How does Zwift figure on trainers that don’t go above 13%?

Hi, so I have a Wahoo kickr snap 2018 model. I recently did a race that went up Epic KOM Reverse then to the tower and back down. When I finished the race Zwift detected a new FTP increase from (estimated) 352 to 392, which jumped me up on Zwift power to 4.5 W/KG. My trainer does not give the correct resistance obviously on higher than 13% ( trainer difficulty set to 100%) I decided to do an actual FTP test full the next day and got a reading of 332 which seems closer to reality for me current fitness and weight. I sent an email with all the current info I have. But what experiences have others had in this situation and are there any things I should do. And yes I did calibrate the trainer before both rides after warming it up.
Thank you

Even if your trainer can only do 13% the power is still power. Watts is watts.

Right, I agree very much that watts are watts, but are they equal watts I guess is my question? If my trainer can “only” simulate 13% but the Zwift gradient it 17% am I getting say a bump in wattage in Zwift thinking I am doing say 500 watts on a 17% grade when I am really only feeling the 13% and can produce it there but could not on the 17% where I would have an even lower cadence and maybe only able to produce 400 watts? This is all new to me since I don’t run power outdoors yet and I am still getting to understand this. Sorry if I am not wording myself correctly and conveying my meaning.

Steve, my two cents.

  1. If in doubt, sent trainer difficulty to 50%. Not sure if you fully understand what this setting does. It will not change your race results - Zwift will send to your trainer only half of the incline, but will read the power which you generate and this, along with the real incline, your weight, etc., will be used to calculate how your avatar moves. So, the change will be only in how often you need to switch gears and how low you need to go.
  2. FTP is estimated as sustained power over 20 minutes, multiplied by 0.95. There may be a few short segments in Zwift which are steeper than 13%, but there is absolutely no sustained climb with such steep incline which you would ride for 20 mins or even 5 minutes! You very rarely see more than 11% in Zwift.
  3. It is totally possible that in a race you are much more motivated to give everything you got. FTP test…a different story. If you did a ramp test, it is good as a quick estimate, but it tends to be inaccurate for many people (those who tend to be sprinters or persuiters are likely to do better on a ramp test). A 20 min sustained ride test, which is the long version of FTP test, is more accurate as it is how FTP is defined when Zwift calcualtes it, but it is mentally difficult. You race for 20 mins with yourself. You might show a lower number than when in a race.
  4. It is unlikely that you ran out of Kickr Snap ability to simulate slope and provide resistance, if your FTP is 392 watts. Your trainer per spec can generate resistance worth 1500 watts. Did you ride all race at 18 watts per kilogram or higher, to exceed 1500 watts ???

Read this article:

The most important point which Gerrie tried to communicate, no matter what incline your trainer can simulate (i.e., what resistance it can create), Zwift calculates your FTP from the power which you have been able to generate during the race. Power is measured by a power meter built into your trainer. For the same resistance, the faster your cadence, the higher is your power output, in the same gear. There is absolutely no way how Zwift can calculate your FTP incorrectly if you did not generate that power for 20 mins, and it has NOTHING to do with the maximum simulated incline of your trainer.

What you are trying to say is, “oh, maybe I ran out of resistance” - well, in this case, your pedals would spin too easily and you would not be able to generate the power and your FTP will measure too low. Just the opposite to what you observed!

An impressive FTP!

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What Gerrie was getting at is that even if your trainer can’t simulate the full grade of the incline you’re riding on Zwift, your power numbers are still accurate and wouldn’t have any affect on your FTP. It’s very possible that the reason you noticed such a noticeable difference in the estimated FTP from the race and from your FTP test is that you did the test the day after a hard race effort. Since your FTP tests are meant to be a full effort workout they’re most accurate when adequately rested and we recommend at least a week between tests if you plan to retake one.

FWIW the advertised gradient simulation numbers are typically based on a 75kg rider, so if you weigh less than that then technically the trainer can simulate more. When I had a trainer that maxed out in gradient simulation at a lesser percentage than the game applies at 100% on the steepest hills, I calculated (using Excel, wahey!) what TD I needed to be at so that those steepest hills would apply the most resistance. This gave me a realistic experience, where the steepest hills felt the steepest, and required me to be in my easiest gear.


I am understanding what everyone is saying and thank you for all the responses. I have a couple of questions but first I would like to put this out first.
First same stats and history of me. I ran track and cross in high school and then in University and up until I was about 35 along with road races. I coached middle distance and distance for about 15 years for high school and six years cross country for a small college. Took up cycling after I just couldn’t keep my Achilles and calf from giving my trouble all the time and started racing not long after. So I have been racing bikes for a bit to Cat 3 and have had some solid results. I have not used power meters because I couldn’t afford one at the time and I could train via heart rate well enough even with it’s limitations, that, and I know my body pretty well to know when I am on or off on days.
I am 49 now, really just getting back into shape these past few months because of family commitments these past years I have trained very little to not at all. I am 188cm and right now between 86 and 87 kilos. Zwift weight 86.7

I did AdZ on stage 2 the week before and was in a race situation if you will even though it was not a race because I am competitive and I hit the bottom of the climb in 80 ish place, I drill that to the top in 43:05 or so and finished in 18th, 13th fastest up which gave me a new estimate of 352. I do the race a week later on reverse Epic and tower and was not pushing on like crazy because I didn’t want or need to to stay in the front pack and I get a new estimate of 392. This is what is confusing me in that my fitness did not jump in one week by 40 points and my trainer was calibrated after 15 minutes of warm up. Is my trainer then not working properly? Or as I have read on many occasions and even in some of the race information descriptions the estimates on the 20 minutes Zwift takes tend to be on the high side?
I will retest this weekend and have some easy days this week and see. I assume a small change will happen but not a huge one.

I think those are rated at +/- 3% accuracy, but that can depend on how tight it is against your tire, tire psi, environmental factors, etc…

Thank you for the info. I read a lot of articles before I bought my trainer and it is a lot to take in and understand the way FTP is done too. When I tested I did the Full FTP test with the 20 minutes. As for motivation I have no more motivation for the race than for the test. I enjoy all challenges and honestly the race was not very difficult in that three us got away and that was that with nothing really difficult until the finish. And as I write below in my response below I had just had a new estimate on AdZwift the week before of 352 in Stage 2 of TdZ, which very racy and competitive in the front group. I wanted to see how many I could pass and went from 80 to 18th. I am just surprised at a 40 watt jump. Problem with trainer? I did not run out of resistance. Certainly not doing 18w/kg… lol

Correct, I keep the tire at about 110psi and 2 full cranks plus a bit as Wahoo recommends, no slippage at all. Calibrate usually around 15 minutes if I have the time, but always after 10. Sometimes calibrate with Zwift and the Wahoo app.

You are doing everything right as it sounds. The only way to be sure is test with another power meter, or if a friend has a direct drive trainer that you can go ride for a bit and see if the numbers match up.

Thanks, I just want to be correct as I start to head into doing real workouts and such. As it jumped me so high it pushed me out of my team on ZRL. Gutted not racing last night with them. But I also want to be in the right cat doing the right stuff. I want to push myself to be fitter and faster even at 49, but I don’t mind getting my head kicked in, metaphorically…… lol, by faster guys. Anyways, I will do the retest and see.
Again thank you and to everyone who has kindly replied.

Well, Steve… If you have not done so, create a (free or paid) account on Strava and an account on Training Peaks (free is enough). Link them to your Zwift accout. Zwift will automatically upload your workout files to these repositories. You can also find .fit files of all your past Zwift rides in /Documents/Zwift/Activities folder on your Windows PC, and you can, with some effort, transfer them manually, definitely to Training Peaks, not so sure about Strava (you may need to research it).

Then, you can either buy a license for WKO5, or, if you are fine with an inferior and less documented tool, you can download and install a freeware program, Golden Cheetah. They both can import fit files from either Traning Peaks or Strava. I use WKO5 and it can only import from Training Peaks. I have very limited experience with Golden Cheetah as I have no reason to use it, as long as I have WKO5.

This will allow you to analyze your rides “to death”. WKO5 will compute your FTP from your rides, you do not even need to do an FTP test. You can do insane amount of analytics, and there are dozens of videos from creators of this program how to use it and how to train using input from these data analyses if you want to learn how to do it. If I had your data from these two rides which you are speaking about, I could tell you why and how your FTP had increased, and your long term data could give you a better estimate of your true FTP. You can do it, too, as all data are on your computer. If you are not clear what to do, do not hesitate to ask.

no need for any spesial software to get FTP. In Free strava you can get the peak 20min power use that and multiply by 0.95 and you have your FTP.

From below 300w x 0.95 = 285w FTP

But we are going off topic here.