Zwift Hub shows incorrect speed on Garmin Edge

I’ve been using my zwift hub for about 6-7 months first with Rouvy and now Trainer Day (similar to Trainer Road) but not with Zwift itself.

I’ve recently started duel recording my rides with my Garmin Edge 840 as Trainer Day does not auto upload to Garmin Connect or pass as much data. Everything works fine but i’ve noticed my speed and therefore my distance is way off on my Garmin - a bit less than half the speed and distance i’d expect and what i see in Trainer Day.

I’m doing structured workouts using the Trainer Day app using ERG mode and it uses an algorithm to based on power to work out the speed which it seems to be doing accurately. The Zwift hub is connected to the Garmin as a sensor and the gear size is set properly in the sensor settings but it showing a much much slower speed. Power readings on the Garmin match Trainer day perfectly.

Does the Zwift hub broadcast speed to the Garmin (if so does anyone know how it works this out?) or does it broadcast something else like RPM that the garmin is then using to workout speed but that i need to alter the settings somehow to make it read correctly?

Obviously I’m not moving anywhere so it’s not super important but it makes my total distances in TrainingPeaks all off and that is mildly irritating so i’m I’ve been investigating but to no avail so far.

I have a similar issue using my Saris H3 paired to my Wahoo Bolt. My distance was approximately 2x even though speed and power were corrrect. I know the Wahoo Bolt works fine with their trainers. Wahoo also has an internal service request to investigate the compatibility with other trainers. Might be worth con tacting Garmin.

The bike computer doesn’t “see” the things influencing the trainer (eg: if you climb a hill in Zwift, the bike computer doesn’t know that - it’ll show a higher speed than Zwift will in game…“because hill”).

I don’t have a Zwift Hub. I do have a Tacx smart trainer which I pair with both Zwift (via Bluetooth) and my Edge 840 (via ANT+), with the latter only as a speed/cadence sensor and power meter to avoid it fighting for control with Zwift.

Next, go into the Speed/Cadence sensor details on the Edge computer, change “Wheel Size” from “Automatic” to manual. I can tell you I’ve set wheel size to 2825mm after some experimentation and it’s fairly close to approximating speed in Zwift but will differ on hills, or with RoboPacers.

You’re never going to get the two to match perfectly. Some people prefer to log only in Zwift and have it sync to Garmin Connect. I dislike the metrics being lost (training load, L/R balance etc) so I double-log in both Zwift and Garmin Connect. Then, I edit my Garmin Connect ride after the fact so the distance, time, and elevation gain match the Zwift ride. (attach a screen shot of the ride report)

Thanks Otto_Destruct.

Sadly not solved my problem.

I disconnected the trainer sensor from the Garmin and just used the speed sensor as the Zwift hub seems to broadcast several ‘sensors’ so this allows you to set wheel size as you say. I played around too and set it to just over 3000 which represented the right speed for a given power (15.8mph for 100w). But then I realised what it was doing which was simply taking an RPM reading from the Zwift Hub and converting it to speed - just like it would on a wheel on trainer.

As a result I’ve not set the wheel size to get the right speed based on a given power; I’d set it based on a given cadence in a given gear.

If you are playing the Zwift game then the trainer RPM does change as you shift gears e.g. slower trainer RPM when Zwift increases resistance to simulate an uphill and you shift up to maintain your cadence etc - so this would give a reasonable approximation excepting downhills I guess but if you’re using ERG mode you stay in one gear and your cadence stays more or less the same but your power changes as the resistance changes which means the trainer RMP stays more or less the same. So 75 RPM at 100w shows the same speed as 75 RPM at 300w - clearly nonsense.

What I need on the Garmin is a power to speed conversion i.e. for X watts of power the speed will be Y mph (which is what the training apps work out). I don’t expect the Zwift Hub to broadcast this so it’s a Garmin thing and I’ll try to figure it out with them.

Any other thoughts gratefully received.

Not to put too fine a point on this but…

Cadence and wheel RPM are two separate things. Cadence is the RPM of your cranks. Speed is calculated on wheel RPM x wheel circumference. Your bike computer and Zwift have no direct indicator of “wheel RPM” - it’s an invisible metric.

Thanks. I appreciate that difference and have set the wheel size on the Garmin to give me a reasonable speed based on the trainer RPM in a given gear at a given cadence but, as in my original post, this doesn’t really help as ERG mode means the trainer RPM is more or less constant but power varies considerably and therefore real world speed (and therefore distance) would vary considerably too.

I think I’ve concluded I need the Garmin to calculate speed based on power not RPM as the Zwift hub doesn’t do this.