I have the TACX Neo Bike rather than the TACX Neo 2T, but the gear I am in affects the ERG Mode. I was told to stay in the center gear.

Not just erg mode, but pretty much anytime. And it can be far more than 12-15W, e.g. during 800W sprint efforts. Particularly at lower cadences. I did my own study after having a frustrating few months underperforming in Zwift races after switching to the Neo 2T. Since then I always use my bike-mounted PM as the power source when "Zwifting."

Yeah I generally don't race on Zwift but that creates a whole other problem. For a $1300 smart trainer that is routinely ranked as one of the best, I expect better performance and accuracy. I'll eventually go back to Wahoo.

Same experience for me. The 2T consistently reads ~10-15w lower than my old Hammer H2, Assioma pedals, and the very old Powertap P1 pedals. AND - you can't even calibrate the thing on Zwift!

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tinman

A measurement downstream at the rear wheel hub should read lower than a measurement upstream at the pedals or crank because drivetrains are not 100% efficient. How much it varies depends on your drivetrain and it's cleanliness. If you take the NEO 2T out of ERG mode, are the differences still there? I notice about 6-10 watts difference between the averages for my SRM and my NEO 2T, but that, to me, is completely normal and expected considering where the measurements are taken.

For me it wasn't about drivetrain cleanliness. I have immacuately waxed drivetrains, and was seeing consistent errors around 10% at low cadence. Clean drivetrain efficiency loss should be on the order of 2-3%. At most.

DC Rainmaker sez that earlier firmware has power issues. But an update seems to have made it all good. Maybe update the firmware? DC sez, after the update, 'What matters is how well it works today, and I have zero issues with it from a power/cadence accuracy standpoint or a reliability standpoint. I do still maintain its a bit edgy (bossy perhaps?) in ERG mode, and I’d love to see Tacx introduce some sort of option in their app to make ERG mode a bit less abrasive (more like the previous NEO units, or perhaps the Saris H3).'

TACX accuracy poops the bed bigly above 20 MPH. But it’s pretty good at slower speeds. As long as you are on the small chainring, TACX power accuracy is serviceable. Many threads on the forum about this.

Why not just use your pedals to drive the workout? Doesn't most software allow that power matching now?

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Interestingly, I have a friend who is a pro triathlete. In contrast to your question, he used to refer to the "Free 20" on the Wahoo Kickr i.e. it read 20W too high so it looks like you're pushing more watts than you really are. And I found that to be true with my Wahoo. When I bought a Tack Neo as my 2nd trainer, it was more inline with my Garmin Vector pedals. Getting multiple power meters synced is always fun.

What does "speed" mean on a smart trainer? You mean like the simulated speed physics done by Zwift? That's really just a surrogate for Wattage. W.r.t. the small ring, yes, the accuracy gets better at higher cadences and apparently lower torque. But having to unnaturally change your preferred torque/cadence in order to get good accuracy is not a great thing.

That's nice for DCR, not my experience. Updated firmware, engaged in a long conversations with the pre-Garmin staff at Tacx who were willling to look at my data, admitted there was likely an issue. Post-Garmin, they stopped responding.

DCR is a world class expert, but he's also just one guy with one piece of equipment. And a guy who'll get immediate response to any issue he has, unlike the rest of us.

I also have the NEO 2T, and mine gives 10-12 less watts than my Power2Max. Before this one, I had the NEO, and it gave 20 less watts than the Power2Max. Nowadays I do not use the ERG mode, as I belive is less realistic than the SLOPE mode. I basically ride "downhill" with my big chainring and smallest cog, and always follow my PM not the TACX, as I compete with my bike and not with the trainer.

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STRAVA INSTAGRAM

My Neo 2T (purchased last year) has been pretty well in line with my other power meters, much like DCR. Currently that's an SRM PM9, and Garmin Vector 3 pedals.

But even if it was different by a fixed amount, as long as it's consistent, then I don't care (for erg). Test with it, set your indoor ftp, go train.

If you're doing Zwift racing and you're 10 watts below every other power meter, then yeah I can see that being annoying - but that hasn't been the case for me.

Makes me wonder if there have been unannounced changes to the hardware side within the 2T itself.

The speed that the Tacx reports, based on its flywheel speed.

Ah....yeah, since the smart trainer doesn't know bodyweight (afaik), must be some kind of strange approximation of bike speed? I've never bothered looking at speed (except in Zwift, which uses its own physics that take into account kg).

It's just the speed the bike would be going based on the rpm of the cassette. Ive seen, and heard from others, that smart trainers don't tend to do well when this speed is too high, especially in erg mode.

I think it is simply cassette RPM times a wheel diameter. Assume 700C default, but it doesn’t matter that much.

When I first noticed the power inaccuracy, I ran a bunch of intervals capturing and comparing Tacx power on TrainerRoad to Assioma power and Tacx speed on Garmin.

For the Neo 2T you can set your body / bike weight in their app. Not sure if that affects Zwift.

Edit: the setting is in the app under "Device Manager", *not* "My Profile" which is your regular Garmin setting. From the description it seems to affect external apps as well. Worth trying for Zwift.

https://imgur.com/a/HSdONZo