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  • RayG

    Sorry. TL;DL.

    • It’s a podcast meant to be listened to in the car, on the train, etc (away from the screen) so we can dive deep into the topic and not skim the surface. You might want to download it on your smartphone and listen during some downtime.

      • winkybiker

        I can read things way, way faster than I can listen to someone read them to me. I also don’t have any significant car time or train time to listen to these things. I like it when podcasts also publish the transcripts.

      • BTD

        (In no way am I complaining about length, content, etc.) It may be worth looking into (web based, like on this page) players that allow the end user to control the speed of the playback for those not willing or able to download the file and use their own player. Even a 1.2x or 1.4x playback speed seems like it would still deliver the content of this conversational-paced discussion/presentation without losing anything.

        Keep up the good work!

  • Yancey Arrington

    Pretty amazing that some of the speakers accept “self-consistency” as good enough. Self consistency is the absolute lowest bar for a measurement device and just barely gets you above a random number generator.

  • Alex Simmons

    Doesn’t play

    • Robert

      Doesn’t matter.

    • Robert

      I’m not sure that the guests answered the underlying question “what
      should a consumer do?” In that case, talking with someone knowledgeable
      who isn’t a manufacturer of power meters might be a good next step.

      There
      are many things that a consumer can do with a power meter, and
      different things demand different levels of accuracy and consistency.
      Training FTP turns out to be one of the least demanding uses for a power
      meter; measuring drag turns out to be one of the most demanding and
      requires a high level of accuracy. In addition, some power meters *can*
      be checked for accuracy by the user, while others make it very difficult
      or impossible to do so.

      I’d endorse Alex Simmons as someone to consider for a followup podcast.

      • tanhalt

        I’d endorse Robert Chung to join Alex in the followup…James might finally get some of the answers to the questions it appears he still has (and also might hopefully reconsider his opinion that “consistency” is more important than accuracy…after all, if a PM is “consistent”, it’s only a calibration away from being accurate ;-)

        • James Huang

          Sounds like a follow-up podcast is in order…

          • Huddo

            Sooner rather than later please. The podcast was a good start, but I’m sure most listeners would agree that the ‘conversation” is far from over :)

  • Chris Capoccia

    5% of 300W = 15W. I can’t think of any situation where I cared about whether my sustained power was ± 15W vs ± 7W. Even ± 30W might be good enough for some people. Unless you’re using some kind of smoothing, the live power number on your head unit jumps all over the place.

    • Robert Merkel

      Have you ever tried to improve your FTP by 15 watts, particularly when you’re already in reasonable form?

      15 watts is huge for anybody racing. Heck, even if you’re just doing Gran Fondos, 15 extra watts while keeping the same weight will make getting up mountains a hell of a lot easier.

      Now, maybe you just want to ride your bike and you don’t care how fast – that’s great. I love getting out on the bike even though I know my current form is appalling by the standards of a couple of years ago. But if you want to race and be competitive even at a weekend warrior level, accurate power meters let you train better, and let you figure out whether that training is actually working.

      Now, if you’ve got a decently long and steep hill nearby, you can get a pretty good estimate of power by timing yourself up that. But not everybody has that luxury.

    • Alex

      If you don’t care about 30 watts you aren’t the target demographic for a power meter. I’ve trained all season to attempt to increase my FTP by 30 watts and only got 20 of those.

  • Lee Gardner

    Can we stop f-ing around here, and just name some names. What are the power meters being tested, and what are the numbers.

    Also, what about the head units? They record at a certain rate, yet some of the power meters (from what I am told by the manufacturers) send or package data faster. If a head unit can capture data on a more frequent basis, that seems more accurate. No?

  • Matt Ghanivand

    Does it really matter? If you have a coach and you use a PM and you know your numbers based on your PM, why does it matter whether its accuracy is 1% 2% or 10%. You guys are always taking the glass half full angle on this podcast… Is your carbon bike safe!? Is your Power Meter accurate! .. whats next … weight scales are wrong, tyres might explode, tubes might explode! Cycling is fun and enjoyable. Talk about productive topics!

    • Yancey Arrington

      Would you accept if you bought a scale that was +/-10%? If you use a +/-10% powermeter and a +/-10% scale you’d never be able to determine your w/kg. At 70kg and 300watts “true” you’d have a possible range of 3.5-5.23 w/kg. One is average and the other may score you ticket to ride pro.

      What if your bike computer was +/-10% on distance? A 100km ride registers 90km or 110km.

      What about +/-10% cadence? Oops… you’ve actually been doing your 100RPM drills at 90rpm. But the reading is consistent, so it is OK.

      Why is consistent acceptable on a powermeter but not for other measurements?

      • Chris Capoccia

        lol… if you’re really pro, you’ll know it by winning races, and that requires a lot more than just getting a high score from a power guesser

        • Alex

          That’s irrelevant. You paid ~$1,000 for a product that says it’ll do something within a certain percentage accuracy. It should be true to it’s claim.

      • Scott

        Accuracy is only important if you’re entering into d-measuring contests with your buddies. So long as the values are consistently inaccurate (IOW, always over- or under-estimating) it won’t matter. Let’s say for the sake of discussion, what would you (or your coach) do differently with “accurate” data vs “inaccurate but consistent” data?

        • Yancey Arrington

          Easy… lets say you are a coach and the client has a consistent power meter that output watts on a 0-100 scale. There are a number of things you can’t do:
          -Can’t determine work rates (KJs) during training or racing to inform fueling.
          -Can’t compare this athlete against other athletes, local or global, to see how strong they are compared to competition.
          ** can you imagine the headache if all 10 of your athletes had significant variances in PM outputs. Each athlete has their own unique power system numbers.
          -Can’t do power profiling (WKO4 or W/KG types)
          -Can’t move to a different power meter and have compatible data… i.e. historical data is only referenced to itself.
          -Can’t ride with others in zwift using the PM(LOL)
          -Can’t use PM to calibrate or control a smart trainer easily without creating a consistent power to true power map.
          -Can’t use best bike split for analysis
          -Can’t do any type of physics analysis (TT times, climb time estimations, aerolab)
          -Can’t determine if the powermeter has linearity/continuity errors.
          -If you can’t confirm accuracy how can you verify changes in fitness are not attributed to drift errors.
          -How can you verify that the powermeter does not drift output over a long period time time (months-years)? If the meter gains 0.25points of output per month how does the user determine this error? The powermeter is consistent today but not consistent over it’s history.

          What if you were a track coach and relied on your athletes to provide performance data from their watches. Each athlete uses a different brand and each brand gives times and distances which are consistent… but not accurate and not consistent with each other. I think you can see the problem. Why is this not acceptable for a running watch but acceptable for a powermeter?

          • Scott

            Makes you wonder how we ever ride bikes at all, or how so many coaches seem to be pretty content to do most all those things, within the accuracy parameters current powermeters provide.

            • Yancey Arrington

              Interesting that each “by the numbers” article on the front page of this website would be pretty useless if the numbers cited were consistent but not accurate. The basic premise of the article would be invalid… that is, giving the average rider a real peek into the physical demands of pro races. We, as article readers, have the expectation that the data given in the article is accurate.

              Of course you can ride, train, and race a bike without a powermeter. The arguement is… IF you choose to use a powermeter for training and racing it should be accurate and consistent.

      • Matt Ghanivand

        you racing for the yellow jersey? i figure not so in the end does it really matter? who would ever know whats absolutely the correct figure for any of those situations you mention.

        • Yancey Arrington

          I have the series leader jersey for my local road race series… so yea, I do have a yellow jersey.

          You know what the correct figures are by using an accurate and precise instrument.

          • Wilson

            pffff

          • Matt Ghanivand

            hey i completely agree, but we live in the real world. Do you weigh your tyres and get them inspected to make sure they match the stats exactly to the gram? Do you inspect the internals of your frame? Do you get your weight scales calibrated regularly to make they are absolutely accurate? Do you get your food inspected before you eat it? Like anything, a number is a number, a gain is a gain if its using the same equipment. You’re winning your local series – good on you. Thats a hard thing to do in any division in this bloody hard sport. Would it make any difference if you had a power meter that was absolutely accurate or off by a half a % … i think not.

  • Cam

    I like to believe my power meter reads about 25% too low and am happy not to be corrected.

    • Nitro

      Count yourself lucky. I have an “Un-power meter” that tells me how much power I’m not generating… It reads off the scale most days…

  • its not accurate at all..

  • Chris Capoccia

    Real test data is available to answer this question, for example: Bouillod et al (2016). “Validity, Sensitivity, Reproducibility and Robustness of the Powertap, Stages and Garmin Vector Power Meters in Comparison With the SRM Device.” International Journal of Sports Physiology and Performance. 1–26 http://dx.doi.org/10.1123/ijspp.2016-0436 Author’s draft copy available here: https://www.researchgate.net/publication/311664675_Validity_Sensitivity_Reproducibility_and_Robustness_of_the_Powertap_Stages_and_Garmin_Vector_Power_Meters_in_Comparison_With_the_SRM_Device

    • Neuron1

      Thanks for the reference. This explains why Garmin cycling team don’t seem to be using Garmin power meters.

  • Gareth

    Very good discussion. As a engineer and a former professional cyclist powermeters are always a hot topic. I’ve been asked “which powermeter should I get?” numerous times and my question back is what do you hope to get from your powermeter? Case in point, I train with a Stages, but I know very well that it only measures my left leg and extrapolates a average power number. But I’m okay with that. As long as the end user is okay with the output +/- 2% is irrelevant to the actual number you see on the head unit. I will say that the guys touched on what I feel is the most important aspect though. A powermeter should be accurate +/- 2% from day to day. I don’t necessarily care if the meter reads 70 watts low or high, but it better be consistently 70 watts low or high (obviously one or the other). Thanks for the listen and I look forward to the next episode.

    • Karl

      That doesn’t really make sense. If it’s consistent, then you just need the right offset ie. zeroing.

  • Charlie Knoll

    This podcast inspired me to do a quick test this morning. I calibrated (spindown) my Wahoo Kicker and also did a zero calibration and magnetic calibration on my Pioneer after a 10 minute warm up. I then did a number of test intervals with the Wahoo Kicker in ERG mode (constant power) and tried to keep my cadence as steady as possible in the seated position. Results:

    2 min @ 200W Kicker ERG, 95 cadence resulted in 183.5W on Pioneer
    2 min @ 300W Kicker ERG, 95 cadence resulted in 278.4W on Pioneer
    2 min @ 300W Kicker ERG, 80 cadence resulted in 280.9W on Pioneer
    1 min @ 400W Kicker ERG, 100 cadence resulted in 365.7W on Pioneer

    It seems the they are off by about 6-8%.

    I used to have a Stages left only power meter and it consistently read 10 – 15% below the Kicker and Computrainer.

    I would have expected Kicker to run lower because it is measured “downstream” from drivetrain losses.

    • William Tsang

      Hmmm… very interesting! I recently did some parallel testing between my BKool smart trainer’s reported power outputs and measurements from my PowerTap P1 pedals, and similarly I saw that the BKool numbers were pretty consistently about 15% less than the PowerTap readings.

      • Charlie Knoll

        Ha ha, it sounds like PowerTap P1 power meter is the way to go if you’re into zwift ;)

        • William Tsang

          Whoops – sorry, got it wrong way around when I typed it up – the PowerTap pedals read ~15% less power than the BKool trainer (fixed the post now) – similar to Charlie Knoll’s experience with the Kickr v. Pioneer.

  • donncha

    The guy from Stages claiming he’s proud of the fact that they’ve never had accuracy issues???

    • De Mac

      Well, THEY at Stages may not have had accuracy issues………

  • cunn1n9

    I agree with Bryan Taylor on one thing – there should be a standard measurement of accuracy for a PM. Infocrank has submitted to an external test agency to do this. This is commendable and it would be great if others did also.

    However he goes too far to classify all his competitors as “power estimators”. He exaggerates and it is very annoying. All crank arm based PMs work by measuring crank deformation and based on this deformation they back calculate the torque being applied by the rider. Essentially a PM has a strain gauge that detects a crank deformation of “X” and then an “algorithm/calculation” that computes what torque must have been applied to result in the measured deformation. There is no magic here and Infocrank, Stages, Pioneer, 4iiii all do this. He is simply overstating things when he says that only Infocrank directly measures torque as it doesn’t. It measures deformation via the strain gauge and then calculates torque like everyone else. Other systems measure deformation at the pedals (Garmin), spider (SRM, Quarq, P2M), hub (Powertap) and then use a calculation to back calculate torque. I agree that the crank is the best place to measure deformation though.

    The only difference is that Infocrank has made their own crank arm so they can control the manufacturing and hence the could legitimately argue that they would be more accurate. Stages, 4iiii, and Pioneer have taken an industry standard crank arm (like Shimano) and analysed it to work out the deformation to toque relationship – its exactly the same thing. So the difference boils down to the manufacturing tolerances of the Infocrank crank arm and the Shimano one. However all the manufacturers will calibrate each individual crank arm to the strain gauges attached to it as part of QC and so there should be no difference in theory between the accuracy of any of these at this level.

    Infocrank claims that it has a special configuration of strain gauges that is called a Wheatstone Bridge that eliminates crank deformations that don’t contribute to power generating torque (ie non-tangential deformation) . However there is nothing stopping anyone else using this same config of strain gauges (and maybe they do) but even if they don’t there are other ways to get the same outcome as they all end up with very similar power readings.

    This brings us to the power readings themselves that he says are “all over the place” with other meters. Well that’s because no one can pedal at a constant power and thats the exact reason most people use 3s ave power to smooth out the real instantaneous power fluctuations. I am sure that if Infocrank is accurately measuring power it too will need to be smoothed as the instant power will be too variable or is his claim that we really ride with a much smoother power and that this variability is error in the PM? I don’t buy it.

    • Russell

      Very much agree with your with you. I’m not sure the comments from the Infocrank guy cleared up any of the misconceptions with power meters. Still a lot of industry hyperbole being spewed about. “Buy our product because it’s more accurate. Promise.”

      The guy from Stages had the more useful comment when he stated look at what the pro teams are using; if it’s accurate enough for them then it must be good enough for the rest of us.

      • Karl

        As in Sky using double-sided Stages that aren’t available to the rest of us yet?

  • Chris Schreck

    The crankarm power meters complicate the issue of accuracy considerably, I’m finding. Let’s assume that the crankarm is functionally accurate–it’ll very closely measure the actual force delivered to the crankarm. But *practical* accuracy would include leg imbalance, which I’m finding is a complicated beast to deal with.

    I have the luxury of comparing my powertap hub numbers with with my 4iiii crankarm and, so long as I’m sitting in the saddle, my right leg is dominant. 4iiii has the ability to model that in by scaling, which is a really neat feature that I appreciate and was the decisive factor for me getting this PM over Stages and Pioneer. So I can have comparable numbers and can use them to confidently evaluate training progress (or if I’m pacing my effort in a breakaway)…at least as long as I’m seated. But my left leg is dominant when I’m standing, by a huge margin (now compounded by the scaling)–my 4iiii wattage values here are considerably higher than the powertap’s readings. In a race it hardly matters since if I’m standing it’s because I’m fighting to hang on or drop people, but when training I don’t want to see numbers that suggest a level of effort that actually isn’t there. You know, Z6 effort when I’m actually in Z5. So, in the field, an accurate power meter can be inaccurate in many situations.

    Luckily I can upgrade my 4iiii to dual sided, which I’m going to do at the end of this racing season.

    • Yancey Arrington

      Summary: Your 4iiii left leg power meter is trash for your application and you wouldn’t have this key insight without comparing it to a total output power meter.

      • Chris Schreck

        Well, yes and no. I can trust it for everything up to Z4, thanks to the adjustable offset. If you’re doing riding up to threshold and value the convenience of being able to move the crankarm from one bike to another, it’s not a bad way to get into power. I do like that feature. But it is rubbish, at least for me, if I’m using it for Z5+.

        …although if I can modify the ranges of my training zones in Golden Cheetah, perhaps I can at least make the wattages go into the correct zones. But dang it sure is a lot of work making these adjustments!

  • Peter

    What a poorly directed discussion on this podcast. The guests spent more time talking up their own products than talking about power meter accuracy. Not one guest could explain what +/-1 or 2% meant for their power meter. Total waste of time.

    Cyclingtips needs to revise the article to show that the podcast didn’t answer a single of their questions:

    “Yet what exactly does “accuracy” mean in this context? Are the claims based on static load tests in a lab, or dynamic ones while actually riding? And how would one independently verify those claims, anyway?”

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