I think I know the answer but wanted to check…
I calibrated my Kickr Snap at the start of a ride (when cold, as suggested by TR). I normally then re-calibrate 10 mins in but this time I forgot! Is the power for the rest of my ride reading higher or lower than if I had recalibrated?
Lower, I’d guess. Magnetic trainers have more resistance when cold. Fluid the opposite.
Edit: I’m wrong, Zahlmiac is right - see below.
I’m pretty sure it reads higher. You are right about the inherent resistance being higher when cold, but I believe that means, that when it warms up, there will be less resistance, while showing the same watts.
I did exactly the same the other day, when borrowing a friend’s Tacx Genius and it resulted in a too easy over/under workout.
I may be missing something, but I have calibrated snap to align with power meter readings once and never touched it since. I use same wheel, tyre, drum pressure every time, the only thing I adjust adjust is tyre pressure to a certain value as it deflates. This gives me same feeling and consistency in workouts…
That’s what I did as well before getting a power meter (strain gauge). As long as everything is consistent, I don’t see a reason why you should recalibrate anything.
If everything is the same, it should be consistent. There will always be a temperature drift, but if the drift is always the same, it should be fine. If the ambient temperature is vastly different, it could be a problem.
I work out in an uninsulated shed, where the temperature can be below freezing. This has a large impact on the accuracy of the trainer (Tacx Vortex) because the difference in starting temperature and “optimal working temperature” is so large. In this condition, 10 minutes is far from enough to warm up the trainer. The last few workouts, I have tested this, by calibrating several times and comparing the numbers. When it is around freezing, it takes around 45 minutes to warm up!
Last winter I did a 2 x 8 test, that gave me a suggested ftp of 316, when other rides indicated it was between 250 and 260! That’s how much temperature drift can affect your numbers.
It’s hard to know for sure and even if you did, there’s no way of knowing by how much. I wouldn’t worry about it.
Why exactly does the resistance change in magnetic trainers? Is this only for wheel-on trainers? I am experiencing some despiriting issues where my Garmin vectors read lower power at the same cadence as the room heats up (it does so around 6 degrees during a 90 minute workout) and I wonder if that’s my trainer producing less resistance. It’s a stac zero though (so it replicates the way a magnetic trainer generates resistance but does so without contact by generating eddy currents in the rims/brake tracks of your wheel rather than through a roller on your tire.