Hello,
I hope I am in the right place in this thread, if not the post can be moved accordingly.

I have a question regarding the FTP in Zwift. I have never done an FTP test and always relied on the Zwift estimates. I have always found them to be quite suitable and that has been enough for me.

In my last race, however, things happened that I can’t explain. It’s about the difference between the FTP value on Zwiftpower / Zwift website and the value that is displayed in-game under “my profile” or in the settings for workouts.

A little explanation: In my last race I achieved a new record, I was even promoted to A Cat. I was speechless at first, but also a bit proud, but I’m a bit insecure. However, after the race I had a message on the screen saying “ftp improvement recognized”. New value: 287. This value is also applied as default for workouts / in the settings.

However, Zwiftpower and the Zwift website now show a zFTP of 297 for my profile.
I assume the value is calculated from the 20 minutes of power, which was 312w in the last race.

Question: Why do the values on the website/Zwiftpower differ from the in-game value? So 287 to 297.

Think of zFTP as more of a “race categorization number” that is not your FTP. The similar naming is unfortunate. zFTP is affected by PRs at a variety of durations whereas the in-game FTP change you got is simply based on 95% of 20 minute power.

When your zFTP is much higher than your FTP, a common reason is a lack of shorter-duration PRs like around 5 minutes. If you increase your 5 minute PR, you may find zFTP goes down.

However, 312 * 0.95 = 296.4
Shouldn’t that also be the “In Game” value?

…Then I should understand better how exactly 20 minutes of power is determined.

Let’s say I ride for 30 minutes. The first 15 minutes max out, 5 minutes super chill, and another 10 minutes hard. In this case, would the first 20 minutes be used to calculate the average? Would the last 10 minutes be ignored in this case as the first 20 minutes would result in a higher average?

When your zFTP is much higher than your FTP, a common reason is a lack of shorter-duration PRs like around 5 minutes. If you increase your 5 minute PR, you may find zFTP goes down.

That sounds a bit counter intuitive. But I’ll just “take your word for it”

Consider the image below. The solid line is my actual power data from some time late last year. The dotted line is the curve from a critical power (CP) model, fitted to my data. A CP model takes two maximum efforts, and it spits out a prediction of the power you should be able to hold at various lengths - it’s accurate from I think 2 minutes out to 30-45 (or so?).

The two circles are the two points used in fitting the model. The model assumes a curve with a certain slope. If you raise my 5-min effort, the slope changes such that the curve’s asymptote will decrease. That asymptote is what they call critical power. My understanding is that it’s usually pretty close to FTP. They aren’t identical. In fact, in one sense, CP is just a mathematical artifact - it’s the asymptote for the power curve, and I don’t think anybody is going to hold their CP for 6 hours.

I understand now. Thank you very much, that’s very interesting.

Maybe that will change soon. Until now I had a “smart”-trainer, which only went up to 700w maximum, and I couldn’t use the high numbers because I had difficulties with the setup in high gears. the trainer was spinning too fast to get real power on the pedals.

Anyway, I just ordered a good trainer with virtual shifting