Different power curves for PM and trainer

Hi Gang,

A while ago I upgraded my old mag trainer with a Tacx flux s. Previously I’ve used the old trainer with a single sided favero PM.

With the Tacx things are different. Power match feels terribly sluggish and oftentimes freezes the trainer, so now I am running my TR workouts on my phone with the trainer in erg. It works fine. However it appears that the trainer reads significantly lower at higher outputs. I’m now running Zwift alongside it on an iPad paired to my power meter and I see a 20-40 watt difference. It is not a linear off set but rather grows bigger the more watts I push. At tempo wattage and lower sweet spot it is almost negligible, beyond threshold it quickly grows to a significant gap. I can run my tempo and sweet spot workouts at the set FTP, but have to reduce workout intensity to varying degrees depending on the workouts intensity when things get tough. I’ve accepted this workaround, but was wondering if anyone has experienced the same and has any idea how to fix this?

Unfortunately the only fix is Power Match, which hasn’t been working for you.

Now with that said, when you talk about “things getting tough”, how much wattage are we talking about here?

The Tacx has drivetrain loss, so it’s going to be lower. Your single sided PM is probably measuring the left side, and many folks are left foot dominant, so that increases the gap even more.

When you say the gap isn’t linear, I think you mean the gap isn’t constant. But it sounds like the power curves are linear, meaning the lines are straight (not curved from an exponential or logarithmic function). I would map out the power and see if this is the case. Easiest way to do this is to record the pedals separately in Zwift or whatever else you use (I like using my Garmin head unit) the next time you do a ramp test.


I’ve got a similar problem with my single sided assioma pedals and my Stac Halcyon trainer. Fortunately there is a setting on the trainer that you can use to align the 2 reported power outputs. Is there anything similar on the Tacx? I know there is on the pedals but as I do the majority of riding outside I didn’t want to mess around with the readings from the pedals.

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You’re right, I phrased that wrong. What I mean is that it appears it is not just offset by a certain value, but the gap appears to be a function of power output. At recovery wattage both readings line up, at 200w it’s maybe ten percent, at vo2 wattage it is closer to 20%.

I’ve recorded the trainer and PM separately for a while now, I’m going to check the data and report back.

It’s my experience that Powermatch is the best way to go. I hated Powermatch when I started using it because it operated and felt different than my Kickr’s erg. I stuck with Powermatch and found it works great. I recently was forced to go back to my Kickr’s erg as my power meter was getting repaired.

I just got my power meter back and decided to run the Kickr’s erg simultaneously with my power meter reporting to my Wahoo Bolt. I was surprised at the results when I analyzed the two power graphs. The two matched early on but then the Kickr started reporting much lower, which made the O/U workout brutal. At times the Kickr was 20+ watts underreporting.


That is eye opening.

Possible thermal issues with the Kickr?

It would be good to do a ‘hot’ calibration and then recheck.

It is possible to resolve the issue. If so, it parallels the recommendations to do calibrations after a good 10 minutes or more of use.

Are you using a left sided only power meter for comparison.

I had some erroneous results comparing my left sided crank with the kickr. Kickr appear to be “underreading” but it is just I put out 55% of my power through my left leg, can be up to 60% at lower wattages.

Not sure what has happened between now and April 2019 when I did the following test workouts, but my Kickr used to follow my Quarq PM more closely. Below workouts from April 2019:

I did that when I got the new trainer, but all it did was freeze the trainer. Most frustrating ting ever when you have to restart your trainer and workout 15 to 20 minutes in. It’s why I stopped using Powermatch and TR calibration altogether.

I quickly pulled some data:

Red Lake +3

Red Lake +3 Powercurve

McAdie -2

My deepest apologies for the formatting and axis, but you get the point. A constant offset would be fine, but the error growing with power output is brutal. Like @MI-XC mentioned, it makes over/unders the worst thing in the world. It appears to be not or only very weakly correlated with temperature, as the error during recovery valleys remains small even near the end of the workout.

I see the same thing, or the opposite actually. Rotor Inpower left-side PM reads the same as my Kickr in zone 1 or 2 but at anything harder it reports 10-12% less than the Kickr. I suspect it may be a left / right imbalance on my part, need to borrow my brother’s Vector pedals to find out for sure. Luckily I think the Rotor PM can be offset to compensate for this.

DC Rainmaker wrote a piece on troubleshooting regarding power meter and trainer accuracy issues. Might be useful.

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