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Crank lenght from activity

Hello,

I tried to get the crank lenght used during a ride on a rental bike where I performed a CP20, and I'm not sure I set it right. But I cannot understand which value was used using https://fit2tcx.runalyze.com.

Files.zip

It was recorded with a Garmin Edge 830 and with Assioma pedals.

I attached the file, if anyone would be kind enough to explain me how can I get the value?

Thanks in advance!

Florent

  • Former Member
    0 Former Member over 4 years ago

    Florent,

    with help from this thread crank-length-data-from-fit-file

    answer is 187,5 = 125 + ( 125 / 2 ) >>> see correct answer below <<< should be 110 + ( 125 / 2 ) = 172,5

    find your connected powermeter in the fit2tcx listing

    search settings for this sensor with value 84640545

    happy & safe sporting

  • First of all thanks a lot for your answer and support.

    However there is something that I can´t quite understand. I was riding 172,5mm crank on this rental bike, while my "home" bike got the 170mm crank.

    Hereunder are 2 activities where I was with my own bike and where I'm sure I used the right setting (so 170mm). The difference between those 2 (if I remember well) is in one case I measured it via Bluetooth (15.02) and the other one via ANT+ (26.02).

    2020-02-26-07-17-21.zip2020-02-15-09-59-53.zip

    When I look at those data, using the formula: crank lenght= unknown9 + unknown9/2 doesn't quite match either. Instead it seems to me that crank lenght = unknown9 + unknown31. Is it something you are familiar with?

    If I apply it to the first file I sent in this post though, that gives me 175mm, value which I never entered in the Garmin, so maybe it's a resolution problem (can't have 122.5 so it writes 125?).

    Another related question: how do you know which device was used? As you see in the file, I've used other device in the past which are all registered in the Garmin, but how do I know which one was use for this activity?

    If anything I explained is not clear, please let me know.

    And again thank you for your support. It seems stupid, but this would set the wrong CP20 for my next weekend of training and I would like to be sure I'm using the right value.

    Thanks!

    Florent

  • Ride 26-Feb was using a Favero power meter with crank length of 172.5 mm

    Ride 15-Feb was using a Favero power meter with a crank length of 170.0 mm.

    Ride 20-Feb was using a Favero power meter with crank length of 172.5 mm

  • The issue in the earlier math is that the constant length that is added to the value that is halved should be 110 mm and not 125 mm

    so 110 + (125/2) = 172.5. 

  • Thanks a lot, then I have used the right lenght for my test, but strangely I didn't set it back correctly today, might be because I switched from ANT+ to BLE.

    Where are the 110 coming from, which line code?

  • The 110 comes from the ANT+ Bike power profile. The value being passed as the crank length is not the total crank length, but rather an offset from the 110 mm. You need to add the static crank length offset to the passed crank length value to get the total value.

    From the specification documented

    The scale of the crank length value is 0.5 mm, and an offset of 110 mm must be added to the received value to calculate full crank length.

    What is being passed is

    CL Value = (Full Crank Length – 110mm)/0.5 

    Examples:
    (1) To indicate that the crank length is 172.5, the value in the Crank Length field is sent as 125.
                (172.5 mm– 110.0 mm)/0.5 mm = 125.
    (2) A value of 130 is received in the Crank Length field. The length of the crank is 175 mm
               130*0.5 mm + 110.0 mm = 175 mm

  • Former Member
    0 Former Member over 4 years ago

    , my humble apologies for the misleading Wink

    , thank you for the kind rectification Slight smile

    learned something today

    happy & safe sporting