Hi Rob and
to the forum.
The Magic Pie Edge motors are the same for 24, 36 or 48V. So if you can find a Magic Pie Edge rear motor with a 16" rim, it will run on 36V with the controller voltage set to 36V.
The smallest Magic Pie Edge wheels that I've seen advertised have 20" spoked rims.
I haven't checked the measurements yet, but it might just be possible to fit a shallow section
(not deep "V") 16" rim with radial laced spokes, but only if the gap between the hub and the rim is large enough
and you can obtain short enough spokes.
All Smart Pie 5 motors are supplied with an internal controller.
The SP2x is wired specifically for use with an external controller:
The standard Smart Pie internal controllers can already supply more current than the motor can really cope with, so the only real benefit of having an external controller fitted to a Smart Pie would be to run the motor at higher voltages than the internal controller can withstand
(60V and above) in order to produce higher speeds.
Alan