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Vector 3 accuracy thread

Teriemer
over 7 years ago
So here we go... This is the accuracy thread for the V3.

Now for my first post here, I need to inform you, that my comparison is with a Kettler Ergoracer S (2016 model). Note this bike has a flywheel which resistance is being controlled electronic. The bike is calibrated according to a DIN std. and the bikes comes along with the calibration sheet. My bike is slightly below actual wattage. As a simpel test, I do get the same result when doing a FTP test either on Kettler or outside with a different powermeter. So the wattage must be about right.

Back when I had Vector 2, one funny thing, I had to set the pedalarm to 187 mm (actual is 170 mm) to get the same wattage as Kettler(!). Never the less is worked quite well. With that setting I got good comparable data from both sources. So when getting V3 in my hands, I started out setting the pedalarm length to 187 mm. Once done riding, I compared the data. To my surprise and happiness, I found that V3 had to be adjusted to 171 mm to perfectly aligned with Kettler. Sweet thing - so I adjusted to 170 mm. However on the following ride, V3 was clearly not showing enough(?!). Going through the same comparison, I calculated the adjusted pedalarm to now 198 mm...!?!

So my very first conclusion here, based on only two rides indoor on my Kettler with V3 is the following. To get aligned wattage, I see pedalarm length varies from 170 to 198 mm. That's a difference about 16%

Check out the charts below and let me hear your thought...

EDIT: can't upload my second picture. I'm gonna try again later...
ciq.forums.garmin.com/.../1280575.png
  • 0 over 7 years ago
    Reading your post on your blog Shane, it's quite obvious to me that V3 suffers some cadence issues.

    Yes - more data is needed - but it's very interesting to see how power output changes when CAD changes. Yet P1 doesn't change in the same test.

    Garmin
    You may want to review your power sampling cadence data...?

    I would assume this is a quick-fix with a software update...
  • 0 over 7 years ago
    Yeah, something is up..... and I'm spending many hours trying to ensure it's not me.

    I want/need these pedals to be 'the answer' to the power meter question. It'll give me something to refer people to and be done with it. More time to spend on indoor trainers.... but something is screwy and I'd love to know what.

  • 0 over 7 years ago
    Vector-Info

    What's your take on the cadence-thing..?
  • Former Member
    0 Former Member over 7 years ago
    More than willing to take a look. gplama Would you be willing to provide the FIT files from your testing? There is a lot of additional information contained in the files that would be good to have access to.

    The link to Test 1 on your blog does give access to the data.
  • 0 over 7 years ago
    Just out of curiosity...

    For what range is 1% error margin defined?

    On Garmin's web, it's only stated +/- 1%.

    But is that for 0-2000W with cadence from 50-120...?

    Or how is that verified?

  • 0 over 7 years ago
    gplama, can you also make available your Quarq data?
  • Former Member
    0 Former Member over 7 years ago
    gplama

    Hi Shane,

    I have a couple of questions for you.

    How does the Quarq data track compared to the trainer?
    Do you have another Edge unit you could use so that all data recording is done on an Edge units rather than a mixture of Edge and Bolt?

    The reason for using Edge units for all data recording is two fold, one is to eliminate any data processing differences. This is especially important at a higher cadence value. I will cover this in detail in my next posting. The second issue is that the Bolt does not appear to be recording the accumulated power data (it could be that the trainer is not sending this data). This information is key to understanding how the unit handles missed messages.
  • 0 over 7 years ago
    Phew - you almost lost me there Vector-Info ;-)

    But - your explanation - what has that to do with accuracy?
  • 0 over 7 years ago
    Sorry guys, I'm putting this one to rest for a few days. It's taking up too much of my time. I really need to figure out what's going on.... I also really need to get back to work so I can pay for these damn things! :)

    I'll loop back when I have the time and get another data set using Edge520 & Edge820. And more data from my 4-5 year old Quarq... which might need a static calibration about now.

  • Former Member
    0 Former Member over 7 years ago
    Understanding how a head unit handles missed messages is key, especially at higher cadence values as as a message is more likely to get missed.

    To understand this you need to appreciate the difference between the data transmission rates, the frequency that the content of those messages gets updated and the data recording rate on the head unit.

    Most power meters are sending basic power messages at 4Hz. (e.g. 4 messages a second). This is a simplification as other message types will periodically replace one or more of these power messages, such as a battery status message, but to keep things simple well will assume that power messages are being sent at 4Hz.

    How often does the content of a message get updated?

    This depends on the power meter implementation. Most power meters such as Vector and SRM are what is known as an event based power meter. They update the message content based on an event, in the case of Vector and SRM this is the completion of a crank revolution.

    So at 60 rpm the message content gets updated once a second so you get four identical messages sent, then it updates it and you will get 4 more repeated messages. Each message has an identifier so that the receiving unit knows if it has processed that message before.

    The other type of power meter is a time based device that updates the message content using a fixed duration, e.g. every second.


    Most power meters do not actually send power data, they actually send torque and cadence data. It is the head unit that computes power from this data. They do send an accumulated power value (Accumulated power is the running sum of the instantaneous power data and is incremented at each update of the update event count). This is key for handling missed messages.


    If we increase the cadence to 120 rpm, then the message content is getting update twice a second on an event based power meter. Each message gets sent twice before it changes.

    The head unit is recording data one per second. At a cadence of 120 rpm it will be getting 2 different message contents in its recording period, what it should do is average those values out and record that. Some units just record the last message they got before the write event rather than averaging the content of the messages.

    With the message content only being available in 2 of the four messages it is possible that the head unit might miss a message. This is where the accumulated power value comes in. Because of the event count on the messages the head unit knows how many messages it has missed and what the delta is between the accumulated power from the previous message it got and the current message. If the number of missing messages is small the head unit can average that power out across the missed messages. This is the correct way to do it. Some head units will just fill in the missed values with the value from the previous record.

    All the above is really an explanation on why you need to recorded data from different power meters on head units that have implemented the same logic, especially at higher cadence. Without knowing the internal works of the Bolt the best way to ensure that the logic is the same is to use 2 Edge units. Anything from the 520 up.