Re: Single speed or multi-speed bike
The jack shaft takes the chain drive from the engine and drives a shaft that then crosses over to the right side and then drives down to you frount sprocket useing a second sprocket which then can drive through the chain drive you now have on the bike.
There is a part that you install in the crank that keeps it from turning while the engine is driveing it. Other words you can still peddle your bike to ride it or start engine but after you are useing the engine your peddles will not be turning. Hope this helps i put this the best i can.