Odd power data depending on gearing

Equipment
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#1

This is a bit of an odd one I think so I’ll cut out as much fluff as I can.

So, when on the trainer and I’m riding in the small chainring the RPE drops by one or two levels if I drop into an easier gear on the back whist maintaining the same power output as reported by my trainer’s power meter to TR/what I see on my laptop.

Curious to see what was going on I decided to record a ride whist the trainer was controlling the power but also to see what my power meter pedals (Assioma duo’s) were reporting.

When the watts we low say during a warmup/rest between sets the trainer (tacx Flux) and the Assiomas power matched close on my head unit to what TR/my trainer was reporting.

Now where it gets weird.

I just finished a threshold workout and during the work sets whilst in an ‘easier’ gear on the back, the Assiomas were reporting my power to be 15 to 20 watts lower than what I was seeing on TrainerRoad/what my trainers power meter was reporting but TR was still telling me I was holding target power.

I shifted into a harder gear, the RPE went up, the power reported by the Assiomas would more or less match what TR was reporting but the power as reported by TR stayed the same - it didn’t change at all.

Dropping back down a couple of gears the power reported by TR stayed where it should be, the RPE dropped and the Assiomas started reporting my power to be 15 to 20 watts lower than TR.

I get that power sources are different etc but I’m pretty sure that my trainer/TR is reporting my power as being 15-20 watts higher than it actually is when riding in a lower gear (I think the Assiomas are reporting correctly).

I’m not sure I’ve explained this well but I have the FIT files and happy to share and if anyone cares to take a look and I can pinpoint the time when I was riding during an interval in an easier vs harder gear so it possible to see the variation, it’s kind of hard to take screenshots to illustrate the point but I’ll give it a go.

So this is the first interval of the workout as report by TR/my trainers power meter and loaded in Golden Cheetah shown overlayed with power zones (its easier to see the difference vs seeing power in watts):

Power zone
Power zone1221×851 161 KB

This is the same interval of the workout as report by the Assiomas power meter and loaded in Golden Cheetah:

power zone
power zone1221×843 12.2 KB

And this is TrainerRoad/My trainers data in TR:

TR
TR1600×648 56.8 KB

So according to the data from my trainer I’m in the right zone, based on the data from my Assiomas I’m in sweet spot.

Does anyone know what’s going on? The variation kicks in when in an easier gear that’s the only difference.
I ride with oval chain rings as well in case that matters.

I know I could use powermatch but I wasn’t over the moon and I would really know what the root cause is.

Any thoughts welcomed!!!

#2

My guess would be that the reason you see a difference between the different gears is because of the inertia. If you’re in an easy gear, the trainers flywheel will spin at a lower speed and thus will not even out the fluctuations in power input during a crank rotation. Sort of the same effect you can feel when riding in a steep uphill, that the “deadzones” of a crank rotation is noted much more than if you go high speed in a high gear on a flat.

Why this effects the power calculations differently might very well be due to the oval chainrings, but it’s hard to say since the companies usually don’t share internal details of how they calculate the power.

EDIT: Seems like the Assioma pedals should be designed to handle oval chainrings: IAV Power Technology - Power Meter City

#3

It could be your Trainer could be inaccurate compared to your pedals by a varying amount depending on the intensity. I went down this rabbit hole trying to figure a similar thing out a few months ago and eventually lost the will to live trying to figure it out and do something about it. One thing I did was to do some 10 minute intervals in Erg mode in the lowest gear. highest gear and a middle one and compare the average power readings to see if it provided some predictable inconsistency on my turbo (Elite Zumo).

1 Like
#4

Thanks for the reply.

I’m pretty sure it’s not inertia, it’s at least 2 RPE levels easier when I drop down the gears so I dont think thats the culprit although I wish it were!!

#5

Yea thanks sense.

At the end of day I just go one gear harder and evreything lines up power wise so I guess I should just suck it up and accept it’s going to hurt more!

#6

Tacx Flux S - Gearing / accuracy - Equipment - TrainerRoad

Take a look at my data :slight_smile:

1 Like
#7

Dude, it’s the same issue huh?

Looks like I should switch to power match.

Did you redo your ramp test?
If so, was there a difference?

I’ve only recently switched to the low chain ring so I’m hoping this bad data is limited and I haven’t been training at a false FTP :confused:

#8

@GPLama Mate, I saw the post where you replied to @SaSha1103 and looks like I have the same issue.

Not sure if you wanted to look any deeper into this but I read in the thread he created it was hard to dig any deeper that due due to only having 1 sidded PM.

I have dual sided PM so if the data files are of any use/interest to you let me know and I’ll ping them over to you.

#9

i’ve picked the gear that the pm and tacx matched and never switched gears anymore :slight_smile:

#10

Thanks for that.

Did you find you needed to retest FTP?

#11

could be a ceiling/floor issue with your trainer. Basically you’re power target is lower than the gearing allows. Although, curious why it would report a different power than you were actually putting out. I would think it would just not go that low.

#12

Nope. Just go by feeling if you just started a block.

#13

I can’t recall that thread… I’ll take a punt - Most smart trainers are well outside their published accuracy spec at higher flywheel speeds. I’ve added this test to my trainer test protocol in the last 12-18 months. Almost every smart trainer fails to hold 225W ERG in the 53/11 at ~90rpm.

You’re on a FLUX, so that unit is four years and two generations (maybe more?) old now. Not much to be gained digging to much into the numbers coming from it from my end. Ideally these are the issues Tacx/Garmin need to be answering.

#14

Thanks for the reply and insights, much apprecaite!

Maybe time to upgrade :grin:

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