While you don't need a smart trainer to use SYSTM, consistency in your trainer setup will ensure consistency in your training. To ensure that your power is always measured in a consistent manner, you'll need to ensure:
- that your tire pressure is always the same
- that the tension on the back wheel is always the same
Variations in tire pressure and wheel tension will cause inaccuracies in the values reported in the app.
If you remove the bike from the trainer, be sure to set it up exactly the same when you put it back on. Depending on the trainer, a quarter turn on the tension knob can add a significant amount of resistance. Follow your manufacturer's instructions for setup.