Power reading / Resistance on Zwift Hub


Since about a year now I have been using the zwift hub to do my indoor riding and training.

Is mostly ride trainings in Erg mode in de the Zwift online environment and I have been loving it.

Since the beginning though I have been having my doubts about the power accuracy of the zwift hub. The power readings always seemed to be quite of of my Garmin rs200 but sometimes it was quite higher and sometimes a lot lower. Also sometimes if I rode at like 250 watts it felt much more doable than on the road or even another ride on the zwift hub. I couldn’t quite understand what was going on until recently.

Tonight I had to do a training of 10 minute intervals at 235 watts at 65 rpm. My FTP is around 250 so that should be doable. The first interval was very hard, the last couple of minutes I was holding on to myself te keep on going and my heartrate rose to levels I only see when I am at Vo2Max. (160+)

The second interval I shifted down from 17t cog to 11t cog back and did the same interval at 235 watts and about 65 rpm. It felt much more doable , even easy, my heartrate didn’t go above 125.

The third interval I was back in 17t cog but speed up my cadence to about 100 rpm, still at 235 watts. The interval was harder than the second one but considerably easier than the first one. My heartrate was between 130 and 140ish.

Concluding it seems that;

  • In higher cadences the resistance is lower than on lower cadences and vice versa although the power reading is the same
  • in smaller gears (11t vs 17t) the resistance is considerably lower than in the bigger gears (17t vs 11t) and vice versa although having the same power reading

The first one could be explained by the fact that I am poorly trained on lower cadences, but the difference is too big for that.

The second one I simply cannot understand. How can the same 235 watts feel so much easier in the 11t cog than on the 17t cog also generating entirely different heartrate data.

To me this can’t be right so please help me to solve this.

Kind regards