Did a workout yesterday and noticed a big difference in difficulty when going either uphill or downhill in ERG. My route included the Alpe and approaching on the flat I was holding 235 watts, all fine and good, then start the climb and the same 235 watts feel a lot easier, in fact the steeper the slope the easier it felt. On coming back down the same 235 watts feels much, much harder. ERG should make hills feel flat, right, but there is definitely a big difference in exertion needed for going either up or down, my heart rate figures confirm this. I am using a 2017 kickr for reference.
Hey Mike, can you send us support conversation and send us your log files for that ride?
Support Conversation: Contact Us
How to find log files: https://support.zwift.com/en_us/how-do-i-find-my-log-files-(cycling)-SyTW0JZSX
We also have some troubleshooting ERG tips in this article: https://support.zwift.com/en_us/-how-to-use-erg-mode-in-workouts-rklSxCkbH7
Did anything ever come of this? I recently commented the same to a fellow rider who said I was imagining it. So I googled it and now I see that someone else had the same experience.
I’ve noticed it too but for me I chalk it up to psychological. In ERG the trainer is holding your watts steady but past experience riding and the visual cues you get don’t match that fact. So when you see 7% grade in the screen your mind expects it to be harder because in real life it is, you have to lay down more watts or slow way down. Same is true downhill, past experience has told your brain it should be easier so holding 200W downhill feels harder psychologically.
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Daniel, I assumed the same. So I took the visual out of the equation. I closed my eyes and had someone in the room with me as I rode. I nailed the changes in grade 95% of the time. It’s a real thing. 
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Same issue here. Using a Gen 1 Kickr the resistance decreases with uphill gradient and increases with downhill gradient in Erg mode. This is the same if I’m using TrainerRoad Erg workout or Zwift. Trainer connected by ANT+ in Zwift and Bluetooth for TR. Very frustrating makes erg workouts in Zwift kinda pointless.
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Ha, I’m not imagining it then. On Tacx NEO2. Every time I do Threshold workouts on Tick-Tock I dread the down slope into the tunnel. It feels so much harder than the rest of the ride. Even though ERG says same power.
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Nope, definitely not imagining it. Drives me nuts as really impacts the Erg Workouts. On the plus side a good way to set a PB up the AdZ 
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Using a Gen 1 Kickr the resistance decreases with uphill gradient and increases with downhill gradient in Erg mode. This is the same if I’m controlling using TrainerRoad Erg workout or Zwift Erg mode. Trainer connected by ANT+ in Zwift and Bluetooth for TR. Power reading on screen remains constant, have tested this blind in case it was some kind of mind trickery, but it is a genuine change in resistance. Very frustrating makes erg workouts in Zwift kinda pointless.
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Hey please check carefully the video from the post before: the one talking about the “spiral of death” Indoor Cycling Smart Trainers: ERG Mode Explained - YouTube by Shane Miller (Explanation in the middle).
This is the explanation to everything, I had exactly the same issue.
So you are suggesting people are finding it easier uphill because they are increasing cadence and harder downhill because they are decreasing cadence?
That would be true but it doesn’t account for the people that say they notice it with their eyes closed.
Those that haven’t tried it with their eyes closed watch your cadence and make it constant, see if that changes the feel.
Also, I wonder if using either the highest or lowest gear will make a difference to this phenomenon (that I haven’t felt because workouts aren’t fun).
I wondered if it was a sub-concious easing off after cresting a hill resulting in the trainer resistance kicking up, but I’m not convinced it is. Seems to happen whether paying attention to the screen or not. I’m intrigued to try it with some power pedals as a secondary power meter source to see if there’s any measurable difference
I just noticed this reply to the thread. I watched Shane’s video and his explanation of the spiral of death. That’s not at all what this is.
The way I see it, you’re actually riding on rolling terrain while in Erg mode. So to keep the wattage requirement consistent, Zwift’s logic has to “zero out” the terrain… to an extent! They still have to use the terrain to compute your speed at the wattage at which you are riding. So my guess is there is a flaw in their logic where they’re overcompensating for the terrain changes in both directions, going negative for uphills and going positive for downhills instead of zeroing out. But the software thinks that it’s doing the right thing, so it displays the consistent power number on the screen.
Someone suggested using a separate power meter on the crank or the hub. I haven’t tried that, but I’ll bet anything it would highlight the change immediately.
Agree @Maurice_Alvarez - I have seen a few posts now of the exact same symptoms and have my own experiences. Yet another problem that I (and no doubts others) have formally logged with support and hear nothing. I get it very rarely luckily.
Hi all, I know this is an old post, but I have exactly the same issue, and have experienced it on both a Tacx flow and now my new Kikr Core, so it’s definitely a Zwift issue. Wattage doesn’t change but the experience is much harder downhill - HR goes up and cadence drops. The opposite is true for uphill - i’d attach an example image showing my cadence against gradient but its not letting me attach to this post
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Same here. I have experienced this since day 1, and today during a workout it went bananas to the extent that I could barely pedal. First time using the Apple TV Zwift app, but I don’t know if this had anything to do with it. I voted for this issue hoping that it will get addressed by the dev team at some point. Anyone know of any other thread describing the same issue that I should go in and vote for as well?
I’ve got the same issue. Apple TV, Kickr Core, Bluetooth devices connected through Zwift Companion on iPhone. It’s incredibly annoying, and wasn’t previously the case.
Same issue for me on Kickr v4. I also stumbled upon someone mentioning it on chat during some watopia free riding. There is something to it…
I reported the issue to Zwift support but after a few weeks just got a “canned” response explaining differences between sim and erg mode (sic!). Reported back with basically the same as in the original report using other words. Hope they take it more seriously then before.
I’d encourage anyone who stumbles upon this thread and is experiencing the same issue to report do Zwift Support. If they get a few similar reports they might take action.
Currently ERG mode is unusable on hilly routes.
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Anyone had any update on this? I took it up with Zwift Support and sent the link to this thread to point out others having the same issue
No responses to my previous emails to support and unfortunately I dont expect them either. Did have it again very recently when playing around on a recovery ride doing the Italian Sprint Loop (Rebel route) - interestingly I had the experience going one direction but not the other. Its only a short loop (2km) so doing that for an hour you get to know the feel very quickly.
Wish I had a spare external PM that I could leave on my bike permanently.