Power power power!

SRM wiresless vs. Taxc Flow vs. PowerTap SL 2.4. All calibrated properly. Discuss..

one-on-one-113-powertap-030

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14 Comments

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14 responses to “Power power power!

  1. T

    The Tacx measurement can be thrown out because it’s calculated instead of actually measured.

    The other two? Shit, I dunno. They’re within about 7% of each other, so it could just be a matter of when the sample was taken and how often the display refreshes.

    My guess would be that you were slowing down ever-so-slightly, and the SRM caught it before the PT. Check the cadance…the SRM is at 87, the PT is at 88. If the cadances were reading the same, the power would likely be closer to the same, or at least withing the spec’d accuracy %.

    Like I fuckin’ know…

  2. timmer

    the cadence was pretty close all the time.. yea, i wrote of the tacx reading a while ago but it’s relative but obviously fluffed up. the PT and SRM were pretty consistently off.. at 300 for the PT it was higher so it’s exponential.. i have yet to down load averages which should give a little more accurate differentiation between the two. In any case I’ll describe my power experiment(s) with one word; distracting!

  3. redBeard

    jest ride yer bike……

  4. Butkiewicz

    Hands down the SRM is the way to go. Can be used with any wheels. Can be used with a trainer or rollers. The only thing that sucks is no Mac program but that can be worked around. The wireless now has a crazy memory so you don’t have to download after every ride plus the extra weight is minimal. Difference between it and the PT readings don’t really matter once you find out what your SRM numbers mean. It will change the way you ride TTs.

  5. I rode a Tacx with my PT this past weekend (Foss’ place). It didn’t quite have that much discrepancy (209 to 265).

    Seemed to be consistently off by a few watts but not that much.

    When you said they were all calibrated, are all taking a measurement at the same interval (i.e., every second, every 3 seconds, etc.)? I know PT allows you to adjust.

  6. Pete

    Yo Timmer,
    SRMs are the way to go if you want power and have the coin. Saying that the Slope calibration from the factory is hit or miss which could account for the discrepancy between the 2.4 and the SRM. The SRM all things being equal should read about 4watts higher than a properly reading PT. You can do a “stop test” to verify the accuracy of the 2.4…
    Not quite sure if the Slope procedure is the same on the wireless SRM as the wired though…

    have fun; your numbers will be really freaking high

  7. Pete

    that should have read “Stomp Test”
    so much for my proof reading skills

  8. timmer

    i sloped that fucker pete.. how’ve you been man? who ya riding for?

    my numbers are low.. but so’s my position so the compromise of power in lieu of aerodynamics is evident..

  9. Ian

    Man, that is f’ed up. I went to a seminar on power a few years back at CU conducted by Dr. Allen Lim. While he is in bed with PT, he was able to show that the two systems were within a reasonable margin of each other. So, I don’t know. If you have a new SRM maybe the strain gauges are still sticking a bit. Were rides done in consistent temperatures? I got so sick of my SRM with my coach that I just stuck tape over it, rode, and blindly sent him the files.

  10. Pete

    The powers that be in Blue (gsca) have given me another chance this year so we’ll see how it goes.

    I’ll see you when it warms up (-25 now yikes)

  11. DaveL

    Hmm. Since the SRM is picking up power upstream of the PT you would expect that to read 3-7% higher not the other way around. So the actual difference here is much greater. No?

  12. mark0

    As above, the exact sample can be off and you get a different reading. If they are set at 1 second intervals, it would be hard to know when each one recorded what part of the power area you were in. I’d do a FTP test for each and see how close you get. If they are still off, it really doesn’t matter as long as it’s consistent. You would like to boast of the higher number, but what’s good for you and the increase is all that matters.

  13. Jeff Burmeister

    I also have the Tacx unit, and last night I was doing an upgrade of the motor brake when I read the following in the Manual.

    Scale setting – The factory-set scale factor ensures that you read the correct power on the computer.
    The value relates directly to the magnet unit of the trainer and can only be calibrated with
    a special power meter. Using the +/- key, you can set the scale factor between 80 and 120.

  14. JT

    my $0.02…

    I wonder if people ever stop to think that they are actually holding themselves back by using powermeters?
    You are subconsciously, physcologically limiting your effort because mentally, you believe you are a “this” level and your PR is “this”. The mind is a powerful tool. Its also a powerful mind-fuck. There are guys out there who are beat before they even get to the line because their SRM numbers weren’t where they usually are during their warm up – so then they start to mind-fuck themselves thinking they are ‘off’ that day.

    Or riding a pursuit or road TT and you are feeling great, and having a pro-day based on your SRM – but your mind starts to fuck with you and you start to think you better float a little more because you have never seen numbers like these and if you stay on this pace you are going to pop like a stuck pig for sure – so you throttle back – and thus miss the opportunity to shatter a PR – because who knows if you would have blown or not? It could have been “your day”. but you shut it down in ‘fear’ of the numbers. Sometimes the ‘unknown’ is fun, uncharted territory is fun – its how you learn – but there are geeks out there who do live and die by those numbers, and in some cases, I believe its even hindering them to succeed to their fullest potential.
    There is a time and place for meters, use them wisely.

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