Humm, I have the opposite problem with my stages, it reads too high. It averages about 5% high and I say that because of the test results I got from full frontal. Sufferfest uses the full 20 min result, Garmin doesn’t know better so it does the 20 min * 0.95 to knock off 5% and these two numbers were the same for me.
What I’ve done is train to the kickr and know that the stages is high and then use that to get into the heads of my riding buddies by telling them the higher stages number when we ride together outside. Amazing how much one little number messes with someone’s head and in turn their performance on a bicycle!!
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It’s called virtual power in SYSTM. It’s the power value from the trainer’s power curve. I ride a Kurt Kinetic RM. The curve is well known. If you know your real speed, you can determine the amount of power, at the wheel, needed to be there. I have a power meter as well on one of the two bicycles used on it. I’ve mounted the bicycle without o e on the trainer and used virtual power for it.
So with them being the same during the warm up, but drifting in the cool down, it could be that fatigue impacts your leg balance. This is fairly common that one leg will work harder at higher intensities and then imbalances are exaggerated for the remainder of the ride. Strength training and single leg work such as split squats can be helpful to resolve this over time. There is also the chance that the crank power meter has a different slope, which could explain the larger difference at higher power
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I’ve had similar issues and journey. Old Kickr gave me great winter form, as they read higher when cold so power numbers always dropped for the summer. I also had a single sided 4iiii that was 40 watts higher, another that was 20 watts higher, and a dual sided that was 30 watts lower (compared to Kickr). Finally have a Kickr V5, Powrlink dual sided pedals, and Quarq which all read within 1% of each other regardless of temperature or fatigue levels (did a lot of work to improve leg imbalances when fatigued). Also had s Gen 1 stages that was weirdly accurate for 6 years given they were know to be random number generators
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