Planes can stall. A plane maintains lift through speed. If the speed drops below a threshold, the plane stalls and begins a downspiral.
Now that many 1x modern bikes are discarding front derailleurs, it’s becoming almost common to see lavishly large rear sprockets. When the teeth number 50-52, it starts to look like a chainring.
At the same time I read, with a bit of amusement mixed with bewilderment, cyclists wondering whether they can swap the (single) 34-tooth chainring on such bikes with a 28-tooth chainring.
This makes me wonder how far the gear ratio can go.
If the ratio can approach 1:3 (front : rear), can a bike be usefully fitted (leaving out derailleur feasibility from this discussion) with a 1:4—or a 1:5—ratio? Will the cyclist at some point not be at too high a risk of stalling. How far would manufacturers go? ("Your Honor, my client cracked his cranium riding; he didn’t expect the maker to assume he’d be an acrobat, not a cyclist.")
But if we ask simply about the minimum speed for a cyclist to maintain balance, then each of us will recall the folks who can seemingly balance on the spot indefinitely while waiting for the lights to turn green. Perhaps you yourself can do it.
Hence the “minimal speed that still permits maintaining balance” is a simple Zero.
A twin question to the minimal speed question is why anyone would want, say, a 1:5 gear ratio. We could speculate. On a stationary bike it’s possible to vary the workout to a precise “n-minutes in HRZ 5, followed by m-minutes cooling down to HRZ 3”. On a moving bike the slopes dictate the workout, and a very wide gear ratio might help design a specific workout despite the terrain.