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  1. #1
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    Powertap "stomp test"? Static torque test?

    All of these threads about powermeter accuracy have gotten me curious. I zero my Powertap regularly but haven't compared it against anything else, and I sold my second PT hub a while back so I don't have anything to compare.

    I've seen mention of a "stomp test", but don't really know what that means.

    My thought is just to level the crank arms, hang some weights from the cranks, set the PT on torque mode and calculate the torque I should be seeing. Do this with a few different weights, plot the line.

    Any better methods?
    Kendall Frederick

    Orange Park, FL

  2. #2
    Burning Matches. ElJamoquio's Avatar
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    That's really the best method.
    Reacting is mind candy; it requires no thought. Thinking is tedious.

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  3. #3
    Burning Matches. ElJamoquio's Avatar
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    Oh, Stomp test means *you* are hanging from the pedal, rather than weights.

    If you want to reduce inaccuracies, make sure you know how heavy your weights really are, by the way, and measure your real crank arm length.
    Reacting is mind candy; it requires no thought. Thinking is tedious.

    Bikeforums 'Group Buy': Kinlin Rims, Sapim/DT Spokes, Formula/Bitex Hubs


    "The only good race pace is suicide pace, and today looks like a good day to die."
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  4. #4
    Quarq shill cslone's Avatar
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    From midweek FAQ

    First, check that the transmission icon is on, and if not, give the rear wheel a spin. Then, enter the torque mode by holding the “Select” button down for 2 seconds or longer (the “WATTS” designation will disappear from the top line.) Apply the rear brake sufficiently to lock up the rear wheel. Now, measure torque as follows: with the cranks exactly horizontal (right crank at 3 o’clock), hang a known weight of at least 50 lbs from the right crank, or simply stand on it – hence the name ‘stomp test’! Measured torque = (weight in lbs) (crank length in mm) (1 in/25.4 mm) (cog teeth/chainring teeth). For a 159 lb rider standing on a 175 mm crank, with the chain on the 39 tooth ring and the 23 tooth cog, 159 lbs 175 mm 1 in/25.4 mm 23/39 = 646 in-lbs. Compare this to the displayed value by calculating % error as (measured torque - displayed torque)/measured torque.
    FS: Fuji SL1 frameset, 55.5cm toptube, excellent condition.

  5. #5
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    Quote Originally Posted by KendallF View Post
    My thought is just to level the crank arms, hang some weights from the cranks, set the PT on torque mode and calculate the torque I should be seeing. Do this with a few different weights, plot the line.
    Better than trying to level the cranks, start with them slighty below horizontal then rotate the wheel backwards while watching the torque until they're a little above level. The maximum value will be when the arms are horizontal.

  6. #6
    umd
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    Quote Originally Posted by KendallF View Post
    All of these threads about powermeter accuracy have gotten me curious. I zero my Powertap regularly but haven't compared it against anything else, and I sold my second PT hub a while back so I don't have anything to compare.

    I've seen mention of a "stomp test", but don't really know what that means.

    My thought is just to level the crank arms, hang some weights from the cranks, set the PT on torque mode and calculate the torque I should be seeing. Do this with a few different weights, plot the line.

    Any better methods?
    That's what I did with the Quarq...

  7. #7
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    Quote Originally Posted by asgelle View Post
    Better than trying to level the cranks, start with them slighty below horizontal then rotate the wheel backwards while watching the torque until they're a little above level. The maximum value will be when the arms are horizontal.
    That's a good idea...thanks!

    I was going to try and do this before leaving for Six Gap, but I don't have time to go chase down a scale tonight. Guess I'll try it some time next week.
    Kendall Frederick

    Orange Park, FL

  8. #8
    Making a kilometer blurry waterrockets's Avatar
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    Note that when rolling the wheel back, you are accelerating the weights, so torque will read higher. Go slow or maybe even just go a cm (at the tire) at a time and stop.

  9. #9
    部門ニ/自転車オタク NomadVW's Avatar
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    Or just have two of them. You'll know pretty well whether or not one is off from the other.
    Envision, Energize, Enable

  10. #10
    ..... Jynx's Avatar
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    Quote Originally Posted by asgelle View Post
    Better than trying to level the cranks, start with them slighty below horizontal then rotate the wheel backwards while watching the torque until they're a little above level. The maximum value will be when the arms are horizontal.
    Correct although is the powertap that sensitive to get readings of just the torque created by the just the crank and pedals?

  11. #11
    smell'n bacon bikeride's Avatar
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    How accurate is Powertap when it comes out of the factory? So far I've only done the torque zeroing thing.

  12. #12
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    Quote Originally Posted by waterrockets View Post
    Note that when rolling the wheel back, you are accelerating the weights, so torque will read higher. Go slow or maybe even just go a cm (at the tire) at a time and stop.
    I'd have to be slinging the wheel around like roulette to get significant centripetal force.

    Quote Originally Posted by Jynx View Post
    Correct although is the powertap that sensitive to get readings of just the torque created by the just the crank and pedals?
    I believe Asgelle's suggesting doing this with the weight attached. Move the wheel slowly and watch the torque readings; when you see a max, that's when the crank's level.
    Kendall Frederick

    Orange Park, FL

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