What is the value of zFTP?

Racing
1

Seeing as this value is public on Zwift Power, I’d like to know more about how it works and what it means.

Officially it’s:

Estimate of your current Functional Threshold Power (FTP), the power you can sustain for a long period of time, i.e. > 40 min

My zFTP is 308

My 40 min power PB from racing is 325

My best 20 min power is 350 (zFTP appears to be 88% of 20 min power)

But my FTP in Zwift is 336

TrainingPeaks has my FTP at 326 (inside, set yesterday)

Wahoo (outside with PM) has 346

I don’t really ride races longer than 30 minutes and rarely use Zwift for longer intervals.

What does zFTP actually tell me?

Why is it going down while my racing score is going up?

My zFTP was 327 a year ago, when I was regularly getting dropped in A races I am now ‘relevant’ in.

My zMAP has gone up by 20 watts in the period in which my zFTP has dropped by 20 watts.

My 1 min power has gone up by 100 watts during the last year, while zFTP has tanked. Is this logical or even possible?

2

I can’t answer your question but this is why I set my FTP in Zwift according to my eFTP from Intervals.

Zwift won’t replace Intervals until all of your questions are answered.

3

This is a common occurrence, improve your 1 - 5 minute power and your zFTP goes down.

4

Its value is that it’s one of three numbers used to determine which racing start pens you are not allowed to enter, and there’s really nothing else useful about it. Zwift doesn’t even use it to estimate your FTP - they developed a system to estimate that for The Grade KOM that they don’t use for racing categorization. I wouldn’t stare too hard at zFTP because that way lies madness.

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5

critical power, if critical power had small print. you can’t read the small print though

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6

In answer to the OP’s question: What is the value of zFTP? [Dark Helmet voice] ABSOLUTELY NOTHING! [/Dark Helmet voice]

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7

It’s bizarre.

Zwift has my zFTP at 85% of my actual FTP and now has my VO2 max watts at 96% of my 10 minute power.

8

I’m not going to defend zGimmick metrics, but the models can only work with the data fed to it.

9

Yeah. If you don’t do long races (there aren’t many) or longer intervals, your 40m power will be lower than what you can actually do.

I get dropped back to B every few months if I don’t hit over 5.1 for 5 minutes.

I never do 5 minute intervals (for VO2 max my coach has me do 4 min intervals or 10 minute hell blocks where you jump between Z4 and Z6.

As a heavier rider, I don’t do races with 5 minute climbs in as taking on the 50kg club (legit or otherwise) is just pointless.

You are totally right - if you don’t train or race in a way that means you hit your max for Zwift’s measurement metrics, your numbers are not reliable.

I do think Zwift should really make zFTP 90-95% of your 20 min power, however and not use 40 mins (this is what I have seen)

I mean, hardly any races are long enough to take 40 minutes.

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10

Estimated metrics are…estimates.

They are good for tracking trends and, many times, are the best data we have but estimates don’t show the whole picture.

If you’re tracking VO2 max but not doing 5 min max intervals, then I assume the data may not match estimates of VO2 max since most formulas estimating VO2 max were derived using that value.

It may be different now.

I had posed this very question to Intervals and this was the unanimous opinion.

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11

The estimations are frankly bizarre.

I mean, I’m clearly a non-sprinter, with comparitively poor anaerobic numbers and comparitively high aerobic numbers.

I can do 420 for 5 mins, 350 for 20 but only 558 for 1 minute.

From this, my zFTP is calculated at 307.

I’m seeing other riders with zFTP almost equal to their 20 minute power, yet have much higher anaerobic ability.

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12

But do we know how zFTP is calculated?

Zwift has not shared the formula and it has probably evolved somewhat from its original form

zFTP should be considered zRP - Zwift Racing Power and it is some metric that Zwift uses to group riders.

I’m ok with the details being secret and evolving over time

I just go where they tell me to go and try to win ir at least do my best.

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13

more or less

the importance of having MAXIMAL efforts at the suggested durations in their faq is understated, since they will not use the data if it doesn’t look right

14

the cold, unfeeling computer doesnt care about if you’re a sprinter or a long jumper. it only cares if the data it uses for the calculation is worth using

15

Yeah it’s like that.

I just find it odd that my zFTP has decreased by 20 watts while my ZRS has increased by 110

Zwift seems to be saying that I have gotten a lot better at racing, while being able to produce fewer watts while racing (despite 5 min and 1 min power increasing and no decline in 20 min averages either)

16

You’re mixing apples and oranges and neither are measuring your fitness.

ZRS has nothing to do with your fitness other than seed score.

Increases and decrease are based on finishing position not fitness.

zFTP is a proprietary Zwift metric that’s probably based on a critical power curve. Improvements in specific intervals may increase or decrease zFTP.

eFTP is an estimated metric that allows you to set workout interval goals.

IRL is were you ride with humans and you see if all this data crap has actually made you a better rider.

Are you setting PR’s or KOM or getting in the top 10 or not getting dropped on the group rides?

IRL rides, do you spend more time feeling like the hammer or the nail?

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17

like how i win 2.3w/kg vs the group avg at 3.0w/kg,

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18

Yeah, since I got much better at (A) racing, my overall averages have gone down, while my 1m / 30s / 15s have all increased.

You do less work when you’re not hanging on at the back and if you can stay near the front you can rest more.

It seems Zwift’s computer thinks this is some sort of decrease in fitness.