2008 Specialized Fatboy Conversion

Discussion in 'Frame Mounted Engines' started by 1968gtcs, Jul 14, 2008.

  1. 1968gtcs

    1968gtcs New Member

    Just picked up this jewel this weekend. Plan on cutting the downtube(seat frame tube) from the bottom bracket and pivoting it about 1 foot further back to accomodate for the motor. Going to weld up all the vent holes and cable routing holes in the faux tank part and just use it for my gas tank(plenty of volume, maybe 1 gallon). I'll document the conversion for all that want to see how to do it.<br>
    <img src="http://www.bikepedia.com/Helpers/readImage.aspx?filename=2008-Specialized-Fatboy-Ti.jpg">
    <img src="http://www.motoredbikes.com/attachment.php?attachmentid=9107&d=1216042565">

    Attached Files:

    Last edited: Jul 14, 2008

  2. azbill

    azbill Active Member

    man, you gonna cut up a 1800$ bike ?
    you'll need to offset the engine quite a bit to clear the HUGE back tire as well :D
    the local LBS has one on display...I drool all over it every time I go :lol:
  3. 1968gtcs

    1968gtcs New Member

    Sure, why not? Its a lot more fun to ride with an engine:)
    Last edited: Jul 14, 2008
  4. Nice bike

    You could save yourself some cutting by setting it up like this.


    That frame would be perfect for this mod. Weld another bottom bracket into the frame just behind the seatpost. Then you could use a quality motor too. The Robin 35, Mit 43, or Honda 50. Look at my build for Zack. You will have to get a wider crank to clear the motor but you can get a Grube wide crank for 50$ & only extend it on the right side like this.

    Last edited by a moderator: Jul 14, 2008
  5. 1968gtcs

    1968gtcs New Member

    would any of those motors fit in that small space behind the seat post? also, do you have to use some sort of freewheel crank? Will normal bicycle chain be strong enough to drive bike or will it snap? Could you draw a little picture on top of my picture of what you have in mind for the mounting? Can you post link to your build for Zack?
    Last edited: Jul 14, 2008
  6. link

    Here is the original thread. This bike has gone through some more mods but the drivetrain remains the same.


    This is another frame a lot like yours but a lot easier to modify than from scratch. I think any of these motors will fit forward of your seatpost.


    The only mod on this frame was the extra b bracket & motor/gearbox mounts.
    Last edited by a moderator: Jul 14, 2008
  7. 1968gtcs

    1968gtcs New Member

    Which motor do you think will fit the best? Also, where is the best plact to buy all the sprockets stuff? Thanks for your advice:)
  8. Motors

    The two strokes are the narrowest. I put a 4 stroke Robin on another bike like this & had to widen the crank on both sides. You can't go wrong with a Mit 43. Zack has a GP 460 now, but if I were starting over I would use the Honda 50, & deal with the width issue with a wider crank. You could use a HT, but after spending 1800 for the bike, why not do it proper? A HT motor is definately the narrowest, but the kit will not fit your bikes frame, and you will have to sacrifice that nice rear disk brake. So you're in for some mods no matter which direction you take. I like FRAME MOUNTS. Your bike deserves a nice one.

    Again - sorry if I offended any of you HT fans out there. But I'm not a fan..............................
  9. 1968gtcs

    1968gtcs New Member

    I spent a lot less than $1800 for the bike i have....I'd shoot myself if I spent that much on this bike.....I really like the way you did the other bike......I'll take more pics of mine around the seat post when i get home to get your advice on where to put the 2nd bottom bracket with the jackshaft.
  10. jackshafts

    If you do choose to go with a HT - that jackshaft kit that GhostO has looks like it uses a lot of the same components that I use & it would be an easier install. I just looked at the specs . Your bike is Aluminum. welding another bottom bracket will be a trick. Can you weld AL? If so then it is possible. It also has a 3 piece crank. The Grubee wide one piece crank won't work on that because your front bottom bracket is smaller in diameter. Are you going to install gears? All these things must be considered before committing to a finished design. I posted before I looked at your bikes man. specs. Sorry bout that. I can't weld AL or I would build one of these too. Very good start for a motored. I really like the way this bike is made.
    Last edited by a moderator: Jul 14, 2008
  11. Jax Rhapsody

    Jax Rhapsody Member

    WHy would you spend 1800 on a bike that doesn't excel in anything but goin slow? I think disc brakes are wasted on it. It's a cool bike and with a powerful motor would be fun to ride. I remember when the sting ray came out I was like I know that back tire can be used for something else.
  12. 1968gtcs

    1968gtcs New Member

    Which jackshaft kit is GhostO's? Do you have a link? Where can I buy the jackshaft kit? Yes, I tig weld aluminum. I'd like to keep to 1 gear because its illegal here in TX to have a geared motored bicycle. The way the bottom part of the frame is designed doesn't look like it gives me clearance to put a jackshaft on it?

    <img src="http://www.motoredbikes.com/attachment.php?attachmentid=9188&stc=1&d=1216102195">

    Attached Files:

    Last edited: Jul 15, 2008
  13. 1968gtcs

    1968gtcs New Member

    It doesn't use a stingray tire.......this bike has 24'' wheels, the stingray has a 20'' back tire. This isn't a kid's bicycle like the stingray. This bike is built with VERY high quality, thats why it is pricey ($1800 was the 2007 Fatboy price, this is a 2008 Fatboy with a MSRP of $2800).
    Last edited: Jul 15, 2008
  14. plenty of room

    You could do it. See how close this one is.



    The problem that I see in doing one like mine is your crank will not be wide enough to clear the motor unless you machine another longer center shaft for your crankarms.

    Here is one of GhostO jackshaft kits.


    He & Pablo are selling these as a kit. Looks pretty impressive. Would work fine on your bike to transfer the drive to the crank sprocket. If you choose this design you will have to use the HT motor.

    That is a really nice looking bike.
    Last edited by a moderator: Jul 15, 2008
  15. 1968gtcs

    1968gtcs New Member

    So with this kit, its just transfers the engine crankshaft sprocket from the N.D.S. to the D.S. of the bike? Isn't the stock sprocket too small to allow any pulling power? Shouldn't I have a 36t sprocket or something that then engine turns, or does that jackshaft connect to a sprocket on the crank sprocket that turns the rear wheel independently from pedals?
    Last edited: Jul 15, 2008
  16. transfer

    Yes all this kit would do for you is transfer the drive side. They use it on shifter bikes, but you don't have to have a shifter to use it. You should ask GhostO about gear ratio options. I think they have the ratios correct the way it is -- Too bad that this kit only fits the Happy Time. It is made to work with the three piece crank.
    Last edited by a moderator: Jul 15, 2008
  17. 1968gtcs

    1968gtcs New Member

    great! This will allow me to keep my disc brake and I won't have to weld canti bosses on the frame for v brakes. Does the crankset come with the jackshaft kit?
    Last edited: Jul 16, 2008
  18. jackshaft

    I think it comes with everything you need. Ask Pablo & GhostO.
  19. 1968gtcs

    1968gtcs New Member

    Using a jackshaft will allow me to keep my rear disc brake. Thanks for pointing me in the right direction.
    Last edited: Jul 16, 2008
  20. 1968gtcs

    1968gtcs New Member

    I've done some research on the jackshaft for my bike and it seems that from my current gearing I've calculated from the following assumtions:

    Engine max RPM under load----5850
    10 tooth 1.6inch diameter engine crank
    18 tooth rear freewheel 2.9 inch diameter
    24 inch wheel

    Top speed came out to be 73.86 mph......which is nonsense because that gearing would never get the bike moving. I've adjusted these assumptions in my model and find that this would work out a lot better:

    8 tooth 1.3 inch diameter engine crank
    23 tooth rear freewheel 3.7 inch diameter

    This gives me a theoretical top speed of 46.2mph, meaning that I think I will be ok on low end torque and still be able to pedal it @ 50 rpm @ 2.08mph.

    Any bigger freewheel and I can't comfortable get the bike up to speed.

    I also think that the kit that guy is selling is going to be very bad on low end torque.......it leaves the front crank spinning at 1330 revs/min which is going to tear that china freewheel crank up very quickly.......and the rear freewheel in the lowest 28t gear is spinning at 2089 revs/min.......26 inch wheel going at 51.4 mph in that gear, going to obliterate a cheap freewheel cassette in no time......in 18t gear bike is going 80mph........this is just not a feasible gearing setup......going to be very interesting to see what people think about the kit. Plus, there isn't really any reason to use a freewheel crank.......why not just use a freewheel on the jackshaft kit and you can use your stock crank?
    Last edited: Jul 16, 2008