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Re: Something borrowed...something FAST! [RChung] [ In reply to ]
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1. Head tube is one of the areas that frame builders have spent some time cleaning up -- this is one of the reasons you might worry that attaching an iBike there could mess up the flow.

Indeed - and that's one of the reasons I mentioned round head tubes. Unfortunately there aren't too many easy ways to mount an ibike that don't interfere with the flow to some extent. I've considered some sort of aero-stalk extending above the rear of the saddle, or attached to the fork and sticking out towards the side, but I'm worried about the vibration issues.

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2. Yaw is an interesting issue. I don't have any experience with this, of course, but I was thinking that if the measured wind speed varied with ground speed then if you can assume the direction was constant you could back out the yaw.

Based on my dabblings in boundary-layer meteorology, I don't think the direction will be constant enough. I could well be mistaken, but my gut feel is that this would only work in some very special circumstances - like flat terrain with uniform surface roughness for considerable distance upstream and down.
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Re: Something borrowed...something FAST! [eb] [ In reply to ]
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2. Yaw is an interesting issue. I don't have any experience with this, of course, but I was thinking that if the measured wind speed varied with ground speed then if you can assume the direction was constant you could back out the yaw.

Based on my dabblings in boundary-layer meteorology, I don't think the direction will be constant enough. I could well be mistaken, but my gut feel is that this would only work in some very special circumstances - like flat terrain with uniform surface roughness for considerable distance upstream and down.
FWIW, the weather vane on our (head-tube-mounted) "whirlgig" device constantly changed direction - it would occasionally settle down and remain in a ~30 deg sector, but only at high speeds/when there was no traffic going past (testing took place using the shoulder of a rolling, multi-lane highway).
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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Another big question is how the ibike responds to yaw. The makers say they've never tested it.

Seriously? I would have thought that they'd have put significant effort into ensuring that the shape of the port was such that the readings were as independent of yaw angle as possible.

Yes, seriously. I emailed them last year and asked a number of wind and yaw related questions. I was very surprised, too.
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Re: Something borrowed...something FAST! [eb] [ In reply to ]
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I know this is a little off topic, but I heard that there's a rumour going around that Powercranks was about to buy out iBike.
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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2. Yaw is an interesting issue. I don't have any experience with this, of course, but I was thinking that if the measured wind speed varied with ground speed then if you can assume the direction was constant you could back out the yaw.

Based on my dabblings in boundary-layer meteorology, I don't think the direction will be constant enough. I could well be mistaken, but my gut feel is that this would only work in some very special circumstances - like flat terrain with uniform surface roughness for considerable distance upstream and down.
FWIW, the weather vane on our (head-tube-mounted) "whirlgig" device constantly changed direction - it would occasionally settle down and remain in a ~30 deg sector, but only at high speeds/when there was no traffic going past (testing took place using the shoulder of a rolling, multi-lane highway).

Very Interesting. That's mostly consistent with what I've observed using a yarn tuft on a shifter tip. Traffic really changes things!

Most of our CdA calculations assume a homogeneous windfield, and that is an enormous assumption that clearly is not strictly correct. The question is how much the lack of homogeneity affects testing results - and I don't pretend to have a good handle on that. A good start would be to measure the 2D windfield (at representative time and length scales), back out CdA as a function of yaw, and compare to tunnel measurements. Any volunteers? ;^>
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Re: Something borrowed...something FAST! [Karl Rove] [ In reply to ]
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I know this is a little off topic, but I heard that there's a rumour going around that Powercranks was about to buy out iBike.

Hey! Careful there! :-)

I like my ibike. It's a cool gadget.

I like my powermeter even more.
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Re: Something borrowed...something FAST! [eb] [ In reply to ]
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Most of our CdA calculations assume a homogeneous windfield, and that is an enormous assumption that clearly is not strictly correct. The question is how much the lack of homogeneity affects testing results - and I don't pretend to have a good handle on that. A good start would be to measure the 2D windfield (at representative time and length scales), back out CdA as a function of yaw, and compare to tunnel measurements. Any volunteers? ;^>

We performed this experiment about 10 y ago. What we found was that we could predict the power requirement of cycling outdoors under very windy, i.e., non-homogeneous flow, conditions to w/in +/- 2 W based on wind tunnel measurements made under (obviously) much more homogenous flow conditions. The corollary to this is that estimates of CdA obtained in the field are essentially just as accurate (although usually not quite as precise) as wind tunnel tests.
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Re: Something borrowed...something FAST! [RChung] [ In reply to ]
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Once the main touch points are fixed, the remaining thing to worry about is head position.

In addition to head position, what about sliding forward or backward on the seat? What about scapular retraction or protraction? What about increasing or decreasing natural spinal curvature? Could these things affect aerodynamics? Do you know what the magnitude of these changes would be, for Tom, on a P3C and P2K? I'm just trying to get an idea of the potential error that could be introduced by these changes in position.

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small changes in head position will affect variance of the estimate, not its location.


You seem to trust that Tom did his best to maintain the same head position on both bikes, and I'm sure he did try, but from your comments above do I correctly interpret that you are ignoring the possibility that Tom consistently held a different head position on the two bikes? You seem to be allowing for the possibility that he moved his head around more on the P3C, but not that he consistently held his head in a different position. Please correct me if I've misinterpreted.

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Tom has worked on his position quite a bit which is evident by his ability to repeat his CdA on his reference frame to about 1%. Since his estimated CdA in this test matches his estimated CdA from other tests, we know he wasn't sitting up in order to make the P2K look bad.
Tom has worked to optimise the repeatability of his CdA, and the data suggests that his position on the P2K in the P2K vs P3C test is similar to his previously reported position. So, as you say, we know he wasn't sitting up on the P2K compared to the position used to produce previously reported data. On the other hand, do we know that this highly repeatable CdA is the very best that Tom could possibly achieve using the contact points that were used for both the P2K and P3C? If it's not, then there would be scope for him to improve his CdA on the P3C.


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Parameter location is determined by the tilt of the profiles; good precision produces lap profiles that look identical, while poor precision distorts the lap profiles. I don't have the data, only the lap profiles from the plots, but I'd say the precision of the P3C laps is only slightly larger than for the P2K laps.
If he consistently used a different position on the P3C to reduce CdA, would this be obvious from the data?
Last edited by: donm: May 28, 08 1:26
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Re: Something borrowed...something FAST! [donm] [ In reply to ]
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"On the other hand, do we know that this highly repeatable CdA is the very best that Tom could possibly achieve using the contact points that were used for both the P2K and P3C?"

Did you miss the part where Tom mentions his refined and "optimized" position over the last few years? It's not like he just woke up a couple of weeks ago and decided to start playing with watts, CdA, etc... Seems to me this "potential source of bias" has already been addressed adequately...as have most of the others, Don.

Because it is a human factor, I think we're all granting you and the others the "possibility" that SOME level of positional difference MAY have occurred. But first, as Chung points out...the potential differences would cover MAYBE 5% or so of the total difference. Second, its also been noted that Tom's repeatability and attention to detail mitigates even that ~5% possibility. (forgive me for not going back in the thread and doing the actual % math on numbers Tom, AC, and RChung have already put out there...but they are there).

So with all else either being the same (wheels, bars, etc.) and the environmental factors calculated for each run to ensure apples to apples numbers...what else is there to conclude? I mean, its not like Cervelo changed the way the cables route on the bikes to, say, enter behind the stem, that might even account for another 2-3% of the CdA difference....

Sorry...but unless someone can point out where the equations supporting the Chung method are just plain wrong...I'm going to believe Tom's numbers are a reasonable approximation of the delta between these two frames...especially because they are closely corroborated by numbers from others' tests using the same method.
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Re: Something borrowed...something FAST! [ In reply to ]
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Look, folks, I think it's obvious that Tom has overestimated the drag savings of a P3C vs. a P2k. Why? Well, because the magnitude of the difference he reports is about the same as what has been measured for a fork, front wheel, and a long, round PVC pipe "head tube" alone! There's just no way that refining the shape of every single tube on the frame could result in such a large difference...or could it? ;-)

Andy ("if you think a P3C is fast, try it w/o gears") Coggan
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Re: Something borrowed...something FAST! [TriBriGuy] [ In reply to ]
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Did you miss the part where Tom mentions his refined and "optimized" position over the last few years?
Optimisation of position for time trialling involves a balance of minimising CdA and maximising power output. Therefore it doesn't necessarily follow that Tom's "optimised" position is the one with the lowest possible CdA. It may be, though, if Tom has chosen a position that is optimal from an aerodynamic standpoint only.

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Re: Something borrowed...something FAST! [donm] [ In reply to ]
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You seem to trust that Tom did his best to maintain the same head position on both bikes, and I'm sure he did try, but from your comments above do I correctly interpret that you are ignoring the possibility that Tom consistently held a different head position on the two bikes? You seem to be allowing for the possibility that he moved his head around more on the P3C, but not that he consistently held his head in a different position. Please correct me if I've misinterpreted.

No, I'm not ignoring it: I specifically brought that possibility up early on. I said that if there were a change, it would have to be a consistent change, held throughout the trial: otherwise, were he shifting about (his head, or on the seat, or moving his spine or neck) it would appear as distortions in the profiles. So any putative change would had to have been held constant. But, as I pointed out in that same early response, why should Tom have hit on exactly the consistently held position change that decreased CdA from the very moment he got on the P3C? How is it that this spontaneous hypothetical change decreased his CdA rather than increasing it, and decreased it by .023 m^2?

So, we know he had great repreatability on the P2K. We know he didn't sit up on the P2K, artificially inflating its value. We know he meticulously transferred the touchpoints to the P3C. We know he used exactly the same wheels and the same PM. We know he couldn't have been squirming. We know that during the course of the test runs themselves he could not have known what the CdA was, or the effect of any positional change on CdA.
We know the only equipment change was the frame. Your theory is that the lack of blinding led him to subconsciously alter his position from the moment he got on the P3C in a way that decreased his CdA by an order of magnitude greater than his usual precision.

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Re: Something borrowed...something FAST! [RChung] [ In reply to ]
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You seem to trust that Tom did his best to maintain the same head position on both bikes, and I'm sure he did try, but from your comments above do I correctly interpret that you are ignoring the possibility that Tom consistently held a different head position on the two bikes? You seem to be allowing for the possibility that he moved his head around more on the P3C, but not that he consistently held his head in a different position. Please correct me if I've misinterpreted.

No, I'm not ignoring it: I specifically brought that possibility up early on. I said that if there were a change, it would have to be a consistent change, held throughout the trial: otherwise, were he shifting about (his head, or on the seat, or moving his spine or neck) it would appear as distortions in the profiles. So any putative change would had to have been held constant. But, as I pointed out in that same early response, why should Tom have hit on exactly the consistently held position change that decreased CdA from the very moment he got on the P3C? How is it that this spontaneous hypothetical change decreased his CdA rather than increasing it, and decreased it by .023 m^2?

So, we know he had great repreatability on the P2K. We know he didn't sit up on the P2K, artificially inflating its value. We know he meticulously transferred the touchpoints to the P3C. We know he used exactly the same wheels and the same PM. We know he couldn't have been squirming. We know that during the course of the test runs themselves he could not have known what the CdA was, or the effect of any positional change on CdA.
We know the only equipment change was the frame. Your theory is that the lack of blinding led him to subconsciously alter his position from the moment he got on the P3C in a way that decreased his CdA by an order of magnitude greater than his usual precision.

My position is, there must be something else we don't know. Why? the decrease seems too large. Why? two reasons come quickly to mind. It appears he was not able to achieve this same CdA when he actually did the time-trial using, supposedly, the same set up. And, the P3C hasn't shown itself to be that much better than other frames in actual time-trial reslts, which should be easily discernible if it were that much beter than other frames.

--------------
Frank,
An original Ironman and the Inventor of PowerCranks
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Re: Something borrowed...something FAST! [RChung] [ In reply to ]
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You seem to trust that Tom did his best to maintain the same head position on both bikes, and I'm sure he did try, but from your comments above do I correctly interpret that you are ignoring the possibility that Tom consistently held a different head position on the two bikes? You seem to be allowing for the possibility that he moved his head around more on the P3C, but not that he consistently held his head in a different position. Please correct me if I've misinterpreted.

No, I'm not ignoring it: I specifically brought that possibility up early on. I said that if there were a change, it would have to be a consistent change, held throughout the trial: otherwise, were he shifting about (his head, or on the seat, or moving his spine or neck) it would appear as distortions in the profiles. So any putative change would had to have been held constant. But, as I pointed out in that same early response, why should Tom have hit on exactly the consistently held position change that decreased CdA from the very moment he got on the P3C? How is it that this spontaneous hypothetical change decreased his CdA rather than increasing it, and decreased it by .023 m^2?

So, we know he had great repreatability on the P2K. We know he didn't sit up on the P2K, artificially inflating its value. We know he meticulously transferred the touchpoints to the P3C. We know he used exactly the same wheels and the same PM. We know he couldn't have been squirming. We know that during the course of the test runs themselves he could not have known what the CdA was, or the effect of any positional change on CdA.
We know the only equipment change was the frame. Your theory is that the lack of blinding led him to subconsciously alter his position from the moment he got on the P3C in a way that decreased his CdA by an order of magnitude greater than his usual precision.

Okay, based on your summary above, I'm willing to acknowledge that my theory is unlikely but, of course, not impossible. Since Tom was a one man band doing this test, an awful lot of the validity of the study rests on his credibility and assumed objectivity.

Also, out of curiosity, what would be the magnitude of the change in CdA for Tom that could be attributed to a systematic, consistent change in head position, scapular retraction/protraction, sliding fore or aft in the saddle, and altered spinal curvatures?
Last edited by: donm: May 28, 08 7:23
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Re: Something borrowed...something FAST! [RChung] [ In reply to ]
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Also, just to clarify, my position has never been that changes in Tom's body position would account for the entire CdA delta, but rather that they could account for some, and therefore invalidate "rule of thumb" estimates of per km time savings.
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Re: Something borrowed...something FAST! [Frank Day] [ In reply to ]
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It appears he was not able to achieve this same CdA when he actually did the time-trial using, supposedly, the same set up.

Again, you're ignoring the fact that Tom made the (conservative) assumption that wind conditions were exactly the same both years.

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the P3C hasn't shown itself to be that much better than other frames in actual time-trial reslts, which should be easily discernible if it were that much beter than other frames.

You're also ignoring my wife's pursuit performance (essentially the same time for 3 km despite an ~80 W reduction in average power), not to mention the fact that the relevant comparison here is P3C vs. P2k, not P3C vs. all other bikes.
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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It appears he was not able to achieve this same CdA when he actually did the time-trial using, supposedly, the same set up.

Again, you're ignoring the fact that Tom made the (conservative) assumption that wind conditions were exactly the same both years.

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the P3C hasn't shown itself to be that much better than other frames in actual time-trial reslts, which should be easily discernible if it were that much beter than other frames.

You're also ignoring my wife's pursuit performance (essentially the same time for 3 km despite an ~80 W reduction in average power), not to mention the fact that the relevant comparison here is P3C vs. P2k, not P3C vs. all other bikes.
What wind assumption would he have to make to get the CdA to turn out the same as measured?

Are you telling us that you wifes numbers are due entirely to the P3C frame? Why would a frame cause her to lose power?

--------------
Frank,
An original Ironman and the Inventor of PowerCranks
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Re: Something borrowed...something FAST! [Frank Day] [ In reply to ]
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What wind assumption would he have to make to get the CdA to turn out the same as measured?

Well duh: that the wind was less favorable this year (and in fact he stated that he thought that it was).

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Are you telling us that you wifes numbers are due entirely to the P3C frame? Why would a frame cause her to lose power?

No, I'm saying that the lack of difference in speed despite a much lower power is largely due to the frame. The lower power itself appears to be at least partially the result of a medical issue.
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Re: Something borrowed...something FAST! [donm] [ In reply to ]
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Also, out of curiosity, what would be the magnitude of the change in CdA for Tom that could be attributed to a systematic, consistent change in head position, scapular retraction/protraction, sliding fore or aft in the saddle, and altered spinal curvatures?

[and, combined from a different post,]

Also, just to clarify, my position has never been that changes in Tom's body position would account for the entire CdA delta, but rather that they could account for some, and therefore invalidate "rule of thumb" estimates of per km time savings.

Tom would have to be the one to answer the first question. I think he's spent a couple of years refining his position but I don't know what it started out at or the changes he's made to get to where he is.

As for the second issue, I pointed to analyticcycling.com where you can do the exact calculation to figure out how a change in m^2 of CdA translates into a change in seconds/km. At Tom's speed and CdA, I think you'll find that the "change in CdA of .01 means about 1 sec/km" rule of thumb is pretty good. As a reminder, Tom's precision on his CdA estimate is around .002 m^2.
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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What wind assumption would he have to make to get the CdA to turn out the same as measured?

Well duh: that the wind was less favorable this year (and in fact he stated that he thought that it was).

Here are the VE plots for both races (2007 and 2008) using the "assumed" Crr and CdA as measured in separate testing. A couple of things jump out at me; first you can see variability in the wind strength/direction just during the run by comparing the first 10K to the last 10K (which are along the same leg of the course). Secondly, you can see that the wind direction and strength in 2007 made those same 2 segments appear "downhill", when in actuality the grade was uphill. Here's a link to the course (w/elevation) on MapMyRide:

http://www.mapmyride.com/...angeles/168322294943





I don't know...which conditions do you think were faster overall?

http://bikeblather.blogspot.com/
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Re: Something borrowed...something FAST! [Frank Day] [ In reply to ]
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As regards the PC studies (Luttrell, Dixon), there is no comparison. They all had n's greater than one, had controls, and reached statistical significance regarding what was being studied. While no study is perfect, that is what a scientific study is supposed to do.

Frank:

You didn't respond to my question in post #359 above so I thought I'd ask again.

Could you describe the controls for the Dixon study, and the hypothesis that was tested?

Could you point to the research -->prior<-- to the Luttrell study that made them think that PCs would affect efficiency?
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Re: Something borrowed...something FAST! [Tom A.] [ In reply to ]
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Here are the VE plots for both races (2007 and 2008) using the "assumed" Crr and CdA as measured in separate testing.

Just to clarify, you were using .228 for 2007 and .205 for 2008?

Cool.

(Graphics geek comment: comparison would be easier if you plotted both lines on same figure so the y-axis scale would be the same).
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Re: Something borrowed...something FAST! [RChung] [ In reply to ]
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(Graphics geek comment: comparison would be easier if you plotted both lines on same figure so the y-axis scale would be the same).

Even nicer would be to plug in the actual profile and calculate the effective head/tail wind as a function of distance (although to do so would require assuming that CdA is independent of yaw angle, when it is not).
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Re: Something borrowed...something FAST! [RChung] [ In reply to ]
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Here are the VE plots for both races (2007 and 2008) using the "assumed" Crr and CdA as measured in separate testing.

Just to clarify, you were using .228 for 2007 and .205 for 2008?

Cool.

(Graphics geek comment: comparison would be easier if you plotted both lines on same figure so the y-axis scale would be the same).

Correct on 2008. For the 2007, I used .225 (what I measured for my Soloist with a borrowed 808 front). Crrs were .0040 and .0045 (temp adjustment) for 2007 and 2008 respectively.

On the comparison plot...maybe later, I might even include the actual elevation as well. Would that be better, or would it start getting "too busy"?

http://bikeblather.blogspot.com/
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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Look, folks, I think it's obvious that Tom has overestimated the drag savings of a P3C vs. a P2k. Why? Well, because the magnitude of the difference he reports is about the same as what has been measured for a fork, front wheel, and a long, round PVC pipe "head tube" alone! There's just no way that refining the shape of every single tube on the frame could result in such a large difference...or could it? ;-)

Andy ("if you think a P3C is fast, try it w/o gears") Coggan
You're such a hater...
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