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Thread: Power Meter

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    Quote Originally Posted by achoo View Post
    Yeah, he almost seems like a shill of some sort.

    And even then, drag Cda's are relative - even if the number from a PowerTap is presumed for some unsupported reason to be more accurate, any power meter that produces consistent results will consistently produce lower Cda numbers for what in reality are lower-drag positions/configurations.

    Which is the point of drag testing, no? Find the lowest drag position/configuration.

    He doesn't even appear able to understand THAT. "Script kiddie" indeed.
    I guess I disagree with you there. I don't doubt he is correct in his determination of what is required for his method. But I do question the value of his method in the practical sense for the typical person asking about a Stages on this forum. In my opinion he should consider his audience a bit more. After having a few seasons under my belt with a PT and a quarq I know I just need something that I can meter out intervals with. I believe a stages is accurate enough (because the power comes from your legs, not the device).

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    Quote Originally Posted by canam73 View Post
    I guess I disagree with you there. I don't doubt he is correct in his determination of what is required for his method. But I do question the value of his method in the practical sense for the typical person asking about a Stages on this forum. In my opinion he should consider his audience a bit more. After having a few seasons under my belt with a PT and a quarq I know I just need something that I can meter out intervals with. I believe a stages is accurate enough (because the power comes from your legs, not the device).
    But when you ask him to show why, say, a PowerTap is more accurate than a Stages, he vapor locks.

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    Quote Originally Posted by achoo View Post
    But when you ask him to show why, say, a PowerTap is more accurate than a Stages, he vapor locks.
    On that I am taking it on his word when he says he tried it with a Stages and it didn't work out. I know my PT is more stable in regards to temperature/pressure changes than either of my quarqs (warranty replacement). That said, I mainly use the quarq as I prefer the ability to change wheels.

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    Quote Originally Posted by achoo View Post
    You'd think though, with the high sampling rate coupled with measurement of the crank angle something would show up. Gotta wonder what's going on inside that SRM unit...
    I take it you don't understand how the SRM measures torque or power. The SRM doesn't measure torque on the individual crank arms. It effectively measures the torque applied to the chainrings through a spider. As I said before the SRM has no way to differentiate whether the torque measured is coming from a pedal being pushed down the opposite pedal being pulled up. It can only measure net torque applied.

    How long was each step? 30 second steps would be a whole lot different from 30 minute steps. The longer the steps, the more reliable I'd consider this.

    If the steps in this study were "short" - I'll put off defining "short" for now because I don't really want to spend much more time on here right now - the higher power numbers would appear to be "failure" numbers from not-so-great cyclists. Getting highly asymmetrical power at 300W because it's a "high" power level isn't indicative of a decent level of competitive cyclist. That's, oh, about a cat 4/5 FTP for a 180-lb rider. In other words, the level of power this study is calling "high" is a level of power that good competitive cyclists can sustain for hours.

    At those power levels, unless those power steps are each "long" - and by long I mean measurable with a sundial if not a calendar (100W?!?! I've averaged over 200W for a double metric century and I'm not all that fast...) - this test is showing us that recreational cyclists have one leg fail before the other.

    And even then, it's not saying that left/right power balance varies on a day-to-day basis for non-failure levels of riding, or that someone who's left-dominant today is going to be right-dominant tomorrow.

    I don't see that it's demonstrating any variance in the fraction of total power provided by the left leg for a given cyclist riding at aerobic power levels.
    Here's the profile of the riders tested:
    Participants:
    Ten cyclists (three female and seven male club riders) with competitive experience in cycling and/or triathlon were invited to participate in the study: age = 30 7 years, body mass = 72.8 13 kg, standing height = 175 12 cm, maximal oxygen uptake 55.6 8.8 ml/kg/min, peak power output = 336 77 W, and peak power per body mass = 4.6 6 W/kg). Lower limb dominance assessed by the Waterloo inventory indicated all ten cyclists were right leg dominant.
    I'm sure your VO2Max is north of 70 ml/kg and you can maintain 350W for hours, but for most people considering a stages powermeter these riders are representative examples.

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    Quote Originally Posted by gregf83 View Post
    I take it you don't understand how the SRM measures torque or power. The SRM doesn't measure torque on the individual crank arms. It effectively measures the torque applied to the chainrings through a spider. As I said before the SRM has no way to differentiate whether the torque measured is coming from a pedal being pushed down the opposite pedal being pulled up. It can only measure net torque applied.

    Here's the profile of the riders tested:

    I'm sure your VO2Max is north of 70 ml/kg and you can maintain 350W for hours, but for most people considering a stages powermeter these riders are representative examples.
    Those riders are fairly strong actually. Certainly not beginners.

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    Quote Originally Posted by achoo View Post
    Sure, it seems like total power should be better than just using left power doubled. But then when you start digging into what would actually make it better, there's really nothing there. The "signal" of the recorded power data is no more consistently correlated with your actual power output for either a Stages unit or a PowerTap.

    And all that matters for training purposes is the consistency of that correlation. Every time you generate X watts your power meter should record "300W", and it really doesn't matter what "X" actually is.
    Quote Originally Posted by achoo View Post
    with no knowledge of left/right power balance, how measuring total power is more accurate than measuring just left power and doubling it.

    "Because it is!" isn't an answer.

    And a consistent number is all you need for training purposes.
    Wrong again, if I'm at 300 watts and bump the power up 10% with just the left leg, stages will tell me I'm at 330. My power tap will tell me I'm at 315. I have limited mobility in my right leg and on longer intervals I'll sometimes get lazy with the right side. Stages would not show this. My power tap does. It's a real issue for me.

    Gregf83 posted a study that shows AI changes with power, so it's not just an N=1 problem. You don't like the study so you just wave your hands claim it's wrong, same thing you accuse others of doing.
    Last edited by thill454; 04-27-14 at 02:38 PM.

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    Quote Originally Posted by thill454 View Post
    Wrong again, if I'm at 300 watts and bump the power up 10% with just the left leg, stages will tell me I'm at 330. My power tap will tell me I'm at 315. I have limited mobility in my right leg and on longer intervals I'll sometimes get lazy with the right side. Stages would not show this. My power tap does. It's a real issue for me.

    Gregf83 posted a study that shows AI changes with power, so it's not just an N=1 problem. You don't like the study so you just wave your hands claim it's wrong, same thing you accuse others of doing.
    Two questions:

    Should other people make their PM decision based on your condition?

    What travesty will ultimately happen if you do a set of intervals at 315 watts when you think you are at 330?

    Is y

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    Quote Originally Posted by canam73 View Post
    Two questions:

    Should other people make their PM decision based on your condition?

    No, they don't need to make decisions based on my problem. You do however fail to mention the study gregf83 posted showing asymetry is a common problem and varies with power. achoo wanted an example of why a power meter that measured total power is more accurate than one that measures one side and doubles it. So I gave him an example.

    315 watts when you think you are at 330?

    No real travesty. There is the potential to reinforce and increase the asymetry.

    I don't disagree that stages is good enough for the vast majority of people, including me. But it's not as good as a meter that measures total power. I have 2 PT G3s so that's what I'll stick with. I would like a PM that measured both sides like the Garmin Vector, but I'll wait a few generations to make they or someone else has all the bugs worked out.


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    Quote Originally Posted by thill454 View Post
    Wrong again, if I'm at 300 watts and bump the power up 10% with just the left leg, stages will tell me I'm at 330. My power tap will tell me I'm at 315. I have limited mobility in my right leg and on longer intervals I'll sometimes get lazy with the right side. Stages would not show this. My power tap does. It's a real issue for me.
    And how would you do that? How would you know you bumped your left-leg-only power up 10%?

    You can't do that with that much accuracy, now can you?

    Gregf83 posted a study that shows AI changes with power, so it's not just an N=1 problem. You don't like the study so you just wave your hands claim it's wrong, same thing you accuse others of doing.
    No, I had substantive comments about the study. Which no matter how you slice it STILL doesn't show that the variation in left/right power is unstable for any single person. It gets higher as power goes up and this group of cyclists approach blowing up.

    Like I said: BFD. That study shows that one leg fails first. It doesn't show what happens during 20-minute intervals at steady power when you're not blowing up, now does it? What happens over a 3 or 4 hour ride? Oh, wait. It doesn't show that, now does it?

    And guess what? It's a single study. There's nothing in that study that couldn't be explained by a problem with the pedals. It's not like there haven't been problems with Garmin Vectors, which I assume is what they used.

    So yeah, in your case it IS an N=1 problem - a single study that out-and-out states up front that it's come to a conclusion that no other study found.

    I'm not waving my hands. Those that claim a Stages unit is less consistent than a PowerTap are.

    When I ask why, I get "Because!!!! TOTAL POWER!!!"

    Which IS hand waving.

    Which is exactly what your response is.

    If you're going to make a claim that a Stages unit is less consistent than a PowerTap, why don't you go through the math, assume a reasonable error at each step of the process used to produce a single number called "power", and demonstrate HOW and WHY using doubled left power is less consistent than total power.

    You wanna make that claim? I'm not hand waving. I'm saying "Prove it!"

    If you're going to make that claim, that's what you have to do.

    If you can't do it - or even make a reasonable effort - you don't know what you're talking about.

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    Quote Originally Posted by gregf83 View Post
    I take it you don't understand how the SRM measures torque or power. The SRM doesn't measure torque on the individual crank arms. It effectively measures the torque applied to the chainrings through a spider. As I said before the SRM has no way to differentiate whether the torque measured is coming from a pedal being pushed down the opposite pedal being pulled up. It can only measure net torque applied.
    I undertand full well how an SRM measures power. But NO variation at all? No one pulls up as hard as they push down - it ain't even close. The other studies alluded to? What did they show with crank-measured power? It'd be interesting to know.

    Here's the profile of the riders tested:

    I'm sure your VO2Max is north of 70 ml/kg and you can maintain 350W for hours, but for most people considering a stages powermeter these riders are representative examples.
    So I asked questions about the study, and you retort with ad hominem attacks?

    And what the hell is "peak power" anyway? 336W PEAK power? That better be measured in tens of minutes....

    You'd think the mere existence of a single Stages power meter were a threat to the very fabric of the universe. Or maybe the perceived valued of the sunk cost of a PowerTap....

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    Quote Originally Posted by reef58 View Post
    Those riders are fairly strong actually. Certainly not beginners.
    How do you know? What's the time frame for the "peak power" mentioned? As I said in my previous post, at the peak power numbers listed it better be measured in tens of minutes at the very least.

    And even then, the study is just ramping up power until failure, and it's noting that one leg tends to fail first. Since in any competition once even one leg starts to fail you're pretty much toast anyway, it's really a "don't care" situation from the data presented.

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    Quote Originally Posted by thill454 View Post
    I don't disagree that stages is good enough for the vast majority of people, including me. But it's not as good as a meter that measures total power. I have 2 PT G3s so that's what I'll stick with. I would like a PM that measured both sides like the Garmin Vector, but I'll wait a few generations to make they or someone else has all the bugs worked out.

    Why? Why is a Stages unit "not as good"?

    Because it doesn't measure total power? That's blatantly a circular argument.

    Amazing how asking that question sets people off.

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    Quote Originally Posted by achoo View Post
    I undertand full well how an SRM measures power. But NO variation at all? No one pulls up as hard as they push down - it ain't even close. The other studies alluded to? What did they show with crank-measured power? It'd be interesting to know.
    I'm not quite sure where your confusion lies. The point of the paper was that the SRM is not an effective tool for measuring pedaling asymmetry. They provided ample evidence to support this assertion and I've explained why it can't separate the force applied by left and right legs. Not sure what else I can do.



    So I asked questions about the study, and you retort with ad hominem attacks?

    And what the hell is "peak power" anyway? 336W PEAK power? That better be measured in tens of minutes....

    You'd think the mere existence of a single Stages power meter were a threat to the very fabric of the universe. Or maybe the perceived valued of the sunk cost of a PowerTap....
    I don't recall making any attacks although I do recall some snide comments you made on the strength of the subjects. Based on your comments I assumed you were an elite rider.

    You can read more about MAP tests here: Alex's Cycle Blog: MAP Testing - where failure is a success...
    It's a commonly used lab tests for cyclists.

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    So I just got a Stages. I don't race, but thought it would be fun to have some power data and do some work on improving it. I only have 4 rides with power so far and I'm not entirely sure how to work with the data but I'm seeing some fairly consistent data even though the 4 rides were very different. The variations make sense given how I rode, length, etc. Kind of like how I monitored HR data over time which gave me a ballpark measure of how I rode. Power data should help me fine tune that some. I'm not sure which statistic to watch (average power, normalized power, max 20min power, etc.). I'm sure training peaks or similar can make more sense of it but not sure I'm ready to delve that deeply.

    Someone mentioned Stages is a good gateway drug. Maybe so. I think it allows people like me (regular and somewhat serious rider but ultimately it's for fun and fitness) to make the leap to power data without a big investment. I almost went with a PowerTap but in the end I didn't want to be tied to a particular wheel. Crank based PMs don't currently work with the Hollowgram Spidering - except Stages.

    Quote Originally Posted by thill454 View Post
    What travesty will ultimately happen if you do a set of intervals at 315 watts when you think you are at 330?

    No real travesty. There is the potential to reinforce and increase the asymetry.
    I'm sorry, but that response doesn't make any sense. Only something that output live L/R data would allow you to monitor and adjust if you were trying to train out any asymmetry. Anything that gives only a total power number has the issue you mention. This is also a very specialized user case and specialized needs required specialized devices.
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    Quote Originally Posted by bikerjp View Post
    So I just got a Stages. I don't race, but thought it would be fun to have some power data and do some work on improving it. I only have 4 rides with power so far and I'm not entirely sure how to work with the data but I'm seeing some fairly consistent data even though the 4 rides were very different. The variations make sense given how I rode, length, etc. Kind of like how I monitored HR data over time which gave me a ballpark measure of how I rode. Power data should help me fine tune that some. I'm not sure which statistic to watch (average power, normalized power, max 20min power, etc.). I'm sure training peaks or similar can make more sense of it but not sure I'm ready to delve that deeply.

    Someone mentioned Stages is a good gateway drug. Maybe so. I think it allows people like me (regular and somewhat serious rider but ultimately it's for fun and fitness) to make the leap to power data without a big investment. I almost went with a PowerTap but in the end I didn't want to be tied to a particular wheel. Crank based PMs don't currently work with the Hollowgram Spidering - except Stages.



    I'm sorry, but that response doesn't make any sense. Only something that output live L/R data would allow you to monitor and adjust if you were trying to train out any asymmetry. Anything that gives only a total power number has the issue you mention. This is also a very specialized user case and specialized needs required specialized devices.
    If you don't have one already, I would get a book like Racing and Training With a Power Meter (Allen & Coggan). It talks you through everything and also has some very good workouts for targeting various power zones. Joe Friel has one called The Power Meter Handbook but I haven't read it. From what I understand it is a little simpler to understand but less thorough than the first one I mentioned.

    I use Golden Cheetah 2.1 to log my training and look at power data. It is free and I find the older 2.1 version more user friendly than the current 3.02 edition.

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    Quote Originally Posted by bikerjp View Post
    I'm sorry, but that response doesn't make any sense. Only something that output live L/R data would allow you to monitor and adjust if you were trying to train out any asymmetry. Anything that gives only a total power number has the issue you mention. This is also a very specialized user case and specialized needs required specialized devices.
    The difference is it doesn't matter how you increase your total power. If you end up using one leg more than the other and it gives you a higher total power that's not necessarily bad. Measuring only one leg, however, presents the opportunity for you to think you are increasing your total power when it's only your one leg that is putting out more power and your total power doesn't actually increase.

    You generally aren't limited by the power in one leg. For example, if your FTP was 300W it's usually possible to do extended single leg intervals at 200W or significantly higher than FTP/2. It's not additive though so if you can do single leg intervals at 200W that doesn't mean you can get 400W with 2 legs.

    The argument is that with a one legged powermeter, subconsciously you may end up increasing the asymmetry in order to raise or maintain your power while your actual total power could be stable or going down.

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    Quote Originally Posted by achoo View Post
    How do you know? What's the time frame for the "peak power" mentioned? As I said in my previous post, at the peak power numbers listed it better be measured in tens of minutes at the very least.

    And even then, the study is just ramping up power until failure, and it's noting that one leg tends to fail first. Since in any competition once even one leg starts to fail you're pretty much toast anyway, it's really a "don't care" situation from the data presented.
    It said 1 minute ramp test which is pretty standard. Maybe I misread it, but one minute is fairly common, and that is what the lab used with me when I tested my VO2 Max.

    And the VO2 Max numbers were also listed, and fairly decent. Seems to me you just like to argue for the sake of it.

    One more thing I made no mention of leg failures or anything else. I just stated they were decently strong riders. You just assumed the rest.
    Last edited by reef58; 04-27-14 at 08:17 PM.

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    Quote Originally Posted by gregf83 View Post
    The argument is that with a one legged powermeter, subconsciously you may end up increasing the asymmetry in order to raise or maintain your power while your actual total power could be stable or going down.
    In the absence of any studies on ones subconscious use of one leg over the other while cycling (and not people with medical issues) I'm inclined to think this is just over-analyzing. While I suppose one could really push with the left leg in order to record artificially high numbers there isn't much point in doing this nor do I think anyone would continue to do this for any length of time - at least no one with any sense.
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    Quote Originally Posted by bikerjp View Post
    In the absence of any studies on ones subconscious use of one leg over the other while cycling (and not people with medical issues) I'm inclined to think this is just over-analyzing. While I suppose one could really push with the left leg in order to record artificially high numbers there isn't much point in doing this nor do I think anyone would continue to do this for any length of time - at least no one with any sense.
    I agree with you but it would be interesting to do some tests. I have a powertap and SRM and sometimes ride with both on one bike to make sure they match up. When I'm doing an interval I don't stare at the power number, rather I have a look every minute or so and make adjustments as necessary. I can usually hit a target power within a watt or two and there isn't a huge variation while riding. I make very small adjustments and can't imagine how I would pedal harder with just one leg but you never know until you try it.

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    Quote Originally Posted by canam73 View Post
    I guess I disagree with you there. I don't doubt he is correct in his determination of what is required for his method. But I do question the value of his method in the practical sense for the typical person asking about a Stages on this forum. In my opinion he should consider his audience a bit more. After having a few seasons under my belt with a PT and a quarq I know I just need something that I can meter out intervals with. I believe a stages is accurate enough (because the power comes from your legs, not the device).
    Hmmm. More than 100 posts ago in post #22 of this thread I wrote "I've also spent some time examining the Stages compared to various other power meters. Like all power meters, it has good points and not-so-good points. Its data quality is not equivalent to a PT or SRM but it's not bad. I wouldn't use it for QA analyses, or for drag estimation, or for sprint training, but not many people do those kinds of things. Sky probably won't be looking at QA analysis or sprint training, and they can afford to send their riders to the wind tunnel for drag estimation." And elsewhere I've said several times that I think there's a place for the Stages; that's especially true for this particular forum. I think it's a good gateway drug. Just like gateway drugs, some people will stay with it, and some will progress on to harder stuff when they need to do the things I listed above. If they never need to do the things I listed above they can probably do just fine with the Stages. However, in your case, the NP you get from a hard one-hour crit when using the Stages probably won't be as good an estimate of FTP as the NP you'd get from that same crit if you were using a PT or Quarq.

    And achoo doesn't know what he doesn't know.

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    Quote Originally Posted by RChung View Post
    Hmmm. More than 100 posts ago in post #22 of this thread I wrote "I've also spent some time examining the Stages compared to various other power meters. Like all power meters, it has good points and not-so-good points. Its data quality is not equivalent to a PT or SRM but it's not bad. I wouldn't use it for QA analyses, or for drag estimation, or for sprint training, but not many people do those kinds of things. Sky probably won't be looking at QA analysis or sprint training, and they can afford to send their riders to the wind tunnel for drag estimation." And elsewhere I've said several times that I think there's a place for the Stages; that's especially true for this particular forum. I think it's a good gateway drug. Just like gateway drugs, some people will stay with it, and some will progress on to harder stuff when they need to do the things I listed above. If they never need to do the things I listed above they can probably do just fine with the Stages. However, in your case, the NP you get from a hard one-hour crit when using the Stages probably won't be as good an estimate of FTP as the NP you'd get from that same crit if you were using a PT or Quarq.

    And achoo doesn't know what he doesn't know.
    This is where I think you need to consider your audience a bit more. Yes, you qualified your answer and I understand what you are saying, but the person asking "what do you think of the Stages?" won't. They haven't a knowledge base of what will or won't be important to them down the road. When you come in saying 'look out there, you won't get good enough data for the rchung method of drag estimation' they haven't a clue if that is important or not. And in most cases it just isn't.

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    Quote Originally Posted by canam73 View Post
    This is where I think you need to consider your audience a bit more. Yes, you qualified your answer and I understand what you are saying, but the person asking "what do you think of the Stages?" won't. They haven't a knowledge base of what will or won't be important to them down the road. When you come in saying 'look out there, you won't get good enough data for the rchung method of drag estimation' they haven't a clue if that is important or not. And in most cases it just isn't.
    Fair enough, but I have never said that it is important for most riders. You can look in this thread or in other threads or in other forums -- I've never said that. I wrote in #22: "I wouldn't use it for QA analyses, or for drag estimation, or for sprint training, but not many people do those kinds of things." What's misleading about that? In post #110 , I also wrote: "It's not hard to show that you can get better measurements of drag from a Power Tap than from a Stages. Whether that's important is a different question and perhaps one worth addressing -- whether the PT is more accurate and more consistent really isn't." What parts of what I wrote in #110 do you think are misleading?

    [Edited to add] On reviewing my posts in this thread, I see that I only brought up my estimation method after thill454 brought it up, and I only listed some of the people who use it to show that it's not just me who thinks it can be done. Or you can believe achoo who continues to think it is impossible -- and he has an MSEE.

    BTW, I think there's a place for the Stages. I think it can be a good gateway drug.
    Last edited by RChung; 04-27-14 at 10:43 PM.

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    Quote Originally Posted by RChung View Post
    Fair enough, but I have never said that it is important for most riders. You can look in this thread or in other threads or in other forums -- I've never said that. I wrote in #22: "I wouldn't use it for QA analyses, or for drag estimation, or for sprint training, but not many people do those kinds of things." What's misleading about that? In post #110 , I also wrote: "It's not hard to show that you can get better measurements of drag from a Power Tap than from a Stages. Whether that's important is a different question and perhaps one worth addressing -- whether the PT is more accurate and more consistent really isn't." What parts of what I wrote in #110 do you think are misleading?
    I think your qualifications can get lost in a thread like this when the discussion is monopolized by proving technical points. I realize you were mostly responding to others, but the shear volume of posts on the matter makes the issue appear more important than I think it should be for most riders.

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    So what you're saying is, there's nothing misleading in what I wrote, it's just that in a long thread you got the wrong idea about a technical point? Woah, that's never happened before.

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    Quote Originally Posted by RChung View Post
    So what you're saying is, there's nothing misleading in what I wrote, it's just that in a long thread you got the wrong idea about a technical point? Woah, that's never happened before.
    Although I have no desire to get into circular arguments on this, I personally think for the average amateur racer/PM user your concern over the ability to do accurate drag testing is small enough you shouldn't bring it into these conversations, qualifications or not. To somebody who is new to this it's mere mention can be misleading enough, even if you have covered you bases here in there with a disclaimer. But hey, it's a public forum and it's your opinion. But just in case it helps a first time PM buyer on a budget make the best decision for him or her, I am going to post my opinions now and again in an attempt to balance the discussion.

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