Inacurate power meter crank on trainer

I use a 4iiii 105 left crank on my roadbike
My FTP last time outside was about 290.
Yesterday i tried indoor trainer (tacx flux smart) for the first time. I had the poweroutput of the trainer on the phonescreen and from my crank on the wahoo bolt.

After the FTPtest the trainer gave me power outputs around 300 watt, and it felt like that was acurate, little harder than the outdoor 290 one.

But, my wahoo and 4iiii said i was doing about 230 watts when trainer says 300. 60-80 watts always below.
I used the crank again outside and for feel riding again at 300 watts it did read 300 watts.
What is happening? Is a left crank bad for indoors on the trainer? It was calibrated and everything, neverhad inacuracy problems before outside

Grtz, Kristof

There can be a discrepancy from a single sided system to a dual side system but that is massive. From what I’ve heard/seen (500+w up the Zwift Alpe :face_with_monocle:) I would put more faith in the crank than the turbo and train of the power meter.

I believe the Tacx Flux trainer is reasonably accurate +/- 2.5% so it should give you some solid numbers but that’s a huge difference between the two. I’m assuming you don’t have a massive leg imbalance so how do you perform outside relative to other riders? As there’s such a huge difference between those numbers, are you a rider that gets dropped a lot or can you hold your own?

Do you have a friends bike with a PM that you could borrow to validate the Tacx numbers?

guys, I suspect that we should focus on the huge variation in readings between the PM indoors and the PM outdoors, rather than the difference between the trainer and the PM


Yup, trainer issue is interesting, but that is not the real problem here.

I get the impression that the OP is “new” to indoor training (guess based on wording and comments in the OP). But if my guess is right, I suspect we have the common indoor vs outdoor issues here:

  1. Cooling (not enough air flow over the rider leading to overheating and lower power limits).
  2. Motivation (totally different inside vs outside and depends heavily on the workout and “drive” of the rider in vs out).
  3. Fixed trainer (lack of motion on a fixed trainer may be an issue, but this is a very low tier issue compared to 1 & 2).

I’d like to hear the specifics of the OP’s workout to learn if these are related, like I bet they are.


My initial thoughts were the same but that’s a big difference, even with a terrible indoor set up.

Interested to hear more specifics


Hi, yes I am new to indoor.
I would say i do fairly good outdoors, tho I don’t ride in groups. So can’t say much about being dropped.

To put the effort in perspective,
I have the KOM outside for the highest climb in belgium, outputting 260W or 4.5W/kg for about half an hour. Wich was a week ago, so fitness wouldnt be much different now. Wasnt absolutely dead at the end (dont take this as bragging, just for reference that i am used to riding very hard for a prolonged effort)
This felt less hard than now on the trainer.
On the trainer I went a lot above 180bpm and set a new highest of 193 bpm.
No way i can believe i was only outputting 240 average for 20 minutes when i felt like i was dieing

About cooling, ihad a fan at max speed running

It’s just so strange that i can do close to 300 acording to the crank on the same day, same bike, outdoors. feeling less difficult than 240 inside

Grtz, Kristof

Can you tell us more about the tests?

Did you use a ramp test or 20 min test protocol or something else for each test?

Can you share the activities with us so we can see the numbers?

There are a lot of potential factors involved so it’s important that you share as much information as possible so that we can help you.

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In addition to those:

  • Were you using ERG mode or Resistance mode on the trainer and app?

  • What gearing were you using on the bike?

  • If you used ERG mode, are you familiar with how it is intended to work? It is something of a learned skill and different than using Resistance mode or an old dumb style trainer. The following video is worth your time if you are new to ERG use:

I used the 20min test of the tacx trainer app, with 20min warmup. It was in resistance mode, simulating a 5% incline.
Outside was just warmup and 20minutes all out. Soi knew what pacing should feel like in the muscles
I only have the data from tacx, since i turned off my wahoo out of frustration after about 10 minutes, everytime i saw the low powernumber and the difference i subconciously tried to go harder to targetpace, which made me go too hard and fall out of cadence.
Gear was in 32 front - somewhere in the smallest cogs rear. 105 cadence average

Here is the powercurve of the inside test, and a powercurve of an outside test a while ago, can’t get a later one it seems, but my fitness has a bit increased since then and i have done one in between those 2 which was a bit higher than this one outside.
both where with the same Wahoo bolt and same 4iiii crank tho
(outside on powermeter 287avg on crank, inside 316avg on tacx. at same time 230avg for 10 minutes on the crank untill i shut it off)

If there Is a big temperature difference inside vs outside and you do not perform a Zero Offset your Power Meter might be somewhat off.

Along with this, was a proper spindown calibration performed on the trainer (after about a 10 minute warm up period)?

  • It’s important to properly ZO the power meter, and calibrate the trainer to have any hope of either giving their best data.

outside like 18-19°C, inside bout the same probably, garage was open
I zero offset the crank every ride
about the trainer calibration, this was probably not done 100% correct, I borrowed the trainer and did not calibrate after warmup
even if the trainer is off because of the calibration, still remains the case that 300W on the trainer felt the same as 300W outside, and most important 230-240W outside doesnt feel really hard, certainly not 170-180+ bpm-hard like when i did that according to the crank.

the tip about the calibration is really helpfull tho, maybe i better try again, with all those parameters definately correct and see again


The other variable we have is climbing vs trainer. Here is an old blog:

This does not explain the tacx reading which should rather be lower than higher (drivetrain losses), but might be a piece of the puzzle.

The effort in the picture from outside was on completely flat and even terrain, on a road without trafic and intersections along a canal, the least amount of change thats possible for a full 20 minutes here. So no climbing

I found somewhere where i can probably borrow a direct drive trainer instead of a wheel-on, and try again

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  • OK… this raises questions. Per your comment in the OP:
  • A Tacx Flux Smart is a wheel-on smart trainer (which is what you said you used):

  • Did you actually use a wheel-on trainer like a Tacx Vortex Smart or Tacx Flow Smart (both look similar to the pic below):

  1. This all matters, because the Vortex and Flow trainers are widely known for being inaccurate power data devices. They are long seen to show different data when compared to power meters. So I would likely disregard the power differential between them. That said, I know that is not the whole issue, so see below for more.
  1. Regarding gearing, performing a test at lower flywheel speed is not a lot like outside riding on a flat road. The typical loading on the body is different when compared to higher gearing and faster flywheel speeds.

  2. Related to #2, the Vortex and Flow trainers have relatively small and light flywheels. They lack proper flywheel inertia that is more common on wheel-off trainers, like the Flux you mention in your OP. That combined with the low gearing is quite a bit different from rolling outside at normal speeds of most active riders.

Finally, I know you said you had a fan, but I strongly encourage using more than you think you need, even if out in a garage. All things equal (temp in particular), you need several really good fans to even approach the cooling effect of rolling outside at typical road speeds. One average fan is underpowered at best for keeping you properly cooled when sitting still.

These are all pieces to a puzzle, and each one contributes a portion to the apparent differences in your in/out FTP.