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🚴 Power drift on a Stages SB20

I’ve been using a Stages SB20 trainer at home for over a year. I love it, jumping in the saddle three or four times a week to tackle a TrainerRoad workout. The idea behind TrainerRoad is that you aim to hit a specific power output for each ‘interval’ within a ride, whilst keeping your feet spinning at an optimal cadence of between 85 and 95 revolutions per minute. For a while, I’ve been noticing that towards the end of a ride I may end up in a higher gear than I started, despite being asked to hit the same measured power output as an interval at the start. I don’t understand what’s going on.

Today I tackled Low Saddle, a 1h45m ride that includes a single 1h33m ‘interval’ at a steady power output. My completed ride is shown in the diagram below. The top of the blue shape represents the power target that TrainerRoad wanted me to hit, the yellow line is my measured power output and the red line is my heart rate.

My completed Low Saddle TrainerRoad ride

My completed Low Saddle TrainerRoad ride

Once I finish a ride it gets automatically uploaded to Strava, which provides further analysis. A ride like Low Saddle is perfect for illustrating the issue. In the charts below you can see that my power output stays roughly the same throughout the ride whereas my ‘speed’ drifts from 25km/h to 29km/h and my cadence from 95rpm to 105rpm. This is a massive difference that I can really feel over the course of a ride. I’m glad it isn’t my mind playing tricks on me.

Power, Heart Rate, Speed and Cadence from the Strava analysis of my ride

Power, Heart Rate, Speed and Cadence from the Strava analysis of my ride

The SB20 trainer has a power meter on each crank. I’m not sure whether the problem is that:

  1. The measured power output from the power meters falls off during a ride, forcing me to push harder to keep the same measured power output, or
  2. The resistance on the trainer eases up over the course of the ride, meaning that I need to turn the cranks faster to maintain the same power output.

Does anyone know if there is something I can do to find out?

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  1. @adoran2 That does sound exhausting. That’s a pretty good cadence increase just to stay at the same power output. All I can think to try would be some experiments at different outputs to see if the drift is consistent — which involves a lot of work and time plus uncertainty!

    Reply to alans

  2. @alans The heart rate increase shows that it’s definitely harder work for me by the end of the ride, which is no bad thing I guess? But that harder work might just be a higher cadence. The trouble with trying different power outputs is that it still won’t prove what the cause is.

    Reply to adoran2

  3. @adoran2 Right, it’s not necessarily bad, but it’s definitely a different kind of ride than ‘just’ endurance. I was thinking with the power output thing that maybe it could help isolate the problem or give you more data that you could take to tech support, which sort of feels like the ultimate destination for this.

    Reply to alans