Stryd Power off - how to adjust rFTP? | 80/20 Endurance

Stryd Power off – how to adjust rFTP?

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  • #14334
    winoria
    Moderator

    Hello 80/20 community!
    Hello David & Matt!

    I’m really hoping that somebody has an idea of what could be going on with my Stryd or maybe somebody else had similar issues in the past?

    Here is what I experienced:
    About 10 days ago, everything was normal. I did my runs as scheduled even though I was on vacation. As most of us, we know what a certain pace on a known course feels and what power output to expect. Or, in other words, what pace to expect at what power (roughly).
    I always wanted to know how the Stryd 9-3 min test compares and ran one on 8/25.

    Here the results:
    9min: 307 Watts, pace 6:16 min/mile
    3min: 322 Watts, pace 5:52 min/mile

    A few days later, my power output is much higher to run a certain pace and I am certain, that my Stryd is off.

    I also ran a 10k race yesterday and the numbers make it quite clear:

    41:41min: 306 Watts, pace 6:43 min/mile – how could that even be possible? I know that the 9min test mentioned above were all out at 307 W and I could never ever have ran another minute at that output, not even thinking of adding another 32 min??????

    I was also looking into numbers from a Half Marathon, even though this race was on May 1st, but still ok for comparison I guess. Interestingly, I remember that the HM was a surprisingly high power output also, as my rFTP tested at 289 Watts which seems low looking at the watts below, but David mentioned that some people have racing and training zones that can differ.

    1h29min: 289 Watts, 6:48 min/mile

    To me, the output of the HM is in line with the 9-3 test. But if you compare the 10k to the HM, there is now way that it takes 17 Watts more to run 5 sec. per mile faster. In my opinion, the Stryd, for some reason, suddenly shows a 15-20 Watts higher output.

    Now, what should I enter as rFTP? The peak 30min of my 10k power output?

    Thoughts? Ideas?

    Thanks
    Winoria

    #14344
    Jeremy Hutchinson
    Participant

    Has the weight setting been changed on the pod, that would be my first thought.

    #14345
    winoria
    Moderator

    Nope. All the same!

    #14353
    David Warden
    Keymaster

    winoria, hmmmm. This may be a better question for Stryd, and I’d love to see their answer. You mentioned of course that it is common for people to race much higher (observed) than when they train (unobserved). This does not apply here. When we discussed this last time, your pace and power were coupled, you just had crazy higher performance in your race than trainnig. In this case, the pace and power are not coupled.

    If I had unlimited funds, I would buy a second Stryd on the other shoe, pair it to a second device, and run for a week with both. Or, if you have Garmin HRM, then you have a second power meter built in. I would not expect the Stryd and HRM to give the same values, but I would expect them go up and down in sync.

    The bottom line is that the numbers can’t be right: you can’t have a 6:43 and 6:16 pace for the exact same power for a given weight unless your form is crazy off. I suspect the device is in error.

    David

    #14355
    Jeremy Hutchinson
    Participant

    Another random thought if the weight has not changed (which can happen automatically on iOS if not turned off), has the calibration factor changed? I’ve heard of this happening after a firmware update turns auto-calibration for the foot pod back on and it scales your pace.

    I’m also assuming air power, if Stryd Wind, and gradient are the same for both comparisons?

    #14357
    winoria
    Moderator

    Yes, almost no wind, same gradient.
    It’s not just those two points of comparison. It’s also the fact that I almost always run the same or similar routes. I leave my house and know that the first 5 min WU at lets say 210 Watts would result in about a 9:15 pace. Suddenly, the 9:15 pace would require a 230 Watts or in other words: the same effort and same pace would show a higher power output. If it was something that would have happened once, I would not worry as it could be wind, etc. But I simply get a higher power reading.
    Regarding reaching out to Stryd: I was trying to ask this very question on their Facebook Support Group but my post was declined and I was asked to reach out to the Stryd support by email. Kind of makes me feel that they don’t want those kinds of issues to become public but rather hide them so that people believe the device is rock solid?

    #14372
    David Warden
    Keymaster

    Yes, something is amiss. I have high confidence in Stryd’s ability to take care of you (if they don’t, I have connections…, let me know).

    David

    #14380
    winoria
    Moderator

    thanks David.
    Here an update as I want to make sure no wrong information is spread.
    I did reach out to Stryd and as it turns out, it is all my fault (shame on me).
    I have one of these smart scales that I step on every morning.
    Obviously, I don’t bring it with me when on vacation so my weight remained unchanged from the last day at home till the first day I came home from vacation.
    I have to commit, that I didn’t know that the scale somehow updates the weight on Stryd..I know it does on TP, Garmin, etc….so the smart scale outsmarted me 🙂
    Anyways, I enjoyed vacation and the food and when I came back and stepped on the scale, I was a bit heavier than before vacation. The new weight was uploaded to Stryd and therefore my power data/reading changed.
    What still puzzles me though is the following:
    If I compare my Stryd to my bike Power Meter, I don’t understand why this would happen. The raw power I create on the bike does not change in dependency of my weight. 300 watts remain 300 watts. If I weigh 150lbs or 300lbs. It’s 300 watts.
    With the Stryd however, this isn’t the case as it is not a real power meter but a device that calculates power by a formula (I assume weight of athlete, elevation, pace, etc.) and comes up with a number. That can be a problem:
    Let’s assume an athlete is running a rFTP test. The outcome is 300 watts.
    Over the course of the next few weeks, the athlete is losing or gaining weight. That does not necessarily mean that his raw power output would change. Only his pace at a given power, right? But as per Stryd, the power output would differ based on weight? Or do I get this wrong?

    thanks
    Winoria

    #14383
    David Warden
    Keymaster

    Wow, this kinda blows my mind. I need to ponder this one. So many implications.

    A bike power meter uses a strain gauge to measure torque and angular speed to come up with power. Therefore, it is independent of the athlete’s weight.

    The Stryd appears to be an accelerometer, which in retrospect, of course it is!

    A strain gauge would require a constant length and path of the “lever” which a bike pedal/ crank/bottom bracket/hub provide. But, an accelerometer would be most applicable to a variable length and path, like a running stride.

    And, to calculate power most accurately from an accelerometer you would need the mass. Of course! Force = mass x acceleration. A Newton is the force required to move 1kg of mass per second, a joule is one newton meter, and a watt is one joule per second… ergo, one needs acceleration AND mass to calculate power from an accelerometer.

    Want to really wonk out? See http://www.georgeron.com/2017/12/stryd-running-power-model.html which is an estimate of the Stryd proprietary run power model, and in part states, “Ground forces are estimated from the acceleration-time signatures in the 3 axes multiplied by mass of the subject runner.”

    The implications here are interesting. Not only will your weight change from Monday to Friday, it’s common for a runner to lose 5+ pounds over the course of a marathon, and more than that over an IM. Therefore, your power becomes more and more inaccurate over a long event/run. At 165 pounds, your power is correct at, say, 280 watts, but at 160 pounds 7 hours later the Stryd is still assuming 165 pounds and the results are wrong.

    I’d still trust a Stryd over using HR and Pace, but need to think about this a bit more.

    Thanks for bringing this to our attention, winoria!

    David

    #14385
    MartinH
    Participant

    I can’t pretend to fully understand the physics, but the advice in the Stryd FB group is to never change your weight in the settings.
    Intuitively this has always felt wrong to me.
    Basically it appears that if you do make a change, you have to retest and establish your curve again at your new weight which is a pita.

    #14390
    winoria
    Moderator

    @MartinH
    By not updating the body weight, the power you are actually running will for sure be off though. E.g.: you weigh in at 165lbs and your rFTP is 300 W. If you never change your weight, your Stryd will keep assuming the same weight for all its calculations. If in reality, your weight is down to 150 W, you will have to run at a much higher force as in order to make up for the force you are missing because of your lower weight?
    As David mentioned, over a full IM race, this could be rather significant.
    Here a copy of the Stryd email I have received:
    I think most of the confusion is arising from the recent weight change you did on your account.

    When you changed your weight from 157 to 163, that scaled your power up about 4%. Your CP test was when your weight was set to 157, so that calculation is based on your older weight setting. If you took your 10k and scaled it back down as if your weight was still set at 157, the avg. watts would be 294W. Does that seem more reasonable to you?

    Based on the Stryd email above, 6lbs would cause a 4% change in power. A 6lbs weight change is easily realistic while racing….

    Very confusing.

    Winoria

    #14396
    MartinH
    Participant

    Yeah I’ve lost about 8lbs in this training block so that would make quite a difference

    #14408
    Jeremy Hutchinson
    Participant

    Tried to post the following a couple of day ago but the forum would not let me:

    If you’re on iOS Stryd app reads your weight from Apple Health, which
    your smart scales will have updated, and you have to go into
    Settings, Health, Data Access and Devices, Stryd and turn of Allow Stryd
    to Read Data Weight there. I’ve also turned off Stryd writing to
    Apple Health as it gets that info from Garmin. Probably something
    similar for Android.

    It’s a shame Stryd analysis is not done in W/kg which would negate
    some of the weight change problem, but I believe this was because the
    watches only work with W as it was the method implemented for cycling.

    As an ultra runner carrying a loaded pack which and consequent weight
    decline I questioned this when I started with Stryd, but most if not
    all indicated that the effects were small and not to be worried about,
    it certainly is better using Stryd than HR or pace.

    Added since then:
    I’m not sure how you could possibly account for changing weight during a long race eg in the IM examples above or for instance over a longish ultra where pack weight changes. Fatigue and other factors are probably taking over the small variations in power at this point.

    #15387
    winoria
    Moderator

    Hi @davidwarden,

    Have you ever thought about this in more detail?
    I still struggle with the fact that a change in weight is impacting power readings.
    Stryd actually recommends to not change the weight. But that doesn’t make any sense either?

    Thanks
    Winoria

    #15395
    Bstarr
    Participant

    Winoria,

    It’s simple physics. You’re moving a mass(weight) a certain distance in a certain amount of time. It takes a certain amount of power to do this. If the mass changes, then the amount of power it takes to move the same distance in the same time changes. So, if you change the weight setting Stryd is using and move at the same pace, the power calculated to do so would change.

    Think of it this way. If weight didn’t affect power, none of us would be worried about gaining 50 lbs. before our next IM and would eat whatever we want.

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