# ERG Mode resistance too low

I recently noticed that the power output in an ERG mode intervall is a few percent below what it should be when calculating the interval power regarding to your ftp.
E.g 90% intervals with FTP 200 will not result in 180W. It will be around 174/175W.
This occurred with a Kickr and a Tacx Bike so zi guess it is a software problem.
Anyone else seen this.?

90% of 200 is 180, 200*0.9 is the calculation

That’s what I’m saying! An 90% interval should make me push 180W (I know the calculation ), but analyzing the workout later I see, that I have pushed 174W, while ERG mode was on.
ERG mode results in less power generated than it should be.

gotcha, that’s not how your original post read

In your example, is Zwift showing 180 as you are riding, but some other program later shows 174 for the same period? Or does Zwift show 174, but you think it should be 180?

Yes, Michael, Zwift shows 180, but later on TrainingPeaks an Garmin Connect show the interval power as 174.
They both got the data via Zwift connection (upload).

Is it a timing offset issue? So instead of taking your, say, 5 minute interval it’s taking, for example, 4m40s of the interval and 20s of the recovery?

Interesting. I checked a few old rides and saw the same average power (or within 1 point) in Zwift and Garmin connect—which makes sense since Garmin connect is just importing the Zwift activity. I’m using a Kickr Core, but I’m not sure why that would matter. What does your Zwift activity show—the summary after the ride,not the heads-up display?

I don’t know the answer to the question, but will speculate. I noticed about a 1% drop similar to what you report in a ERG mode ride yesterday. I believe this has to do with how quickly the trainer is able to keep up with pulses in power throughout your pedal stroke.

Power = rpms * force, so for your trainer to hold you at a fixed power, as your cadence pulses the trainer essentially has to change the resistance in real time as you are pedaling. Imagine you’re supposed to be holding 180 watts, but maybe you’re pushing harder with your quads and not doing much to pull up on the pedals (assuming you are clipped in). This might mean that when the pedals are at 3 and 9 o’clock positions you are actually pushing a little harder and at 12 and 6 o’clock a little softer. The effect of this is that just after 3/9 o’clock the trainer will recognize you’ve sped up slightly and lower the resistance, whereas at 12/6 it will recognize you’ve slowed down slightly and increase the resistance (to keep the power level steady). This effect is always lagging behind slightly since it has to react to what you do (and do a little computation to figure out the new force).

I think this means that it will be a little off. I’m not sure why it appears to be a little off in the negative percent instead of uniformly at random, and I’m not sure why Zwift seems to report the ERG value instead of the recorded actual value. It may have something to do with how Zwift is smoothing out the data to show you a number (it isn’t showing you instantaneous feedback–it only gets updates from your trainer once a second).