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Different distance between 945 and Zwiftplatform

spruitp
over 3 years ago

i did an indoor ride with my Garmin 945 on a suggested workout. I performed the workout on a Tacx Neo connected to Zwift. I rode a flat course to keep the watts in range of the workout.

on Zwift the distance was 33,6 km. https://www.strava.com/activities/4616205273
on Garmin the distance was 22,9 both in 65 minutes https://connect.garmin.com/modern/activity/6102966787

this is a huge difference in distance. Does anyone knows what causes this difference. I personally think the truth is somewhere in the middle. Garmin distance feels too short and Zwift too long. 

  • 0 over 3 years ago

    well, for example there might be a case when in zwift you'll go crazy speed downhill and the 945 won't register that; or you draft other players...just record the activity with both devices and erase the garmin one afterwards - or - via the fitfilerepairtool merge both activities

  • 0 over 3 years ago

    The Tacx measures axle speed, and varies resistance to met the power requirements.

    As such, which gear you are in will affect the Tacx reported speed. Put it in the big ring and a small cog, and it will report a much faster "speed" than in the small ring. A bit like the real world.

  • 0 over 3 years ago

    Zwift is virtual distance. Assuming you are getting your distance on the 945 from a properly set up speed sensor then the Garmin distance will be the real distance.

  • 0 over 3 years ago in reply to Dr Phil
    Dr Phil said:
    the real distance

    The real distance is zero. You are still sitting in your pain cave and have gone nowhere. Grin

  • 0 over 3 years ago

    Thanks for the insights. Power and speed comes from the Tacx Neo which is paired with my Garmin via ant+

  • 0 over 3 years ago

    The only thing you need to determine distance on a training bike is a speed sensor on your wheel. Since the number of times it rotates X the circumference of your wheel is how far you've traveled.

    Zwift emulates distance based on the virtual world you're in so if you're going downhill you travel "further" even if your wheel spins the same amount and when you go uphill you travel a "shorter" distance per wheel. Zwift also does some other things to adjust things, like modifying your speed to help keep you with the pack or drafting things etc. Zwift also has an annoying tendency to drop ANT/Bluetooth signals occasionally so I don't trust it as much to maintain the data.

    I expect that Garmin's speed sensor is the accurate one here because it would only use the information provided from the Neo in terms of rotations. This wouldn't be affected by the resistance applied by the Neo, smart or not, nor the gear you were using to make those rotations.

    That being said if you trust the power readings on your Neo (especially since the Garmin one and the Zwift one report basically the same thing), a power of about 125W should net you about 27.9km/hr, so if you want to call it in the middle this way you could. (But first check that Garmin tire size is the same on your next run)

  • 0 over 3 years ago in reply to NauticalHamster
    NauticalHamster said:
    a power of about 125W should net you about 27.9km/hr,

    Not necessarily. You could be generating 125W going 10km/hr up a hill. There is no direct correlation between power and speed except perhaps on the flat.

  • 0 over 3 years ago in reply to Dr Phil

    https://www.gribble.org/cycling/power_v_speed.html

    I based it on this for a flat course with the default bike weights and drag coefficients.

    The courses he posted were relatively flat, but there is a direct correlation between speed and power, it is non-linear however and affected by other factors such as slope, drag, wind etc. In general though, if you have less power you have less speed for given factors and more power = more speed all else considered.

  • 0 over 3 years ago

    A good read for this topic is the standard Zwift Insider article.

    Why Your Garmin Speed and Distance Don’t Match Zwift’s