Virtual Power vs. Trainer Power - Dumb Trainer vs. Smart Trainer

So i recently upgraded from a TACX Flow dumb trainer to a KICKR Core smart trainer. Iv’e always wondered how different the power would feel between the 2 or how different my FTP would be measured.
I’ve heard a lot of opinions that virtual power is no where near accurate as the power settings on a smart trainer.
I’ve always disagreed as my TACX head unit displayed a power readout of about 8 - 10 watts below what the app indicated i was pushing out - at my FTP thats about 5% so it can’t be that inaccurate - Smart trainer specs advertise a 2% accuracy.

So changing to a smart trainer, 1st thing that is needed is a new FTP test. I never expected any increase as i’m just finishing Traditional Base 1 going into TB 2. I did the ramp test twice and both tests on the Kickr were within 2 watts of my current FTP done on virtual power.

I suppose a true test would have been the 1st on the dumb trainer and the 2nd on the Kickr but i doubt my FTP is any higher than my last test on the dumb trainer done 4 weeks back and then completing TB 1 mid volume.

My point is, Dumb trainers and virtual power is on point and just as good as a smart trainer. Anyone thinking they not getting the full benefit because they not on a smart trainer is wrong.


I don’t think anyone has really said that you don’t get full benefit from a dumb trainer.
With that said, I’m sure that your results of virtual power to “smart” power are not typical.
When I switched from virtual power (Elite qubo fliud) to smart (used kicker snap initially, now hammer2) my FTP dropped on the order of 30% (330 to 230).

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Whether VP works well or not depends on the trainer. Your results may not be the same for other people. Some models are better than others and there even can be variations among individiual trainers of the same model. Then there is the whole issue of consistency in the power curve. Just because VP on a fluid trainer matches a real power meter at 200w does not mean they will match at 400w, etc. And all that is before you get to consistency of set up issues i.e. repeatability from ride to ride.

FWIW - my FTP test result instantly changed by 30% when I got a power meter and it was also painfully obvious the entire power curve on my worn out trainer was nowhere close to the model TR was using (my trainer was jacked, TR was not the issue).

Virtual power was a fantastic invention and of great use when power meters cost $2000+ and complicated and expensive Computrainers were the only thing close to a modern smart trainer.

Virtual power is still a valuable tool for folks without access to real power reading but by no means did you waste your money getting a smart trainer with real power.


There are two issues you hint at but don’t state clearly.

  1. Training Effectiveness:

    • I don’t see people claiming that VP is not an effective training tool. In fact, I see plenty of people (myself included) state that VP can and does work well as intended.
    • When used with consistent setup of the bike and trainer, you can get repeatable data for setting training zones and executing workouts.
    • This leads to performing workouts with consistent info and can lead to effective training (when done as part of a properly executed training plan).
    • So, there is no need to “justify” VP because most of us recognize how useful of a tool it really is. Ignore any dissenters because they likely misunderstand the reality of the tool and it’s proper use.
  2. Accuracy (compared to “true power values”):

    • VP is an estimate of power. It is based on testing performed at the TR facility, on a specific bike, with a specific tire, at a specific tire pressure, and a specific trainer (with it’s associated roller pressure). They measure wheel speed and power output from multiple power devices (power meter pedals, cranks and/or hubs). They correlate all that data into a “Power Curve” associated with that trainer. Your measured wheel speed is then “converted” to a Virtual Power value (an estimate of “real power”).
    • Why state all of that? Because if you change any one of those variables, you may experience different results in the data reported. A single difference in tire pressure, roller pressure, actual tire used, etc will all affect the VP value.
    • As such, when you survey VP users who get a power meter, you see reports of examples that show VP as higher, lower, or roughly equal to the real power data. In many ways, it’s much ado about nothing. The desire to have VP be “accurate” is largely secondary to the desire for it to be consistent (the real need for training purposes).
    • The issues arise when people try to make or expect VP to be “accurate”. It may well be so, but there is no guarantee without verification using a proper power meter.
    • Your case is great and interesting that it might have matched (I question your method and conclusion). But we can find many examples where that is not the case. No single example proves that VP is “accurate”, because we can see many more where that is not the case.

In your case, you are comparing info from different points in time and that could well lead to variation. I wouldn’t take that as any validation. Even if you do back to back tests, you introduce the before/after timing and other variables that are likely to skew results.

The only good way to do a VP to Smart Trainer comparison is to use a 3rd device (power meter installed on the bike) and run through a range of power outputs. Correlate the data from each trainer to the power meter (as the “measuring stick”) and see how they compare across a range of power outputs. They may align at one point or many, or they may be offset. No one knows and it is pure guessing without proper testing.

Be happy that you got good training from your VP use and move on to the smart trainer use. There is not much benefit in trying to back validate your VP training. It was what it was, and the numbers are largely irrelevant now.

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I raised the post mainly because most of my mates still us HR based training on the old POLAR plan. I’ve tried to get them to move to Power based but they don’t have smart trainers or power meters, and when i show them VP they say its a waste of time. So my post is more directed to them or those that think this way. I’m a true believer in VP and now very happy with my spend on a smart trainer - Don’t get me wrong, and i’m sure other who made the move had different results, this is just my take and experience.

That’s great to try and educate people. But I think the attempt to validate VP with respect to RP is unnecessary.

  1. The real test of VP is the results that any person pulls from it. If you can show that as better than HR, RPE or other methods (besides VP), you make a stronger case.

  2. Pinning the success of VP with it’s relationship to RP will fail in many cases, and acts to undermine the overall case from a reference that is not beneficial (VP vs RP accuracy).

In short, judge VP based on the results of the users who apply it as part of their overall training plan.

VP is most definitely not a waste of time. I trained with VP for several years and it was very effective (even with the errors I described above). Anyone who does not have a power meter or smart trainer should definitely make the very small investment necessary to get up and running with VP. Power is a great training tool AND understanding power by riding with it also gives huge insights into pacing and how “going fast” and training actually works. If you ride with power, real or virtual, you will be both a more fit cyclist and a better, smarter cyclist and racer.

All that being said, real power meters, whether as part of a smart trainer or on the bike are more accurate and better tools than VP if you have it in the budget to make the necessary purchase.

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