Re: direct drive an 18T fixie sprocket?
Probably too much gear for the motor. That small a rear sprocket is a really really big gear.
I've often thought a track cog and lockring would be the best solution- particularly for the kind of riding I do onm the motorized bicycle
but I searched the net a few years back and the largest track cog I could find was I think a 22-
there's also a problem with the lockring then- which is designed to lock on the PEDAL side- and the force is pulling it from the opposite direction then on the motor side.