Has someone built a penny-farthing (high-wheel/ordinal) with a gearbox in the hub? If not, are there obvious reasons one cannot do such a thing?
(I recognize this is silly, but that never stopped people from doing all sorts of other things.)
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Yes, there exists at least one which was made/improvised with a Schlumpf Innovations bidirectionally-fixed internal-gear unicycle hub. If I recall correctly, it had a 36 inch unicycle wheel, giving an effective diameter in high gear of around 54 gear inches, fairly typical for larger penny.
The structure and silhoutte of course were odd, with the relatively small physical wheel it might be termed a "mini-penny", the actual fork was an old Nimbus Nightfox unicycle frame squeezed to fit the 100mm bearing spacing of the Schlumpf hub vs the 125mm spacing of the Nimbus Unicycle hub it was designed to hold.
Edit: build description from the creators, alas, no pictures.
There also is/was the "bicymple" which has a fairly small difference in wheel size for a "penny" but does have the bigger wheel up front. It freewheels, but one of the models incorporates a freewheeling version of the Schlumpf hub.
The raison d'etre of the Penny-Farthing was to go faster by using a larger circumference direct-drive front wheel in absence of geared bicycles. Once geared "safety" bicycles came onto the scene and solved the problem of going faster more elegantly- and safely- through the use of gearing in lieu using a huge front wheel it all but killed-off the much beloved PF.
Peddling pretty briskly on my 50" Penny-Farthing, I can maintain a nippy 15.5 MPH for 30 miles; just under 2 hours riding. Even if I could double the speed of my PF through gearing, I'd be disinclined from doing so as I'm sat with my weight over a direct-drive wheel. Firstly, there's the risk of doing a "header"- it was the Penny-Farthing where the term was coined- and secondly, stopping! Imagine being presented with a hazard on a PF at 25-30 MPH on a PF. Ohhh, doesn't bear thinking about ;-)
Given the problem "safety" bicycles solved- going faster more safely- it was counter-intuitive gearing a Penny-Farthing considering the dangers of riding them even at slower speeds. I wouldn't be interested in buying a geared PF even if they sold them. This is why you're probably struggling to find PF's with gearing I'd imagine.