Any way to measure power output (watts) without powermeter?

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  • neeb
    neeb Posts: 4,473
    So the percentage effect of air resistance with a 2m/s wind roughly halves when you go from a 6% gradient to a 10% one. Will that relationship be exponential, i.e. if you go to 12% gradient will there be a bigger drop in the influence of air resistance?

    All very nerdy I know, but I like to get to the bottom of things when I'm wrong... :wink:
  • desweller
    desweller Posts: 5,175
    FWIW, the power required to overcome aerodynamic resistance is a cube law with speed, i.e. if you double your speed you will require eight times as much power to overcome the resulting air resistance. Hence at low speeds aero is not important, and low speeds are generally what people are doing up hills.

    I've always wondered why no-one's ever made a cycle computer with a basic pitot tube. It wouldn't be that hard to do, would it? It would look cool as hell to have one of those sticking out in front of your handlebars. The data could be used for all kinds of things, and it would answer questions like this very effectively.
    - - - - - - - - - -
    On Strava.{/url}
  • Alex_Simmons/RST
    Alex_Simmons/RST Posts: 4,161
    DesWeller wrote:
    I've always wondered why no-one's ever made a cycle computer with a basic pitot tube. It wouldn't be that hard to do, would it? It would look cool as hell to have one of those sticking out in front of your handlebars. The data could be used for all kinds of things, and it would answer questions like this very effectively.
    They are. A project I have involvement with:
    http://andyfroncioni.com/2010/03/the-ch ... k-project/
    RST Sport : Coaching - Training Plans - Powermeters -- My blog -- AeroCoach Australia
  • Alex_Simmons/RST
    Alex_Simmons/RST Posts: 4,161
    DesWeller wrote:
    FWIW, the power required to overcome aerodynamic resistance is a cube law with speed, i.e. if you double your speed you will require eight times as much power to overcome the resulting air resistance. Hence at low speeds aero is not important, and low speeds are generally what people are doing up hills.
    Yes - it's a cubic equation. On flatter gradients the cube part of the equation is more dominant, on steep inclines the linear part of the equation is dominant.

    For the example above with 10% gradient and 2m/s headwind, the power for steady state speed goes to overcoming the resistance forces in the following ratios:

    Air resistance: 6.0%
    Change in kinetic energy (acceleration): 0%
    Rolling resistance (tyres): 4.5%
    Bearing friction: 0.1%
    Change in potential energy (gravity): 89.4%
    RST Sport : Coaching - Training Plans - Powermeters -- My blog -- AeroCoach Australia
  • Team Banana Spokesman
    Team Banana Spokesman Posts: 538
    Jeff Jones wrote:
    I've found similar results for climbs around here. 7'15 (light tailwind) and 7'40 (light headwind) for the same climb @ exactly the same power on two different occasions.

    It's not a shallow climb either, averaging 7.4% for 2.2km.

    almost the same for me too. theres an 8 min climb which comes down to about 7:30 in a tailwind.

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