OK, nobody has any ideas of cantilevered rear wheel bikes other than the Goldwing and the BMW?
If no one knows about the cantilevered rear wheels, maybe someone can off a 90 degree gearbox that would withstand the torque of the Harley motor with the load of drive sprocket to the rear wheel.
It does not have to be a reducer gearbox as I can play with my ratios with the sprockets.
Any ideas?
This is the new Morgan Three Wheeler.

This is a '30s Morgan Three Wheeler. I plan on making mine a bit more like this with doors rather and climbing into it. A Harley "B" motor will be used. An '06 or older to get the 4-bolt primary flange rather than the new '07 and later bolt pattern. I can buy the "bell housing" through a Morgan dealer and then make up my own compensator to mate the two together.

Here is the S&S 2L with the Miata 5 speed mated to it.

Same but different view.

Inside driveshaft connected from the trans to their 90 degree gearbox.

Rear view of their 90 degree gearbox mounted with their drive sprocket installed.

Front view of their gearbox mounted.

So if don't do the Goldwing GL1800 complete swing arm assembly, I'll be doing my own swing arm and have to have a 90 degree gearbox for my drive.
Like I said, the Goldwing final drive is stout enough on the output side because it supports the weight of the bike, no worries mounting a sprocket to that.
Here's a picture of just the final drive on the Goldwing. It has a nice 4-bolt mount on the front that bolts to the swing arm housing, so it should be more than strong enough to mount in the chassis. It has a 85mm 5-bolt wheel pattern, so making a hub and sprocket mount would not be a big deal. I can draw up in Cad and then send to our machine shop to have it made up. There's no oil in the front shaft area, and I do have a drive shaft that slides right onto it that I can modify down to a mounting flange with bearing support. This would give me something to bolt my main driveshaft to.

If anyone has any ideas or suggestions, please feel free to interject at this point...
