FTP Test and Virtual Power

 Wanted to give some feedback to people with regards to virtual power and how it may be off. 

I joined Zwift a month + ago.    At that time I was using a dumb trainer and letting Zwift calculate my power.    I then got a Tacx Neo unit so my power measurements should be ±1% now (if we can believe the Tacx literature).   

Tonight I did my second FTP test, this time on the Neo.   Needless to say I was a bit surprised (and to be honest, disappointed) to see that my FTP as calculated by the Neo unit was 50 watts lower then what I had recorded on the dumb trainer.

It must be very difficult to calculate virtual power so I’m not critical of it…  Just wanted to pass along the information to others that may be in a similar situation.


Another odd thing I noticed that when I finished with the FTP test Zwift gave me my number (209 watts).   However, when I pulled the file into Golden Cheetah and did a detailed analisys of that 20 minute effort block Golden Cheetah calculated it at 220 watts.       

It’s all just numbers so I’m not too concerned (other then the fact that I now have real evidence that I suck :slight_smile:  )      I’ll also keep the recommended Zwift number plugged in as future workouts and I assume effort levels will be based on this.   

Two things that could be affecting my numbers are:

  1.  I have been on an aggressive weight loss program for the last 2 months… (8kg dropped) so this may have pulled down some of my power.

  2.  I have been concentrating on base miles only for the last month with hardly any efforts to speak of.  

I think you were looking at average power over the 20 minutes in golden cheetah.

FTP is the max power you can output in 1 hour.

The test is only 20 minutes long. However most people can do around 95% of their 20 minute power for 1 hour.

95% of 220W is 220 x 0.95 = 209W

Ah…  Thanks Douglas…   

Yeah, that makes perfect sense and I guess that’s what I’m seeing.    Thanks for the education.