I did an initial FTP test in December after several months off the bike and then did another instance of the same FTP test a few days ago after consistently training 4x per week in between tests. As expected, the more recent test was significantly stronger and the power outputs recorded are demonstrably higher than those of the December test yet Zwift indicated my FTP was lower after the recent test than it was after the first test in December.
What’s up with that? How can the app record a higher power output throughout the recent test but give a lower FTP score?
Full disclosure, I’m on an old school fluid trainer not a smart trainer and don’t have a power meter so I fully recognize the FTP is just an estimate. But that said, the data the app used to calculate my FTP in December showed significantly lower power output than the data Zwift recoeded in the recent test so I again ask how it can calculate a lower FTP for the more powerful ride?
Where are you seeing this new, lower, FTP? Do you mean the zFTP shown on your Zwift activity feed, or something else?
When I completed the FTP test there was a message on the screen saying something to the effect of “Your new FTP is ###” and the value was actually lower than my previous FTP value. Now when I go into my profile it displays that lower value as my FTP.
I don’t have much/any experience with 20 min FTP test. A couple of things it might have been:
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Any chance you had accidently lowered your FTP over the past 3 months?
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This is shown within Zwift’s FTP information: “At the end of the test, Zwift will display the option to update the FTP value on your profile with the one from the test.” To me this indicates that the new test figure will be displayed even if it is lower and should give you the option to update to the lower figure.
I notice for some of your recent workouts you have been increasing the workout bias. If you feel that the new FTP figure on your profile is a little too low you can change it on that profile screen.
EDIT: sorry didn’t read your first post carefully enough. You say that you think FTP of second test was higher than first test - are you sure? In first test you were averaging 118w with 4 minutes to go and finished well. On second test you were averaging 111w with 9 minutes to go and finished well. Are you sure you boosted your average on second test to above first test.
I am fairly sure that the FTP test will not give you a lower FTP result from a higher power.
Sometimes it is easy to confuse the average power you are seeing on screen with that actual FTP result; being 95% of the average power.
That’s part of what doesn’t make sense to me, the numbers just don’t align with the number that displayed while riding and in the graphs from the FTP Test portion of the ride. More on this below but first to answer your questions…
No, I didn’t touch the FTP since it was set in December via the FTP test.
Yes, I have been bumping up the bias on some recent workouts to make them a little harder but without touching FTP. I just wanted to raise the difficulty of those rides but I didn’t want to modify the FTP value until I actually did the test and then use that value.
Yes, I feel quite sure that the March test was stronger than the December test and I can’t explain the numbers you’re seeing. I am confident not only based on my memory of the prior test and comparing the numbers in my head but also from the power graphs I see in the Zwift Companion app. When I move the slider across the graph of each ride, with the exception of the first 5 minutes from the initial test where I went out too strong and had to dial my effort down, the March test is consistently stronger for the last 15 minutes. Comparing the two, the first 5 minutes of the December test was stronger, but not massively stronger, whereas I’d say the last 15 minutes of the recent test were significantly stronger than same time period of the first test.
If I’m successful in uploading the image you’ll see I added a red and blue line across them to make it easier to visualize how much stronger the second ride was. The red line essentially skims across the first 5 minutes of the recent test and clearly shows that the majority of the recent test, again with the small exception of the first few minutes of the first ride, was significantly stronger than the earlier test. The blue line marks the approximate start of the final 5-6 minutes of the recent test and shows clearly how much longer and stronger the finish was vs. the December test.
I remain confident that the second ride was the stronger of the two rides and remain perplexed as to how it produced a lower FTP value.
Also, thank you for taking the time to look at the data and provide a detailed and considered response…it’s greatly appreciated!!
Unfortunately the system at work is blocking the image upload. I’ll recreate and try from my personal computer when I have a chance.
Thanks again!!
Oh, and I’m confident I’m getting stronger because my outdoor rides, weather permitting, have been getting stronger/faster and my times on the Strava segments I have starred for the purposes of tracking progress have all been improving. I remain welllllll off my personal bests but relative to rides in Feb and early March I am improving. All the more reason I’m confident the recent FTP test was stronger than the prior test…I gave it my all and I’m confident I have more horsepower now than I did in December. It’ll be a long road to get back to where I was in 2020 but that’s life as we get older and body parts have to be replaced…lol
As I mentioned above, I took these power graphs from Zwift and added the red and blue lines to even more clearly illustrate how much stronger the recent FTP test was than the one I did in December. Am I misinterpreting the data?
I remain baffled as to how Zwift calculated a lower FTP value for the March test than it gave for the December test.
Those graphs are relative to whatever your FTP was set to during the test, so it’s unclear from the pictures. But if you look at the power numbers during the final minute of the ramp it should be apparent. If you upload FIT files for your biggest efforts to intervals.icu you’d probably get a better estimate of FTP. If you link your Zwift account to Strava you can have the data imported to intervals.icu automatically.
Fair point in that the FTP before the first test was a few points lower before the test. I now understand that the graphs won’t necessarily be comparable because they’re relative to the FTP value prior to the tests and since they were run with different FTP values they don’t necessarily align…thanks for explaining that.
Perhaps it’s just the limitations of the app when you’re not using a smart trainer or power meter…it’s just not going to be consistent from one test to another in how it estimates and captures power data. In the grand scheme, the most important thing is that I know the work I’ve been doing this winter has made me a stronger rider because my outdoor rides have been getting stronger / faster and I’ve even started to push myself up some hills which is something I was struggling with far more in Feb and early March. The FTP number in Zwift isn’t all that important when I look at it in the context of my real world rides so I’m not gonna sweat this too much at this point.
Thanks to all for pitching in on this one!
Ohhh yes if you don’t have a power meter the results are highly suspect. It’s the limitation of the hardware more than the app. Your guess is probably much better than the measurement.
If you’re interested in getting a power meter for a reasonable price, a used left-side crank arm power meter on eBay is usually the way to go. Not as good as dual-sided but much better than nothing.
Thanks Paul.
I’ve thought about a power meter but have decided not to pull the trigger at this point. I gather from your comment that you prefer a dual sided meter and my thinking has been along the same lines…do it right if I’m going to do it. Right now though I’m just not prepared to plunk down that much $ for good one and I have enough macro-level work to do just getting my body back in shape after a couple of seasons of modest riding following a hip replacement. Adding a power meter would be overkill right now…lol Maybe when I start setting new PR’s in Strava on the big climbs I track I’ll start thinking about it again.
Thanks for the feedback…much appreciated!!
Oh, and as for the accuracy of the Zwift measurement without a power meter, yeah, I knew it wasn’t accurate but I was at least hoping it was consistent. Accuracy lets me compare myself with other riders but right now I don’t care about how I stack up…consistency on the other hand would have at least allowed me to compare myself against my own past tests but it seems that was not to be…oh well, no biggie.
Now to get stronger and wait for the technology prices to drop 