Curiosity got the better of me with my last test. I was curious to see why the TACX Vortex Smart had an accuracy rating of +/- 10 percent. After I completed the TACX Calibration Review, I had a few people ask me to do the same thing with the KICKR Snap. As chance would have it, my new puppy ate my Vortex Smart power cord. This gave me the opportunity to test the spin down calibration of the Wahoo KICKR Snap. However, before we begin, we should look at what Wahoo says about doing the Spin Down Calibration for this trainer. Wahoo covers this in their article “How to get the best results from you new KICKR Snap.” The one difference in my testing is, I did the advanced spin down each time I changed something and wanted to measure it.
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If you want to skip all of the reading, you can watch my video below.
Baseline Power Curve
The first thing I wanted to establish is the baseline power curve for the KICKR Snap. Every mid-range trainer will have a baseline virtual power curve. As you can see from the image below, the KICKR Snap’s large flywheel gives it a power curve very similar to that of a Fluid based trainer. Mag trainers have steady increases in power over speed, where fluid trainers get progressively harder as you go faster resulting in a power curve that starts bending upwards.
The graph above is a simple scatter plot running my Stages Power Meter on the KICKR Snap. I just convert the Garmin file into a .csv file using Golden Cheetah. If you want to do this yourself, I talk about that in this article “Creating and Understanding Your Own Virtual Power Curve.” Everything below the red line is where you cannot go. You cannot get below the base line power curve. I have some users who complain that they get stuck at 160 watts and ERG mode is supposed to keep them at 135 watts. That is because the wheel speed is too high and must be decreased. Most of these mid-range trainers will need some shifting to meet targets. Everything above the red line in green, is the extra resistance that the Wahoo KICKR Snap will add on with the Electro-magnets.
I do some pretty simple testing. Even though the testing is simple, it does take me hours to do this and compile all of the information. I also wouldn’t consider this a scientific result. There are still other variables to consider, like manufacturer tolerances and tire types. I used the S-Works Turbo tire in this test. Someone with a Conti Gator Skin may have a slightly different result. You have to keep in mind, tire contact and friction will have an influence on these results.
Part 1, Advanced Spin Down
The first thing I did with the Wahoo KICKR Snap is go to the Wahoo Fitness App and conduct an advanced spin down. That means two full turns on the resistance knob after the tire makes contact. I then jumped on the Sufferfest Training Centre App and did the video workout “Ignitor”. As you can see from the chart below, the KICKR Snap over reported power by about 15 to 20 watts.
Part 2, Marking The Resistance Knob
For part two, I needed a way to measure how much I was turning the resistance knob. This is the one thing Wahoo could improve on. If they marked the knob somehow, it would be unnecessary for me to use the black marker. So, to ensure that I was making a full turn of resistance, I broke out the black permanent marker. You can see that I just simply marked the knob at the 12 o’clock position after I had my two full turns of resistance on.
Part 3, One Full Turn Up In Tire Contact
After I marked the resistance knob, I added one full turn of tire resistance. This would increase friction and make the trainer report closer to my power meter. What I found, is that one turn of resistance now had the KICKR Snap reporting near 20 watts lower than my Stages Power meter.
Part 4, Advanced Spin Down With 1 Full Additional Turn
The next reasonable question was, “How does the full turn resistance affect the calibration after another advanced spin down?” With three full turns of contact on the tire, I did another advanced spin down. Surprisingly, that advanced spin down brought the trainer right back to its original calibration; the KICKR Snap was again reporting 20 watts higher than my Stages power meter. This shows me that Wahoo’s advanced spin down calibration does take into effect the amount of resistance that the tire contact is creating. That is pretty much how I thought it would work.
Part 5, 1/2 Turn Resistance After Advanced Spin Down
For this last part with the Sufferfest Training Centre, I put the tire back to two full turns and checked to see if it was going to over report. Surprisingly, the KICKR Snap doesn’t over report if you take off tire contact. So, I couldn’t see doping this trainer on Zwift at all besides the additional 20 watts you get in the beginning. Next, I did the advanced spin down one more time with 2 turns of resistance. After that was complete, I added only 1/2 turn of resistance. The image below shows that this put my KICKR Snap reporting right in line with my power meter. The trend lines are almost right on top of each other.
Part 6, Testing the 1/2 Turn Theory on Zwift
The last thing I did here was to go on Zwift and test out my theory. I did another Advanced Spin Down Calibration with the two full turns of resistance (this was getting exhausting). Then, I connected to Zwift and rode for 6 minutes. The resistance felt correct in the beginning but I could tell it was drifting a bit. Now, the trend line won’t be as accurate here. You can see in the image below where the app was stuck on the FE-C call. At 3:15, Zwift kept reporting 225 watts while you can see the power numbers on the Stages drop. This is very typical of all apps, but it could easily be missed when riding around in Zwift. It is more apparent in other Apps like The Sufferfest Training Centre and Trainer Road. In those Apps, you are dying on intervals and desperate if it doesn’t release wattage off the interval, or you feel a bit discouraged when it doesn’t add resistance coming on an interval.
I stopped my ride at 6 minutes, then got off the bike to add the 1/2 turn of resistance. In the image below, you can see that the power numbers were more closely aligned, or at least moving in that direction at the end.
In the end, we see again that even the best built trainers are still not as accurate as a Power Meters. The best thing we can do is make a judgement call when we are riding our trainer. If you feel like the resistance is too easy, then give your KICKR Snap a half turn up in resistance after you do your spin down calibration. That worked for me. However, your mileage may vary. Again, tire type and inflation pressure will dictate how much resistance needs to be added. I’m not willing to test every tire out their. However, I am curious and if I ever found the time, I wouldn’t mind seeing what a Gator Skin tire or a tire specifically built for indoor training looked like.
The key here is to be consistent each time you get on your trainer. I would argue that being consistent is more important than being accurate; consistency in numbers builds consistency in training.
Here is what I would do:
- Do your advanced spin down first with the two turns of resistance.
- Mark your resistance knob to ensure you can measure how many turns or 1/2 turns you are putting on.
- Then add 1/2 turn of extra resistance.
- After you complete a workout or ride, make sure you remember how many turns you backed off to get the roller off your tire. Then, just add that many turns when you get back on.
- Make sure your tire pressure is the same each time you get on your trainer.
Having a power meter makes all of this pretty irrelevant. Most training apps are moving towards Power Match (most are already there). Power Match allows you to read the power from your power meter and then control the trainer with that power reading. I wouldn’t be surprised if we start seeing some companies that make indoor trainers, start to construct power meters or pair up with existing companies to package power meters with indoor trainers.
Thanks for reading everyone! If you would like to read my review of the Wahoo KICKR Snap, you can follow the links below.