large power difference between kurt kinetic inride and zwift zPower via bike-equipped ANT+

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1

I saw several similar posts but not quite the same so I decided to post a new one.

TL;DR:  Zwift zPower from bike-equipped Garmin ANT+ speed/cadence kit consistently at about 60% of inRide reported app from smart KK trainer.

I am considering buying a kurt kinetic road machine, and so borrowed a friend’s to try it out. It is a KK Road Machine 2.0 fluid trainer, smart version, with built in BT power meter. I also was convinced to try zwift, which I’m loving.

Zwift works perfectly with my ant+ sensors. For curiosity sake, I installed the inRide app, calibrated the power estimate with my bike setup, and ran both in parallel. To my surprise, the zwift reported power was consistenly about 60% of the inRide reported power.

 

In the end it doesn’t matter individually, as day to day it seems consistent. However, I have lost confidence in comparing my speed to others on the track.

I will be buying a KK Road Machine 2, not smart version, and I would be much happier if I could have consistent power output.

Is there anything I can try to rectify this situation? I am happy to help trouble shoot, as a computer scientist I am comfortable with debug software, etc.

2

To make sure your zPower setup isn’t reading low, your setup should match all the guidelines we outline here: https://support.zwift.com/hc/en-us/articles/203968635–Setting-up-zPower-Classic-Trainer-Rollers?section=200993429

If your setup matches ours, the readings should be very close to the same.

3

Thanks Jason for the comment. 

Here’s what I did

  • I made sure my bike is centered on the KK

  • I checked the inflation pressure, it was a little low, so topped up to 100psi

  • the tire is a 700x23 training tire

  • I moved the ant+ sensor very close to the bike

  • I re-checked the KK manual and correctly set the tension against the bike tire

 

After doing all of this, I still have this problem. E.g., when maintaining a steady cadence my inRide is reporting 100W, while zwift is reporting 60W

I picked up my own KK today (this is a loaner), but it’s not a smart version, so it will be hard to compare.

The only other thing that comes to mind is that this is a KK road machine 2.0, while all your SW / lists just says KK Road Machine. I wonder if they’re that different.

Thanks for your help. I only have a few days left of my zwift trial and still deciding if I’ll go for it.

 

4

Quick update:

I did a short ride (~10min) to warm up the KK, and then re-calibrated it, in case it was calibrated cold. I got the exact same calibration number as before. For what it’s worth, it gave me 1.54

 

It isn’t the most expensive bike, I wonder if my wheel has additional resistance (that is, a problem) that the KK picks up in calibration and translates into the additional power. That would at least logically make sense, as zwift relies on the wheel speed only, and KK factors in the actual resistance. 

 

It would be nice if zwift had a spin-down calibration as well, but I imagine that’s not high on the list…