Hi everybody, so I was just letting my thoughts drift a way for a second I came across this ridiculous idea. Geared Unicycles already exists and freewheelers exists too, but a combination of those two, does that exist? Is it really possible to manufactory one? And if so could I use the back wheel/hub from an old bike and then make a unicycle out of that? Any ideas or help would be appreciated, thanks!
Not sure how you would mount it in a unicycle, but the Truvativ HammerSchmidt AM Chainset is in the direction with planetary gears: Lying at the heart of the Truvativ HammerSchmidt two-speed crankset is a set of planetary gears. A trio of pawls engages or disengages those gears instantly with a flick of the trigger-style shifter to swap between gear ratios of 1:1 or an ‘OverDrive’ ratio of 1:1.6.
Almost the same gearing as a Schlumpf (1:1 & 1:1.55).
Originally cost something like $700-$800, not sure if there’s still being made and price point.
So technically I think it should be possible, but not sure if there’s anything available in the right hub/package to mount in a unicycle as the Hammerschmidt is designed for a crankset + chainring.
I think riding in high gear might be pretty sketchy as with a freewheel you fall off the back quite a lot and not sure I’d want to do that at 20+km/h. On the Schlumpf in high gear you usually do a “controlled” dismount off the front into a run. But then the freewheel itself is pretty sketchy so I guess that’s the same.
But why? I haven’t ridden a freewheel enough, but I would think a 29"+ with shortish (say 125mm or 114) cranks would be fast enough for most things without a higher gear?? For the flat not sure what the advantage/motivation would be. I guess for something like advanced downhill flow moderate speed riding in a bikepark could be pretty awesome (I had a 29" Schlumpf with 137mm cranks in a bike park last summer and that was way fun although physically taxing).
Mine does some brief, automatic freewheeling when I shift. Does that count? 
I feel like I might have answered this on Reddit earlier!
The MC2 and Kervelo hubs are probably our best bets:
https://www.kervelo.com/transmissions/gearhub-kernel-hub/
They’ve both recently become available (in theory, I’m still trying to get a price for a Kervelo).
The MC2 is 1:1 and 1:3 (yes 1:3), and the Kervelo is up to 12 gears 1:0.6 to 1:3.5 (if I’m reading the spec sheet correctly).
MUCFreerider, I currently ride a 29" freewheel and on technical offroad it’s a bit on the large side, but on light offroad/on-road I do fancy going larger.
I’ve got a 32" wheel to throw a freewheel hub into but no suitable frame right now.