Using external power meter - not recommended


Wrote this a few days ago, as I was really struggling with my workouts. Solution here is ofc to respect my recovery days.

However another huge difference I figured today is that I tried changing so I used HT powermeter data instead of my external powermeter (4iii precision 3 L),

WHAT a difference!! Did an anaerobic threshold today and everything felt a lot “smoother” and sustainable. I have about 18% difference (check 20 min ftp test) when comparing my external power meter with my HT (tacx flux s) and I think this somehow really causes issues if I use my external power meter as power source, when using ERG mode.

with Tacx Flux S as Power source: Log In to TrainerRoad
Similar wo With 4iii as power source: Log In to TrainerRoad

So… Using my powermeter from hometrainer from now on, eventhough I must admit it’s going to be hard in Zwift races knowing that it shows way too low. However most important is to get the correct training and completing my workouts imo.

Downside is that when doing rides outside, I guess my TSS will be much higher and my Endurance rides will not be recognized as “base” training, but “tempo” rides. Any of you have similar issues and is there a fix? (other than buying a new hometrainer ofc?). It would be greate if I could set Power zones based on what Power Source used…


@Janrknudsen your trainer measures power at the rear hub. Your crank-based power meter measures power at the crank. Most of your drive train losses happen between those two points…so the power measured at your crank-based power meter should always be a little more than the power measured at your rear wheel hub. The difference is drive-train losses.

If your crank-based power meter has proper zero-offset taken at the current ambient temperature AND your trainer is properly calibrated and warmed up for 15 minutes…then the difference between those two powers should be about your drive train loss. So my first reaction is that your drive train is a little on the inefficient side.

Another thing to think about is that your crank-based power meter is not too great at dealing with temperature. So if you zero the power meter at a chilly temp and ride it at a warmer temp it’s gonna read high. If you zero the power meter at a hot temp and ride it at a chilly temp it’s gonna read low. So that’s one more thing to think about.


You have to redo your ftp if you change the power meter, and even more so if you change the location of the power meter (from cranks to pedals for example).

There are variances that might make your wo feel easier than they should. Also, make sure your trainer is properly calibrated.

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Hi Brennus,

Thanks for the info. I have been changing crank, cassette, chain etc when troubleshooting on this previously and think the difference between the two power meters could be smaller also not but there is still a huge difference which make it proplematic hitting the “right” zones when riding outside.

But still it’s most important to hit my zones correct in workouts so I will use the zones from my hometrainer and live with them being a bit misleading outside where I just have to go nuts in a race or ride at zone 2 for endurance rides.

I’ll do a ramp test in a weeks time in my “rest week” so FTP is set correctly but still the last workout was a whole other experience using the internal power meter which I find really great.

I experienced something similar some time ago, although it could have been me doing something wrong.

I have a Saris H3 trainer, and Powertap P1 Dual pedals. 99.9% of the time I use a PC to ride on TR, and connect via ANT+ to everything. I also record the workout on my Edge 530, connected via NT+ to the same pedals.
When I experienced the issue, I was having problems with my PC so used my Android tablet instead, which doesn’t have ANT+, and possibly it connected via Bluetooth to only one of the pedals (probably me being stupid). The workout felt more difficult, and on my Garmin the power was showing about 10W more than what was displayed on TR.

This is probably not quite the same issue as what you have; in my case if I need to use my tablet again I will deliberately not connect to the pedals.