Thursday 31 March 2022

Quick and dirty cross-calibration of Stages and Power2Max power meters

I will make this a short blog post, because the chart to the left speaks for itself.

Finally, after about 4 years of  using exclusively single sided poer meters on my bikes (Stages left hand crank-based PMs), I decided to buy myself a dual-sided power meter.  This new power meter was to be fitted onto my hardtail mountain bike, a Scott Scale, which until now never had a power meter installed on it.

After some research, I decided to by a Power2max NG-eco spider-based power meter.  Strictly speaking, this measures total power, rather than both sides because the strain gauge measurements in the crank spider aren't able to differentiate whether the measured torque is coming from the left or the right side.

Before test riding my mountain bike with Power2Max power meter installed on it, I was keen to first check how its readings compared to the 2nd generation Stages power meter installed on my road bike, which I use for the majority of my training.

Unfortunately, it wasn't possible to fit both power meters on the same bike, because the the Stages Shimano 105 left hand crank arm wasn't compatible with the XTR crankset on my MTB, with the splines being different.  Instead, I used a technique I've used previously (described here) to compare power meters, whereby I used my Wahoo Kickr trainer as the 'balance'.

The method involves testing one bike, then the other, on the Kickr, with the Kickr target (ERG) power profile controlled via an app using Bluetooth, in my case the TrainerRoad app.  The actual power meter measurements are then recorded via ANT+ on my Garmin head unit.

The plot to the left shows the power meter readings versus time for the same Wahoo Kickr power profile.

It's clear that the Stages power meter reads significantly higher that the Wahoo Kickr power, whereas the Power2max power meter tracks very closely with the Wahoo Kickr.  The plot at the top of the page shows the average power over 100 seconds for each interval.  The Stages power meter reads between 15-25 Watts higher than the Kickr (5-14%), whereas the Power2max power meter is within 2-3 Watts.

There's no way to say for sure which power measurement is closest to 'the truth', but given that two power measurements match very well and the odd one out is a single-sided power meter, I think it's highly likely that the Stages power meter is the wrong one out of the three.

I think it's most likely that my left/right leg balance isn't 50/50, which is an assumption that the Stages power meter makes in it's calculation of total power from the left-hand power measurement.  A leg/right balance of 55/45, for example, would result in a 10% over-estimation of power by the Stages. 


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