Yeah, it seems odd that the new 15min power would drop things, though your 5min is pretty high which probably puts downward pressure on it.
My 15min is 315W and my 5min is 365W and it has me at 281W zFTP while intervals has me at 293W FTP.
i suspect your zftp was just overestimated previously because of a lack of data between the 12-16 min range. if the data for a given interval seems off or strange, CE seems to just ignore it and fill in the gap with some sort of estimate instead
i’m speculating though, as always
Thanks for this explanation! This happened to me today. 5 min power went up, longer power stayed the same and my zFTP went down. Normally I wouldn’t care about the zFTP numbers but it’s causing me to yo-yo back and forth over the Cat cut offs in the midst of ZRL which makes planning which team to race with complicated. Can’t wait till there a results based system!
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Not sure where your 15 min power (now 300w) was previously, but presumably it had to have been somewhere between 299-296 watts. In other words, nothing was drastically changed.
Looking at intervals.icu it was 298 previously.
So if you check out the numbers in my earlier post those values gave me a zFTP of 266W which was really a lot lower than I would expect for having a 30min effort at 296W.
I did notice I didn’t have any hard efforts in the 12-15min range, so I did a 15min block at 315W (not gonna lie, that kinda sucked).
With that effort the zFTP went up from 266W to 281W as you can see below:
Now intervals.icu shows me at 293W eFTP, I just did a ramp test which had me at 306W FTP (though I know those tend to overestimate for me), and 95% of my 20min power is just over 283W (though that 20 mins was during an event so not a max effort). So is 281W for zFTP reasonable? Not sure, but it’s probably closer than 266W was.
I think of zFTP as a magic number that sometimes has no close relationship to FTP. Without opening the algorithm to inspection, it will sometimes be incomprehensible. Reading the tea leaves gives us some understanding of the levers but there may be strange choices baked in, or exceptions to handle edge cases, or bugs.
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Follow up question: I got upgraded to Cat C on about November 14 and then downgraded yesterday on December 6. I thought your cat upgrade was supposed to stick for at least some minimum number of days before downgrading. Has that changed? Seems like a The only thing that changed in my feed numbers was improvement in 3 and 5 minute power. If anyone could read my tea leaves I would be grateful.
Edit: I think I am definitely as edge case as my zFTP went down by just two watts and I was bumped down. Adding to the interest, I did a weigh in todayand it turns I lost one kg so I’m wondering what that will do (if anything). I’m not fussed about the numbers, it’s just that are women’s only D team has fewer than six for ZRL Tuesday so if Zwift and WTRL say D, I can help them out…
A 3-5min improvement could easily drop zFTP down a few Watts.
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Nothing until you set a PR at the new weight. Then the weight you were at that new PR will be averaged in with the weights you were at the rest of the PRs. So changing 1kg will not have a huge effect. In your situation it seems risky to race in D having just been C and trending lighter.
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Thanks. I understand that but I thought once you upgraded you had to stay for at least 30 days (or 60/90). Is that not the case?
You’re only stuck in the new pen category for 90 days if your power curve doesn’t change in a way that demotes you to a lower pen.
I got a zMAP promotion to C back in late August, from what I suspect were slightly inflated numbers due to infrequent calibration of my Saris H3. I’ve been back in D for just over a week.
zMAP promotions mean you are promoted for 90 days with no real possibility of escape unless it was bad power readings that ZwiftHGQ take pity on (I didn’t bother appealing, I’ve been an edge case most of this year on a very good day). zFTP promotions are much more dynamic.
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Ahhh, I see. This means if my power curve numbers change in a way that trips the wires, I could theoretically ping pong up and down cats frequently — where’s that results based system?
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I’m all for trying a results based system, providing it has much better logic than Zwift Racing Score a few months back! 
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There’s no guarantee that a results based system would be less volatile to someone on a category boundary. They haven’t explained how it will work so that remains unknown. Depending on how it’s implemented, it could be more volatile than a power based metric.
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That’s true, but hopefully it will be well done and feel less arbitrary. I got upgraded for P16 finish out of 60, where some of my teammates with better numbers (w/kg) and finish placement on the day did not (weights similar). The behind the scenes math and magic of zFTP feels sorta random. I don’t mind being upgraded or downgraded, just want to sign up for a team in a Cat that the system says is right and have clear logical, consistent reason for being bounced.
Thanks for all of the help and insights everyone!
Same thing just happened to me - in my latest event, all power went up: 40 min, 20 min and 5 min, yet my zFTP went down. I don’t get it.
not many of us do due to the very strange calculations that zwift use
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same here. my power went up this week for 3 min, 5 min, 12 min and 15 min and my zFTP dropped down for 10 watts. 2 - 3 weeks ago I set a new pr for 20 and 30 minutes and zFTP dropped down for 5 watts.
my zFTP dropped down for 15w although I recorded new power records.
The duration of all of those efforts is below FTP duration, and so {speculation begins} the model probably figures something like “Aha! It seems that your anaerobic capacity has increased. Excuse me while I remodel the curve to reflect that.”
The consequence being that it shifts down at the other end of the duration spectrum.
And to answer your next comment: Yes. That does seem a bit silly, but that’s why “all models are wrong, but some are useful” is a saying.
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