When do folks use ERG mode during TR workouts on Zwift? Is there a rule of thumb when to turn it off? Thanks
Personal preference.
I leave it on all the time but many people turn it off for VO2max and above.
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During zwift? never since I don’t use it 
For me though in TR it is on for pretty much anything but vo2max but I even did one in ERG yesterday because I was having issues with resistance mode. I not only had a problem with my shifter cable, but recently switched to tablet and adjusting resistance setting is far more annoying that in the desktop app. So I’ll probably actually do those on my laptop going forward, once I fix my shifting.
I also spent a few years on a trainer that was not compatible with TR in anything but ERG mode so I did 100% of my workouts during that time in ERG and had no issues other than maybe doing vo2 max stuff “wrong”.
I personally prefer ERG because I like that I can remove the mental load of trying to hold a target or even just not even thinking about it at all. Everyone has a differing opinion on it including saying that you MUST think about it or you can’t learn how to make/hold power. So just do what works for you and your training. There is no right or wrong.
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I think it depends a bit too on the quality of trainer. In the ideal world I’d have ERG switched on for all but VO2max (when I want to push beyond the power zone to my true oxygen based max) but I also tend to have it switched off for anything near threshold. I don’t know if my single sided PM meter triggers it, but I’ve seen the trainer drop power or fluctuate wildly, or hang at the end of an interval (stay 106% after 15mins instead of dropping to recovery) and drag me into the spiral of death and quite often I’d get a calf strain or something.
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I never use ERG. I’ve tried it but I like slope mode. The shorter the interval the more ERG sucks IMO. It’s fine for longer intervals.
I also think it’s not appropriate for certain types of intervals like hard start vo2max where the goal is breathing like a fish out of water. But if you want to do 105% intervals and call them vo2max then ERG is fine for that.
Zwift’s erg mode is pretty terrible. I had much better experiences with using TrainerRoad or using my Wahoo Bolt to control trainer power for workouts. It’s a bit complex to set up, but I would download my workout to the Bolt, then just have Zwift read power (not control it), so I could have the eye candy.
Zwift riding can be good for stochastic intervals (spikey power) or you can do good sustained work on any of the long climbs without erg mode and it’s closer to riding outside.
In general, as long as you’re meeting the intent of the workout, you don’t ever need to use erg… just like outside riding you just need to pick a good route for the training you intend to do.
A while ago I posted my general rules for trainer modes:
Endurance/tempo/Sweet spot - erg is OK.
Threshold - resistance/slope/standard preferred, but erg is OK.
Supra-threshold/VO2s (longer intervals of a few minutes) - Resistance or Standard/Slope.
Short/shorts - Standard/Slope.
Any time you’re using erg, you need to be dialed in to RPE and not just zone out as you can easily ride too hard for how you feel that day just by blindly following erg. The lower the power, the more “viable” erg is in my opinion.
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Usually only for “ERG and chill” sessions i.e. endurance rides where I want to turn the brain off and zone out, and those are usually done directly in TR since, as others have said, Zwift’s ERG mode is not great. Other endurance sessions are done in free ride mode. For higher intensity workouts I’m mostly using Zwift’s incline mode.
My ex coach said similar when I got a smart trainer.
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I think the only time I have used erg recently is during a recovery ride to keep the power low…
IMO it’s best avoided, however if you want to use it during tempo/sst/ftp sessions it’s not really an issue (assuming your ftp is actually accurate and not over estimated) but should be turned off for VO2 max and other all out style efforts.
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I almost exclusively use ERG mode. If the workout is correct for you, then following along in ERG won’t create any issues.
It’s really rare for me to struggle with a workout, and in those cases, it’s usually because I hand-picked it or selected an alternate that was too tough.
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I think this is a bit strong - I respect the opinion of people who preffer to turn it off for short high intensity efforts - but it is not “incorrect” or “wrong” to leave it on…. in my opinion…. 
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I use ERG all the time for TR rides. I prefer the accuracy of it and the fact that you cannot cheat the intervals one tiny bit if you’re on ERG. You either do the power or you don’t. People mention that using ERG somehow makes you unable to know how to ride or hold power in the real world, but I’ve never come out of winter training and felt in any way confused by riding outside and controlling my power.
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With my trainer, Jetblack Volt v1, I always use ERG mode unless the intervals are over 500W, when I switch to Level mode to stop the sudden jump in power causing instant erg spiral of doom.
Not if you have a Suito, it might of improved firmware wise, but when I first used mine it couldn’t cope and would hang at 130% 
The argument against Erg for VO2 or high power intervals is that those intervals should essentially be maximum efforts and not strictly based on a % of FTP. Anaerobic capabilities vary widely from athlete to athlete and basing those intervals off of functional threshold power is a mistake.
I agree with the others saying the issues with riding in erg otherwise are overblown. I never had an issue coming out of a SST progression block done entirely in erg and applying power on the road. I understand Kolie’s take on erg, largely agree with it, and was coached for one season by an Empirical coach. I think as long as you are dialed in to RPE and willing to make adjustments to power, using erg for sub-threshold and threshold work is fine…. and my Empirical coach at the time agreed and uses it in his training. 
Erg is fine, but IMO its use cases should be intentional, not just “default”.
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I fell in love with ERG mode when first started using it on Computrainers ~15 years ago. I use it for just about all structured workouts. It’s a merciless taskmaster. There is no easing off. For some workouts with short max effort intervals or sprints, level mode can be preferable, and some FTP tests require level mode. Though I’ve ridden Zwift and Rouvy in the past, I have not used TR in Zwift preferring cycling (mtb) povs for workouts rather than cartoon worlds. .
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I’m in the always use ERG mode group.
I’ve had long periods where I used only ERG and long periods where I never used it, but these days, whether I’m using ERG mostly depends on how mentally invested I am in the workout in the moment. When I’m excited to get on the bike and feeling energetic, I typically don’t use it, especially for high intensity work. I like the engagement and feeling of chasing a power target. That said, it takes mental energy and focus to maintain the right power through shifting, and sometimes I’m tired and just want to get the work done and not think about it so much. In those cases I turn ERG on and let it do its thing.
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Erg mode all the time. Occasional Resistance mode for in between times when I want to stand and stretch the legs, and the watts are too low.
Erg keeps the dose right on target, and I feel I can dig deeper on hard intervals. When I’m having my doubts about finishing, I can focus on breathing, cadence, and just keep pedaling. It’s like I’m trying to hold someone’s wheel, or trying to break someone lose from mine.
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