Power meter sudden accuracy issue on dumb trainer

Hi guys

Can you please help problem solve this issue.

So I started a ramp test with virtual power scored 164, trained for a week then upgraded to favero assioma single side PM, redone the ramp test with PM and scored 139 (ouch). i trained for 2 weeks then today i took the bike off the dumb trainer to clean my drivetrain, i readjusted the thru axle and put it back on the trainer.

Started todays workout and it just felt too easy, so easy that i couldnt hit the 70 watts as per TR instructions without spinning at something like 30rpm on the easiest gear. and i could spin at 260w easier than spinning at 140w, compared to before taking the bike off the trainer.

i calibrated the PM 3x, from the assioma app and on TR. no changes.

Whats going on ? What did i do ?

bike is cannondale synapse with disc brake, trainer is a mag dumb trainer, favero assioma PM

Check tire pressure and make sure the trainer roller is applied to the tire at the same tension

  • This may be key here. What exactly did you do?

  • Assuming you are still connected to the power meter in the TR app and getting data from that (not the old VirtualPower), this sounds like some level of friction or resistance changed between your old and new install on the trainer.

  • If there was an issue above, and you now have different conditions, it may well be necessary to repeat your FTP test with the current setup. This might have been a result of an overly tight or improperly installed axle, creating too much friction at the rear hub, but that is a guess.

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i didnt like where it was pointed, so I undone it and tightened it.

yes, its the PM data, not the virtual power.

I am thinking the same regards to doing another ramp test, on one hand my FTP will be higher for sure, but i have no idea how removing the rearwheel thru axle and re applying can make such a difference with how the wheel spins.

  • It’s possible to overtighten an axle enough to impact loading on the hub bearings. Maybe not likely to result in the changes in power you mention, but it is a consideration. In an absolute worst case, enough compression with certain bikes may lead to contact between the hub/cassette and the frame, resulting in more resistance. Not saying that happened here, but again a possibility.

  • Additionally, since you have a through axle (and most likely disc brake), is there any chance you had some brake drag that was resolved by moving the wheel, resetting the brake pad pots, etc? Essentially, if you had a pad dragging and didn’t notice it, that would result in more resistance compared to proper pad clearance from the rotor.

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is there any chance you had some brake drag that was resolved by moving the wheel, resetting the brake pad pots, etc? Essentially, if you had a pad dragging and didn’t notice it, that would result in more resistance compared to proper pad clearance from the rotor.

I was thinking of this as a possibility, but how can I check in the future if the “brake pad” is rubbing ?

Sound is one way. Spin the wheel and listen close to the disc rotor and caliper. If you hear a little “tick” or “hiss”, that is a sign of pad and rotor contact.

Sight is the other. If you look dead through the rotor, looking at the sides of the pads where the rotor passes through, you should see a gap between the pads and rotors when the wheel is spinning. Using a white piece of paper or a flashlight behind your rotor and disc can help see the gap (or lack there of).

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You have a pedal based power meter. The tire/rear axel, etc will have nothing to do with the reading (unless like others have mentioned, TR is actually still getting the data from your virtual power). The power is measured when you apply force to the pedal. You might have a faulty PM or have installed them incorrectly. You should reach out to the manufacturer with your questions.

Its pretty obv that the power output is wrong given you state its easier to spin 260w than 140w and you have to have 30rpm to ride at 70w.

Forgetting about the power numbers, does the trainer feel the same to ride after your adjustment? I mean to say, if you were to ride based on perceived effort, does it feel the same? If yes, then its an issue with 1) your meter itself, 2) the way your meter its transmitting the data to TR, or 3) the way TR is receiving your data.

It is unsurprising the virtual power you used prior is inaccurate (they usually over-estimate).

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