Monday 7 December 2020

Stages Power Meter Cross-Calibration


Cross-calibration of Stages Power MeterI recently bought a new Stages 3rd generation power meter (PM) for my new gravel bike.

This is the third power meter I now own, with the other two also being Stages PMs. The other two PMs I own are on my road bike and my time trial bike (a 1st gen and 2nd gen Stages PM respectively).

I decided to buy another Stages, partly because Stages PMs are good value for money (this third one was less than £300 brand new), and partly because I was keen to have the same brand of PM on all bikes.  My hope was that by having the same brand of PM, they would deliver consistent power values between the three devices.  However, I wanted to check how consistent they actually are...


The best way to do this, I decided was to use my Wahoo Kickr Smart trainer as the 'balance'.  In general, the power numbers from smart trainers are not considered to be as accurate as those from dedicated power meters. However, with some care, the power values from the my Kickr smart trainer should at least be consistent, meaning that my Kickr could be used a common power source to enable the other devices to be compared with each other.  The agreement with the values from the Kickr is unimportant.

Some care was taken to ensure consistent testing of the three devices
  • Testing was done in the same gear, 50t front, 16t cassette, and at the same cadence (85rpm)
  • The same pedals and RH crank/chainrings were use for each PM test.
  • All PMs were stored in the same place (same temperature) before installation.
  • The Kickr smart trainer was connected via bluetooth to my iphone.  The power meter was connected with ANT+ to my Garmin Edge 520, to provide independent recording methods.
  • Exactly the same PM installation sequence was done each time.
  • The same testing protocol was done for each PM.
The protocol for the testing was as follows:
  1. With no power meter installed, I warmed up the smart trainer for 15 minutes at 140-170 Watts.
  2. The smart trainer was then calibrated. No further re-calibration of the trainer was done.
Then, for each of the three power meters, the following was done:
  1. Install the first power meter. LH crank bolts tightened to 12Nm.
  2. Power meter calibrated (zero offset) via the Stages app.
  3. Collected power data at the first power setting, 150W:
    • Use TrainerRoad app to set the smart trainer target power to 150W in ERG mode.
    • Pedal for 1 minute to allow everything to stabilise.
    • Press the lap button, and pedal for a further 2 minutes to collect power data.
    • Record the average power from the power meter (Garmin) over the two minutes.
  4. Collect power data at second power setting, 215W, using the same steps.
  5. Collect power data at third power setting, 280W, using the same steps.
  6. Optional: Collect repeat power data at 215W and 150W if time, energy, and motivation permit.
The order of testing the power meters was:
  1) Stages Shimano 105 7000 Gen 3 power meter
  2) Stages Shimano 105 5800 Gen 2 power meter
  3) Stages Shimano Ultegra 6700 Gen 1 power meter

4) A fourth run was done with a repeat of (1), the 105 7000 Gen 3 power meter, to check the repeatability after a PM re-installation.


Cross calibration of Stages power meter using Wahoo Kickr


You can see from the plot above, that there's a fair amount of variation seen in the results, not just between the three power meters but also some significant changes seen in the repeats.  Ideally, the repeats should be very close to the first recordings, so what can we learn from this?

Starting with the repeats, the installation repeat run gave some differences of 2-11 Watts between run (1) and run (4) for the same power meter. There are several possible explanations for this difference that I can think of:

Explanation 1) The Stages power meter calibration drifted between runs 1 and 4.  This is unlikely because both runs were done with exactly the same protocol, so there is no additional usage of the PM that should cause a drift. For example, it was no warmer on the 4th run, having been inactive for 45-60 minutes.  Also, the differences between the powers are not consistent across the range, so it's not a parallel offset that would otherwise indicate a calibration drift.

Explanation 2)  The Wahoo Smart trainer has drifted.  Again, I think this is unlikely because if this were true, I'd again expect a constant parallel offset to be seen between runs 1 and 4.

Explanation 3) The most likely hypothesis is, I think, that both runs are subject to small changes in my left/right leg balance, with random changes affecting the results.  For example, if at 150W I have a 48/52% left/right leg split, then the left leg produces 72W, the right leg produces 78W to create the 150W (real) power total. However, the Stages PM always assumes the right leg also produces the same as the left, 72W, so the Stages PM would give a total power of 144W. If that L/R balance then swings by 4% to 52/48% L/R instead, then the left leg produces 78W, the right leg produces 72W, but the Stages PM gives a total power of 156W.  So a swing of 2% asymmetry to the left, to 2% assymmetry to the right, will create an apparent power difference of 12W for the same actual 150W power total in both cases.  If the target power is higher, say 280W, that same swing would be higher, 22.4W.

I think this is third explanation, of random small changes to the L/R leg imbalance, is the most plausible explanation of the 2-11 Watt variation seen in the installation repeat. It also explains why similarly large differences are seen in the repeats that were done before uninstalling each power meter (step 6 in the method).

Power meter differences

The differences between the power meter results has to be considered with a view of these repeatability results.  However, even considering the repeatability, and the uncertainty that it introduces, I feel the Gen 2 105 power meter (the red data points) is reading higher that the other two PMs.  Although its difficult to be conclusive, I would say the the Gen 2 105 PM is reading around 10-Watts higher than the other two.

This isn't a huge difference, but it is large enough that I'll need to take it into consideration when riding close to my limit, close to threshold.  This Gen 3 105 PM will go onto my gravel bike, with the Gen 2 staying on my Road bike, which I use for most training, FTP tests etc.  I will therefore have to keep in mind that the power values on my gravel bike may read a little low compared with what I am used to on my road bike, for the same effort level. 


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