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Power Meter Calibration Differences

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    #32
    doozerie
    Registered Users Posts: 4,830 ✭✭✭doozerie


    nilhg wrote: »
    The Neo couldn't be using your cadence to calculate power, it would have to be measuring rpm of the shaft the cassette is on and measuring torque somewhere in the same area, whether by a strain guage or some calculation based on the magnetic forces involved in the resistance unit

    Thing is though, the formula for power that I've seen cited most often (from multiple sources) is: power = torque x cadence

    Sometimes some static factor is cited as a multiplier on the right hand side of that equation too, I imagine some factor is always used but just not mentioned most of the time for simplicity.

    The simplicity of the formula appeals to my simple mind. Take away cadence and that formula no longer works. And if you measure the rotational speed of the rear axle/spindle/cassette you are obviously not measuring cadence since different cadence will give you the same rotational speed at the rear for different gear combinations.

    There is probably a simple explanation that I am missing here. Maybe the "torque x cadence" formula is itself a simplification of something more complex that is measurable directly within the heavily controlled environment of a turbo trainer i.e. maybe cadence is only used when you don't have access to the rotational speed of the wheel/flywheel? I guess that would fit with your suggestion above, and also how PowerTap hubs work as Crocked mentioned.


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    #33
    nilhg
    Registered Users Posts: 5,144 ✭✭✭nilhg


    doozerie wrote: »
    Thing is though, the formula for power that I've seen cited most often (from multiple sources) is: power = torque x cadence

    Sometimes some static factor is cited as a multiplier on the right hand side of that equation too, I imagine some factor is always used but just not mentioned most of the time for simplicity.

    The simplicity of the formula appeals to my simple mind. Take away cadence and that formula no longer works. And if you measure the rotational speed of the rear axle/spindle/cassette you are obviously not measuring cadence since different cadence will give you the same rotational speed at the rear for different gear combinations.

    There is probably a simple explanation that I am missing here. Maybe the "torque x cadence" formula is itself a simplification of something more complex that is measurable directly within the heavily controlled environment of a turbo trainer i.e. maybe cadence is only used when you don't have access to the rotational speed of the wheel/flywheel? I guess that would fit with your suggestion above, and also how PowerTap hubs work as Crocked mentioned.

    The formula i was thought in leaving cert physics was power = torque x speed, so in a rotational system like a crank then speed is in rpm, or as cyclists say cadence.

    That formula only applies if all the inputs are working on the same system so you can't take the cadence figure from your crank and apply it to the torque generated in the trainer, only the cadence/rpm of the trainer shaft will work.

    AFAIK turbo trainers derive a cadence figure from tiny changes in the power applies as you go through your pedal stroke, ie they count the dead spots and divide by two so if you hit a sweet spot where your stroke is particularity efficient then it will struggle to report correctly.


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    #34
    harringtonp
    Registered Users Posts: 2,568 ✭✭✭harringtonp


    Chatting to a club mate last night who has a stages which consistently under reads by 40W across a wide spectrum of power levels. Unlike me he was able to test it against a turbo with power and garmin vector pedals and saw the constant difference as he increased power.

    I know of other riders who have had Stages that I felt over read (compared to my P2M) and I suspect difference versions and firmware can behave differently.

    The leg difference was cited to him too from multiple sources but he did a watt bike test and there was little discernible difference.

    When I think about how fickle the mechanism is I'm not at all suprised... you are measuring deflection at various points in the arc of the pedal stroke and trying to map these amounts to torque values


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    #35
    tuxy
    Closed Accounts Posts: 14,983 ✭✭✭✭tuxy


    Constantly under/over reading is fine for most people if you're training to power with just one PM the the numbers don't matter once it can be replicated.

    dc rainmaker did do a tests on a one sided 4iiii and found it to be very accurate or at least as accurate as a one sided setup could be.
    https://www.dcrainmaker.com/2015/12/4iiii-precision-review.html


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