I have done the same 27.3 mile race almost every weekend for the last 8 or so weeks. I always average between 140 and 155 Watts. Today I noticed that the indicated Watts was much higher during the race with about the same effort. At the end my average was 222 Watts! A 70 Watt increase. The only difference was that I installed a cadence sensor yesterday. I also ran the Zwift calibration for the first time a few days ago. Could the addition of a cadence reading affect my Watts that much? What about the calibration? I have a Kickr Snap smart-trainer purchased new in June 2020.
With a wheel-on trainer you should warm up the trainer at least 10min. and then do a spindown before every ride. And even then the power readings are only about 10% accurate.
Without calibration the power reading of a wheel-on trainer is a wild guess.
And no, the cadence sensor has nothing to do with it.
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Check your tire pressure. A low tire pressure will add watts.
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The tire pressure was set to 110 lbs just before the race began.
I did notice that I had been turning the knob on the trainer too far and the spin down times were too short, 6.8 seconds. Yesterday I made an adjustment and got the spin down time up to 12 sec. I used the same turn setting before the ride today.
Today I set the tire pressure to 110 lbs. I turned the knob 2 5/8ths turns past the wheel touch point. I was using 3.5 turns previously. I then did a spin down with an 11.8sec spin-down reading. I then did a Zwift Calibration and started a self directed ride. Watts were again higher than previously with 6.8 sec spin down reading, but believable given the effort. Bottom line: I think I was turning the wheel too far previously which caused my old wattage readings to be low.
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It can be hard to know which reading is correct.
You said you ride 27.4 miles and have averaged 160 w and 222 w.
Based on your wt, which ride gives you a 60 - 90 minute W/KG average that you believe is correct for your fitness level and agrees with your outdoor rides?
The wheel-on trainers are VERY sensitive to the exact position of the the tire. If you push the tire too far into the drive you’ll produce more friction, which means you have to pedal harder for the same watts. You’ll also wear the tire out very quickly. The difference in watts can be completely explained by this effect.
I also know from personal experience with the Snap that the new, higher wattage may not represent real world wattage (but who cares about that, in the end it’s all about improving your fitness…)