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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).

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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.
80 watts improvement is "largely" due to the frame. OK folks. Dr. Coggan said that, not me.

--------------
Frank,
An original Ironman and the Inventor of PowerCranks
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Re: Something borrowed...something FAST! [roady] [ 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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Re: Something borrowed...something FAST! [Tom A.] [ In reply to ]
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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"?

Depends on what you're trying to show. If what you're trying to show is that the wind was different in 2007 and 2008, it's not essential. I think I'd put it in the "nice if you have time" bucket.
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Re: Something borrowed...something FAST! [Frank Day] [ In reply to ]
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80 watts improvement is "largely" due to the frame. OK folks. Dr. Coggan said that, not me.

Yes, I did. I'd break it all down for you, but it would be a waste of time since you'd apparently have felt quite at home in the White House in the first week after Hurricane Katrina. ;-)
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Re: Something borrowed...something FAST! [RChung] [ In reply to ]
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We know he used exactly the same wheels and the same PM.

There is one potential variable here (a big one): the accuracy of the PM. Even in mild temperatures, I've noticed that it takes a while for the PT to fully 'acclimatize', so that it gives a lower reading during the first few steady state efforts (even if the torque is zeroed at the start of the effort). Given that he did the P3C run first, I wonder if the power isn't underestimated on that one?

This has been a consistent observation on my part, after riding around with an SRM and a PT for the better part of the last 3 months. The one nice thing about the SRM is that the zero offset can be checked to measure the actual drift, where as this can't be done on the Power Tap (unless you're using both and comparing the files of the 2 devices, and you've consistently checked the SRM's zero offset during the ride).

I'm not saying that this IS the case, but I do think it's a possibility that some of the difference between the bikes could be PM measurement error.
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Re: Something borrowed...something FAST! [roady] [ In reply to ]
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We know he used exactly the same wheels and the same PM.

There is one potential variable here (a big one): the accuracy of the PM. Even in mild temperatures, I've noticed that it takes a while for the PT to fully 'acclimatize', so that it gives a lower reading during the first few steady state efforts (even if the torque is zeroed at the start of the effort). Given that he did the P3C run first, I wonder if the power isn't underestimated on that one?

This has been a consistent observation on my part, after riding around with an SRM and a PT for the better part of the last 3 months. The one nice thing about the SRM is that the zero offset can be checked to measure the actual drift, where as this can't be done on the Power Tap (unless you're using both and comparing the files of the 2 devices, and you've consistently checked the SRM's zero offset during the ride).

I'm not saying that this IS the case, but I do think it's a possibility that some of the difference between the bikes could be PM measurement error.

Indeed, when tested "straight up" the strain gages of the PowerTap appear to be a bit more temperature-sensitive than those of the SRM. However, you're assuming that Tom's hub wasn't already equilibrated to the environmental temperature when he began his tests, and/or that the temperature was increasing significantly throughout them.
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Re: Something borrowed...something FAST! [roady] [ In reply to ]
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We know he used exactly the same wheels and the same PM.

There is one potential variable here (a big one): the accuracy of the PM. Even in mild temperatures, I've noticed that it takes a while for the PT to fully 'acclimatize', so that it gives a lower reading during the first few steady state efforts (even if the torque is zeroed at the start of the effort). Given that he did the P3C run first, I wonder if the power isn't underestimated on that one?

This has been a consistent observation on my part, after riding around with an SRM and a PT for the better part of the last 3 months. The one nice thing about the SRM is that the zero offset can be checked to measure the actual drift, where as this can't be done on the Power Tap (unless you're using both and comparing the files of the 2 devices, and you've consistently checked the SRM's zero offset during the ride).

I'm not saying that this IS the case, but I do think it's a possibility that some of the difference between the bikes could be PM measurement error.

Can you "ballpark" quantify "a while"? If this helps, I zeroed the PT before heading out my driveway and it took ~5 minutes to get to the venue plus assorted "riding around" on it making sure I was all set before starting the run. The PT is set to "auto zero" and there was plenty of coasting time that would have triggered that.

I'm open to that being a possibility...but I was just wondering if in your experience it was likely based on the above extra information?

http://bikeblather.blogspot.com/
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Re: Something borrowed...something FAST! [RChung] [ 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?
Well, regarding the prior "research" I guess there were probably three they "knew" about. First, was the work I did when I determined that a 40% power improvement claim was reasonable to make. Second, I suspect they did a little pilot study, perhaps on himself before determining what protocol he might want to use. And, third, he probably researched the internet and saw the anecdotal claims people were making regarding the product. His was the first published research study on the product.

Here is what Luttrell wrote that I feel best describes his "hypothesis" for the study. ". . . Because the PowerCranks are designed for training purposes only, it is important to investigate whether any early phase physiological adaptation might occur after utilizing this device. Therefore, the purpose of this study was to compare the effects of 6 weeks of training with PowerCranks to a control group training with normal bicycle cranks on VO2max and AT during a graded exercise test (GXT), and heart rate (HR), oxygen consumption (VO2), respiratory exchange ration (RER), and gross efficiency (GE) during a 1 hour sub-maximal ride."



The Luttrell controls: "Following the pretraining testing, the male subjects were paired in accordance to their pretraining VO2max values. The female subjects were paired together (ed, there were two). One subject of each pair was randomly placed into the control group and trained using regular bicycle cranks (normal ranks), while the other subject was placed into the experimental group and trained using PowerCranks. . . . Subjects were instructed to adhere to their normal diets thorught the training period. Subjects trained 3 dwk at an intensity that corresponded to a workload of 7-% of VO2max (19). Heart rate telemetry units were used to monitor exercise intensity every 5 minutes during training. As the subjects became better trained resistance on the ergometer was increased to maintain HR at the desired training intensity. A cadence of 80 rev-min was maintained during all training sessions. . . . Each pair of subjects was given a similar training schedule in an effort tocontrol training outside the laboratory. Weekly mileage was matched between each pair of cyclists and kept similar throught the study. Subjects completed a weekly training log of total training volume during the study."

I look forward to you comments.

--------------
Frank,
An original Ironman and the Inventor of PowerCranks
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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well, I'm not assuming that--I'm more wondering it out loud. I'll admit though that I looked at the time of the tests though and wondered if the bike went from a warmer car to a cooler outside temperature for the first test.

As far as the PT being 'more sensitive', that's a tricky one. My observation would be 'sort of'. It seems to take longer for the PT to stabilize, but once it HAS stabilized, it seems to be less sensitive to changes in temperature during a ride.
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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80 watts improvement is "largely" due to the frame. OK folks. Dr. Coggan said that, not me.

Yes, I did. I'd break it all down for you, but it would be a waste of time since you'd apparently have felt quite at home in the White House in the first week after Hurricane Katrina. ;-)
I am amazed that you would find my claim that it is possible to increase power about 80 watts (40% of 200) in one season by training differently so "outlandish" when you claim the same improvement can come about largely by simply changing the frame. :-)

--------------
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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80 watts improvement is "largely" due to the frame. OK folks. Dr. Coggan said that, not me.

Yes, I did. I'd break it all down for you, but it would be a waste of time since you'd apparently have felt quite at home in the White House in the first week after Hurricane Katrina. ;-)
I am amazed that you would find my claim that it is possible to increase power about 80 watts (40% of 200) in one season by training differently so "outlandish" when you claim the same improvement can come about largely by simply changing the frame. :-)


Apparently you've failed to absorb the "take home" message of this article by my friends Asker and Jim:

http://www.ncbi.nlm.nih.gov/...anel.Pubmed_RVDocSum

(BTW, please take note that I said "...essentially the same time..." and "...largely due to...". IOW, I did not make the claim that a P3C would allow someone to ride exactly the same 3 km time as on a P2T while producing exactly 80 fewer watts.)
Last edited by: Andrew Coggan: May 28, 08 9:39
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Re: Something borrowed...something FAST! [roady] [ In reply to ]
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well, I'm not assuming that--I'm more wondering it out loud. I'll admit though that I looked at the time of the tests though and wondered if the bike went from a warmer car to a cooler outside temperature for the first test.

Aaah...nope, no car involved. In fact, the bike went from non-heated garage (actually on a stand right near the door), to outdoors, PT zeroed, then ~5 minute ride to venue (with coasting enough to trigger additional auto-zeroing)....

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?
Well, regarding the prior "research" I guess there were probably three they "knew" about. First, was the work I did when I determined that a 40% power improvement claim was reasonable to make. Second, I suspect they did a little pilot study, perhaps on himself before determining what protocol he might want to use. And, third, he probably researched the internet and saw the anecdotal claims people were making regarding the product. His was the first published research study on the product.

Here is what Luttrell wrote that I feel best describes his "hypothesis" for the study. ". . . Because the PowerCranks are designed for training purposes only, it is important to investigate whether any early phase physiological adaptation might occur after utilizing this device. Therefore, the purpose of this study was to compare the effects of 6 weeks of training with PowerCranks to a control group training with normal bicycle cranks on VO2max and AT during a graded exercise test (GXT), and heart rate (HR), oxygen consumption (VO2), respiratory exchange ration (RER), and gross efficiency (GE) during a 1 hour sub-maximal ride."



The Luttrell controls: "Following the pretraining testing, the male subjects were paired in accordance to their pretraining VO2max values. The female subjects were paired together (ed, there were two). One subject of each pair was randomly placed into the control group and trained using regular bicycle cranks (normal ranks), while the other subject was placed into the experimental group and trained using PowerCranks. . . . Subjects were instructed to adhere to their normal diets thorught the training period. Subjects trained 3 dwk at an intensity that corresponded to a workload of 7-% of VO2max (19). Heart rate telemetry units were used to monitor exercise intensity every 5 minutes during training. As the subjects became better trained resistance on the ergometer was increased to maintain HR at the desired training intensity. A cadence of 80 rev-min was maintained during all training sessions. . . . Each pair of subjects was given a similar training schedule in an effort tocontrol training outside the laboratory. Weekly mileage was matched between each pair of cyclists and kept similar throught the study. Subjects completed a weekly training log of total training volume during the study."

I look forward to you comments.

Frank:

I didn't ask about the controls for the Luttrell and Potteiger study. You said that the Dixon study was controlled, so I was asking about that. It appears from the abstract that the Dixon study was not controlled. In addition, the Luttrell study, which was controlled, showed no difference in VO2Max or AT (and the controlled Bohm study showed no difference in power at AT).

As for the Luttrell and Potteiger hypothesis, that still puzzles me. You're saying your "40% power improvement" claim was known to them but instead of testing power they tested GE? What was the basis for that hypothesis?
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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80 watts improvement is "largely" due to the frame. OK folks. Dr. Coggan said that, not me.

Yes, I did. I'd break it all down for you, but it would be a waste of time since you'd apparently have felt quite at home in the White House in the first week after Hurricane Katrina. ;-)
I am amazed that you would find my claim that it is possible to increase power about 80 watts (40% of 200) in one season by training differently so "outlandish" when you claim the same improvement can come about largely by simply changing the frame. :-)


Apparently you've failed to absorb the "take home" message of this article by my friends Asker and Jim:

http://www.ncbi.nlm.nih.gov/...anel.Pubmed_RVDocSum

(BTW, please take note that I said "...essentially the same time..." and "...largely due to...". IOW, I did not make the claim that a P3C would allow someone to ride exactly the same 3 km time as on a P2T while producing exactly 80 fewer watts.)
From the abstract
Quote:
Training is a first and most obvious way to improve power production and was predicted to have the potential to improve 40 km time trial performance by 1 to 10% (1 to 7 minutes) . . . An aerodynamic frame saved the modelled riders 1:17 to 1:44 min:sec. . . . From the analysis in this article it becomes clear that novice cyclists can benefit more from the suggested alterations in position, equipment, nutrition and training compared with elite cyclists. Training seems to be the most important factor, but sometimes large improvements can be made by relatively small changes in body position.
1:17 to 1:44 is the equivalent of 80 watts???? Tom measured an improvement that goes further than the most optimistic improvement predicted for frames in this study, which I presume is the difference between a highly aerodynamic frame and one with no aerodynamic enhancements and he was comparing a highly optimized frame to one that supposedly has some optimization.

I especially enjoyed the part "sometimes large improvements can be made by relatively small changes in body position" as it pertains to this thread.

What part did I fail to absorb or understand?

--------------
Frank,
An original Ironman and the Inventor of PowerCranks
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Re: Something borrowed...something FAST! [Tom A.] [ In reply to ]
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We know he used exactly the same wheels and the same PM.

There is one potential variable here (a big one): the accuracy of the PM. Even in mild temperatures, I've noticed that it takes a while for the PT to fully 'acclimatize', so that it gives a lower reading during the first few steady state efforts (even if the torque is zeroed at the start of the effort). Given that he did the P3C run first, I wonder if the power isn't underestimated on that one?

This has been a consistent observation on my part, after riding around with an SRM and a PT for the better part of the last 3 months. The one nice thing about the SRM is that the zero offset can be checked to measure the actual drift, where as this can't be done on the Power Tap (unless you're using both and comparing the files of the 2 devices, and you've consistently checked the SRM's zero offset during the ride).

I'm not saying that this IS the case, but I do think it's a possibility that some of the difference between the bikes could be PM measurement error.

Can you "ballpark" quantify "a while"? If this helps, I zeroed the PT before heading out my driveway and it took ~5 minutes to get to the venue plus assorted "riding around" on it making sure I was all set before starting the run. The PT is set to "auto zero" and there was plenty of coasting time that would have triggered that.

I'm open to that being a possibility...but I was just wondering if in your experience it was likely based on the above extra information?
Yeah, here's a pretty typical scenario: I leave my house (bike kept inside @ room temperature, 65 degrees or so), and ride for 18 minutes to get to the venue for my 15 minute intervals. Outside temp is about 60 degrees as reported by my SRM. It's midday, the outside temp. stays about the same. I set the ZO on the SRM at the start of the ride, then check it again at 18 minutes (it's drifted about 10 points, and I reset it). I manually zero the PT torque. I start my 15 minute intervals, and they go about like this: SRM: 302/298/305, PT 287/299/304. The first 2-3 minutes of the first interval track almost perfectly, then it starts on a downward slope. There's no coasting during the intervals, btw--but I coast immediately after, and the auto-zero function on the PT kicks in the the torque is re-set. I check the ZO on the SRM after each interval, and it's it's within 1-3 points (1 watt difference).

I should mention that this isn't a single ride--this happens almost every time on this ride, and the colder it is out side, the bigger the difference, and the longer it takes for the PT to stabilize. Even in 60 degree weather, it seems to take about 40 minutes for it to really stabilize. Of course, if you're doing shorter efforts, the auto-zero works, and there are no issues.
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Re: Something borrowed...something FAST! [RChung] [ 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?
Well, regarding the prior "research" I guess there were probably three they "knew" about. First, was the work I did when I determined that a 40% power improvement claim was reasonable to make. Second, I suspect they did a little pilot study, perhaps on himself before determining what protocol he might want to use. And, third, he probably researched the internet and saw the anecdotal claims people were making regarding the product. His was the first published research study on the product.

Here is what Luttrell wrote that I feel best describes his "hypothesis" for the study. ". . . Because the PowerCranks are designed for training purposes only, it is important to investigate whether any early phase physiological adaptation might occur after utilizing this device. Therefore, the purpose of this study was to compare the effects of 6 weeks of training with PowerCranks to a control group training with normal bicycle cranks on VO2max and AT during a graded exercise test (GXT), and heart rate (HR), oxygen consumption (VO2), respiratory exchange ration (RER), and gross efficiency (GE) during a 1 hour sub-maximal ride."



The Luttrell controls: "Following the pretraining testing, the male subjects were paired in accordance to their pretraining VO2max values. The female subjects were paired together (ed, there were two). One subject of each pair was randomly placed into the control group and trained using regular bicycle cranks (normal ranks), while the other subject was placed into the experimental group and trained using PowerCranks. . . . Subjects were instructed to adhere to their normal diets thorught the training period. Subjects trained 3 dwk at an intensity that corresponded to a workload of 7-% of VO2max (19). Heart rate telemetry units were used to monitor exercise intensity every 5 minutes during training. As the subjects became better trained resistance on the ergometer was increased to maintain HR at the desired training intensity. A cadence of 80 rev-min was maintained during all training sessions. . . . Each pair of subjects was given a similar training schedule in an effort tocontrol training outside the laboratory. Weekly mileage was matched between each pair of cyclists and kept similar throught the study. Subjects completed a weekly training log of total training volume during the study."

I look forward to you comments.

Frank:

I didn't ask about the controls for the Luttrell and Potteiger study. You said that the Dixon study was controlled, so I was asking about that. It appears from the abstract that the Dixon study was not controlled. In addition, the Luttrell study, which was controlled, showed no difference in VO2Max or AT (and the controlled Bohm study showed no difference in power at AT).

As for the Luttrell and Potteiger hypothesis, that still puzzles me. You're saying your "40% power improvement" claim was known to them but instead of testing power they tested GE? What was the basis for that hypothesis?
Dixon's subjects were self controlled. The study was conducted at the end of the racing season. The hypothesis was that the performance of each participant would be at a maximum at that time they started the study and that the expectation is that these participants would normally either maintain or lose performance at this time of the year normally using the training intensity studied, such that any improvement seen could be assumed to come from the PC intervention.

You would have to ask Luttrell why they did what they did if you really want to know for sure but as I read it, our power claims required a 6 to 9 months intervention, so they only decided to look at early changes that might eventually lead to such improvements. If no changes were seen in 6 weeks it is unlikely they would suddenly appear at 3 or 6 months. Such a study is much more doable, don't you think.

--------------
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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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.
But that rule of thumb rests on the assumption that the entire CdA delta between the P2K and P3C is attributable to the frame alone, rather than to any systematic changes in Tom's position while on the two frames, correct?
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Re: Something borrowed...something FAST! [donm] [ In reply to ]
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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.
But that rule of thumb rests on the assumption that the entire CdA delta between the P2K and P3C is attributable to the frame alone, rather than to any systematic changes in Tom's position while on the two frames, correct?

???. That rule of thumb depends only the power equation.
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Re: Something borrowed...something FAST! [roady] [ In reply to ]
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Yeah, here's a pretty typical scenario: I leave my house (bike kept inside @ room temperature, 65 degrees or so), and ride for 18 minutes to get to the venue for my 15 minute intervals. Outside temp is about 60 degrees as reported by my SRM. It's midday, the outside temp. stays about the same. I set the ZO on the SRM at the start of the ride, then check it again at 18 minutes (it's drifted about 10 points, and I reset it). I manually zero the PT torque. I start my 15 minute intervals, and they go about like this: SRM: 302/298/305, PT 287/299/304. The first 2-3 minutes of the first interval track almost perfectly, then it starts on a downward slope. There's no coasting during the intervals, btw--but I coast immediately after, and the auto-zero function on the PT kicks in the the torque is re-set. I check the ZO on the SRM after each interval, and it's it's within 1-3 points (1 watt difference).

I should mention that this isn't a single ride--this happens almost every time on this ride, and the colder it is out side, the bigger the difference, and the longer it takes for the PT to stabilize. Even in 60 degree weather, it seems to take about 40 minutes for it to really stabilize. Of course, if you're doing shorter efforts, the auto-zero works, and there are no issues.

That sounds like a problem with your PowerTap to me.
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Re: Something borrowed...something FAST! [roady] [ In reply to ]
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In Reply To:

Yeah, here's a pretty typical scenario: I leave my house (bike kept inside @ room temperature, 65 degrees or so), and ride for 18 minutes to get to the venue for my 15 minute intervals. Outside temp is about 60 degrees as reported by my SRM. It's midday, the outside temp. stays about the same. I set the ZO on the SRM at the start of the ride, then check it again at 18 minutes (it's drifted about 10 points, and I reset it). I manually zero the PT torque. I start my 15 minute intervals, and they go about like this: SRM: 302/298/305, PT 287/299/304. The first 2-3 minutes of the first interval track almost perfectly, then it starts on a downward slope. There's no coasting during the intervals, btw--but I coast immediately after, and the auto-zero function on the PT kicks in the the torque is re-set. I check the ZO on the SRM after each interval, and it's it's within 1-3 points (1 watt difference).

I should mention that this isn't a single ride--this happens almost every time on this ride, and the colder it is out side, the bigger the difference, and the longer it takes for the PT to stabilize. Even in 60 degree weather, it seems to take about 40 minutes for it to really stabilize. Of course, if you're doing shorter efforts, the auto-zero works, and there are no issues.

OK...thanks, I'll have to keep that in mind in future testing to see if that's something I observe as well with my unit. Actually, I have some older PT vs. Polar data I might take a look at to see if I observe such changes as well.

Another option would be, rather than making sure I was constantly pedaling throughout each run like I normally do, I can just make sure I coast at some point to trigger the auto-zero within each lap...

http://bikeblather.blogspot.com/
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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Yeah, here's a pretty typical scenario: I leave my house (bike kept inside @ room temperature, 65 degrees or so), and ride for 18 minutes to get to the venue for my 15 minute intervals. Outside temp is about 60 degrees as reported by my SRM. It's midday, the outside temp. stays about the same. I set the ZO on the SRM at the start of the ride, then check it again at 18 minutes (it's drifted about 10 points, and I reset it). I manually zero the PT torque. I start my 15 minute intervals, and they go about like this: SRM: 302/298/305, PT 287/299/304. The first 2-3 minutes of the first interval track almost perfectly, then it starts on a downward slope. There's no coasting during the intervals, btw--but I coast immediately after, and the auto-zero function on the PT kicks in the the torque is re-set. I check the ZO on the SRM after each interval, and it's it's within 1-3 points (1 watt difference).

I should mention that this isn't a single ride--this happens almost every time on this ride, and the colder it is out side, the bigger the difference, and the longer it takes for the PT to stabilize. Even in 60 degree weather, it seems to take about 40 minutes for it to really stabilize. Of course, if you're doing shorter efforts, the auto-zero works, and there are no issues.

That sounds like a problem with your PowerTap to me.

You'd think so--but suppose that same scenario was duplicated while using 2 different computer heads, and 4 different wheels?
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Re: Something borrowed...something FAST! [Frank Day] [ In reply to ]
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1:17 to 1:44 is the equivalent of 80 watts????

77-104 s/40 km = 1.9-2.6 s/km = approx. 19-26 W. However, that's 1) not at pursuit speed, and 2) is based on the benefit provided by now-outdistanced aerodynamic frames, e.g., the Hooker. You're also again overlooking the fact that I said "...essentially..." and "...largely...", not "...exactly..." and "...entirely...", but no surprise there: as I said before, it's crystal-clear that you'd have fit in quite well in the Bush Administration. ;-)

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What part did I fail to absorb or understand?

That for trained cyclists, there's quite often more to be gained by focussing on reducing aerodynamic drag than there is to be gained by attempting to further increase power output.
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Re: Something borrowed...something FAST! [Andrew Coggan] [ In reply to ]
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Yeah, here's a pretty typical scenario: I leave my house (bike kept inside @ room temperature, 65 degrees or so), and ride for 18 minutes to get to the venue for my 15 minute intervals. Outside temp is about 60 degrees as reported by my SRM. It's midday, the outside temp. stays about the same. I set the ZO on the SRM at the start of the ride, then check it again at 18 minutes (it's drifted about 10 points, and I reset it). I manually zero the PT torque. I start my 15 minute intervals, and they go about like this: SRM: 302/298/305, PT 287/299/304. The first 2-3 minutes of the first interval track almost perfectly, then it starts on a downward slope. There's no coasting during the intervals, btw--but I coast immediately after, and the auto-zero function on the PT kicks in the the torque is re-set. I check the ZO on the SRM after each interval, and it's it's within 1-3 points (1 watt difference).

I should mention that this isn't a single ride--this happens almost every time on this ride, and the colder it is out side, the bigger the difference, and the longer it takes for the PT to stabilize. Even in 60 degree weather, it seems to take about 40 minutes for it to really stabilize. Of course, if you're doing shorter efforts, the auto-zero works, and there are no issues.

That sounds like a problem with your PowerTap to me.

But...it does bring up a good point. I'm fairly confident (due to multiple instances of comparing it to other PMs and using it in previous testing) that my own PT Pro hub most likely doesn't suffer from that same problem. HOWEVER, this testing was done with a hub that I don't have as much experience with since it was borrowed. Maybe it suffers from the same issue...maybe not. I don't have enough info to say one way or the other.

I'm thinking that if I get a chance to repeat the testing, I might just use my own PT Pro wheel since I have a higher level of confidence in it's repeatability from previous testing experience.

edit: I'll also see about making sure I get in a few minutes long of steady-effort on the hub prior to the first run to make sure everything is "settled in".

http://bikeblather.blogspot.com/
Last edited by: Tom A.: May 28, 08 10:36
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Re: Something borrowed...something FAST! [Tom A.] [ In reply to ]
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Yeah, here's a pretty typical scenario: I leave my house (bike kept inside @ room temperature, 65 degrees or so), and ride for 18 minutes to get to the venue for my 15 minute intervals. Outside temp is about 60 degrees as reported by my SRM. It's midday, the outside temp. stays about the same. I set the ZO on the SRM at the start of the ride, then check it again at 18 minutes (it's drifted about 10 points, and I reset it). I manually zero the PT torque. I start my 15 minute intervals, and they go about like this: SRM: 302/298/305, PT 287/299/304. The first 2-3 minutes of the first interval track almost perfectly, then it starts on a downward slope. There's no coasting during the intervals, btw--but I coast immediately after, and the auto-zero function on the PT kicks in the the torque is re-set. I check the ZO on the SRM after each interval, and it's it's within 1-3 points (1 watt difference).

I should mention that this isn't a single ride--this happens almost every time on this ride, and the colder it is out side, the bigger the difference, and the longer it takes for the PT to stabilize. Even in 60 degree weather, it seems to take about 40 minutes for it to really stabilize. Of course, if you're doing shorter efforts, the auto-zero works, and there are no issues.

OK...thanks, I'll have to keep that in mind in future testing to see if that's something I observe as well with my unit. Actually, I have some older PT vs. Polar data I might take a look at to see if I observe such changes as well.

Another option would be, rather than making sure I was constantly pedaling throughout each run like I normally do, I can just make sure I coast at some point to trigger the auto-zero within each lap...

I think unobserved (or unobservable!) wind (especially x-wind) is a more likely source of error. Next time, hold all traffic and repeat the baseline please :-)
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Re: Something borrowed...something FAST! [roady] [ In reply to ]
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suppose that same scenario was duplicated while using 2 different computer heads, and 4 different wheels?

Then I'd be a bit more concerned. However, the fact that some version or versions of the PowerTap hub aren't as stable as they should be is fairly well known, so the first questions I'd ask are 1) did you ever have yours checked by Saris, and 2) is it the same model as the one Tom used?
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