I’m wondering if it’s at all possible to build a unicycle with a Shimano Alfine hub as an internal gearing system. That’s to say, build one with a freewheel, disc brakes, and a Shimano Alfine hub, without using a cassette at all.
Just curious, I was reading up about Chinese wheelbarrows (single, large wheel in center) and I thought: hey, maybe unicycles, properly built and ridden, could be superior to bicycles as a mode of transportation.
You don’t need a cassette but can use a single sprocket (I have one on my speed bike with no cassette). However, the Alfine assumes a chain and as far as I know has no way to attach the pedals directly. I don’t know much about it, but I’ve seen GUNI designs with a chain that would probably work with the Alfine. But even then you would still have a freewheel, so it is not trivial to ride (see a recent thread about freewheel experiments, I forgot from whom).
I’d imagine there’s an extremely elegant way to link the pedals to the Alfine; use a chainring with a 1:1 sprocket size and ratio to the Alfine, then directly connect each element of the chainring to the Alfine’s sprocket. Presto, no chain necessary, unless you’re aiming to use a specialized chain with wide gearing to do the connection.
What I’m more worried about is that the Alfine won’t permit pedals to be threaded through the hub, however, or that having three independently moving units (wheel, frame, pedals) will require custom work.
Okay, so far, what I have is that it looks viable to connect the 23T sprocket on an Alfine to a 23T chainring for a pedal. The sprocket is 1/8 - 3/16 inches wide, and there are chains, albeit not bicycle chains, that are produced in 1/4 to 3/8th inch widths, so it should be possible to link the sprocket to the chainring. However, you will need to remove the chainguard and install your chainring in lieu of the chainguard.
The challenge now is, how do you link the chainring to the pedals?
The architecture looks like this:
frame attachment -> Disc brake -> spokes -> hub -> spokes -> sprocket -> chainring -> frame attachment -> pedals
Conventional bicycle pedals, however, assume that there are no obstructions between the chainring and the pedals. I need to find some kind of pedal that can directly attach to the chainring without obstructing the frame attachment. How do I do that?
Still looking at the problem; I think the way it could work would be if you’d set up the skewer to function as a transmitter for the crankset, connecting the crankset to the chainring, but that would be challenging because skewers typically aren’t designed to withstand crankset forces. Beyond that, you’d need a bracket construction to connect the skewer to the frame instead of directly being able to connect the frame to the skewer… this probably is not doable with off-the-shelf parts.
There is the show stopper for trying to use the hub in this way. There simply isn’t space to fit anything wider through the hub, and no material which will provide adequate strength and stiffness to transmit drive torque with a shaft that thin.
Which only leaves the options of completely rebuilding the hub to make more space for an axle (I don’t know if that is even possible whatever tools you might have), or having the hub on a jackshaft, which is how geared bicycle hubs have previously been used on unicycles.
I’m not sure, how do freewheeled-unicycles work, or geared unicycles for that matter?
The way I understand it right now is that if you consider existing unicycles, it’s probable that there are chainsets that work off skewers and are compatible with existing hubs, because that’s needed for giraffe unicycles and freewheeled-unicycles. The only thing I can think of that still poses a significant challenge is the ability to connect the chainring to the skewer, but I suspect that issue has already been solved somewhere.
It is Waaalrus that documented his experimentations & progresses with freewheeling unicycles.
And JustinLE posted his experimentations with bike internal geared hubs with along with plenty of pics & tech details. I his case, he got rid of the freewheeling part to avoid making things even harder to ride. The results are fun but you need some craftmaship & proper tooling.
They use hubs with an axle through the middle designed to transmit drive torque - in the case of the freewheel projects, I think they’re hubs from front drive tricycles. A Schlumpf has an ISIS axle through the middle. These axles are much larger diameter than a skewer.
Nope. There is no chainset which works with a skewer or anything of similar thickness - as I said above there is no material which will result in sufficient strength and stiffness to transmit drive torque through that narrow a rod. I don’t know how you think giraffes work, but in general they actually use parts which are fairly close to standard bike parts - the cranks are attached to a standard bottom bracket, and the hub doesn’t need to transmit drive torque through the middle.
I’ll do a quick calc for you in case you still think you can connect cranks with a skewer:
A skewer is 5mm diameter, let’s say 150mm cranks an 80kg rider and a 100mm long “axle” skewer. When he stands up on the pedals there is 40kg on each pedal…
torque = 409.8.15 = 59Nm
moment of inertia of skewer = pi * d^4 / 32 = pi * (0.005 ^ 4) / 32 = 6.1 * 10^-11 m4
angle of twist = (torque * length) / (moment of inertia * modulus of rigidity)
modulus of rigidity of steel = 7.9 * 10^10 Pa
so angle of twist = (590.1) / (6.110^-11 * 7.9*10^10) = 1.2 radians = 69 degrees!
I think you’ll agree that the cranks twisting relative to each other by 69 degrees the you stand on the pedals isn’t very desirable! I suppose you could use a diamond “skewer” which would result in only 11 degrees twist.
For reference a standard square taper BB axle is 17mm diameter, which would result in about half a degree of twist for a steel axle.
Superior in size and weight (smaller; lighter) but not in speed. So depending on your needs, a unicycle can be an excellent substitute for a bike. But if you need to cover a lot of ground in not a lot of time, the bike is going to win no matter how much tech you put on the uni. Not counting self-balancing systems, of course, but self-powered geared drive systems get more and more touchy as the gear ratio goes up, to the point of being pretty unstable when you get in to bike gearing ranges.