A few thoughts...

- The Direto has a 'power floor' or 'wattage floor' or whatever they call it which means ERG works better in the small front chainring for most people. 25-29km/h flywheel speed is the zone it works best iirc.. (I'm at 31,000 feet and limited bandwidth to check). So maybe try the FTP step test in the little ring and mid cassette up the back.

- Pair both the Controllable Trainer and Power Source as the Direto. Power matching other power sources "works".. but introduces lag/delay into the process in a number of cases. That plus the stepper motor Elite trainers use can be a wavy path to power smoothness as you've outlined.

- iirc the Elite MyETraining App (which is also their config tool/app) has a 'power smoothness' config option under Advanced Config. You can bump that up so the instantaneous power reported is smoother. Zwift workout mode shows 1s power, not 3s, even when that's chosen for free rides / events. This option in the MyETraining app is purely cosmetic.

- Maybe try Bluetooth over ANT+ to see if there is any difference in the experience. Zwift direct Bluetooth to Windows machines is now a few months into beta, I'd hope it's capable of trainer control by now (I haven't checked).

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Shane Miller - GPLama
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Pretty much every FTP test that i'm aware of uses Slope mode. Otherwise, it will more or less just set your FTP based on whatever you set the power to be in Erg mode. More than likely Zwift ignores the Erg mode setting when doing an FTP test and just puts you in slope mode.

Step/Ramp tests are best done with ERG due to their design. These are very different to maximal x minute power tests (typically 2x 8min or 1x 20min) which will use slope/level.

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Shane Miller - GPLama
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Thanks Shane, I’ll give those ideas a shot.

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Taco cat spelled backwards is....taco cat.

Zwift also does allowable ranges for power (it's in the +/- 20 or 25w range) so there will be variability (as your power outlay is not constant either, so the trainer is constantly micro-adjusting resistance to what you are laying down). That said, for the ramp test, definitely let the trainer set the power range for you and keep it in an easier gear... I got too aggressive gearing wise when I tried the ramp test, and ended up in the ERG death spiral, and bombed out way earlier than I should have...

And for sure I would never try erg mode for the first time on something like the ramp test... I would try a few workouts first to get a feel for it, since it's a different way of modulating your power than riding outside...