Cycling Power

Discussion in 'Cycling Training' started by LookAt, Nov 24, 2005.

  1. LookAt

    LookAt New Member

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    I have noticed that quite a few questions in this forum address the problem of how to verify generated Power (Watts), and-or training fitness.
    I would like to share my own experience on this subject.
    I have recently purchased a BC-401 cyclocomputer (www.clino.it) with real-time road gradient indication (it is based on a micro-accelerometer), that can also display the power generated, albeit only on uphill stretches of road. To me this is not a serious limitation, as I am anyway able to test how much power my body can generate, and I do not really care whether I can do this only while cycling uphill ... that's sufficient to verify my training fitness and how much power I can deliver for different styles of pedalling. I am using it in combination with a conventional Heart Rate Monitor, so that I can even cross-correlate BPMs with generated Watts ! On the market one can also find more professional power meters (based on torque measurement at the wheel), which allow the display of the generated power in a any condition (flat road, uphill, downhill), but which are very expensive indeed (around 1000 USD).
    OK, that with the BC-401 I can do this only while riding uphill might be a limitation, but at 1/10 the cost I do not mind at all, it's just a great instrument !
    It would be interesting to share our personal experiences also about other types of cyclocomputers featuring a power meter function.
     
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  2. frenchyge

    frenchyge New Member

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    My computer is doing funny things after clicking on that link. Believe virus or search engine hijacking spyware is involved. Beware.
     
  3. superVan

    superVan New Member

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    strange, I also clicked the link, but everything was absolutely normal. I
    am running Norton Internet Security, and LavaSoft spyware malware detector,
    and they did not detect any danger.
    Are you sure the source of your problem is not somewhere else?
     
  4. superVan

    superVan New Member

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    strange, I also clicked the link, but everything was absolutely normal. I
    am running Norton Internet Security, and LavaSoft spyware malware detector,
    and they did not detect any danger.
    Are you sure the source of your problem is not somewhere else?
     
  5. IronClaude

    IronClaude New Member

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    The item BionClino 401 is cool !! Great instrument, I clicked the web site www.clino.it no problem
     
  6. IronClaude

    IronClaude New Member

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    BionClino401 is a great instrument, cool !!!! I wish to try one of this new powerclinometer in future. I clicked on www.clino.it absolutely no problem
     
  7. dhk

    dhk New Member

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    If you can find the gradient of a climb, it's easy to estimate power required to go up the hill at a constant speed, ignoring he small aero drag for speeds of 10 mph or less:

    Power (watts) = (2 x weight(lbs) x % grade x mph) + 25w (friction losses)

    EG, for a bike and rider weighing 200 lbs, on a 10% grade @ 5 mph:

    Power = (2 x 200 x 0.1 x 5) +25 = 225 Watts
     
  8. Pureshot78

    Pureshot78 New Member

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    That equasion doesn't make sense.. a flat road would require 25w regardless of speed. Where is this equasion derived?
     
  9. dkrenik

    dkrenik Member

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  10. RapDaddyo

    RapDaddyo Active Member

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    I'm curious. When you ride on a hill of known grade at a given speed, how does the power reading of your bike computer compare with that computed by analyticcycling.com? I have a lot of confidence in this tool kit, especially for upgrades, based on comparisons with my PowerTap SL. Unfortunately, I don't have as much confidence in the kreuzotter toolkit. If convenient, it would be interesting to see the comparison at several different road grades. Thanks.
     
  11. squidwranglr

    squidwranglr New Member

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    Rap,

    Out of curiosity, why are you saying that you don't have as much confidence in the kreuzotter power toolkit? Are you saying it just because you haven't used it as much or because you've seen inconsistencies between it and analyticcycling?

    I find kreuzooter's user interface ten times more usable than analyticcycling's for quick calculations...

    Berend
     
  12. whoawhoa

    whoawhoa New Member

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    On another thread, riders with powermeters noted that it overestimated power compared to their pm. It also estimates cda based on body size rather than letting the user enter those figures. Cda is probably overestimated in that system, leading to the overestimated power outputs.
     
  13. RapDaddyo

    RapDaddyo Active Member

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    The kreuzotter interface is nice. I like the interface better than that of ac.com. But, when I have ridden hills of known grade and have looked at my PT power numbers, they are quite different from what I get when I plug in my data into kreuzotter. On the other hand, the numbers I get from ac.com are very, very close to my actual (measured) power numbers. My only issue with ac.com is that my speed on the flat is a little less than predicted by ac.com, even after trying to be pretty careful with the frontal area assumption. It may that the rest of my equipment (e.g., wheels) is not as aero as ac.com assumes. But, that is a very small inconvenience. Bottom line, I don't trust the power numbers I get from kreuzotter, but I do trust the numbers I get from ac.com.
     
  14. whoawhoa

    whoawhoa New Member

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    The default cd on ac is .5, I've followed the advice of some on the btr website and gone with .7.
     
  15. wiredued

    wiredued New Member

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    analyticcycling gave me acurate watts I double checked my flat watts with mountain watts and they were within 6 watts of each other if I guessed my frontal area at .69 instead of .7 I think they would sync up completely. I'm 6' 1" 195 lbs on the brake hoods wearing baggy shorts. If you need to get elevations of a hill or mountain google earth maps has a beta that works pretty good it does occasionally crash though dsl or broadband is required.

    http://earth.google.com/
     
  16. squidwranglr

    squidwranglr New Member

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    Interesting. I've used it mostly to see if the power measured by my Polar sensor has been accurate during climbs and it has generally come out within +/- 3%. These are climbs longer than 1 mile with grades greater than 5%, so my speed is usually less than 12 mph, so I think that the aerodynamic drag (and therefore CdA) component of the power estimate is a very minor component. Perhaps that's why I've seen better correlation - I'll look at some flat sections to see if the correlation gets worse.

    Berend
     
  17. dhk

    dhk New Member

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    The basic formula is just calculating the power required to lift your body and bike, and converting it to watts. You can of course use SI units, newton-meters/sec (watts), or english (HP = 550 ft-lbs/sec; watts = HP x 746. Using english units happens to work out close to the easy conversion factor of 2, so it's easy to remember.

    The +25 watts is a factor for minimum drivetrain and rolling friction losses. On a flat road, 25 watts should be good for around 5-7 mph. As I said, it ignores aero resistance totally, so is no good above 10 mph anyway. Actually, as the power input increases on a hill, the losses increase as well, without adding anything to your speed. Perhaps using a 10% factor would be more appropriate. EG, for 250 watts to the rear wheel, you need to input 250 x 1.1, or 275W to the crank.

    The major uncertainty and error is always in estimating the gradient anyway, so it doesn't matter much if you use 25 W, 50 W, or a factor of 1.1 to account for the drivetrain losses.
     
  18. LookAt

    LookAt New Member

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    With reference to all those questions about: how is power computed ?

    I have actually met the guys of BC401 at the Milan's (Italy) 2005 Bicycle Fair, this is what I recall of their explanations:
    - yes, as many have guessed the Watts are simply computed from
    Watts=(speed)*(weight's parallel component)
    - and indeed, BC401 allows the user to set the total weight cyclist+bicycle
    - the road gradient is measured by means of a micro-accelerometer, which senses the parallel component of the gravity acceleration, and therefore the above (weight's parallel component) can be easily computed by the instrument
    - the fact that a micro-accelerometer is used, instead of the usual barometric altimeter, allows to detect true real-time gradient (try it while holding BC401 in your hand: by changing its inclination, you immediately see the dispaly indication being updatad), whereas gradient measurements inferred from barometric altimeters need you to travel at least few hundred meters to obtain a reasonably accurate MEAN gradient
    - they further explained to me that BC401 uses a look-up table for adding also a correction factor for aerodynamic drag, which anyway would become typically negligible at road gradients larger than about 6 or 7 %
    - this aerodynamic drag compensation would mostly be important to improve accuracy for gradient between 2% and said 6-7%
    - below 2%, the Watts display goes blank (showing "--"), just to indicate to the user that Watts measurement are no longer displayed, as they would not be sufficiently accurate (because aerodynamic drag would start to dominate, and therefore differences in users' aerodynamic penetration coefficient, CX, would become too important)

    Overall, I am very happy of the smooth feeling one gets by observing variations in road gradient and power, while riding uphill.

    I find great fun trying to match the Vertical Ascent Rate of great champions such as Lance Armstrong:
    he is capable of 1700 m/h, try it to actually feel what it means ... this guy must really be a monster ! myself I have almost passed out, but I could not get more than 1100 m/h ...
     
  19. wiredued

    wiredued New Member

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    Why don't they make it so you can enter your frontal area and continue getting watts on the flats and downhills?

     
  20. LookAt

    LookAt New Member

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    Hello Wiredued,

    that's becoming too detailed for me to have an answer, but it is an interesting question, and so I have forwarded it to the guys at www.clino.it, informing them that people at cyclingforums might be interested in more technical details about the BC401 instrument.
    If they'll find the on-going discussion interesting, they might reply directly on this forum ...
     
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