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Indoor profile - speed and distance calculation when on a turbo trainer

GLM1989
over 1 year ago

Hi

I've recently started duel recording my indoor sessions using my Edge 840 to benefit from the richer data captured (I also have a forerunner and make good use of Garmin Connect) and plus Garmin does not support the automatic upload of sessions from my training app - Trainer Day - resulting in manual faff I can't be bothered with. 

My issue is that the Garmin reports the wrong speed and therefore distance. I say wrong but what I mean is simplistic and therefore not good enough for my needs.

After a little investigation it seems like the Garmin is simply basing its speed reading on the RMP of the trainer using the speed sensor. I use a Zwift Hub (but not the Zwift game) and the Garmin can 'see' various sensors broadcast by the trainer (speed, power, the trainer itself). The issue with this simplistic RPM based calculation is that if you are using the trainer in ERG mode to run structured training programs based on power via an app you essentially stay in the same gear and keep your cadence more or less constant. This means the RPM measured by the trainer is more or less constant despite your power varying widely which, using Garmin's simple calculation, means your speed is more or less constant - or to put it another way - wrong!

What I need the Garmin to do it calculate speed based on power not on trainer RPM - 300w at 75rpm is clearly a much higher speed that 100w at 75rpm. There are well known simple formulas for doing this and you can compensate for weight and drag and all sorts of other factors and it's what the training apps use themselves so I'm sure Garmin must have this too but I'm yet to figure it out.

Any help greatly appreciate  

  • 0 over 1 year ago in reply to Kruszon

    the solution would be a "virtual speed sensor" by any app (Zwift, MyWhoos, Tacx App), what you can pair to the head unit as a normal sensor (but i think for this you probably would need 2 ANT+ sticks, or BT sticks on PC...), and this virtual speed sensor has exactly the same speed value what you have in the used app in given moment because the app is continously communicating as a (speed) sensor with your head unit. And in this case both speed and distance values (the app's and the head unit's one) will be totally similar, and wont be any complaints.

  • 0 over 1 year ago in reply to Luko79

    That's right, 100%! I have a Spec Diverge connected to the trainer, and with a small front sprocket the results will be far from what Zwift shows. That's why, after a few tests on Zwift, I use a large gear at the front and it's more or less OK in the base and pace zones. Of course, it doesn't make sense for professional use, but many people find numbers fun ;)

    Luko79 explained the topic clearly, thanks!

  • 0 11 months ago in reply to GLM1989

    I want this too!

    Also I often do workouts just on the Garmin and for instance for chain wear I use distance. Also for that reason “true”(r) distance would be nice. This week for instance I did 7 hours on the trainer and I can’t compare it to my usual wear measuring. I could ofc start counting hours for chain wear, but that wouldn’t take strain into account same way distance does. 

  • 0 10 months ago

    Am I correct in thinking that if ERG mode is NOT used, then, assuming the turbo trainer is setup to mimick the resistance you would encounter on flat terrain in the real world, then it would give you an accurate distance (albeit based on perfect conditions)?

  • 0 10 months ago in reply to @mikeyclarky

    if it is in SIM mode then there is difference too (could be smaller or larger), even with the own Tacx videos also. With Zwift, much bigger. (especially if there are tons of users on the routes and the draft is huge). maybe if you disconnect your internet (or ride alone in a meetup), then a bit closer on a full flat route (like Tempus) and with 100% trainer difficulty setting (because even the Tempus has small uphills)