The belt shouldn't rub on either side of the pulley flanges. It should run on the middle of the pulley.
Its a bit of a process to true the belt. First loosen the axle nut then use the two adjustment nuts on the end of the swing arm to adjust the angle of the wheel assembly until the belt runs in the middle of the pulley.
Either spin the wheel by hand or run the car with the rear wheel elevated in first gear. Make adjustments and repeat the process until the belt runs true.
Finally check belt tension and confirm the belt is still running true, then tighten everything up.
