An idea

[Jwestie15]

Has anyone ever used the bottom bracket tube as a jackshaft bearing carrier, seems like it would be the best way to build a bike that shifts.

 

[gary55]

Has anyone ever used the bottom bracket tube as a jackshaft bearing carrier, seems like it would be the best way to build a bike that shifts.

I don't know about the best way, but it's pretty hard to find a better, more reliable, and easy to install jackshaft kit than the one sold by SBP.

 

[FurryOnTheInside]

Has anyone ever used the bottom bracket tube as a jackshaft bearing carrier, seems like it would be the best way to build a bike that shifts.

As far as I know, not with functioning pedals.
I thought there may be a way to do without the jackshaft and just use the rhs of the freewheel cranks for ratio reduction and freewheeling, but that would require running the engine in reverse and being able to get it positioned just right, perhaps using extra tubes welded into the frame. You'd have to deal with intake pointing down and forward and the exhaust pointing at your nuts.