Is my ‘smart’ trainer limiting my gains?

A few posts recently on big cog vs little cog in erg mode and different feels and power.
I have done a few interval workouts now and recorded the workout using my trainer in TR and then separately from my stages LS to my Garmin.

Results show that for intervals which are say 350 watts it will always show around that number within TR, but from the Garmin it can vary a lot depending on the gear. If in the small gear at the front middle back it will generally be 25-30 watts lower and sometimes more. If it’s in the big front middle back it can match the 350 but my cadence then affects it. For example if cadence is 100 it’s roughly right but less and the power is less. I try to keep the cadence smooth and let the power catch up so to speak so no sudden surges but still the crappy results. This is all further exaggerated the higher the power. Yesterday’s intervalsincluded some at 400 watts which in Garmin were way off.

I’m thinking that this is limiting me in most HI workouts as unknowingly my watts are actually less than what’s prescribed and I’m not being pushed hard enough during these hard efforts (even though in TR it’s telling me that they are)

I’m using a Tacx flux and tempted to upgrade to a neo. I just want erg mode on the trainer to take care of the exact power requirements without me thinking about changing gears, certain cadences etc.
Any thoughts or advice welcome.

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I believe you based on a recent experience during Pettit: Form Sprints in ERG mode - in that thread you’ll read that during Pettit I changed gears from big to little chainring and that made it feel like I was riding down a hill. So based on the alone, it sure felt like that 15 minute interval at 144W was much lower, definitely below 125W.

Everyone claims the difference is between little and big chainring in Erg mode is the difference between optimizing road feel for climbing vs flat roads… So if that is true, why did a recent 35 minute climb at 93% (224W) and 71rpm average cadence feel the same as doing intervals in Erg mode / big chainring?

Will try and test again this week, because so far I’d have to say I agree with what you are observed.

I find that in the big ring is harder, I have in the past had to change to the small ring during intervals to be able to finish the workout. Now I know that in the big ring I am more than likely pushing higher watts. It can even be higher than the prescribed watts shown in TR without realising.
A smart trainer in erg mode should just allow you to pedal for the duration of the workout and actual power should match the workout. Mine does not.

Couple of thoughts.

  1. Are you certain the power being reported from both devices match up?
  2. Power from a Left only PM at the crank will be different from that at the rear. Also do you have any Left/Right imbalances?

I have the same issue. I have correlated the power between my Flux and Power2Max PM. See this post for more details.

But as long as it is consistent I’m fine with it. I do however record the data with the Power2Max and don’t use power match.

Let’s set aside the small vs. large chainring thing for a second, because that doesn’t seem to be your problem. Regardless of what gear you’re in, the Stages LS and the Flux should be relatively close…not sure what the calibration procedure is for both of those but have you zeroed the Stages and done a spindown on the Flux recently? My Kickr was way off from my power2max recently but after I did a spindown it tracked really well.


power reported by Stages matches Kickr in big chainring, at 100rpm. Doesn’t match in little chainring.

I wasn’t aware the Flux has a faulty power curve… Sounded like this was turning into yet another discussion about the benefits of slow vs. fast flywheel speeds. I stand corrected

Sorry, I thought he said Kickr. You are correct, it’s a Tacx Flux.

My experience is flux is close enough above 200w but a little off below. That was my old broken/faulty one. I haven’t had a chance to compare my new replacement flux S.

Sounds to me like you are hitting resistance floors or ceilings inherent within the trainer. At larger powers I believe you simply need the higher flywheel speed to help smooth out the variation inherent in everyone’s pedal stroke that simply becomes more apparent when you are doing big power in the small chain ring (think hill climb Vs flat TT for a real world analogy)

At sweetspot intervals (around 240 - 260) the two align closely enough.

The problems seems to be at higher powers.

I just think that the trainer is flawed by the looks of it.

I don’t think that it is consistent that’s the problem. At around 200 - 260 it tracks close enough but then more power equals less and less tracking.

I need to test with the ramp test maybe to see how it matches up.

I just think that where I’m supposed to be hitting high numbers it’s not high enough and therefore limiting me?

I think that you are correct.

My theory is that even in erg mode each gear has an upper and lower power limit.

I.e. in extreme example. Large front ring and small back if TR is 150 watts at 100 rpm my actual watts would be way way higher.

This is not how erg mode should work as far as I’m concerned. I ride mostly in the flats but I want to be able to vary my cadence and use either the big or small ring depending on my training and for the power to be correct.

Anyone have a neo that can confirm if that does this?

The problem is that rather than measuring power, the flux estimates it. Lot’s of good information out there about the quirks of this - DC Rainmaker’s review is a good place to start. If I’m honest, if you have the option, always use a real power meter.

Makes sense for me. I notice that when the trainer is “cold” it reads way too high. It takes a while to settle in. I’ve learned to calibrate it (the Flux) always at the end of a pass, when it is warmed up, and not at the beginning.

I’m going to treat myself to some Favero Assimona pedals for Christmas, but in the meantime, the overs and unders are plenty, plenty tough, so the Tacx Flux is serving its purpose well.

This is news to me! We’re allowed to use the little ring in the front??? :exploding_head::biking_man:

I’m always big ring (52) in the front and middle 3-cogs in the back. I track against a Quarq (on Elite Drivo) with PowerMatch. It all feels like pain… just sometimes with a little faster cadence.


I also use a Quarq Dzero with power match on my Kickr. I agree all i feel is pain.

Before i setup power match, my Kickr was ± 14 watts higher than Quarq when running together .

I share the same problem with my Tacx Flux - power in erg mode is typically 15 to 25W higher than the measurement from my Stages, which drives me absolutely nuts. Over-unders are just not difficult at all, which is just wrong! Furthermore, I get massive power drift over the course of an interval. For example, I was doing anaerobic 90s intervals this evening and although the power being displayed by the software suggests I am on target, the power measured using the Stages just drifts right down across the course of that interval, such that I’m actually just doing a VO2 max interval. This is damn frustrating since clearly I’m not getting the training benefit that I am attempting to achieve. This is a fairly recent thing as the Flux used to correlate quite well with the Stages, now they are quite different and its making me crazy! I’ve tried contacting Tacx for some support but no response (a week on).

I take it that you are not using PowerMatch?

This is one example that shows why that tool exists. It may not be perfect (there are several discussions about some issues with PowerMatch), but it is meant to handle this in particular, to keep your power and resistance based on the Stages PM that you use outside.


Thanks for the quick response on this.

To be honest, I think I just need to ditch the smart trainer altogether and just refer to the Stages as it appears to have developed a number of gremlins. I was initially quite pleased as my ftp was up on the same time last year. Now I just think it’s down to a duff result from an over-optimistic outcome of the ramp test :sob:.

When I use power match with the Stages in erg mode, nothing happens, that is, I still have to change the resistance manually - another gremlin!

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