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  1. #1
    Rides again HiYoSilver's Avatar
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    What's the watts power formula?

    Ok you whizzes, what's the formula for watts given

    1. distance
    2. avg speed
    3. total bike/rider weight
    4. avg wind
    5. elapsed time
    6. avg slope
    7. gears used

    Or is my only option buying a power hub or different fancy dancey cyclometer?
    Hi 'o Silver away

  2. #2
    Senior Member SirScott's Avatar
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    FTW Racing Crew

  3. #3
    Senior Member DannoXYZ's Avatar
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    It's really hard to compute because you must know the wind-drag of the bike+rider combo. One person going at 30mph may require 400w while another with that power can only achieve 25mph. Power meter's the only accurate way.

  4. #4
    Overacting because I can SpongeDad's Avatar
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    http://www.kreuzotter.de/english/espeed.htm

    Calculator (at least for ball parking) and some equations on calculation of power.
    “Courage is going from failure to failure without losing enthusiasm." (Churchill)

    "I am a courageous cyclist." (SpongeDad)

  5. #5
    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by CrimsonEclipse
    How does a power meter work?

    CE
    It's one of these types of strain-gauge devices that measures instantaneous force. That measurement is then used in a integration equation with RPM and time to calculate power. Similar to how a dyno works for a car that measures instantaneous torque at the wheels. That is used to integrate with RPM vs. time to come up with power (area under the curve).

    CycleOps Powertap
    SRM power meter

  6. #6
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    Quote Originally Posted by DannoXYZ
    That measurement is then used in a integration equation with RPM and time to calculate power.
    Goodness, I hope my bike made better grades in Calculus then I did.

  7. #7
    Rides again HiYoSilver's Avatar
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    Quote Originally Posted by SpongeDad
    http://www.kreuzotter.de/english/espeed.htm

    Calculator (at least for ball parking) and some equations on calculation of power.
    Forgot about this one. This is perfect. Thank you.

    Now how do I figure slope? I only need beginning elevation, ending elevation and distance, right?
    Hi 'o Silver away

  8. #8
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    You have got it wrong in trying to calculate the watts required for this or that conditions. Riders have a limitted sustainable power output and have to use what they have. What you really want to know is how many watts per heartbeat. As you become more efficient, you may be able to nudge this figure up a little.

    If you meet a headwind or a slope, you can't increase your power you just have to slow down.

    The other side of the problem is getting as much speed as possible for your power output by choice of position, equipment, riding style etc.

  9. #9
    Rides again HiYoSilver's Avatar
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    This was more of a conditioning question. I thought it might be better motivation to use rough guage of watts created than speed. Speed increments are starting to come slower and do not account at all for winds. So I thought starting to focus on watts would be better. I've never been a sprinter, but more of a plugger. Also don't have a HRM, so the idea of using watts at a given HR doesn't appeal to me. I ride up to a certain level and then stop pushing it. I do work on conditioning, but also have ideal of riding as fast as can without sweating.
    Hi 'o Silver away

  10. #10
    Senior Member DannoXYZ's Avatar
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    Yeah, watts may be better because it measures your exertion. If you ride into a headwind, up or down a slight incline, the resistance will change along with speed. But if you monitor watts and keep that constant, you can maintain a more steady exertion level for more even workout. It's a more instantaneous reading without the time-delay of HR. But without getting an SRM or Powertap that would measure actual watts output, I wouldn't bother with those simulated watt displays. Getting a HRM would be much more helpful.

  11. #11
    Rides again HiYoSilver's Avatar
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    Quote Originally Posted by DannoXYZ
    Getting a HRM would be much more helpful.
    How so? Slower heart rate can either mean in better condition or not trying as hard. Higher heart rate can mean sickie or out of shape or trying hard.
    Hi 'o Silver away

  12. #12
    Senior Member DannoXYZ's Avatar
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    Well, if you've got a meter that reads out a calculated 400w when you're actually pushing anywhere from 350-450watts, it's not much use compared to a HRM that's indicating a true 160bpm +/- 2bpm. Once you've measured your actual max-HR with biking and treadmill tests, you can also determine your LT through the 2x20 test. With this piece of data, you can then structure your training to gain maximum improvement rates.

    You have to remember without an actual connection to the crank or rear-hub, those meters are only give you an "estimated" wattage output, like a guy from the electric company sitting at the kerb looking at your house and saying, "Well, 3000sq.ft.. should have 5-6 bd/3-4 bath/2-car garage/kitchen... that should be 1000-kilowatt/hr per month... let's charge him $250 this month..". Only by actually measuring the actual energy-flow through the system can you come up with a realistic power-output figure.

    A HRM measures the actual BPM that your heart is doing...

  13. #13
    Rides again HiYoSilver's Avatar
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    Ok, makes sense now. FYI I'ld guess my watts are about 250 and target HR is closer to 130 or 140 max. Thanks for the explanation.
    Hi 'o Silver away

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