FWIW, The 20" Black Domina 2 I bought at Maijer.com came with 135mm cranks. 125s would have felt real short, but I can see how they would have appeal as I get better.
Reflecting on my limited experience so far (15-20 hrs practice, can consistently ride 50 yds, usually turn, and almost free mount) I’m changing my opinion on what the ideal first unicycle should be. If you’re committed, and willing to do the 20 odd hrs it seems to take most people to reach any semblance of “riding” a unicycle, you’re going to figure out how to ride on pretty much anything. A good seat will make the journey more comfortable, smaller wheel/low friction tire/longer cranks might make learning a little bit faster, but you’re going to get there.
But, once you can say “I can ride a unicycle” what is going to keep you riding? Tricks? Showing off to friends? Getting a friend to ride too? The ability to ride to the corner store, or local trails? I think the answer to these questions is quite a bit more important in what you select than what’s going to get you through those first 20 hrs fastest.
Of course, if your goal is to convince someone else to learn too than you perhaps do want to focus on the easiest learning bike. In this case I know what I’d get: a 20 incher, and I’d buy a set of 150mm cranks from Unicycle.com - they’re only $20.00 for a cheap set, which is all you’re going to need to start with.
I’m certainly no expert, but I might be able to clear some confusion surrounding crank arm length and wheel size. At the very least any misinformation I give should cause the real experts to come out of the woodwork to put me in my place and answer your question in the process.
As far as ease is concerned I believe it’s an issue of the ratio between the crank arm and the wheel size. So if the wheel size remains constant longer cranks will make it a bit easier to scale hills and/or overcome obstacles. If the crank size remains constant smaller wheel size would make it a bit easier to scale hills and/or overcome obstacles.
Now in your example you changed wheel sizes. Larger wheel sizes typically come with longer stock cranks to maintain what I’d imagine to be a default/sweet spot ratio, so a 20" uni with 125mm cranks might have a similar crank length:wheel diameter ratio as a 24" uni with 140mm cranks, so it comes out being roughly the same… if any of that makes sense. Probably not.
I’ve heard that it is easier to learn on a 20" unicycle, but I wouldn’t know. I learned on a 24" with 5.5" cranks (we used inches back in the day and we liked it!). And yeah, ^ was good advice. Might as well get the unicycle based on what you are interested in since you should be able to learn just fine on a 20" or 24". Trails or trials.
I think there’s another aspect to consider, particularly with us beginners: a longer crank arm gives you more turning leverage, regardless of wheel size. This seemed to help keep me from falling over sideways because the uni “wobbled” more as I pedaled, and I could correct by how hard I pedaled.
I mentioned this in another thread, but I’m currently practicing on my son’s SUN 20" and I believe the cranks are 114mm (center to center). My feet keep slipping off the pedal because it feel like my legs expect a larger diameter of motion. Thus, my question about longer cranks.
I noticed on some unicycles that there is an option for the KH cranks that have two holes (e.g. 125mm/150mm) as on the Nimbus Muni cycle. Might this be a good idea?
I’m thinking, too, that a muni would handle the abuse and I can always change to a less knobby tire for street use…
I think this is true. 20" or 24" wheel and 125mm or 150mm cranks are fine. Any combination will work.
Since you already have some idea of what you want to ride, you should go for it. Don’t worry about how long it will take to learn. It’ll take a while no matter what you buy. Besides, learning is a pretty small blip on the radar compared to how long you’ll be riding. If you practice every day, you’ll be able ride around in a few weeks. If you’re into muni, you’ll be out on the trail in a few months. So get the unicycle you want to ride. The Nimbus muni you mentioned with 150mm cranks would be a good choice.