Issue with Zwift Hub: Watt Variations During ERG Mode Workout

Hello everyone,

I hope someone can help me with an issue I’m experiencing with my Zwift Hub. I recently purchased the device and noticed a peculiar behavior when trying to follow a workout in ERG mode with a specific watt target.

The Problem: When I set a target, for example, 350W, my Zwift Hub continuously fluctuates between 310W and 380W instead of staying steady at 350W as it should.

Additional Information: My Zwift Hub has been updated to the latest available firmware. Furthermore, it’s important to note that my Zwift Hub is connected to my Windows 10 computer using the Bluetooth protocol.

I was wondering if anyone else has had a similar issue or if there’s a solution I could try. I’ve searched online but haven’t found a clear answer.

Thanks in advance for any help or suggestions you can provide!

Hello @Tiziano_Speranza, welcome to the forums.

While I’m not sure if the 30 watt fluctuation is “normal” for a Hub or other trainer, the reality is that no trainer can hold an exact watt target. Wahoo trainers have what they call ERG mode smoothing, so the power graph on the screen looks like a nice even line with no fluctuations, but that is just a visual display so you feel good and doesn’t reflect what power is actually being reported.

I would assume also that the higher the target the more fluctuation there is as you have to exert more effort with each pedal stroke.

One thing you can try is to make sure you are in a “middle” gear, with a nice straight chain line. Some people think using the small cog on the front chain ring and a middle cog in the rear cassette is the best option (assuming you have a double chain ring in the front). Try to pedal as smooth as possible and see if that helps reduce the fluctuation in power, but don’t expect it to be perfect.

Thank you for your feedback.
Do you think that with an ant+ usb is better?
Because previously I used a tacx flow on-wheel and I didnt have that fluctuation.
Thank you

no, it shouldn’t make any difference if you are on ant+ or bluetooth. Bluetooth is known to react faster to changes in power however (sends info from trainer to Zwift faster that is).

make sure you are in a small gear (little ring up front then somewhere in the middle at the back to give a straight chain line)

does it stabalise eventually? and just to check you don’t change gears or anything and try and maintain a consistent pedal stroke?

When I opted for the Zwift Hub device, I didn’t expect the experience in ERG mode to be so inconsistent. Even when I change gears, I notice that the values fluctuate significantly, making the pedaling more challenging than necessary. Unlike other smart trainers like WHAOO or TACX, I can’t maintain a fixed value like 300 watts, and this shortcoming makes the training experience more stressful than I’d like.

It would have certainly been helpful to know about this limitation before making the purchase, as I could have considered alternatives more suited to my specific needs. I hope Zwift works on these issues and improves the accuracy of the ERG mode in the future. In the meantime, I’m trying to adapt to this situation and find ways to still optimize my training routine.

EDIT: I add a picture of a session where the fluctuating are consistent.

First off,. don’t change gears in ERG mode, because changing gears will mean it takes longer for your trainer to settle at the resistance level required to provide the target power output at your given cadence. Also try to keep your cadence constant.

Are you comparing your power trace to ones on other trainers which show a flat line? If so, then those other trainers are lying to you via their smoothing function which reports the target power and not what you’re actually producing. You can toggle that function off/on on other trainers, but as far as I know the Zwift Hub doesn’t have such a function.

I’m not changing gear while doing the interval! :slight_smile:
By the way, I attach a picture of TACX wheel-on trainer that was definitely much cheaper than the zwift hub and the power is more stable!

This is a perfectly normal power trace, I think.

But it’s simple to check. Just do the same workout with ERG off and try to hold smooth power. See if you can get it as smooth or smoother.

Think about it - if your ERG mode trace is smoother than you can do naturally … it’s a big old lie.


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That isn’t quite true, at least not in all cases, they often show a smoothed average power, it is still what you’re doing but averaged over a few seconds so it irons out the bumps. but, yes, it is just a visual thing and not real time what you are actually putting out.