Originally Posted by Bill in Oregon
I was hoping someone would define jackshaft ...
Actually I think the term Jackshaft comes from the mechanical means of 'Highjacking' a given motors output via a shaft to change the side and/or direction of the motors output.
In the case of motorized bikes that would be highjacking the left side counter clockwise turning motor output via a shaft and 2 sprockets over to the right side where your pedal cranks and reversing it to clockwise like your pedals so you can tie the two together.
Why? So your motor can use the bicycle drive chain and gears ;-}
If you have a clockwise spinning right side mount motor you don't need one, all you need is the freewheel pedal crank assembly and sprocket to hook it up.
Here are 2 examples.
This bike is a 4-stroke left side CC turning motor with a Jackshaft.
This bike is an electric right side clockwise turning motor that doesn't need to be 'highjacked'.
In both cases the bicycles fixed pedal cranks are replaced with freewheeling cranks.
It would be nice if you get a cheap 2-stroke motorized bicycle motor that was like that electric then with the just the freewheel you would have a bike with just one drive chain from the pedal side.