Power meter vs trainer

Hi fellow zwifters, recently discovered on a rzwift meetup with some friends that you can gain 3-4% increase in power output from connecting a power meter to control zwift rather than relying on your trainer. I understand this difference is due mainly to drivetrain losses between reading power at the crank (meter) opposed to the cassette (trainer). question i have is do zwift have a way of levelling this difference up in a race as this seems to be an unfair advantage? In real life you get drivetrain losses. On a direct drive trainer the same… unless you spend £400-700 on a power meter and then you effectively get a constant power boost! Just interested to hear the views of the forum.

And now you know why the really serious races need validation from two power source, even go as far as insisting on using the same model of trainer in some cases.

But I race for fun. I know that cheating is rife. Plenty lying about their height and weight. Plenty riding kit they unlocked without doing the required challenges.

I accept that that will be constantly there at a background level and difficult for Zwift to do anything about it. Price of being able to rock up to a race at minutes notice at any time of day. More interested in having fun and getting fit.

There is enough variability between different trainers and they way they calculate power. Smart trainers tend not to react very well on quick power changes.

See this comparison.


It is likely more the trainers accuracy or single sided power meter being on the more dominant leg than anything to do with drive train efficiency.

How you decide which is the more accurate requires another source to test against, if 2 are similar go with one of those. If they are all different you’re back to square one!

I have Favero Asioma’s & a Wahoo KIckr18.
During regular rides, the difference is 1-3 watts (average), the Assioma’s being higher.
But in ERG mode, the difference is 7-10 watts, again Assioma’s being higher.
I’m not sure why the difference is greater in ERG mode.

It’s a family sized can of worms this one. It might also be worth considering the number or racers on poorly calibrated wheel on trainers that are probably reading 10-20% higher than they should be.

I have at least three people in my IRL club that smash me on zwift. However get them on the road and they are nowhere near my level. Common factor? Wheel on trainers.

FWIW i run a Tacx Flux and a crank power meter.


Computrainer user here. I’m happy to use this trainer as its been repeatedly validated in several dozens of studies in research literature. I don’t know about other trainers but a calibrated Computrainer’s accuracy is pretty spot on.

Just among people who use direct drives, a change in trainer suddenly nets them +/-5% in power.

Regardless of what you use, I find f you don’t calibrate and check your equipment, you’ll tend to see anomalies that cannot be explained as a genuine performance marker.

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Update on this:
I had been using “ERG power smoothing”, and I believe that’s what led to the 7w-10w difference instead of the 1w-3w difference. The 1w-3w is expected due to drivetrain loss.
I turned ERG Power Smoothing off, and had 3w-4w difference, pedals being higher.