Did the zFTP algorithm just change?

My zFTP increased by 4W yesterday after doing a race where my event power curve was below my 90 power curve across all time intervals:

It definitely went up yesterday, and it seems unlikely that it happened during my warm-up, particularly since all of the “peak power” numbers in my Zwift profile are from early April or earlier:

So the only explanation that makes sense to me is that maybe the algorithm used for zFTP has changed? Like, maybe they’ve started including longer time intervals (I tend to do better in the 20-60 minute range).

Anecdotally, another woman I often race with got upgraded to B cat yesterday, and her event power curve was also below her best. In both our cases, our new zFTP doesn’t look unreasonable, it’s just weird that it would go up from an unexceptional performance.

you sure nothing dropped off your curve?

as an example, my 5min power dropped by 21w and my zFTP went up by 15w

Guessing here… but more than likely one of your longer power numbers dropped off the 90 day window, 12 - 40 minute power.

The calculation hasn’t changed, not that any of us know of at least.

Hi @Jessica_Leigh welcome to the Forums! I’m Oscar from Zwift.
@Rich and @Mike_Rowe1 thanks for helping y’all.

I appreciate the information and screenshot shared, I’d like to let you know that so far we haven’t made any changes with zFTP, and the intervals keep including the length we had, now, you can definitely contact us and we will take a look at your account and give you one-on-one support and see what is happening with the discrepancies you are seeing.

Stay safe!

no. the category system hasn’t been touched in well over 6 months. probably a lot longer, but i’m only vaguely recalling a staff post from a while ago so i’ll just say 6 months. the FAQ stresses an importance on making sure you have strong efforts across a range of intervals to get an accurate category… you can find the listed intervals in the “more info” tab next to your fitness metrics

Thanks. I contacted support, and they said to do a spin down on my trainer.

No idea how my trainer could change my zFTP without affecting my actual power values, but whatever, I’ll do it.

This is a plausible explanation, although I don’t completely get it. I’m aware that the shorter intervals that are used for zMAP can have a counter-intuitive effect on zFTP. I’d like to know more about the curve they fit! Maybe dropping some of the lower values changes the shape of the curve so that some of my longer interval values have a bigger impact on the estimate than they did before?

I don’t think those shorter values have dropped off. Definitely the 3 and 5 minute values haven’t changed, but some of the shorter ones might have. There are some values in the 15-20s range where my 90 day peak is well below my all time peak, so yeah, possible.

that’s why I figured the longer ones may have changed, perhaps some higher values dropped off and were replaced with lower values changing the shape of the curve, resulting in higher shorter values and increasing the zFTP?

Zftp isn’t affected by anything under the 5min curve apparently

Might be worth signing up for intervals.icu and then you can use dates to see your power curve over time and compare from a few days ago

Looking at the dates, I think you can expect your zFTP to change again next week.