Power slightly too low in ERG mode

When doing workouts in ERG mode, I’ve noticed some puzzling behaviour and I can’t quite determine if it is a feature or a bug…

My setup is a Tacx Vortex smart trainer connected to my Macbook Pro using Bluetooth (and the Companion app on Android). I calibrate the trainer on a regular basis with the Tacx app and keep my tire well inflated.

The issue is that often the power recorded is ever so slightly (a few Watts) lower than the target. For example, yesterday I did a workout with some 10-minute intervals at 135W, 85 rpm. I settled in to a steady 85-86 rpm cadence, and the power bounced between 131-133 W but never actually reached the target 135 W.

What’s even stranger is that my trainer is certainly capable of quickly zeroing in on the correct power, because it hits the target numbers dead-on during warm-up and cool-down ramps. So why is it failing to hit the target during the workout? I’ve also noticed that the problem is the worst in the Tempo (green) zone. I can’t tell whether it’s me, a “feature” of the workout mode, a wonky calibration curve on the trainer, or something else…

This is driving me absolutely crazy since I spend entire workouts with the target power just barely out of reach!

Did you ever figure this out? I’m experiencing the same thing. Watts are slightly lower than what shows as target. If I bump up the bias 1 or 2 % it seems to help. It seems like the target, even though it’s 1 set #, has a range that is slightly below and slightly above. Bumping the bias seems to move within that range even though the target watts displayed may not change at all.

If you use Strava you could look at your workouts analysis and confirm what wattage you were holding as compared to the target wattage. I am thinking along the lines that you are holding the correct wattage just it is not being displayed accurately.

I do use Strava & TrainingPeaks, both show watts 2-3 under target. I suspect it’s the range thing - my workouts come from TrainingPeaks and are a percent of FTP range. So it might show watts between 224 & 251 in TrainingPeaks, but when loaded to Zwift it has to pick a specific watt and is generally about halfway between the range, or say 240 in this example. Target shows as 240 but in ERG mode it holds 236-237 no matter what. I can bump up the bias 1-2% and the 240 is still the target but then it has me holding 240. A few watts is not a big deal, it just drives me crazy that it shows a target but then doesn’t hold that.

Strava also shows the power is below the target. Unlike @Karen_Richardson_PTz below I am using Zwift workouts, not TrainingPeaks, so the FTP should be a precise value. I am wondering if it has something to do with improper application of a floor / ceiling operator (not rounding to the nearest multiple of 5 W), though… Either that or the calibration curve is just wonky.

I was looking for something else and bumped into this thread. If you’re still interested it’s because Zwift rounds the target to the nearest 5W in the workout list on the left, but doesn’t round the actual power during the workout. Why it doesn’t display the unrounded watts in the list I don’t know, but it doesn’t.

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Thank you! Makes sense but it was driving me crazy! Appreciate the response.

Thanks, @ishay, so it’s more or less as I suspected. I can understand rounding the target power in the list for simplicity, but it’s just lazy not to do the same for the actual power. (Although not surprising, given how terrifying the Zwift codebase seems to be…) Good to know!