When you turn off ERG, another mode takes hold. Depending on the trainer and connection method in use, that is either Resistance or Standard mode (some trainers offer both). These modes require shifting for normal use in order to access easier or harder resistance & pedaling effort.
Per the above, if you turn off ERG and end up in Resistance, there is also a setting for that in a 0-100% range. Lower values are “easy” with higher values being “hard”. The same is true in Standard but it is a 0-9 scale.
So, the key here is which mode took hold once you turned off ERG, and the related setting for that mode. You can change the settings for either mode, and then you need to add in shifting (even virtual via the smart bike shifter) in order to use these other 2 modes.
The TR app tends to hold the prior mode level setting, so just opening the app, then Devices and looking at the smart bike there may answer the question of what value it had when you swapped modes.
I had the same thing happen the first time I did it mid ride.
My trainer switched to resistance mode and the setting was at 50% and made it impossible to turn the pedals.
Some trial an error, after reading threads etc, I’ve finally settled in around 26%(yes that specific) for resistance mode. That Specific % allows me the most efficient shifting without me needing to vary my cadence by 20rpm to hit the wattage targets.
I did my resistance mode ‘training’ during endurance rides until I got the hang of it so as not to stuff up my ‘work’.
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Definitely check the resistance settings. It’s been some time since the first time I switched from ERG to resistance mode, but if memory serves when I changed it it defaulted to 100%, making it VERY difficult to turn the pedals, just as you noted…
Once you change this setting, it will default to that setting moving forward. I believe I’ve settled at 27-28%!