Value of FTP


I have a question.

Yesterday I did a Time Trial. I have finished in 18:48 but once time I was arrived at finish line, I continued pushing to improve my w/kg for 20 min. I get a power average of 284 of race and the 285 for w/kg for 20 minutes. These values was showed in table of Zwift when you finish a ride.

The surprise was that Zwift says me that I improve FTP for 242 to 250. It was great but I Thought that I was improved until 270 (95% of 284).

Is there any wrong?.

And now I don’t understand in the table of my best values of power in Zwift appears 263 w/g for 20 minutes but when just finished the race the value was 285. What’s is the reason that value change?

Number of my activity: 747913004896199760 in Zwift.
Activity was first 20 minutes to full to improve FTP (including 18 min of timetrial) and the rest relax

What’s the reason?

Anybody can help me please?

Thanks in advance.
Best Regards

Did you check your power-duration curve after this ride? Zwift shows it in one of the summary tabs at the end of the ride. You need to move your cursor to 20 mins, check what maximum power you were able to maintain continuously over 20 mins, and multiply it by 0.95.

You see, if your total ride duration was exactly 20 min, but during these 20 mins you were riding for half a minute at a lower power, say at 260 watts, you will not have 20 mins of continuous power at 285 watts. Your maximum power which you were able to sustain through the whole 20 mins will be only 260 watts, whereas time at 285 watts can be just 19.5 minutes!

Next time just ride a couple of extra minutes, and you will get what you want.

I hear what you are saying, average power over 20 mins trial multiplied by 95%, but this might easily be not the whole story (or just a simplified story). Zwift to my knowledge has not disclosed their exact algorithm of how they calculate FTP. Yes, average power over 20 mins used to be the method to calculate FTP before more sophisticated programs for data analysis were developed. Professional bike training sites, like Training Peaks and affiliated with them WKO5 software package, calculate FTP from modeling power duration curve. I use WKO5 and no longer see a need in even doing FTP tests. It is recalculated for me after every ride, with a bunch of other metrics.

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