TACX Vortex Smart Review Part 2: Spin Down

Hello Everyone,

I wanted to take a minute to talk about the spin down and calibration of the TACX Vortex Smart.  I normally do 2 posts when I review a trainer.  The first post covers build and setup (TACX Vortex Smart Review Part 1: The Build) and the third post covers the ride, performance, and my final thoughts (TACX Vortex Smart Review Part 3: The Ride!).  Because the TACX Vortex Smart is a smart trainer that uses a virtual power curve, it is really important that the spin down is done correctly.  Otherwise, you would think this trainer is wildly inaccurate and not a good purchase.  I will also be using this article to reference other trainers that may be setup the same way.  If you would like to skip all of the reading, I summarize everything in my video below.   I’m using the same graphics and images.

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Mag Trainer + Smart Control

The TACX Vortex smart uses magnets to create a baseline resistance.  It is no different than a non-smart Mag trainer.  They use 8 neodymium magnets to set thes baseline mag resistance and then augment that with 8 electromagnets for smart trainer control.  It is all pretty “Smart” actually and I’m impressed with the idea and design.  You can look below and see inside the trainer and what I am talking about.


I was curious on what the baseline resistance looked like with the Neomagnets so I rode the trainer unplugged while recording power with my Stages power meter.  From this, I was able to use a scatter plot and figure out the baseline virtual power curve for the “Mag” resistance of this trainer.  This is the same curve that TACX might have programmed into their trainer.  Using a setup like this will have some disadvantages though.

slide1We cannot go below our virtual power curve (the red line).  You can look at the image above and we have power graphed against wheel speed.  You will not be able to go down below your (red line) baseline mag resistance.  Resistance can be added above your baseline resistance through the use of the electromagnets, but we cannot go below.  The only way to reduce resistance is to lower wheel speed.  This means the TACX Vortex Smart may require some shifting to stay on target.  I used the warmup video “Ignitor” from the Sufferfest Training Centre to test this theory out.  As you can see from the power profile below, at a 28 mph wheel speed I was above my target power.  This was in ERG mode.   I needed to shift up a few gears so I could stay on my ERG target.



Okay, so now it is time to talk about the variables when we combine virtual power with smart trainer resistance. You can follow along as I went through and tested a few variables.  After I setup my Vortex Smart, I did a cold spin down.  I used the TACX App and adjusted my tire contact to get a good reading. You can look at the examples below of a bad spin down and two good spin downs.

I then went into the Sufferfest Training Centre with the cold spin down and good tire contact.  I measured power with the App and with my Stages power meter connected to my Garmin 520.  As you can see from the power graph below, the difference in power reporting was around 40 watts.  I also plotted the trend line for the Vortex Smart and Stages power meter.  This way we can see the gap between both trainers and if there is any drift over time.  The blue line is the Vortex smart reporting the “Smart” virtual power recorded by the Sufferfest App.  The orange line is the Stages power meter after a zero calibration with the Stages App.

We have roughly 40 watts difference in power reporting with a cold spin down.  There is also a bit of power drift over time.

I then stopped my workout and did a warm spin down using the Sufferfest Training Centre.  As you can see, just by warming up the trainer we are almost in line with the Stages power meter.  It shows us how important it is to warm up this trainer before we do our calibration.

After a warmup, we are within 5 to 10 watts.  That is really good for a smart trainer without a strain gauge.

I finished that quick workout and then did another spin down with the TACX App.  Here I was just curious to see if the TACX App would also give me an accurate reading after being warmed up.  The image below also shows a pretty accurate reading.

TACX Spin down after being warmed up.  Again, we are about 10 watts off of the Stages Power Meter.

I think these images show how important it is to warm up your TACX Vortex Smart for 5 minutes before you do your Spin Down.  I don’t believe you need to do the spin down every time before you do a workout.  You could probably do this once a month.  However, when you do the spin down, it is important to not only get tire contact correct, but warm the trainer up also.

Zwift Doping

My mind got the best of me in this next part.  I wondered how easy it was to screw up reporting after you did a good spin down.  I then went on Zwift, but before I started I wanted to simulate a condition where things have gotten a bit sloppy.  So, I backed the trainer off the wheel by just a little bit and then pulled some air out of my tire.  These are two conditions that could happen over a couple of weeks if you are not paying attention to your setup.  The results I found were remarkable.

By letting my tire contact get a bit loose, I gained 80 to 100 watts on my FTP.

It was amazing that I instantly gained 80 to 100 watts on my FTP.  I was crushing it in Watopia, and  on virtual EPO!  This is why it is so important to keep that tire pressure and tire contact identical each time you do a workout.  Virtual power curves are heavily influenced by tire pressure and roller contact.  Those two variables have the greatest influence due to friction.  I don’t think anyone on Zwift is purposely doing this, but I could see people ignorant of ignorant of the variables.  If you have never trained with a power meter, you may never know that you are hitting Elite levels when it comes to power.  Very few of us can actually do that.  I am not one of those people.  I sit just above the average.

Do It Right Each Time

You need to calibrate this trainer the correct way and You need to ensure that everything stays consistent before the next calibration.  Consistency in our power numbers is important if we want to train the right way.  Follow my steps below and you to can get your TACX Vortex Smart to report nearly as accurate as a power meter.

Step 1: Warm up your trainer for 5 minutes.  This is something that smart trainers need.  That is why Wahoo Fitness has started using an advanced spin down function that takes 5 minutes.  It creates more reliable numbers.

Step 2: Use the TACX App to ensure you have the right tire pressure and contact.

Step 3: Make sure that you have the correct tire pressure and contact each time you get back on your trainer.  You don’t need to do a spin down every time you get on this trainer but you do need to validate that your setup is the same as when you calibrated it.


As you can see from the evidence I provided, that you need to spend a little extra time getting your TACX Vortex Smart setup correctly.  If you do this, there is no reason this trainer cannot have an accuracy of within +/- 2%.  I believe that the advertised +/- 10% for this trainer is due solely to the expectation of user error.  I’m thinking the folks at TACX are a some pretty smart people.  That is a variable I wouldn’t think a trainer company would inject into their stats.  However, it shows they have great integrity.  They are not trying to falsely advertised this product.  They are being very honest about it, maybe even a bit too honest.  (whisper* It’s +/- 10% because you might screw up the calibration)

This week I’ll spend a good amount of time riding this trainer.  This will let me know what those outer performance edges are and how good this trainer is.  I had to add this extra part because it is easy to screw up the power calibration.  If I got that wrong, I could have mistakenly thought this trainer was much worse than it actually is.

Aaron Johnson

Aaron Johnson

Aaron served in the military for 20 years. Multiple tours in Iraq and Afghanistan. He retired from the military after 20 years of service to take care of our three small children in 2013 as a Stay At Home Dad.

athletictechreview has 60 posts and counting.See all posts by athletictechreview

7 thoughts on “TACX Vortex Smart Review Part 2: Spin Down

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  • January 14, 2017 at 12:50 am

    Very useful post and it helped me get my Vortex calibrated. Just a tip, after calibrating the device warm, you can run the calibration again cold (without actually changing the grip pressure) just to see where it should be when cold. I’ve found it to be pretty reproducible and on my Vortex it’s about 7/8 of the way across the middle green zone, toward the right end. Now for a quick cold startup check I know pretty close to where it should be, at least with these tires and pressure.

    This position might be different for others though depending on different tires and pressure I suspect. So you need to center the calibration when the tire and machine are warmed up, then let it all cool down and come back and run the calibration cold to see where it ends up for you.

    Of course this will make you slower in Zwift, so maybe people will wish they hadn’t read this article.

  • November 6, 2017 at 6:31 am

    Actually your claim: “Resistance can be added above your baseline resistance through the use of the electromagnets, but we cannot go below.” is not correct. With electromagnets you can reduce trainer resistance by magnetize electromagnets to opposite polarity vs neodymium magnets. This is quite old technique which Tacx have been using since Tacx Flow

  • January 27, 2018 at 3:49 am

    Thank you for this really helpfull review. Thank to you I finely bought one. As I have no power meter I’m still not convince on the accuracy even following the advice for calibration (really easy compare to road @ same speed / elevation). Nevertheless it’s a very good trainer (ERG mode, etc…). I would just like a bluetooth connection less capricious !


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