power output

rdavighi
rdavighi Posts: 43
edited December 1969 in Workshop
I thought I would try and use my cycle computer and Excel to find my average power output for a ride. However, the results I'm getting seem quite high and I suspect there could be some error in what I've done.

My cycle computer gives me kilocalories for any given ride.

I then multiply by 4184 to convert into Joules (1 kcal = 4184 J)

I then convert my ride time into seconds and to work out
power in watts = energy in joules divided by time in seconds.

However, this gives average power outputs of e.g.

11.8 miles at 18.8mph average --> power output = 1195 watts

I'm not sure but this sounds quite high.
Any thoughts?

Comments

  • sloe
    sloe Posts: 17
    Check out

    http://www.kreuzotter.de/english/espeed.htm

    TBH I've not a scooby how accurate it is but it looks the business. And then I think you have to add on 2500 calories for baseline life-support.
  • vernonlevy
    vernonlevy Posts: 969
    <blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by rdavighi</i>

    I thought I would try and use my cycle computer and Excel to find my average power output for a ride. However, the results I'm getting seem quite high and I suspect there could be some error in what I've done.

    My cycle computer gives me kilocalories for any given ride.

    I then multiply by 4184 to convert into Joules (1 kcal = 4184 J)

    I then convert my ride time into seconds and to work out
    power in watts = energy in joules divided by time in seconds.

    However, this gives average power outputs of e.g.

    11.8 miles at 18.8mph average --> power output = 1195 watts

    I'm not sure but this sounds quite high.
    Any thoughts?
    <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

    Yep thats very optimistic!!

    I'd be very impressed if your work rate was 350W

    Having said that:

    I apparently used 2000Kcals in 6 hours

    Calc:

    2000 x 1000 x 4.184 = 8368000 J

    6hrs = 6 x 60 x 60 = 21600 secs

    8368000 / 21600 = 387W

    This is also a tad high but I am moveing a lot of lard around so it might be right though I think Lance Armstrong would be very happy with that sort of sustained power output...

    [:p]

    Your calorie calculator might be very optimistic.

    Try plotting your route on mapmyride.com and use the online calulator there to see what it estimates as your calorie burn.
  • simmers
    simmers Posts: 92
    Doesn't this assume that calories consumed by the body are converted with 100% efficiency to pedal the bike? Also, what assumptions are these calorie calculators using to come up with these estimates?
  • ut_o_cykla
    ut_o_cykla Posts: 58
    I think conversion efficiency is reckoned to be about 20-25% ie 1000 kcal on a bike is 250 on to the road. Heartrate monitors are programmed with an algorhthm (only passed O level maths in 1976!!) a in part according to intended use ie 'running' /'cyling' are slightly differnt (due to weight bearing aspects?) They are not truly accurate (too many variables) but provide repeatable information for any given individual (and the personal info you prgramme in).(all tis according to teh most helpful lady at heartrate monitors.

    Sites like Kreutzotter allow for more of the variables but in the end its only (good?) estimates based on sound maths.

    I think 387W sounds far too high high, 2000 kcal in 6 hours is 333kcal /hour - not much more than estimates for walking. For the default settings on Kreuzotter this would be about 23km/h at about 100W per hour.

    But all this with the above note about O levl maths in mind....:-)

    pousse moi s'il vous plait
    pousse moi s\'il vous plait
  • ASC1951
    ASC1951 Posts: 992
    <blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by rdavighi</i>
    However, this gives average power outputs of e.g.

    11.8 miles at 18.8mph average --> power output = 1195 watts

    I'm not sure but this sounds quite high.
    <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">Yes, that's far too high. I don't remember where I saw the figures - a delve around a TdF site might find them - but IIRC the peak power output for one of the big sprinters was about 1250 watts. Most of the Tour Pros couldn't get anywhere near that, even at peak power, and the average sustainable power at race speeds was about 400 watts.
  • sloboy
    sloboy Posts: 1,139
    You'll find nearly all estimates of this kind use an assumption of 25% conversion efficiency between the "food replacement" energy estimate they give you and the work done.
  • Unknown
    Unknown
    What a load of rubbish!

    18mph you're likely to be doing less than 200w, that is unless you're riding up Alpe d'huez where... it'd be a lot.

    You won't find any rider with an average sustainable power of 400w for a whole race! My mate who raced some of the spring classics said they averaged about 215w, and it is possible in some cases to finish a stage of the Tour at under 100w average. Rutland-Melton Classic on the other hand was 250w average and you'll find a French Elite National to be about 300w average, at least for the first couple of hours.

    As for sprinting, the top sprinters will push out a lot more that 1250w, more like 1600w. I can do 1350w and I can't sprint to save my life!
  • Adam Tranter
    Adam Tranter Posts: 621
    What would the rough average power from one ride tell you anyway?
  • hangsheadinshame
    hangsheadinshame Posts: 299
    It would tell you your rough average power for that ride!
  • Anonymous
    Anonymous Posts: 79,667
    power output = how big the smile is after the ride[:D]

    Non Omnis Moriar
  • Steve I
    Steve I Posts: 428
    I've got a Tacx ergo trainer with power output indication. For an average heart rate of 140-145 over an hour I usually average between 225-245 watts, though I don't know if it's an accurate power reading. On the road bike, the same average heart rate translates into 19-20 miles for 1 hour. The same average heart rate takes me about 8.5 miles running in 1 hour. The only racing I do is a hill climb once a year, to train for that I do a set of 4 minute intervals, I've never sustained more than an average of 360 watts over the 4 minutes (and that hurts a lot) so I'd say your estimate of power output is a way too high. The calorie readings on some HRMs are very high and can be treated with some scepticism. If you run, I'd imagine it's easier to work out power output, I've read that running uses roughly 100 calories a mile.
  • Greenbank
    Greenbank Posts: 731
    The human body is only about 25% efficient when cycling. In other words, to produce 200W of power output it is consuming another 600W of energy to pump the blood around your body, metabolise fat into energy, sweat in order to keep you cool, etc. This figure of 25% will be different for every person and is highly dependent on so many factors you could imagine (temperature, weight, etc).

    Sticking the following into google:-

    (200 watts) * (11.8 / 18.5) * hours in kcal

    gives: 109 kilocalories.

    (1 kcal is what most people call "a calorie").

    Your cycling computer is probably multiplying this by 4 to give something around 436 kcal.

    --
    If I had a baby elephant signature, I'd use that.
    --
    If I had a baby elephant signature, I\'d use that.
  • rdavighi
    rdavighi Posts: 43
    So, if I build an efficiency factor of say 20% to account for conversion of calorific intake to power output of bike I might be more on the right lines.

    In terms of what it tells me - well I'm just curious really.

    Thanks for advice - especially www.mapmyroute.com - really excellent.
  • Greenbank
    Greenbank Posts: 731
    <blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by rdavighi</i>

    So, if I build an efficiency factor of say 20% to account for conversion of calorific intake to power output of bike I might be more on the right lines.

    In terms of what it tells me - well I'm just curious really.
    <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

    It gives you a guess based on your speed and distance and a huge amount of assumptions. Don't compare it to anything else except other rides using the same bike and computer.

    Think about it. Did you enter any of the following information:-

    Age? Weight? Sex? VO2max (or some other magic "fitness" indicator) ?

    Maybe you did but, does your cycle computer measure:-

    Wind speed? Tailwind or Headwind?
    How aerodynamic you are?
    How hilly the ride was?
    Whether you were drafting anyone?
    The weight of your bike?
    Rolling resistence? (Imagine you a brake rubbing or a flat tyre)
    etc, etc, etc.

    --
    If I had a baby elephant signature, I'd use that.
    --
    If I had a baby elephant signature, I\'d use that.
  • rdavighi
    rdavighi Posts: 43
    I accept your point totally Greenbank about not using this data for any real comparison purposes.
    It just makes me happy doing this kind of thing! I certainly will not be declaring power ratings to all and sundry.
    I did have to input data into cycle computer when it was set up - height, weight, resting pulse etc.
  • Unknown
    Unknown
    Just a point to note: Tacx trainers do display a large inaccuracy when compared with a true power meter, I've had them showing 50-60w higher than the actual power.

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