As to the accuracy of power meters on a tandem, I can say that Garmin Vectors appear pretty accurate used on a tandem both Captain and Stoker. I’ started train with power 15 years ago, and used a power tap, quarqs, and vector, so hub crank and pedal power meters.
my stoker has a power tap on her single bike, and used vectors on the tandem until recently.
based on years of data for us, our tandem numbers are what you’d expect from our single bike results.
More importantly, for training purposes, the tandem power numbers are consistent over time. And for a power meter to be useful, it doesn’t need to be 100% accurate, it just needs to give consistent results.
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You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.