I want to keep the crankset :
-in case of failure, I want to be able to go back home
-I want continue to pedal, it's good for health and grow the autonomy
-I want to keep the derailleur as gearbox for the electric motorization (for big ascent)
-for the police, it must look like a bicycle (I will find some stickers 250w on Ebay)
For the double stage belt reduction, here is my opinion :
Tyre : 26 x 1.00 (OD Ø609mm, developped 1910mm)
Speed required : 50km/h = 436RPM at the wheel
Battery : 44v
Motor : 130kv (5700rpm at 44v)
I don't want to use 219 chain, to much strident noise, not smoothy, sin effect and vibration with small diameter cog.
I prefere use HTD5M-20 pulley/belt drive, with minimum 18t pulley to keep a very quiet and torquey mid drive.
-1st and 2nd stage are 18t/60t pulley : (60/18)² = 1:11
(If I want to have this reduction with only one stage, I let you imagine the size of the big pulley, not standard)
-3th stage is : 14t freewheel and 42t for the crankset : 42/14 = 1:3
Total reduction at the crankset : 11x3 = 1:33 (5700/33 = 172rpm)
-4th stage (5 speed with derailleur) :
32/13 = 2.46 - Total ratio : 33/2.46 = 1:13 = 438rpm = 50km/h
32/16 = 2 - Total ratio : 33/2 = 1:16 = 356rpm = 40km/h
32/21 = 1.52 - Total ratio : 33/1.52 = 1:21 = 271rpm = 31km/h
32/26 = 1.23 - Total ratio : 33/1.23 = 1:27 = 211rpm = 24km/h
32/32 = 1 - Total ratio : 33/1 = 1:33 = 172rpm = 20km/h (big torque for big ascent)
The idea is that if you have a gearbox, you can manage/optimize the consumption/acceleration/ascent on all the speed range, not the case without derailleur...
I know what I mean ?