79cc Predator Fat Bike - First Build

Hello Everybody!

I've wanted to build a motorized bike for several decades and I am finally able to get started. I know that I will have lots of questions since this is my first time building one. I hope that you guys can save a bunch of headache and hassle.

About my ride... it's a 29" steel-framed fat bike cruiser from AZ Beach Bikes. It has a 3-speed hub shifter and disk brakes. I have the SBP shift kit / jackshaft which came with a very sturdy motor mount. I plan to use the Harbor Freight 79cc Predator motor. Still need to figure out which belt transmission to use. Lot's of building and parts still needed.

Here's an image of what I have in mind (but with the motor facing the right way, lol)...



FIRST ISSUE!

Bottom bracket. I need to have a wider pedal crank spindle so there will be room for the extra sprocket and freewheel. The problem is the case is about 5" wide to start with so the spindle will need to be 8" - 10" long. I'm not able to find such a thing and don't have a very good idea about what I'm looking for exactly. Outboard bearings? Aluminum cups? Threaded? Press fit? Square taper?

Any help or suggestions would be greatly appreciated.

 

[ColdCruiser]

I got a few more suggestions and ideas...

Jim at Sick Bike Parts suggested their "precision sealed bearing for the SBP bottom bracket cartridge which can be used to further extend the bottom bracket cartridge for use with the bottom bracket adapter or fat tire bikes." ...along with threading the crank arm directly into the freewheel instead of into the included adapter to save ~.5". I'm still trying to check the numbers and figure out if it will work.

I also found "Wide Bicycle Pedal Crank Sets for Clearing 4 Stroke Motors 25T" from custommotoredbicycles which is a whopping 13" wide!! They have others from 9.75" - 12" as Cottered American instead of square taper.

When it is that wide should I look for shorter crank arms to help reduce the scraping on corners issue mentioned above?




I've also updated my design comp for a more clear idea of where I'm heading...

 

[ColdCruiser]

I took Jim's suggestion and got the extra sealed bearings. Then I cut off part of the cassette tube with a Dremel. There are a total of three bearings on the left with 2 teflon spacers and two bearings on the right with one teflon spacer (plus the cassette tube).

This seems like it will work well. I can change the sprocket position by adjusting the spacers. As I tighten the left BB cup, the spindle gets tighter (harder to turn). My spacers causing friction maybe? Don't tighten the left cup?