Need Help Interpreting Power data

Discussion in 'Power Training' started by tmctguer, Jun 26, 2007.

  1. tmctguer

    tmctguer New Member

    Sep 9, 2003
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    I have two bikes outfitted with two different power meters and have done a 30 minute Tempo/LT ride on each bike. I am trying to understand why there are slight variations between the two nearly identical efforts.

    First the hardware configurations:

    1). 2004 Cannondale CADD 7, 54 cm frame, 172 mm cranks, full aluminum bike, 53/39 and 12/26 Campy with Powertap SL.

    2). 2005 Specialized Roubaix Pro, 54 cm frame, 172 mm cranks, full carbon fiber bike, 53/39 and 13/26 Campy with Ergomo Pro.

    Next the ride information:

    A 30 minute Tempo ride 1 week apart on the same indoor trainer. I have attached a Word file that lists key measures of both rides along with a min/max/avg comparison for other key measures. To my untrained eye, the ride statistics are very similar in terms of HR, cadence, and pace. However, I am seeing a 12 watt power difference between the two bikes. The aluminum Cannondale requires 12 less watts to produce .3 MPH higher speed at a virtually identical cadence than the Specialized.

    Here is where I need help:

    1). Could the more stiff aluminum frame of the Cannondale account for lower watts yielding a greater speed over the more pliant carbon frame of the Specialized?

    2). Could the slight variation in rear cassette gearing cause the difference? I don’t know which rear gear I was using on either ride, but I was in the 53 front ring on both rides.

    3). Could the variance be explained by the difference in accuracy between the two power meters?

    4). Is the variance so small that it is insignificant and immaterial?

    5). Could the rides truly be slightly different enough to account for the slight power and speed variation?

    6). Do I need to repeat two more rides and attempt to exactly duplicate conditions before any meaningful conclusions should be reached?

    A little background info……..I had ridden both bikes with Polar Power Meters since the bikes were purchased new. Even with the Polar units, I noticed I produced less watts but similar or higher speeds on the Cannondale than the Specialized during rides. Back then, I chalked this up to a potential quirkiness of the Polar units.

    However, about 9 months ago, I converted my Specialized to the Ergomo and noticed that the Ergomo was yielding similar power results than the Polar I just removed. 9 months later, after converting my Cannondale to a Powertap, I see that it is yielding slightly lower numbers than the Specialized similar to the Polar. as a side note, I think I’ve proven at least to myself that the Polar Power Meter is a reasonably accurate meter for the price.

    The reason for my post was after watching the DVD that came with my Powertap, I heard Dr. Lim explain that anything that causes you to produce the same speed with less watts is a GOOD thing. While this is usually attributed to aerodynamic positioning, it dawned on me that the Cannondale might be a better bike for me because I could use less watts to produce a faster speed than when riding my Specialized. Up until I heard this concept, I thought that my Cannondale with less watts might actually be a bad thing.

    So now that I have two good meters, can the more astute power gurus of the group help me answer the 6 questions posed above? Or, perhaps help clear up what I may need to do in order to clear up this mystery?

  2. frenchyge

    frenchyge New Member

    Apr 3, 2005
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    I'd guess it's mostly due to the fact that the Ergomo measures torque at the bottom bracket, whereas the PT SL measures at the hub (and therefore downstream of frictional losses in the bottom bracket bearings and chain). Even if they were both on the same bike at the same time, the hub meter would read lower.

    Some other possibilities could be:
    1) differences in meter accuracy and calibration (still within tolerances)
    2) differences in tire rolling resistance caused by inflation, wear, and pressure against the resistance unit of the trainer
    3) differences in trainer temperature between the two runs or differences in warmup times

    12 watts would be a significant difference in my view. I'd stick with one meter for all my trainer efforts.
  3. Alex Simmons

    Alex Simmons Member

    Mar 12, 2006
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    I'd suggest sticking the Powertap on the ergomo bike and do a ramp test and check the calibration of each meter. 12W doesn't sound a lot though given the circumstances. You could adjust the ergomo to match the PT and then obtain consistent readings no matter which bike you ride.
  4. NomadVW

    NomadVW New Member

    Aug 12, 2005
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    The fact that there's a 69 watt difference in the minimum power can't go unnoticed. Enough of these outlier's in your data will skew the average for sure.

    How long was the warm up to the tempo ride will change the warm up of the trainer and cause drops in power requirements for equal speed on even the best trainers. Not the same wheels, and assuming not the same tires - was the pressure in the tires identical and the contact with the tires equal between the two rides?

    Take 10 PSI out of the tires, and put the trainer back on. You'll see a significant difference in speed vs power relationship.

    All of these variables come into play and aren't expounded on in your information.
  5. TamMan

    TamMan New Member

    Aug 28, 2006
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    I highly recommend to put the PT on the Ergomo bike. I also have both and found that my Ergomo wasn't reading correctly (ended up re-installing the Ergomo). I believe that you will get much better data riding both on the same bike for a while. Also, I would probably hit the road for this comparison exercise rather than using the trainer. You are certain to get a lot of jealous looks from fellow bikers with two PMs on the bike :cool:
  6. peterwright

    peterwright New Member

    Mar 5, 2003
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    I had a similar experince with the Ergomo on my PT equipped bike - the Ergomo was 4-15% low dependent on power levels. I was unable to get the two to track correctly by adjusting K Factor (either they tracked at power ~<FTP or ~>FTP but not both) I also found that the Ergomo would read power output of 30-90w while simply turning the pedals over while coasting downhill (Ergomo say that as long as you are putting torque through the pedals then there is power output) - this skews AP substantially over a longer ride.

    I would concur with the post re sticking to one (accurate) PM.