Free Watts with Virtual Shifting

I have now replicated this bug on multiple Kickr Core units within Zwift (including one brand new unit out of the box). I can send video if necessary, but it should be easy to confirm. I don’t have access to any other models of smart trainer, or dual power recording, but at this point I am pretty certain it is a Zwift bug related to something within the virtual shifting mechanics.

I am using a single speed bike. I have a large chainring (60T) and a large cog (20T) with virtual shifting enabled.

If I am pedaling at a moderately high cadence 80+rpm and then I stop pedaling, my wattage in Zwift will initially drop to zero. However, as I continue to coast, after about 1-2 seconds, my Zwift wattage will jump back up to between 20-40 watts and my recorded cadence in Zwift will increase to ~90rpm, then 100, then 110-120. Both the wattage and cadence will stay this way for approximately 10 seconds, sometimes longer, even though I am not pedaling, before finally dropping back towards zero.

I’m wondering if maybe this issue is caused by having an unusually large chainring/cog combo and the way the virtual shifting calculations are done? Just a guess. I did not have this issue when using my normal road bike with the same trainer.

I do not believe this is a Wahoo bug because for the first couple minutes when I first set up the bike and trainer on Zwift it will not do this. It is only once the first virtual shifting autocalibration within Zwift occurs that it starts to take place.

I have tried doing spindowns (both basic and factory spindowns) within the Wahoo app, and it does not fix the problem.

My wattage and cadence generally seem correct when pedaling. It is only when coasting that things get out of whack (it also makes it impossible to super tuck since when I am coasting Zwift is still recording me as pedaling at 110+ rpms, even though wattage is very low).

First, I really like the old school notion that a 20t cog is ‘large’ :smiley:

Are you shifting when any of this is happening (doesn’t sound like it)? Have you tried not connecting your virtual shifting gear (Click or Play) to see if it happens then?

FWIW, this is something (or something similar) that I and other Saris H3 riders notice. Mine doesn’t go down to 0 and then go back up, but my wattage takes a lot time to zero out. It drops to more or less meaningless numbers (20w) pretty quickly, but stays there for a while. No virtual shifting here. I don’t get your cadence issue though.

I have seen this with the KicKR V6. I’ve found its not isolated to virtual shifting but also happens with manual shifting.

It happens when coasting from very fast flywheel speeds (power doesn’t matter) and it happens in Zwift, Wahoo App, and in indieVelo.

Wahoo advised, I do a factory spin down and the issue mostly went away. I also found that nipping the belt tension up stops the problem.

There is another post on it here and also on Reddit.

I suggest reaching out to Wahoo and reporting back. Especially if it is caused by poorly tensioned belts