Does anybody know of 18"-24" unicycles with fixed gears, eg to make them pedal / speed like a 36" unicycle?
My goal would be to commute with it, and then I would like to have some more speed than I can get out of the 20". On the other hand, I would love to keep the unicycle sufficiently compact to take it along on a train / plane.
What would be the difference between biking eg an 18" unicycle with a 1:2 gear and a 36" unicycle? What would be the cruising speed I could get out of it biking normally?
This came up in the recent thread about giraffes recently (linked below). I also posted a video of my penguin (a geared 19" trials wheel, 2:1 ratio with 150mm cranks, similar to a 36er with 125 cranks) as well. There is no off-the-shelf solution. If you want the speed of a 36 in a more compact package, a schlumpf hub is the way to go if you can afford it. It’s very close to a 1:1.5 ratio, so a 24" wheel is geared like a 36. They are more difficult to ride than a typical ungeared big wheel, but not unreasonably so. If you want a cheap, portable, durable solution, a 29" wheel with short cranks is a great option. My penguin is slower than my 36er, but it will cruise above 10 mph.
A schlumpf only has a 1:1 and a 1:1.5, so you don’t get a ton of value out of a smaller wheel, but someone on the forums slapped on in within this year. That thread shouldn’t be too hard to hunt down. It basically turns a 20" into a 29er.
The third option is a jackshaft, which I haven’t seen done on such a small wheel. (you would have to mount the cranks above the hub, where on most jackshafts, they are mounted below to lower the center of gravity I would guess.) Unicycle.com sold a 26" jackshaft called a Huni-Rex I believe, but the cranks were so low to the ground that pedal strikes while turning was a problem. You can still find them on sale used every once in a while.
I have found that despite what wheel you’re on, your cruising speed will be limited by how brazen you are, and the terrain you ride. For smooth roads, and a geared 36er (likely the fastest solution outside the jackshaft with large gearing), you can get it up above 20 mph, but I was never able to warrant keeping that speed for too long. I generally kept it around 16-18 on smooth bike path. TL;DR: never as fast as a real road bike, but enough to go on long rides or get you around town.
There’s at least one CVT uni with an electric assist that Justin created, but I’m willing to wager nobody is going to consider that as an actual commuting solution.
here’s a few links that you may find helpful:
edit: for clarification, the jackshaft in the first link actually has the pedals in the same location they normally would be and uses a different mechanism than the huni-rex, which, only having one chain on each side of the wheel, forces you to move the cranks. See the link for a more detailed explanation of the jackshaft. The huni-rex is pretty self-explanatory from pictures.
You kind of covered it later in your post with your link, but a few people have made jackshaft unis with the cranks in the usual place - it’s not that technically difficult a solution, nowhere near as complex as a planetary geared hub.
There is a You Tube video of the Unigeezer in a high-speed, head-on collision with the bottom of a stairway on a 36. He just rides straight up the stairs! Trying this trick on an 18" wheel would obviously give you a very different result! That’s the main difference I can think of, and it’s why geared hubs are usually only installed on 29" or 36" wheels.
When I went from A G26 to a G29+ I noticed a gain in stability in high gear, so it seems that the bigger the wheel , the less sensitive to bumps and other floor irregularities (so easier to ride will the the guni be) , that’s why I’m considering putting my schlumpf hub on a 36er one of these days.
So I don’t know if gearing up a small wheel is “great” to cruise faster, I don’t think so, but some guys like Corbin make miracles on a G24 in muni conditions, so… not impossible, but like others say it’s not like a real 36er.
Thanks for all the reactions, links and information!
Summarising, the experience on a “fast” 20" will never be as good as on a real 36". Could that be e.g. compared to the experience riding a Brompton foldable bike (16" wheel size), and a normal size bicycle?
I really enjoy the different options people come up with to gear up their unicycle. Nevertheless, I would be in favor of a planteray (hub) approach, as this would spoil the least to the visual simplicity (for me, I guess this is personal).
In the meantime, I have seen an original concept to add a planetary gear to a trike. Would there be any chance of getting this to work on a unicycle (especially given the high torque I read about everywhere)?
To your first question: no.
A small wheels foldable bike basically behaves like a normal bike, it’s just not as fast.
A geared unicycle doesn’t behaves exactly the same way as a non geared one, since the wheel gives this feeling that it “wants” to go in front of the fork, as you push to compensate it wants to go behind the fork and so on.
This phenomenon is obvious when you freemount a g26 in high gear for exemple, until you go above a certain speed.
In the 0-7mph range it feels like practicing rodeo.
When a certain cruising speed is reached it starts to feel more natural but the balance window is still thiner than if you ride a un geared big wheel.
And this is on a perfectly flat and even floor.
To this you have to add the bumps and other irregularities of the terrain that will perturb the rotating speed of a small wheel more than a big wheel.
Using a disk brake as a clutch seems like a cool way to do the shifting.
It is, though the design seems to be missing something as you need 2 “brakes” locking different elements if you want to be able to shift as otherwise one part will just spin and you’ll get no drive when you release the brake.
In answer to the question of getting that to work on a unicycle - yes it would, as it’s almost exactly how a Schlumpf hub works. Though in that case the elements are used differently with the sun braked and the hub attached to the ring gear. That makes for easier packaging inside a hub shell as the hub shell works as the outer part of the gearing system.
On his website, Mohsen Saleh talks about adding a ratchet to make sure that, without brakes, the ring gear doesn’t run faster than the cranks.
Thanks - I’d missed that bit, and it does make it an ingenious idea - though equally that makes it completely useless for unicycles where you have to be able to apply torque in both directions.
And I overlooked the fact you have to apply torque in both dimensions on a unicycle…
The Schlumpf switches gear by having two sets of internal pins that either lock the drive shaft to the hub shell or the planet gear carrier. There is a good picture of it in How does the Schlumpf hub work? thread
This system could potentially be adapted for other gear systems, though I haven’t heard of anyone even attempting it yet.
There is a glaring error in the design. “The gears are 42/18/12”.
Well designed gears use “coprime” ratios so that every tooth meshes with every other tooth on the other gear. The greatest common divisor of two meshing gears should be 1.
I see a bigger error than that…You cold fix that part, but not the bigger errors: It just does not work, as others pointed out you need a second clutch or a ratchet mechanism (which will give it freewheling in low gear,and no freewheeling in high gear, no pedaling backwards in high, if my thoughts are right.) I’d find that akward even on a trike. The designer of this has forgotten one thing: to be cheap, it’s easiest to use already available parts. This gearset would likely be as expensive as a crankset, chains and a mid tier shimano geared hub.
Honestly, as much as I love to dream about having alternatives, the Schlumpf hub will stay the best option as long as it is made. 1:1.56 is pretty good as a ratio for unicycles, if anything I would want the ratios closer together. A 20" Unicycle at 36" speeds is a terrifying idea to me, so sensible to bumps. I do not think there will be anything cheaper that is shiftable and not a jackshaft design.
Fixed gear hubs would be possible, I’d start with a 24" unicycle, and add some planetary gears, but unless you are looking for an engineering and fabrication challenge, a Schlumpf hub will be better, even if you are not looking to use the shifting function.
http://unicyclist.com/forums/album.php?albumid=42&pictureid=221
http://unicyclist.com/forums/album.php?albumid=42&pictureid=220
http://unicyclist.com/forums/album.php?albumid=42&pictureid=224
if you build one i will let you know how to shift 
Yes and no. Those two bikes definitely ride differently, but the main difference is the small wheels vs. large wheels, making for a harsher ride on the Brompton. The difference is much larger between the two unicycles. Not only do you have a larger change in wheel size (and weight), but the main effect you feel is the backlash and general sluggishness of pushing the gear without that “other wheel” to compensate. For a unicycle, it makes it considerably less stable, especially at low speeds.
G20
That was me. I haven’t ridden it so much recently because the cranks kept coming loose after tightening the crank bolts to spec. Also, the hub occasionally free-wheeled. Florian sent an email out to owners of the 800 series hubs regarding possible mechanical issues with them.
Another poster mentioned the stability of high gear on a larger wheel. The G20, conversely, feels pretty unstable. I went on some long, XC/muni rides on the G20. I had to concentrate very hard. Installing small bar ends helped with stability.
I wanted to learn how to operate a Schlumpf without riding at face plant speeds. I succeeded in that. After trying it, I don’t recommend it. If the hub ends up needing servicing, I’ll probably install it in a 24" or 26".