in intervals what time have you got it to to calculate eFTP? I have mine set to 20 minutes and after stage 4 (when my zFTP went up 30odd watts) it went up by FTP +1 to 271w from 20m at 284w which is 1w short of what zwift has for 20minutes in the last 60 days so they are using a shorter time period and basing it off that, the calculations are seriously flawed
as a quick test I changed my time that intervals should use for eFTP to 12minutes and it gave me an eFTP of 294 for that same ride so getting closer to what Zwift believes my zFTP is based off that one single hill climb so i am going to say Zwift is guessing an artificial number for zFTP not that Zwift will come out with any numbers as usual
another quick change in intervals to 10 minutes and guess what eFTP shows? 297 matching the zFTP number that zwift calculate…Zwift is using 10 minutes to base zFTP? Seriously?
Have you tried changing the intervals.icu power model from the Options menu to either Morton’s 3 parameter CP or eCP: single max effort and Morton’s 3P?
They have both given me consistent results and are still within 2W of the zFTP estimates. Zwift is probably using something based on Morton’s 3 parameter model with a curve fit to look for what it thinks are true max efforts up to 50 minutes and discarding anything else that looks non-max.
eFTP: single max effort and Morton’s 3P - 272w
mortons 3 parameter gives a CP - 243w
monod and sherrer’s 2 parameter CP - 300w
eCP: single max effort and Morton’s 3P - 266w
so the only method of getting it closer to Zwifts value is using the monod calculation
what time value do you have set for intervals to give you your eFTP?
Would you be OK to say which of these you feel are closest to real max efforts and which you are confident are lower than your max? If Zwift is thinks some of your sub-max powers are really max efforts and it is fitting the wrong points that could explain its zFTP result.
i will never use intervals.icu for anything but i just put my numbers in there and the “eftp” calculates your TTE at that power too (mine is embarassingly short) so the numbers are going to look different to CE, which i assume uses a fixed point (in their words, 40mins+) for everyone.
Very sure zFTP tries to fit the CP equation to several points and not just one fixed point, CP is one of the equation parameters and is the asymptote for longer durations which is where the 40+ mins quote comes from.
the description of zftp if you click on “more info” in your zwift feed CE status. anyway, to further confuse everyone in the thread, my zftp actually went down by a couple of watts the other day after i set a 60 day 12 min PR.
oh, i have no idea what data points they’re using. 3 mins i’m sure of, everything else, i dunno. my zftp is pretty close to my hour power though so i don’t think it’s a bad estimate.
depends on how time you are going for max efforts, I wouldn’t be able to sustain 297w for 20 minutes and certainly not for > 40min which is the wording that zFTP uses
it makes no sense that I do a 12 min effort (334w and I was blowing out my ■■■■ at the end of the climb so that was a max effort) and my zFTP jumped from 263 to 297
so if we take 40 minutes, the last time I got close to anything near zFTP of 297w is a race back in may where I held 281w for 39 minutes so how is the calculation made? if you wont provide the calculation on what its based on explain to me why my zFTP went up after a 12min effort
I have not set any specific user points, I just let the programs use the default settings.
I didn’t know I could define them.
Still, my zFTP and eFTP were within 2 watts of each other and whenever there was a change in one, the other always showed a corresponding change of the same degree.
Now, after my last ride, I got an increase but zFTP is + 15 and eFTP is plus 2.
The course was Gotham Grind and I race that course every week, granted not 1 lap.
It seems odd that the zFTP has suddenly showed a departure from the previous tracking.
Thus the question, "has the formula changed?
If it has changed, my guess from what is being reported is that they have either changed the way they choose what they think are genuine max efforts or have changed the time ranges in the curve fit to include more points at shorter (under 3 mins) durations.
I was thinking of your 30s up to 5 mins in the Zwift table. I can get a zFTP for you of 280W with a Pmax of 812 and AWC of 32kJ but your stated powers at those durations would need to be higher than those reported on your Zwift feed. So that’s why I was asking about whether you thought any were also true maxes or less.
my 5 minute is currently 356w (13/12/2022) but my max 5m is 389w due to the socks4watts hill climb series but that 5m wattage hasn’t changed, the only value to change that increased my zFTP has been the 12min effort and seeing as its happened to @Tim_Camden_C i’d suggest something has changed with their calculation or their system is miscalculating it
I mean this value here under settings in intervals.icu if I alter this to 600s it matches zFTP perfectly on the ride that caused it, if i leave it it 1200s it simply increased my eFTP by 1
something has changed or the calculation is flawed but we wont ever be told if this is the case as zwift wont divulge this information
