So my back tyre pressure was quite low and I was reaching higher watts than ever before, thought my fitness was just getting much better but I pumped up my back tyre and now my watts are at an all time low!
I use a garmin tacx boost. It’s not a smart trainer or anything I just have the garmin device on my wheel/ pedal to capture my cadence etc
Is this a fault or does that happen?
To be expected if the tyre was slipping due to under inflation.
Estimated power uses your speed to estimate power for a trainer of known characteristics.
If you apply a roller to a tire and use a given number of turns on a knob that will depress the tire a certain distance. Tire pressure determines the work needed to depress the tire: force is proportional to pressure. So double the tire pressure doubles the work. That means double the work needed, and double the power. There’s other sources of resistance so power doesn’t actually double but it will increase.
So for a power estimation to be accurate you need to maintain the recommended tire pressure for that trainer.
This is different than riding outside. Outside the tire depresses less when tire pressure is increased. This actually reduces resistance rather than increasing it because force over less distance takes less work. It’s why on a smooth track riders use high pressure.