the major ones...as Andy says "Asked and answered".



Ummm...from going faster?



No. The ambient atmospheric values in the original posting are correct.

The error was made in that I posted a screenshot of an analysis of a TT I had run in comparing my spreadsheet to another rider's spreadsheet using his values. We were comparing them and I forgot to change the ambient values to the one's I had used. Even so, that "error" only changed the calculated CdA value by .002 m^2.



It could...if you were WAY off on the inputs. But, as I said above, even that "error" only changed the result by .002 m^2...which is ~ the typical precision quoted for wind tunnel testing.

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http://bikeblather.blogspot.com/

I'm shocked...shocked, I say! Frank Day not understanding physics?!? Next thing you'll be telling me is that the Easter Bunny isn't real! ;-)

(BTW, is it just me, or do others here often wonder whether Frank Day and Tom Kunich are related?)

Actually, even just a "quarter pipe" with braking on the bottom portion (i.e. flat) should work pretty well...that might expand your course possibilities...

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http://bikeblather.blogspot.com/

Yes. Which is why I stated it looked like a great technique for the individual to assess themselves.
Ugh, the purpose of blinding is to prevent the imposition of inadvertent bias into the results. Perhaps the rider "wants" the P3 to test better so while on it does things with their head, body, steering, or something else, without being consciously aware of same, that they "know" will make it look better or the P2 to look worse.
Well, since you were not blinded to the tests we can't know if there is any inadvertent bias that you injected to the results based upon your expectations. I don't doubt you did not intend to do so. But, it cannot be categorically excluded that it did not occur because you say "trust me". It is simply good experimental design. Now, it is not always possible to blind the participants to a study. For instance, people certainly know the difference between PowerCranks and regular cranks. But, without blinding the results are always somewhat more suspect when compared to blinding or double blinding. For instance, a current PC study going on right now the person doing the testing is blinded at to what person is doing what. The participants are not blinded, obviously, but the tester/evaluator is. We think that will make for a stronger study. In this instance it seems no one was blinded to anything.
see above
Let me rephrase the question, your honor. Where is the "going faster" coming from?

But, that is not the only potential source of error. How about the accuracy or precision of the power meter? Or, the potential positioning changes? All these potential errors can add up to be large (if they are all in the same direction) or small (if they are all in different directions). Unfortunately, one cannot tell on a single trial.

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Frank,
An original Ironman and the Inventor of PowerCranks

Attorney: "Your Honor!!"

Judge: "Say no more. Move on, Mr. Day, or I will find you in contempt of court."

Frank RTFM

Now why should he do that, when he can just sit on the sidelines and ask inane questions that have already been addressed ad nauseum? :-)

But, that is not the only potential source of error. How about the accuracy or precision of the power meter?

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Tom used the same power meter so even if there was an error in the meter it wouldn't explain the difference between the estimated CdAs.[/reply]

i admit i haven't read all the posts......but how exactly can the positions be identical?

i would assume the two frames have different TT/ST/HT/Seatstay/BB drop lengths and angles, so depending on a person's body dimensions couldn't this vary wildly, anywhere from way better to way worse?

Yes, why indeed? Somewhat similar to answering the same old stuff about PC's coming from those who have never tried them, let alone trained on them yet who seemingly claim to know what they can or cannot do. :-)

Here is my problem in a nutshell. Supposedly this result shows that the aerodynamic improvement seen changing from one Cervelo bike to the P3C would predict a 2 minute savings in a 40K TT. I presume that people have actually raced on this upgraded bicycle like at places like Kona. How come this improvement is not being reflected in the times there?

This result just seems so different than the racing reality. Where is this so-called difference being seen in the real world?

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Frank,
An original Ironman and the Inventor of PowerCranks

Oh, I dunno...maybe Tom actually took the time to, e.g., raise or lower the seat and/or move it back and forth to make certain that they were? Just a thought....

Judge: "Okay, Mr. Day, that's it! I'm finding you in contempt of court and fining you $1000 for repeatedly asking questions that have already been answered in this thread. Do it again, and I'll have the bailiff lock you up!"

gee thanks for the jack ass answer to my legitimate question....did you perhaps take the time to read the second part of my question?

I did.

I have to agree with you. Even if it was a very stupid question, Andrew Coggan was totally out of line when using such a sarcastic tone. Especially because, being the question so stupid, there is the danger that you are not intelligent enough to understand his sarcasm.

Would this have helped?

[/reply]
I have to agree with you. Even if it was a very stupid question, Andrew Coggan was totally out of line when using such a sarcastic tone. Especially because, being the question so stupid, there is the danger that you are not intelligent enough to understand his sarcasm.[/reply]
didn't sound sarcastic, and didn't at all answer my question. point is there's no way you're going to get the exact same results on two different bikes that have different dimensions and geometries even with the "same position", particularly for riders of different dimensions. especially not 2 minutes worth. maybe it did work for Tom, but not a rule of thumb by any stretch. funny how none of this applies in the real world

I have to agree with you. Even if it was a very stupid question, Andrew Coggan was totally out of line when using such a sarcastic tone. Especially because, being the question so stupid, there is the danger that you are not intelligent enough to understand his sarcasm.[/reply]
didn't sound sarcastic, and didn't at all answer my question. point is there's no way you're going to get the exact same results on two different bikes that have different dimensions and geometries even with the "same position", particularly for riders of different dimensions. especially not 2 minutes worth. maybe it did work for Tom, but not a rule of thumb by any stretch. funny how none of this applies in the real world[/reply] So despite talking about the rider's position, and how the geometries of the bikes may have been different, by "this" you really meant the time differential itself? Now that certainly makes a lot of sense...

I have to agree with you. Even if it was a very stupid question, Andrew Coggan was totally out of line when using such a sarcastic tone. Especially because, being the question so stupid, there is the danger that you are not intelligent enough to understand his sarcasm.[/reply]
didn't sound sarcastic, and didn't at all answer my question. point is there's no way you're going to get the exact same results on two different bikes that have different dimensions and geometries even with the "same position", particularly for riders of different dimensions. especially not 2 minutes worth. maybe it did work for Tom, but not a rule of thumb by any stretch. funny how none of this applies in the real world[/reply] Have you ever recorded the x/y coordinates from the bottom bracket to the seat and from the bottom bracket to the arm rests.

4 numbers are all that is required to set up an identical position on two bikes.

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Erik
Strava

It could, if the precision of the meter is not perfect. Since no measurement device is perfect, we can presume some error could come from this. Only question is, how much? What is the precision of the meter he used?

But, more than this. He is attributing all of the change to the bicycle. This is ludicrous in view of the fact we are talking about humans on a bicycle. It is simply impossible for them to be exactly the same on these different trials. The overall result could be entirely accurate but to attribute the change entirely to the difference in bicycle frames seems a bit of a stretch.[/reply]

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Frank,
An original Ironman and the Inventor of PowerCranks

[/reply] So despite talking about the rider's position, and how the geometries of the bikes may have been different, by "this" you really meant the time differential itself? Now that certainly makes a lot of sense...[/reply]
so you're implying that the height of the head tube makes no difference, the fact that one seat tube and down tube is longer makes no difference, the fact that one seat tube is longer makes no difference........as long as his position is the same who cares? what if he had say chosen a 56cm instead of a 54cm etc frame? it's interesting, but certainly doesn't tell the whole story, and certainly not for every rider, just in the specific instance. which is fine, i was just trying to bring up a possible conflict when applying this broad spectrum

[/reply] Have you ever recorded the x/y coordinates from the bottom bracket to the seat and from the bottom bracket to the arm rests.

4 numbers are all that is required to set up an identical position on two bikes.[/reply]
yes, i'm agreeing you can achieve the same body position (maybe i should have worded it differently in the first place) but that same body position isn't going to apply the same on a totally different frame.

for a real world example, i went from a P3C last year to an Al P3 this year just to save some money, identical body position, and i'm definitely not slower

So despite talking about the rider's position, and how the geometries of the bikes may have been different, by "this" you really meant the time differential itself? Now that certainly makes a lot of sense...[/reply] so you're implying that the height of the head tube makes no difference, the fact that one seat tube and down tube is longer makes no difference, the fact that one seat tube is longer makes no difference........as long as his position is the same who cares? what if he had say chosen a 56cm instead of a 54cm etc frame? it's interesting, but certainly doesn't tell the whole story, and certainly not for every rider, just in the specific instance. which is fine, i was just trying to bring up a possible conflict when applying this broad spectrum[/reply]
I agree. If the bikes are not exactly the same, and only the positions are the same, what is the point of all this testing??? Seriously...

[/reply] But, more than this. He is attributing all of the change to the bicycle. This is ludicrous in view of the fact we are talking about humans on a bicycle. It is simply impossible for them to be exactly the same on these different trials. The overall result could be entirely accurate but to attribute the change entirely to the difference in bicycle frames seems a bit of a stretch.[/reply]
that's what i'm trying to say

[/reply]
I agree. If the bikes are not exactly the same, and only the positions are the same, what is the point of all this testing??? Seriously...[/reply] your attempts at sarcasm are interesting, but i think you're missing my point. i'm talking about the geometries of the bikes, not the tube shapes etc. no doubt the P3C tube shapes are likely faster, but i'm talking about tube lengths etc. so you think if the P3C's tubes were 50m long they'd still be faster than the P2K's just bc they're better shaped?

Have you ever recorded the x/y coordinates from the bottom bracket to the seat and from the bottom bracket to the arm rests.

4 numbers are all that is required to set up an identical position on two bikes.[/reply]
yes, i'm agreeing you can achieve the same body position (maybe i should have worded it differently in the first place) but that same body position isn't going to apply the same on a totally different frame.

for a real world example, i went from a P3C last year to an Al P3 this year just to save some money, identical body position, and i'm definitely not slower[/reply] I wouldn't even agree that one can achieve the "same" body position. One can achieve "similar body positions". but the same (identical) position, no way. How much the shoulders sag, the neck sags, the finger positioning, are all going to vary from ride to ride and even during the same ride. It is my understanding that wind tunnel testing has shown that very small differences in body position can have quite large influences on drag, both positively and negatively. Even though attempts were made to keep body position similar, it is impossible to know that they are. It simply is not reasonable to attribute all these aerodynamic changes to the bicycle frame alone.

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Frank,
An original Ironman and the Inventor of PowerCranks

again please RTFM over and over until you can recite it by chapter and verse. The quiz will be on Monday .. But to make one point, each trial is effectively multiple trials (laps) and us visual monkeys are darned good at picking out patterns that match (or do not!). Something about the jungle I suppose ... So while I did query the magnitude of the delta, the noise ain't nearly as bad you suggest. Repeatability is ~0.001 m^2 or ~1W. Yes, that implies the PM is repeatable to around 1W. I have about five million step tests vs. my PT Pro and CT and two million with my SRM and CT from 150 to 450W and they show the same thing. Repeatability is considerably better than the stated accuracy. I have no problem with that ... Anyhow, carry on ...

[/reply] I wouldn't even agree that one can achieve the "same" body position. One can achieve "similar body positions". but the same (identical) position, no way. How much the shoulders sag, the neck sags, the finger positioning, are all going to vary from ride to ride and even during the same ride. It is my understanding that wind tunnel testing has shown that very small differences in body position can have quite large influences on drag, both positively and negatively. Even though attempts were made to keep body position similar, it is impossible to know that they are. It simply is not reasonable to attribute all these aerodynamic changes to the bicycle frame alone.[/reply]
again i agree.......when Cobb talks about a long drink straw being much less aero than a short one or cables being very non-aero etc, you know tiny changes make the difference. there is no way a human could completely accurately have the exact same overall position, nor can you tell me tube dimensions, materials, etc don't play a factor.

i'm not knocking Tom at all for doing this, it is pretty interesting, just saying it's no rule of thumb

I will accept your assessment of the accuracy of this result. Do you believe it is correct to assign all the change seen to the bicycle?

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Frank,
An original Ironman and the Inventor of PowerCranks