Surprising FTP increase: overstated power?

I’m trying to work out what my ‘actual’ FTP is. Now, I know the answer is ‘it doesn’t matter’, but I’m just curious.

The background: I had my first FTP number (259) in April of this year using the dual-sided powermeter (Ultegra) on my Giant TCR. I used the Coggan test (warm up, hard 5 mins, 10 min spin, 20m flat out), and rode outside.

When I sold that bike a few weeks later, I put a stages left crank meter on my new bike (a Mason Definition). I always felt the Stages underread relative to the TCR (not saying it was wrong, just different) and my first FTP test with it after a successful block of training was 248. Not that it’s terribly scientific, but I’d got quicker on all my regular Strava segments during that time.

I have had issues over the last couple of months with power ‘drifting’ downward on the Stages – i.e. power averages trending downwards over time despite faster average speeds. Whenever I recalibrated it, the first ride or 2 it would always massively overread (suggesting ludicrous numbers like 280w at a moderate spin in a middling gear) so it went back to the shop 2 days ago for them to look at and if necessary warranty.

Anyway, to get to the point: I did a ramp test last night using the power readings from my Tacx Vortex for the first time. I’ve always avoided using this power source because of its reputation for inaccuracy (and used the stages instead).

Also relevant: it was the second time I’ve been back on a bike after having Covid (though CV19 was very mild for me).

Here’s the result:

I would LOVE this to be true, but I suspect it’s overstated. I do not think I’m a 4w/kg rider.

The question is: how much is it overstated by?

For reference/context, I weigh 77kg and am 38. I’ve been cycling 25 years and was a very good CX racer in my late teens and a decent (but nothing spectacular) time triallist in my early 20s. I hardly rode at all between 25 and 35, and while I have ridden regularly again for a few years, I’ve only been systematically training again for about 9 months. I typically average about 29-30kph riding on my own over fairly hilly terrain for 60-80km. I can just about hang with the A group on (infrequent) club rides, but I’m really working not to get dropped.

Any thoughts/guesses appreciated.

I have my own guess but I’ll keep it to myself until some others have chimed in!

Wau 30min Ramptest respect.

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But if i have to guess i think he Vortex may be off by about 10%

I went through this. :laughing: :cry: My Vortex can read 20-70 watts high, and the higher the wattage the more it’s off. My FTP took a 50 watt chop when I got off the Vortex. I made a topic comparing my stages and Vortex.

One thing that set my Stages straight was doing a factory reset. That means removing the battery and shorting the terminals for 20 seconds. You can install the battery upside down for 20 seconds to accomplish the same thing.

This is a very informative video regarding the Tacx Vortex:

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That’s really helpful, thankyou.

If I was to guesstimate, I’d say my ‘real’ FTP is most likely 270-275. The TR workouts I did with FTP set at 260 were initially VERY hard, but I didn’t change them after the 248 result, and the most recent workouts I did before catching Covid were tough but definitely manageable, making me think that somewhere a little north of 260 is likely.

It did make me think that 4w/kg might be possible at some point in the not too distant future, which is encouraging.

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Took ages for Elite to get my trainer reading similar to a Power Meter.

If you really want to know your FTP you have to accept degrees of confidence in your equipment. Either you have confidence in your turbo within its pertains parameters. Or you align the trainer to a PM. And/or you FTP test with both. And/or the same with a second power meter.

Then you need to settle on or accept what FTP actually means. Iirc a quasi-threshold like state which you can hold for something like 45mins to an hour. Above which effort levels last dramatically shorter, and below which can be held for vastly longer.

And finally, accept that this output varies by environment and time, even within the day.

So a bit like your weight, I might say I’m 78kg, but this evening I could be 80kg, or after a run 77kg. My FTP is around 293. I might test 287 tomorrow and yet still be performing in races just as well as today. I might test 300 and actually be no faster.


Sorry to say, my Vortex was over reporting by around 20% :grimacing:


Yep, 10-20% over is normal on a Vortex

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I have a Tacx Flow Smart Trainer, very similar to the Vortex. All the reviews and tests I read put the power accuracy within spec, of ±5% (see @GPLama and @dcrainmaker reviews). Nothing close to 10-20%. So I’m curious to know how you got such a large discrepancy.

It wasn’t until I got a stages left crank PM and tried to do outdoor workouts that I realised I had no business trying to hold 290+ watts for a 3x20 Galena etc. I have since replaced it with an Elite suito and ramp tested 20ish% lower.


Same for me. it’s such a blow to the ego when you realize that your Tacx Vortex was so off. I thought my 20 minute power was nearing 300. :frowning:

All my outdoor FTP tests were total failures because I was nowhere near the power I thought I should be at thus I was starting off way too hot.

If you look at the “Stages vs the world” topic I started, my Vortex sometimes could be off by 10 watts, then 20 watts, then 30, 50, or even 70 watts on a standing hard effort. My Vortex would also drift. The more it was warmed up, the more it would be off from the Stages and Powertap.


Do you think this a problem limited to the vortex, or does the Flow has the same issue?

Hard for me to imagine a less expensive trainer would perform any better (Flow < Vortex), but that is a guess on my part.

As with many trainers, some do fall within claimed tolerances, but we see more than a small amount outside that too. We don’t always know if people are following proper trainer setup and calibration practices either, which can have a huge impact on results.

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Its just weird because I see a lot of this reports from users claiming the trainer overestimates by 20%, but the reviews say it falls within spec of 5%.

I don’t have another powermeter to compare or to use outside, so I’ll just trust the values the Flow gives me, and as long as they are consistent, the training effect will be the same. But I just can’t put much trust in the absolute numbers.


Absolutely you can use that value and it’s no reason to stop training. I used mine for around 18 months and saw great results, it was just hard for the ego not to be bummed by the new lower number.
Now my goal is just to reach those numbers for real :muscle:

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Well I tried using those numbers on an Elite Suito today.

I got about 45 minutes into Glassy and was donald ducked.

New ramp test on the Elite tomorrow (if I am not still donald-ed tomorrow).

Are you just using the Stages, or do you have a smart trainer.
It sounds a bit of a nightmare to me. I think you might be due an early Christmas present.
Some good ones are the Wahoo Kickr ,the Tacx Neo pro and the Cyclops Hammer (I have the latter) having torn my hair out with the innacuracies and inconsistency of the KK.
DC Rainmaker has some detailed reviews if you want good info.

Fwiw, I had a Vortex which overread by about 20%. I’ve seen this several times over the years.

Switched to Stages, which is in line with the Neo.

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Funny you should say that - I treated myself to an Elite Suito this weekend :rofl:

The shop (reluctantly) warrantied the Stages in the end. They did the battery reset and claimed it worked fine, but within 24 hours it was back to silly numbers. I just rang up and said basically it’s a refund or replacement at this point, and as they couldn’t offer me a replacement in a reasonable time frame, we agreed on a store credit. That meant for a fairly small outlay I could get the Elite (which they had in stock).

As my other half has wanted to try Zwift for a while, and it comes with a 1 month free subscription, she was onside as well.

It took me about an hour to get everything connected up, calibrated, talking to each other, etc, but so far so good.

My flow reads high and higher for higher power, but not 20%, more like 5-10%. Did a dual-recorded ramp test once. It seems fairly consistent, but can’t say for sure because I haven’t used it’s power numbers in a while.

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