For one you shouldn’t expect absolute power values to be comparable between different trainers. They are not. Absolute power numbers of smart trainers have zero absolute credibility. And even if absolute values where meaningful, there are 10 or 20 watts difference between what the Vortex would measure at its drum and the Flux at the hub. As you have your FTP set to the same value I’d start off by hammering out a new ramp test and see where the cards fall.
Secondly as a former Flux S user I can confirm that the trainer has a weird power slope. It seems to read (relatively) lower with increasing power as if an error between true and displayed power would grow the more resistance it put up. I had trouble with the extended vo2 workouts on TR because they drifted towards sprint intensity. Mills or Shortoff still give me PTSD, while anything below threshold was perfectly fine.
Third, a smart trainer does not have an unlimited range of resistance and the Flux seems to have a particularly narrow range. This means that in any given gear you can only reach a certain power range. In TR workouts you’ll frequently have to choose between running too much power in recovery or spinning out in power intervals. Unless you shift between intervals, which is annoying in ERG.
Finally, the Flux has little inertia and a not so great ERG algorithm. Little inertia means that in small gears your virtual rear wheel slows down everytime your pedal stroke hits a dead spot at 6 and 12 and the trainer increases power. It can feel like an oval chainring from hell. The poor ERG algo (or maybe its the hardware) hits you in high gears with high flywheel speed. At some point it feels like the smallest possible step to adjust your power is not small enough to correct resistance to meet your target. Any correction will result in a target miss of opposite direction and sends you into endless oscillations around your target power. It seems like someone programmed the deadband around target power too narrow for the hardware and as a result your interval at 120% becomes a constant over/under at 115/125 which is not the same at all. Given that the Vortex was a wheel-on trainer, where the natural fluctuations in your pedal stroke are blunted when they reach the trainers power meter this is probably hitting you the hardest.
So what to do? Retest your FTP. Figure out the power slope and find gear ratios that work okay for certain intensities. Or just buy a Direto X, a Neo or Kickr like everybody else.
There’s no way that Flux is working as it should in trainer road. You simply shouldn’t see that kind of variation in your power from the target power. What you see from the Vortex is really what all trainers in ERG mode should look like.
Comparing HR between rides really highlights the difference in effort. Much higher average with the Flux and there’s no recovery between intervals
So, the natural question is if the Flux is not in ERG mode, but rather SLOPE mode. Is Trainer road actually controlling the Flux (ERG) or is the trainer simply operating like any other dumb trainer and just reporting power (SLOPE). I’m not sure if you’re shifting gears between intevals, but given your cadence remains relatively constant between easy/hard intervals and your power does drop, I’d guess you are in ERG mode, but it’s just not working right.
I see here that you are not alone (although it looks better than your case). Their solution was to increase trainer speed. But given how difficult you say it is to increase speed, that might not be possible to do.
“Just a quick update.
A member of the Tacx Flux owner-group on Facebook suggested using a taller gear, as it was his experience that it stabilized the oscillations.
And behold, last night I tried it in the 36/18 gear instead, and the oscillations were as good as gone.
I’m thinking the Flux might have problems with low flywheel-speeds, so a taller gear will bring higher speeds and more controlled resistance.
The only downside is that I have to switch a few gears lower in the rest-segments between intervals, or else i’m hitting the “wattage floor†of the trainer with my preferred cadence.”
Even the tacx vortex has this low gear oscillation problem (I had one). So it’s odd that you’re getting this in the flux, but not the vortex. …hmmm?
You might try…
to use the tacx app to control the trainer, decrease resistance all the way down, and see if you can get into a bigger (faster) gear to calibrate the trainer.
to use different training software to see if the issue persists on other platforms. Zwift has a a free trial (easy to use). I have come to love Golden Cheetah as a free alternative to Trainerroad (but it’s not as easy to set up initially). …so maybe zwift is your better go-to comparison.
If the problem persists after calibrating in the tacx app and another platform, I can’t see any other option other than calling this Flux a ‘lemon’. Return it (if possible) and go for a KICKR core (which has a very smooth feel to the resistance and a large fly wheel). If you want to stick with Tacx, reach out to customer support for a warranty fix.
Here you go. The first was on the Tacx Votrex and the second on the Flux S.