Harbor Freight 79cc jackshaft build

Discussion in 'Frame Mounted Engines' started by atombikes, Jan 10, 2010.

  1. atombikes

    atombikes Member

    Hi All,

    Very new to the forum, but have been a framebuilder for a number of years. Most of my work has evolved around recumbents. I recently purchased one of these Lifan 79.5cc Harbor Freight Greyhound engines on sale for $71.99. I anticipate a period of planning and development, and hope to have something at least in a prototype stage in a couple months.

    So here goes: I plan on building a frame that will enable me to use some sort of jackshaft thru a second bottom bracket on my custom frame. Will a normal 3 piece crank be adequate for the forces, and if so, how do I do this? Using a jackshaft like this will enable me to run the engine thru a seven speed freehub on the rear wheel.

    I plan on using freewheeling cranks, so the cranks will freewheel when the engine is under power.

    Does this sound like a plan?

    Here is a pic of my early CAD drawing. My plan is to try to use the HF engine as intact as possible, to make the build easier.

    <img src="http://lh6.ggpht.com/__jCR7Bfw3Fk/S0oRm3OsfzI/AAAAAAAAAI4/qZJSxg_owjw/s912/FUSION_MOTORED.jpg" />
    Last edited: Jan 10, 2010

  2. Elmo

    Elmo Member

    It is going to be difficult to get the gear reduction that you need with a single jackshaft. You will need about a 9 to 1 reduction. You could possibly use a reduction like MaxTorque offers as a primary 2 to 1 reduction. http://www.maxtorque.com/html/reducers.html
    I am building a prototype single speed drive but it is a different idea than yours.

    Attached Files:

    • gh6.JPG
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  3. atombikes

    atombikes Member

    Elmo, so your drive will be to the left side of the rear wheel? And your engine will be in the space behind the seat? That's pretty good. I think I would have more space to mount the engine if I put it in a recumbent frame, but I would like this build to be more "traditional", mainly because I want the seat height for visibility in traffic.
  4. atombikes

    atombikes Member

    Here is an alternate config that shows the chainline (or tries to).

    <img src="http://lh5.ggpht.com/__jCR7Bfw3Fk/S0pClTehEoI/AAAAAAAAAJY/BxznqGyLUfs/FUSION_MOTORED2.jpg" />
  5. Elmo

    Elmo Member

    Yeah the engine chain is going to be on the left. Normally I ride recumbents and this is the first diamond frame that i have worked on in about 15 years. If it works out well I am going to build another toureasy type recumbent and try running it through the gears as you are doing. The recumbents are so much more comfortable that it is no choice for me. With an engine who cares if it is a little slower to pedal up a hill. Besides that I have hit 50 mph downhill on my pedal toureasy clone and it was rock solid.
  6. GhettoBike

    GhettoBike Member