We have never had much luck mounting
US Digital encoders to flywheels. We also haven't had luck with using another motor as a tachometer. This year and last year for flywheels we used
photoelectric sensors pointed at retro-reflective tape strips stuck to the wheels. We have 6 strips of the tape stuck evenly around the wheel this year. 6 ticks = 1 revolution. Our speed control has had quick responses and accuracy.