79cc Predator Fat Bike - First Build

Discussion in 'Photos & Bicycle Builds' started by ColdCruiser, Oct 12, 2015.

  1. ColdCruiser

    ColdCruiser Member

    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)...



    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. :)

  2. ColdCruiser

    ColdCruiser Member

    AZ Beach Bikes - Fat Bike


    ...mine is black on black.
  3. Wolfshoes

    Wolfshoes Member

    Here is a link to a floating smooth spindle available in different lengths.


    This may not match your bottom bracket or pedal crank arms. I did bracket work like that in Jr High back in 1969. From that ancient experience I could remind that as the pedals go further out, they are more likely to bottom out in a turn and the arms may need to be shorter.
    ColdCruiser likes this.
  4. 2old2learn

    2old2learn Member

    It will likely take longer than you think to get it totally finished. But once completed it won't matter. That should be a very capable build and one that you will truly enjoy.
    ColdCruiser likes this.
  5. ColdCruiser

    ColdCruiser Member

    Thanks Wolfshoes, I did not consider the pedals scraping on turns. Also, the link you gave looks like what I might need except the spindle is still way too short.
  6. ColdCruiser

    ColdCruiser Member

    Can I cut the shell and re-thread it to fit a more narrow bottom bracket? Any other suggestions?

  7. Wolfshoes

    Wolfshoes Member

    This is out of my pay grade since I have almost never worked with bottom brackets, but here is a suggestion. The smooth spindle in the White Industries assembly is mentioned as being 17 mm. If 17 mm is the spindle size you need; google "17 mm bar stock". Hardened precision steel rod would be the best. Drill rod may be too brittle, Chrome or stainless steel too hard to grind. Either a machine shop could shape the ends or you could do it yourself, but it would not be easy. Hardened steel will grind with common silicon carbide wheels. The piece could be cut to length with a grinding wheel in a circle saw, then be clamped to a board. A grinding wheel installed in a table saw could be used to grind the square ends, but since the square ends are at a taper, the piece clamped to a board would also have to be at the taper angle.
    ColdCruiser likes this.
  8. butterbean

    butterbean Well-Known Member

    You don't want a narrower bottom bracket. You want it as wide as possible to clear the engine and transmission.
    ColdCruiser likes this.
  9. ColdCruiser

    ColdCruiser Member

    Butterbean, thanks for answering but please explain why it would make any difference if the bottom bracket is more narrow if the spindle is the same length or longer?

    Wolfshoes, I contacted white industries about the bottom bracket suggested and they replied that they are unable to help me with a custom spindle length and should contact Phil Wood. I then contacted Phil Wood and they said if I ship them my bike they would make a custom spindle for $320 plus shipping.


    I have some basic hand tools and power tools but I highly doubt I'd be able to fabricate a spindle myself. Also, $320 to have it done is not a reasonable option either.

    Can I cut the spindle and weld in a piece to make it longer?

    Would narrowing the BB be more or less difficult than fabricating a new steel spindle?

    Can I cut off the current BB and weld in a new more narrow one?

    Bottom-Bracket-SM.jpg IMG_2366-SM.jpg p1pp_phantom_bb.jpg
  10. butterbean

    butterbean Well-Known Member

    My apologies, I misunderstood. You can cut out the bottom bracket and weld in a narrower one. However, what determines whether the pedals will clear the engine and trans is something called the Q factor. The Q factor is the distance between the crank arms at their widest point. I personally think the spinle you have will give you the clearance you need, but if it does not, you can buy offset crank arms. Sick bike parts sells them I believe. An offset crank arm is one that will fit a standard spindle but still have a wider q factor because the arms are further out at the ends.
    ColdCruiser likes this.
  11. ColdCruiser

    ColdCruiser Member

    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?

    Custom Motored Bicycle CRANK AXLE KIT.jpg

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

    ColdCruiser Member

    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?

    IMG_2377-SM.jpg IMG_2380-SM.jpg