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Trainer and PM disagree

Posted: Sun Nov 22, 2020 10:57 am
by lemos
So, I have a Tacx Flux S Smart and Favero BePro pedals. Getting instant readings from both, on a stable pace, I get something like a 20% deviation, which is freakin huge.

How do I know which one is right? I've calibrated both at the same time.

Trainer and PM disagree

Posted: Sun Nov 22, 2020 10:57 am
by Weenie

Re: Trainer and PM disagree

Posted: Sun Nov 22, 2020 11:29 am
by TobinHatesYou
lemos wrote:
Sun Nov 22, 2020 10:57 am
So, I have a Tacx Flux S Smart and Favero BePro pedals. Getting instant readings from both, on a stable pace, I get something like a 20% deviation, which is freakin huge.

How do I know which one is right? I've calibrated both at the same time.

Record the same ride with both, upload to ZwiftPower Analysis or analyze.dcrainmaker.com.

Observing instant power differences visually on different devices is not that useful. You’re better off using 3s or even 10s power to get a good gauge on how far off one PM is. As for knowing which is more accurate...you’re going to need a third power source.

Re: Trainer and PM disagree

Posted: Sun Nov 22, 2020 11:46 am
by brearley
I'd just take it for granted the flux pm is wrong.
Mine was almost always 10% low of crank based.

Take into account the flux doesnt actually have a pm inside it, power is worked from an algorithm based on cadence and flywheel speed

Re: Trainer and PM disagree

Posted: Sun Nov 22, 2020 11:54 am
by demoCRIT
brearley wrote:
Sun Nov 22, 2020 11:46 am
I'd just take it for granted the flux pm is wrong.
Mine was almost always 10% low of crank based.

Take into account the flux doesnt actually have a pm inside it, power is worked from an algorithm based on cadence and flywheel speed
Had fluex for over a year - was pretty close to my garmin vector pedals.
Neo (currently owned) reads 2% lower vs pedals.

Re: Trainer and PM disagree

Posted: Sun Nov 22, 2020 3:45 pm
by Lucendi
There will always be some difference when comparing any two power meters. 20 % is not an insignificant difference though!

But generally a few percent difference does not matter one bit as even if the reading is slightly high or low, the power meter should still be reliable (showing consistent results).

As long as a power meter is consisten it is a useful tool for training.

Re: Trainer and PM disagree

Posted: Sun Nov 22, 2020 4:11 pm
by TribesMan
Smart trainer is not a "power meter", it just estimates the power depending on the flywheel speed.

And because the estimation depends a lot on temperature and how (or when) the spin down is performed, it can be very inaccurate.

Pedals on the other hand should be very accurate since they measure torque and cadence directly.
Only thing that could influence that is wrong crank length setting.

Also you should be albe to check the accuracy of the pedals using some weights attached to the pedals (at least you can do that with the Assioma)

Re: Trainer and PM disagree

Posted: Sun Nov 22, 2020 6:12 pm
by TobinHatesYou
My smart trainer (Hammer) is pretty damned accurate. All of my trainer sessions are recorded on two additional power meters.

Re: Trainer and PM disagree

Posted: Mon Nov 23, 2020 11:20 am
by GothicCastle
Why do you think they are off? Just looking at a head unit, or have you examined ride files? I'd suggest riding for 10 minutes before calibrating the Flux, then zero offset the pedals according to the instructions. If workout files differ significantly after that, you have something to analyze/ discuss with Tacx.

Re: Trainer and PM disagree

Posted: Mon Nov 23, 2020 4:38 pm
by Stueys
TribesMan wrote:
Sun Nov 22, 2020 4:11 pm
Smart trainer is not a "power meter", it just estimates the power depending on the flywheel speed.

And because the estimation depends a lot on temperature and how (or when) the spin down is performed, it can be very inaccurate.
Not entirely convinced that's true across the board, for instance the Neo is well regarded as having a highly accurate power meter.

With anything PM related I tend to use the DCrainmaker analyser toolset, gives you a nice visualisation capability. I would say that comparing two devices is fairly pointless though (unless you have a proven baseline on one of them), you need 3 devices to start to determine what is actually correct.

Re: Trainer and PM disagree

Posted: Mon Nov 23, 2020 8:52 pm
by ParisCarbon
Stueys wrote:
Mon Nov 23, 2020 4:38 pm
TribesMan wrote:
Sun Nov 22, 2020 4:11 pm
Smart trainer is not a "power meter", it just estimates the power depending on the flywheel speed.

And because the estimation depends a lot on temperature and how (or when) the spin down is performed, it can be very inaccurate.
Not entirely convinced that's true across the board, for instance the Neo is well regarded as having a highly accurate power meter.

With anything PM related I tend to use the DCrainmaker analyser toolset, gives you a nice visualisation capability. I would say that comparing two devices is fairly pointless though (unless you have a proven baseline on one of them), you need 3 devices to start to determine what is actually correct.
I agree with Stueys 100% on this.. my Neo is dead accurate with 3 SRMs and a P2MaxNG... the original Neo is the most reliable of the units...
20% quoted above is actually a HUGE difference.. especially in the online racing world.. 20% in the wrong direction @ 280 watts means you are now doing 330+ watts .. probably explains though why there are so many cat 4 A+ category riders on Zwift..

Re: Trainer and PM disagree

Posted: Mon Nov 23, 2020 9:37 pm
by brearley
As I said earlier, the flux doesn't have a power meter. It is one of the reasons it is cheaper than the neo which does.
It is possible to use a bit of trial and error on the flux with spindown to get power reading in line with your pm

Basically either stop the flywheel early so the spindown finishes earlier so the power is higher, or keep it spinning for lower numbers

Another thing to check out on the flux is for belt slipping which can be cured with a new belt or a bit of belt spray and there are also some alignment issues also which affect the power numbers.
All this requires taking the unit apart and make sure you put the screws back in the same holes theyre different lengths

Ps I do not advocate doing this to cheat on zwift :lol:

Re: Trainer and PM disagree

Posted: Mon Nov 23, 2020 11:56 pm
by TobinHatesYou
brearley wrote:
Mon Nov 23, 2020 9:37 pm
As I said earlier, the flux doesn't have a power meter. It is one of the reasons it is cheaper than the neo which does.
It is possible to use a bit of trial and error on the flux with spindown to get power reading in line with your pm

Basically either stop the flywheel early so the spindown finishes earlier so the power is higher, or keep it spinning for lower numbers

Another thing to check out on the flux is for belt slipping which can be cured with a new belt or a bit of belt spray and there are also some alignment issues also which affect the power numbers.
All this requires taking the unit apart and make sure you put the screws back in the same holes theyre different lengths

Ps I do not advocate doing this to cheat on zwift :lol:

All modern trainers use RPM vs resistance vs acceleration to estimate your power, including the Neo. The last notable trainer to have an actual strain gauge power meter was the original 2014 KICKR, which grew less accurate over time as the module loosened up on the frame.

Re: Trainer and PM disagree

Posted: Mon Nov 23, 2020 11:56 pm
by Weenie