Why does Zwift slow you down?

So I smash out 400 watts and rip a KOM jersey. I rest on the downhill and start spinning again and I notice my output has gone down by 1 w/kg. ‘OK’ says I.
I cruise to the sprint on 360 bridge, looking to do a Sagan, and I put the hammer down on what feels like 990 watts. I look up as blood (sweat) drips from my nose and Zwift has me at 790 watts and at a virtual stand-still whilst I’m still creaming the cassette! I go over the line at walking speed when I’m reading 500 watts for a 17 second slap in the face for my efforts - WHATS GOING ON BOYS & GIRLS???

Are you saying Zwift isn’t giving you your full wattage, all of your watts?

  • Over 800W (?) the graph of power, if displayed, will flatten or look like it’s being clipped. That is only a display thing. If you look at the actual wattage in the top left or in the Companion app, the wattage can continue to climb.
  • The displayed wattage can be set to either 3s avg or instantaneous.
  • You said, “…feels like 990 watts.” Are you using a power meter or virtual power? If the latter, power can vary a lot depending on a number of things (trainer warmup, tire pressure, trainer power curve, etc…)
  • If you’re “…creaming the cassette…” and at a virtual stand still, then I would suggest you make sure your devices are paired properly and not experiencing signal dropout.