erg mode with external power meter implementation

i  did not dive into the F-EC specification yet, I am using an external power meter as the power source and control my smart trainer. On my experiment if i don’t calibrate my trainer, the ERG mode still runs ok for my setting. So can I assume that the ERG mode implemented by zwift is just sending the resistance level control to the trainer and will not send the target power to the trainer? so that the ERG mode will not effected by the wrong power produced by the trainer. Is that correct?If i am correct so maybe i can totally ignore the trainer’s spin down calibrate in the future. thanks

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If you use a power meter as your power source then have your smart trainer paired as a controllable trainer and do a workout Zwift uses power matching i.e. it reads the power from your power meter and changes the resistance of the trainer to match the power target.

In practice what happens is say the power target is 250 watts. The trainer goes to what it thinks is 250 watts. The power meter might read 240 watts. The resistance is then increased slightly. The effect is the power will oscillate over and under the target power.

You don’t need to do a spin down calibration every ride but you should do a zero offset on power meter every ride (if it has that option). The closer the power accuracy of your trainer is compared to your power meter means power matching will work better. If there’s a big difference then doing the likes of V02 max intervals may be challenging as it does take the trainer some time to settle down to the correct power.

Hope this helps. For me I use power matching almost 100% of the time. My Hammer trainer reads 5 - 10 watts higher than my power meter depending on the target power.


Thanks Nigel, I did a test today.

On the first 10 min I deliberately “not calibrate” the trainer, you can see the trainer’s power(blue) is way up than the power meter(purple). The target power in zwift is 100w, and the ERG mode runs well according the power reading from the power meter.

So i presume the real underneath what the ERG mode on zwift is like the scheme below:

zwift -> trainer: set target to 100w!

trainer raise the resistance, I keep pedaling, the trainer’s power is exceeding 100w, but zwift reading is still below, so zwift->trainer: more resistant!

now zwift reading reached 100w, and the trainer’s power is approximately already 150w… but now zwift->trainer: keep that, now is ok. so trainer will ignore it’s own power and keep the resistant.

So maybe the whole internal procedure is just like that,  but  I think you are right, if the power from the power meter and the trainer is very closer, and  I think will reduce the 2 way transmission, that will give a more stable and smooth ERG experiment.



What’s your trainer? If it’s a Wahoo Kickr, have you done an “Advanced Spindown”?

No… it is a cheap wheel-on smart trainer.  Of cause it also has the “spin down calibrate” feature and after that power is very accurate with my PM. but since it is a wheel-on trainer, so the spin down calibrate is very troublesome. So that the reason why i raise this post, if i can totally ignore to calibrate the trainer when i using external pm to control the trainer.

Finally i found an article about this, so i think zwift is implemented the same way.