I don’t. And more importantly there isn’t space between the flywheel and the casing to add any sort cadence sensor / magnet.
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Seems you are best to ask Stages at this point.
It was more of a curiosity than a need for a solid answer 
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I am super curious as well. Would love to know, but not sure they’d answer a non-owner’s question like that.
I submitted a ticket to Stages asking this question. Will report back when I get a response
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Here’s Stages response:
So you were correct
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Great to confirm. Thanks for asking them and sharing here 
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Flywheel speed is proportional to foot speed…the SB20 is physically a single-speed. If resistance goes up and flywheel speed stays the same, then you’ve also raised your power. If resistance goes up and power stays the same, then flywheel speed goes down.
There may be some kind of technical wizardry like PWM giving some smartbikes more realistic “momentum” under virtual gear changes. The 50lb flywheel in the SB20 likely makes it more difficult to implement something like that.
IOW, when you shift gears in real life and maintain power, your wheel speed stays the same. On the SB20, the wheel speed drops…you’ve basically added gravity or modified the slope you’re riding on.
Have been eyeing the SB20 now when I saw that they have a sale (2200 euros) on it from Stages themselves.
One issue though that keeps popping up and keeps me from buying it is the whole power meter issue with it reading oddly if forces are applied that are no “straight down” if that makes sense?
Is this something anyone has had issues with?
Or any other issues related to the PMs that might be a reoccurring issue?
FWIW my SB20 agrees pretty closely with both Assioma Duo and Vector 3. Typically within 2-3 watts. Yes, I don’t have a weird pedaling style. Garmin says my PCO is -2 left, +4 right.
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This is completely insane and is clearly happening to more than 1% of the population.
I wish they would detail the proper foot position.
Depending on where my cleat is on my shoe can change the reading of the Stages by 20-30 watts.
I know people have gone into this deeply, but here’s what I’ve observed for me:
-The wider the Q factor, the higher the power (the more I move my cleats in the higher the power)
-Possibly lower cadence/higher torque, the higher the power
-Seat height may make a slight difference
-Didn’t notice much difference going to a shorter crank length
-Discrepancy seems worse, the higher the power.
Stages did tell me they replaced a lot of the power units on the early releases and they are sending me new power units. We will see if that’s better.
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Who uses pedals that are 65mm? Jesus.
All I know is that my fit has me push my cleats as far back and inside as I can (increasing my Q factor and making my PCO +10ish). At 300 watts according to both Vectors and Assiomas, the SB20 ends up being 20-35 watts high.
Got the replacement meters. In addition to being kind of a PITA to install (well, the right side anyway), they provide nearly identical results to the first set. With my cleat set up as instructed by my bike fitter, the Stages is 10% higher than Assioma Duos at 300ish watts calibrated by me via static weight test.
Unfortunately, the degree to which the Stages is out seems to be torque dependent, so I’m not sure how useful it actually is as a training tool.
It’s really too bad because (aside from the shifting mechanics) I actually really love the bike.
Hopefully Stages will do something to make this right for folks who are experiencing this problem.
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Do you have access to a set of Vector 3 or Rally pedals to isolate the SB20 as the issue?
Edit: The reason I ask is that the Assioma readings can be influenced by cleat placement / PCO.
What do you mean by this?
Yes. In fact, most of my testing has been done with Vector 3 pedals with Rally SPD-SL bodies. Both sets of pedals seem to generally agree with one another. At around 300 watts, the Stages is 20-35 watts high. Here are some samples with the Assiomas and the V3s:
https://analyze.dcrainmaker.com/#/public/519328e6-3704-4085-5182-446665a394cf
The closest I’ve gotten is with the V3s with my cleats moved roughly to the center of the shoe on a longer ride with slightly lower effort (PCO: L +8, R +3. For me, normally L +15, R +7) Still 10+ watts out:
https://analyze.dcrainmaker.com/#/public/ee6847dc-6ee0-43a7-58a1-aafd882b0d1c
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Hitting you on on DM. Cheers.
I would like to hear more about this. I am so frustrated with my SB20 that I use it less and less these days and have reverted to my Kickr or rollers when I actually care about measuring watts accurately.
Do you own the Duos? Can you not ride the SB20 with these if you do? That is what I do and have no issues.
I recognise the inaccuracies of the SB20 power meter but the above negates this and will still give you the benefits of the bike you like