Originally Posted by Sheldon Brown
Talk is easy, doing it is not so easy.
Shimano has never made an 8-speed freewheel, and freewheels don't fit splined drivers.
If you really want to get the big numbers, you use a second internal gear hub as a jackshaft.
The Sprinter would be a poor choice for this due to its fragility and the difficulty of obtaining spare parts.
Now you could use a threaded driver on a Sturmey-Archer 5-speed hub, stick an 8-speed freewheel onto that. If you used one at the wheel, another at the jackshaft, you would have theoretically 40 at the wheel X 40 at the jackshaft X 3 chainrings...4800 speeds.
If you could figure a way to extend the axle past all of the sprockets, you could use Rohloff 14 speeds instead...
My 63 speed actually works.
Sheldon "Mostly Riding Nexus 8 Speeds These Days" Brown
No such thing as an 8 speed freewheel? I could have sworn that I heard someone say that 8-speeds are available in freewheel form, just not too common. Another legend...
I had considered pointing out a jackshaft drive as an example, but I didn't - technically, so long as you can keep jamming hubs into makeshift dropouts on the frame, you'll wind up with a wholly ridiculous amount of speeds, not to mention a ridiculous bike.
I was attempting to keep things in proportion of the maximum amount of gears someone could possibly mount upon most bicycles with minor modification.
True, if Rohloff made an extended-axle model, a 14+7+4 would be possible, but they don't. I wouldn't care to find out what the stress on that axle would be. Heck, make it a cassette, and stick a Shimano 10-speed cluster on it. 14+10+4. Five-hundred and sixty gears. Enough to bring every boneheaded Craigslister within 560 miles crawling on their hands and knees to purchase it.
-Kurt
P.S.: How many internal gears does the SRAM Dual-Drive setup have? Would probably be easier to base a crazy conversion on that, provided there are 6/6+ internal gears - anything less, and the Sturmey S5 becomes the best base hub for such a project.