One Trainer, Two Cyclists

Hi all !

Due to COVID-19, my girlfriend has started doing some indoor workouts using my Kickr Snap, instead of going to her spin classes.

She loves it, but we suspect the power numbers to be inaccurate, due to the fact that we use two different bikes on the same trainer:

  • my BMC has an 11-speed Shimano 105 groupset and is equipped with an Elite Coperton trainer tire;
    *her Lapierre has a 10-speed Tiagra groupset and is equipped with regular road tires;
  • we both inflate our rear tire at the recommended tire pressure of 110 PSI;
  • I weigh 12 kg more than her;
  • one of us performs a calibration spindown once a week;
  • between us, the trainer is used almost every day.

Do we need to calibrate the trainer every time we switch our bikes ? Is a factory spindown required every time we switch ? If so, is it even possible to share a wheel-on trainer between two different cyclists ?

Thanks in advance for your recommendations !

  • Rider weight is irrelevant and not part of any calibration or power data from the trainer.
  • This is not acceptable if you aim to get “accurate” power data.
  • Ideally, yes.

  • The whole point of the spindown calibration is to account for the current tire pressure, trainer roller pressure, and the tire itself. These matter because a change in any or all of those values will lead to altered power data.

  • No, you only need to do the “Factory” spindown on rare occasions.
  • You can do the “Regular” spindown as your normal one each time you workout (after the warm up).
  • Of course. However, the difficulty in doing so lies in how accurate you expect your data to be. If you want the most accurate, you need to calibrate each time you swap at a minimum.

  • Typical manufacturer recommendations are that you should calibrate a wheel-on trainer (like the Snap) each time you do a workout, even if you have the same bike and tire installed. This eliminates the risk of slightly different tire pressure and roller pressure from session to session.

  • If you don’t care about the data (which is true for people who use a different power meter on the bike for their power), then you can skip the calibration all together. But that is not true if you are relying on the trainer for power data.

  • Garbage in, garbage out. These tools have a proper process to use them if you want to get the best info from them. Wheel-on in particular requires a bit more work for that goal, so it’s one reason people like the wheel-off option instead.


Accuracy is indeed important to both of us, so it looks like we have no choice but to calibrate every time we swap our bikes.

It is a bit of a pain, especially since I mostly do workouts in ERG mode and they usually don’t incorporate a 10-minute warm-up. Oh well, I guess I just have to get used to it !

I was also thinking that I could use the “Control with ANT+ power meter” in the Wahoo App, which would get rid of the necessity to do a spindown, but apparently Android phones don’t have that option !

I don’t know about budgets etc but equipping both bikes with power meters - or a shared set of power pedals - strikes me as the neatest solution to this problem.

I’d said buy another trainer! For the same price as two power meters you can both be spinning at the same time.

Do you have a power meter in one or both bikes?

You didn’t mention that in your OP.

Yes, I forgot to mention that I have the Garmin Vector 3S pedals.

I tried to use the power matching feature, and while it’s OK for non-structured indoor workouts, ERG is a different story: it doesn’t work as good as having the trainer use its own power meter to adjust its resistance. I just couldn’t get the trainer to match the target power, it was always above it (something like 10% over). The power numbers were also much more variable (I’m used to the power smoothing feature of the Snap). I guess the power feedback loop is better when you’re not relying on an external power meter !

I wanted to try the Wahoo built-in power matching (the “Control w/ ANT+ power meter” option in the Wahoo app), but like I said, it’s not available on Android, and I don’t have an iOS device.

So it does seem like I need to calibrate at the start of every workout, or buy a second trainer !

Just perform the spindown… It doesn’t take more than a minute.

Yes, the spindown takes one minute, but Wahoo recommends a 10-min warm-up, during which power can be quite inaccurate, so I can’t start a proper workout in ERG mode until I have performed the spindown. 10 minutes of “aimless riding” might not seem that much, but when you’re trying to squeeze a 1-hour workout in an already busy day, it can make a difference !

I know, I know, I’m nitpicking ! I’ll just have to get used to it if I want accurate data !

Thanks for your help guys !

It’s not a pain at all. Do what I do, ride the trainer for 10 minutes before you start a workout. Spindown. Start workout. It’s simple.

I also have a mark on the tension knob so I can set the same tension every time. I usually leave the tire under tension and have had zero issues with deformity over 3 years of using the same tire.

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If you are following TR workouts, there is a warm up for each workout, then a small recovery Valley before the main sets.

Just do your calibration after the WU and before the WO.

This also works. Realistically, if you’re in an indoor environment with a high ambient temp you probably don’t need to go the full 10 minutes for things to warm up. I would just make sure the warm up time is consistent. It could be 5 minutes, 7, 10. Just make it consistent each time.