Wheel on trainer - how long do you ride before calibrating?


I have a Cycleops Magnus wheel on trainer which I have combined with my Stages PM over the last few years. The Stages accuracy I am happy with but to enable me to keep that on my main bike and use a second bike on the trainer I have been using the Magnus as the power source.

Comparing power tonight on some 5x5 VO2 max intervals it’s clear the Magnus over reads the power by 20-25W, this backs up my feeling from earlier in the week when I did 5x5 (using Magnus as Power source) and my heart rate was too low for the power output.

So to my question, I generally do a 7 minute warm up, calibrate, then do my session. If I did a longer warm up would i likely see a closer match or is it always going to be wide in terms of “matching”?

Tyre pressure always at 120psi, clutch nob on turbo so consistent in that area. The other potential for wide variables is I could have one leg more dominant, but that’s just guessing.

Keen to hear others experiences.

  • There is no stated time from Saris, but 5-10 minutes warm up is a common recommendation related to wheel-on trainers. So I think you are likely good.
  • Best to just test it.
  • Do your normal 7 min warm up and calibration. Compare to your power meter and do the workout.
  • After 30 or more minutes, do another calibration. Compare to your power meter again and see if it gets any closer.

As you note, single-sided power is lacking and is a variable that must be considered. But assuming equal power in a workout before and after the later calibration, one would hope you can give consistent output to the point of a general comparison to satisfy your curiousity.

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Yeah I guess the only sticking point here is that if 30 minutes makes it match closely my session time might become unworkable :grinning:

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Yeah, far from ideal. But you gotta test to even know what will happen. Then you can decide if it matters, or how much work it’s worth.

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I doubt calibration is going to bring you back 20 watts but as Chad said, you have to test. With a wheel on trainer you can try running it loser - lower tire pressure or less pressure on the tire from the roller. That might lower the wattage.

Does the spindown result time affect anything? It’s generally in a range of 3.9-4.8s.

Keep in mind that the roller and braking mechanism also need to heat up, not just the wheel of your bike. That can take a loooooooooong time before things are truly stable. In fact, Saris used to market a stationary bike with a built-in PowerTap hub that had a reputation for constant drift, because the big metal flywheel in which the electronics were embedded would just keep getting warmer and warmer and warmer.

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I did some testing on this. Tacx Genius, Schwalbe Pro One 25mm, butyl tube, 110psi in tyre before starting.
Measured against an FSA Powerbox.

Took 30 minutes to truly stabilise but was within 5% after 20 mins at 10 degrees Celcius and 15 minutes at 15 degrees Celcius. 20+ air temp and the difference was minimal.

Note that I setup the tension after a good 30 minutes riding, used the same cold tyre pressure each time.

In warmer temps it started within 5%.

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I have the kickr snap - gets a calibration every session - warm up - spin down - load session. Works well for me…I’m nearly 52 and the TR warm ups are to short for me - so some extra spinning first is welcome! :smile:

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The spin down time is everything isn’t it ?
The software should use the time to slow down as a measure of the resistance presented by the setup and therefore the power required to spin the wheel for a given speed.

Yes but if you find a good ‘warm’ setting, the temperature doesn’t vary much and you start with same tension and pressure cold it should be pretty repeatable.

Means you check pressure in tyre then just get on and ride/train.

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