Which power is lying to me? | 80/20 Endurance

Which power is lying to me?

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  • #17350
    Hal9000
    Participant

    Hello,
    I’m wondering which power meter is lying to me or inacurate. I have been using a BKool smart trainer that is wheel on for over 3 years now. I have used it with other training programs and use the built in FTP test to determine my threshholds. About 2 months ago I bought some Favero Asiamo Duo power pedals. I have used the pedals on the real road and power definitley feels different then the trainer. Both connect to my Garmin FR945. Today I decided to mount my pedals to the bike I have mounted on my trainer. I diconnected the trainer from my Garmin and used only the pedals for power during my CCI18 workout. I had imported the workout into the Bkool trainer program and ran that workout in unison. During the workout my Garmin and pedals were anywhere from 15 to 20 watts higher then the smart trainer reading. So back to the question which power is lying to me? Both workouts are still in my training peaks.

    #17351

    Hi – that will almost certainly be your wheel on trainer (just for clarity wheel on means that you put the bikes rear wheel tire onto a resistance cylinder and then ride?)

    Whilst the trainer itself will be precise in terms of its power measurement, these types of trainers are fairly innacurate when it comes to power output and riding because there are so many variables involved (tire temperature, tire pressure, tire material, resistance setting, frequency of calibration). Direct drive smart trainers (where the trainer is the rear wheel) remove this problem entirely and are very accurate with power….However they are much more expensive.

    Consequently your pedal power meters are likely to be most accurate.

    Hope that helps

    Gareth

    #17374

    Hal9000,

    I would say neither are lying to you. Power Meters will all have a slight difference in accuracy and calibration through the way they gather RPM’s and Force. Power = Force x Distance / Time/.

    What you are looking for from a power meter is accuracy and consistency. So if both of these power meter are consistent they can both be doing their job, but one may be more accurate than the other. It depends on a lot of factors and calibration is also very important. If you are going to be switching around between power meters then you need to know the accuracy of each one and the best way to do this is an FTP test that comes from both – but this becomes problematic when trying to train as you would have to switch your fTP number when using the different power meters. AS you have the power pedals you have and easy solve in that you can use them as your single source of power info for both indoor and outdoor riding. So get your FTP from the pedals, use that for your zones and ensure that you calibrate them when shifting from indoors to outdoors to improve accuracy.

    Leyla

    #17377
    Hal9000
    Participant

    Thank you both for the insight and input. Leyla as you said I will likely use the pedals as the source for my FTP. Since it was recently purchased and that is the beauty of them I can switch easily between bikes. As you said Favero also recommends calibrating each time I swap between bikes. I’m a little excited to see what my FTP is on the trainer when using the pedals as my power output device.

    Hal

    #17381
    winoria
    Moderator

    Hi Hal,

    a few thoughts and recommendations:
    As Leyla said, it’s not uncommon to see differences in power meter readings. Most power meters read within 1-2% accuracy, but if one reads -2% and the other +2% you have a gap of 4%. So if your trainer shows 200 watts, it could be that your pedals show 208 Watts.
    Now, I personally think there are a few solutions for that. I have a similar situation – I use a direct drive trainer (Saris S3) which I consider to be my “benchmark”. When doing an FTP test, I record the workout using the Saris readings but at the same time, I either use my bike head unit or watch to also record the readings from my Garmin Vector 3. Doing a FTP test with a few surges at higher power output as well as some Zone 1&2 during WU plus the 20min effort for your actual test will give a a good comparison of both devices across different zones.
    In my case, I know that my Vector read 8 watts lower than my saris. So I simply adjust my zones when riding outdoors according to that.
    The other option would be to use the bike power meter all the time and only use the trainer as your “adjustable”. Platforms like Zwift or Trainer Road will let you do that. You only need to hook up the trainer for power and as adjustable and once done, you change the power meter from your trainer to your bike power meter.
    The second option has the benefit that you always use the same power meter but has the disadvantage that you potentially need to swap the power meter when riding your regular road bike, your TT bike or your trainer.
    Should you have several bikes with several different power meters, I recommend to make your trainer your “benchmark” and use all different power meters/bikes on the trainer for comparison. It doesn’t always need to be an FTP Test to compart the meters, but it should be a workout that includes easy zones as well as hard zones. A mixed interval workout is ideal as you would ride Zone 1/2 but also some zone 3 and 4.
    If you do the first proposal, you need to adjust for the different power zones in TP after a ride and recalculate the ride to avoid a wrong TSS and a PMC that lies to you (see recourses about the PMC that lies to you:-)

    Happy cycling
    Winoria

    #17389
    Hal9000
    Participant

    Thank you Winoria. Very good discussion here. I have figured out the best option. Today I completed the CF9 workout. I was able to successfully connect my power meter pedals to the Bkool trainer app. This along with the trainer conected worked great. My pedals were the power and both my fr945 and the app were in sync. Previously there was a difference of 20 watts between the trainer power and the pedals. CCI18 workout the trainer was reading 140w and pedals were 160w. I’m really kind of excited to complete a new FTP test next Tuesday when I have a CT workout. I would imagine I’m going to see a large improvement in the numbers. In regards to swapping the pedals between bikes that is actually very simple and one of the primary reasons I went with the Favero’s and pedal based. Thanks again everyone!

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