Power meter drifting lower during intervals

I have a Favero Assioma Uno power meter. Recently I attempted to transfer the pedals from my primary bike, which doesn’t fit on my budget Travel Trac fluid trainer (wide thru axles), to my less than ideal indoor hybrid bike. Was previously using virtual power, but there’s no reason to use virtual power anymore. Made sure to calibrate the Faveros once connected to TR.

My problem is with power drifting lower holding cadence constant. After about a minute and 30s into the interval, I have to increase my cadence by 1 or 2 rpms. As more time passes in the interval, it continues to drift lower. So where at the start I might measure 210 watts at 90 rpm, later that same 210 watts will require 96 rpm and so on. I’ve failed Carpathian Peak twice as a result of this…legs toasted.

I ran the data on my Garmin head unit and the power/cadence were identical and the same power drifting phenomenon occurred. I’ve never had this problem outdoors. Do you think this is a problem with the Faveros? Could it be the trainer? Is there a solution? Don’t have the budget for a new trainer right now.

I suspect it’s your wheel-on trainer.

I had similar issues with a Cycleops Fluid 2 and often had to adjust my cadence to keep at the required wattage. A proper trainer tyre pumped up to 100psi might help things.

The fluid changes its resistance curve as it heats up. Not your power meter.

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I wouldn’t completely rule out the PM as the cause as it could be a symptom of temperature drift.

Try recalibrating the PM between each interval to see what happens to the power figures. Some PM’s will see that sort of drift which can become less as the temperature wherever you are training stabilises. At least then you can rule it out.

As the others mentioned, the fluid trainer will change resistance as it heats up. Trust your power meter and just shift gears as needed.

My (old) Performance fluid trainer would get quite a bit harder to pedal at the same cadence after ~10 minutes. My current fluid trainer (Kurt Kinetic) is more stable, but gets slightly easier to pedal after it warms up. In either case, I pay attention to what my powermeter says, and shift accordingly to find my desired cadence/power.