Stages power vs Tacx Neo in workout mode

(M T NEO) #1

First up - here’s my setup:

  • Tacx Neo with latest firmware
  • Zwift on Mac with Ant+ dongle
  • Stages power meter - latest firmware 
  • Garmin Edge 820 - latest firmware

When doing workouts, I get very different readings from my Stages power meter - paired to the Garmin - than the Tacx/Zwift setup. But only when I get above 200 watts. 

I can ride a recovery part of a workout for two minutes and hit exactly the same numbers on the Garmin and on Zwift. But as soon as an interval kicks in (ex. 3 min @ 350 watt) the Garmin shows 30-40 watts lower average power than Zwift/Neo. Could this be how ERG-mode affects my pedal stroke? Or is there any other possible issues? Just got a new Stages arm. My old one had same problem. 

The image with the lowest average power is the Stages. The higher number is the Zwift/Neo.

(Chris B (D)) #2

A friend has Stages (i.e. left leg only) and Neo. He also has a discrepancy like yours. He did a left leg only comparison test, and in left leg only, the Neo matched the Stages. Therefore his error is due to a leg imbalance.

Not saying this is your issue to, just that it would be a useful quick test. Warm up (with both legs), then do maybe 15 second at 50W, then 100W etc and see if it lines up.

An imbalance could explain yours too,  as your error is at higher watts, and it’s believable that a leg imbalance would increase as you put more effort in.

Good luck.

(M T NEO) #3

Hi Steven. Thanks for your reply. I use a Power2Max power meter on my outdoor bike (Stages is on my summerbike, which I use indoors) and it states I have a 52/48 power balance on average. That is, 52pct on my right leg. That small imbalance shouldn’t result in that big a difference. 

I’ll try and make a test though. Might solve the mystery.

(Chris B (D)) #4

It is really odd. You are using two completely separate/independent systems, and both are rated to like +/- 1 or 2% … so something isn’t right. 

Apparently there is a Stages app that lets you hang a specified weight off the pedal to confirm that the stages is reading it correctly. That could be worth a go too.

(M T NEO) #5

Stages issue is related to specific device issues. Rather it might be a general issue with Hollowgram-arms from Stages.

Basically I’m down to these possible explanations: 

  1. The left/right-balance my Power2Max gives me (2% lower on left leg) is not right.
  2. The ERG-mode in the Tacx Neo does something that doesn’t require as hard an effort as actually measured by the device. As in: 350 Watt in Zwift/Neo ERG-mode is not really 350 watt.
  3. The Stages can’t handle the way the ERG mode affects my pedalling. Thus, the Stages measures lower.
  4. All Stages power meters just handle loads above 200’ish watts badly

I hope this might be solved via a firmware update all of a sudden. But who knows.

I just checked some power data from my Power2Max rides outside. And I have, without problems, been doing 8-10 minute intervals with higher power than I have trouble doing indoors, should I trust my Stages-data from the Neo/Zwift-setup.