Cadence, really?

 For a few weeks now, Zwift has been showing my cadence, which is very curious because I don’t have a cadence sensor.  Is it calculating/estimating cadence from my CycleOps Magnus?  If so, how does it work?

Many smart trainers calculate cadence, including the Magnus.  If you pair the magnus as a cadence sensor in Zwift (which it appears you are), it’ll show cadence.  It is a recent addition to the Magnus firmware.   Did you update it recently?

Yes, there was an update recently. I understand how it might use resistance and wheel speed to calculate, but wouldn’t you need to know the gear ratio to get pedal RPM?

Your power delivery is not constant throughout the pedal stroke. It’s higher in certain crank position (e.g., cranks parallel to the ground) and lower in other crank positions (e.g., cranks perpendicular to the ground). They use this knowledge and the power to determine your cadence.

OK cool, I’m impressed!

Don’t be too impressed. 

In my experience calculated (as opposed to sensor-measured) cadence can be wildly inaccurate.

Cadence on my Neo calculated by Zwift and actually cadence measured from Garmin 820 with a crank sensor is within a couple of rpm which I find pretty impressive. 

I don’t have another device to compare, but as a lifelong cyclist (~50 years riding) the cadence numbers seem very real. My avatar even stands up when I do!

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My Zwift was also displaying cadence when I was using the iPhone (displaying on the big screen TV).  But I didn’t like using the iPhone so bought an Apple TV 4K.  Now, cadence no longer shows.  

Actually, on the iPhone, it was pretty accurate!  

So I’ve purchased a new Garmin 520 with HR and Cadence bundle.  It hasn’t arrived yet and I have no idea how to hook it up.  

Will actual cadence input to Zwift make any effect on wattage and (hopefully) drafting?  I did a group workout last week and had to maintain like 150+ rpm (estimated) to get the power that the Zwift training program was instructing.  

Very interesting, was just debating how my Magnus now shows cadence on Zwift, and appears fairly accurate - without a cadence sensor.
Yes, I updated firmware.

@Ken Ridout

" I did a group workout last week and had to maintain like 150+ rpm (estimated) to get the power that the Zwift training program was instructing.  "

I’m assuming that you aren’t using a (controllable) smart trainer in ERG mode? 

If that’s the case, you need to shift gears to increase your power output while maintaining a more ‘normal’ cadence or, if your trainer has variable resistance - increase it to achieve the same result. 



Joe, Daknis, my comment was 5 months ago.  When my Garmin cadence stuff came a few days later, I installed it and got reliable cadence when using the AppleTV.