Erg Mode vs Slope/Resistance Mode
Building on the information found in our Erg Mode Explained article, assessing FTP on a smart trainer has to be done in the appropriate mode: Slope or resistance mode (depending on your particular trainer).
Our preferred testing format, The Ramp Test, should be performed in ERG mode if you're using a smart trainer.
Why Erg Mode is Bad for Assessment
If a smart trainer is left in Erg mode, any test workouts won't assess FTP improvement because the workout will scale the assessment interval(s) to the FTP listed in your profile.
To avoid this, the assessment needs to take place in Slope/Resistance mode in order to allow a rider to work at his/her own pace rather than having the trainer determine a pace based on the input FTP.
In the case of a first-time assessment for a new TrainerRoad athlete, using Erg mode could be even more problematic since the default value for a new rider's FTP is 200. This standard value will likely be too high or too low unless your fitness happily coincides with the default 200 watts.
When you assess your sustainable power using one of our two test formats, TrainerRoad's workout player will automatically switch to Slope/Resistance mode just prior to each test interval.
If you wish to use a higher or lower Slope/Resistance setting, you're free to adjust it yourself and it makes no difference if you use different Slope/Resistance settings from interval to interval or test to test.
By switching to Slope/Resistance mode, you're allowed to go hard without being forced to maintain the calculated Target Power that would be forced on your legs in Erg mode. You can pace according to your current capabilities such that you ride as strongly as possible for the assessment interval(s).
Ignore the Graph
During the test intervals, totally ignore the graph's recommendations.
Whether you're above, below or exactly in line with the graph makes no difference. It's important that these assessment efforts are not restricted or guided in any way.
We're simply trying to get a reasonably-close estimate of your Functional Threshold Power, something we can't do if your smart trainer is limiting your power.