Why is my zFTP calculation so low?

I suppose I’m asking more from a place of intellectual curiosity, but within the last 10 days or so, my zFTP dropped quite a bit and I was downgraded to B (women’s) after I’ve been racing A consistently and successfully enough. Just curious if there’s any idea what’s going on here.

Current zFTP calculation is 244W. My 20 minute peak power in the middle of a race just this last Tuesday (so not a true AO effort) was 271W (I know it’s not using 95% of 20 min, but just for reference 95% of that would be 257) and I have non-max efforts in the last 45 days that exceed my zFTP up to 51 minutes.

Just seems very odd. Zwift itself has me at 257, intervals.icu says 276 (too high I think). Why is CE calculating me so low? I won’t race in women’s B races but just curious.

Trying to upload a screen shot…

zFTP is a prediction of what you can do for 40mins.

Given you actually did 252W for 40mins on 7th March (i.e. within the last 60 days) just on Zwiftpower, which won’t show your activities that aren’t races; workouts or group rides…

It’s curious why your zFTP is 244W. :thinking:

Is there a minimum zFTP of 250W to be in women’s A pen, just like for the men?

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James confirmed in the Category Update thread that changes to zFTP were made last week.

The outcome is that many have seen a reduced zFTP, and for some (like me too), that means a lower category. Effectively the category boundaries are now higher than before, because it will take more power to get to that zFTP level again.

No, no watt minimum in the women’s, it’s just a minimum 3.88 w/kg, which this lower zFTP puts me right under.

Yeah, I get that, I guess I’m just wondering what about my curve would be pulling the zFTP so low. I see some people talking about a big delta between 5 min w/kg and 12 min w/kg pulling the curve steeper and lowering the zFTP, but I don’t think I have a particularly strong 1 min, 5 min, 12 minute in comparison to my 20 minute.

Suppose it’s not really all that important in the real world, and plenty of others are seeing similar, but as a low-level data nerd I’m just curious how this could work.

Yes, but it seems outwardly odd to have a zFTP value that is lower than current 40min power.


I’ve had the same happen to me. I’d been a low A under the 20-min power system (FTP ~4.25w/kg) but my zFTP has dropped from 314W to 296W - until I came down with a cold a fortnight ago, my power had been gradually improving in all but the shortest sprints, so I can’t really imagine that my critical performances were dropping out of the back of the 60-day window…

Several people surveyed in my club reported that their zFTP went down, sometimes by a lot (50W). Nobody reported it going up or staying constant.

Honestly, I think the short answer to the subject question is, because they have messed up the formulas behind cat enforcement

At the very least the new CE comms have been non existent. People suddenly dropping lots of places in races will have no idea why unless they are diving into the depths of these forums. And even then you need a PHD to understand it. At first I thought I had inexplicably dropped fitness.

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But I still love Zwift and Zwift racing.