my dream offroad ebike built up from scratch

Although weight will suck a big problem with every ebike build is bicycle tyres are not really suitable and especially knobby versions.  They just get eaten too quickly.

The next option is to put a pitbike tyre on a bicycle wheel or just use the whole pitbike wheel as the starting point since it will be strong and the freewheels don't cut it on bicycles anyway for the amount of power we want to run.

The issues are the widths, axles etc.

12"  12 x 1.85 - 32 Spoke Rim, 7.5" (76mm) Wave Rotor Brake Disc ,39 Tooth 420 Sprocket, Kenda Carlsbad 80-100 - 12" Tire, Accepts 15mm Axle

Since most these bikes take a 420 sprocket it may be a good choice to go with a 19mm jackshaft with a keyed 12t 420 sprocket with something like pillow blocks.

http://www.starproducts.com.au/products/jackshaft-kit-drive-shaft-19mm-12-tooth-41-pitch-sprocket-pillow-blocks/

The front end box would be designed to take something like 5 of these.  At 20s 72v nominal 50a is more than enough to rip through extremely steep terrain.

a 4s pack is 200 x 90 x 40mm so times that by 5 and you have a square battery box of 
20cm x 20cm with a width between the legs of less than 10cm!

http://www.hobbyking.com/hobbyking/store/__80906__Multistar_High_Capacity_4S_20000mAh_Multi_Rotor_Lipo_Pack_AU_Warehouse_.html

I do like this adapter for a freewheel and disc brake holes













I wonder what kind of geometry I could make out of carbon.  

Electric Bike review 2016 mid drive vs hub motor

Love my electric bike videos.