There are lot’s of discussions around the accuracy of Tacx Fortius & i-magic wattage outputs and their calibration. Last week i did my own comparison, by doing an LDR VCF Ride and recording wattage in addition to my Fortius’s own recording with a Powertap powermeter. The ride was paced (until 5km to go) and all calibration & scale settings have been untouched, which is an obligation when riding in virtual leagues. For the sake of easier comparison the Powertap file has been flattened by using 5 sec power averages (which is the format of the Fortius HRM file too).
Fortius shows always more watts than Powertap does. Overall averages have been: Fortius 274W, Powertap 210W, 65W average difference.
Difference in watts Powertap to Fortius:
Difference is low in the beginning (warm-up phase of Fortius break) and increases constantly until first climb is over. Biggest differences of over 100W in steep downhill sections, where i – basically – didn’t pedal at all.
Zoom into a hilly section with lot’s of power spikes:
Difference small on power spikes, because Fortius is too slow to follow those spikes so that Powertap catches up to generally too high Fortius power outputs. Difference is high, when going downhill.
Comparison of flat sections:
Little difference in 1st flat part during warm-up phase of the Fortius break. Difference seems to depend on speed, because it’s less, on the 4th, slower section. Interesting is comparison of Powertap output on 1st & 3rd section, which both have been at same avg. speed: Looks like not just displayed wattage of Fortius is going up, but also resistance of the Fortius break is becoming less over time, thus requiring less power to ride at the same speed, when the break is warmed-up.