I’m getting a 24 in sun, I was wondering what crank arm length would be good for commuting to and from work, I’ve never bought a uni before, do you get multiple crank arms with it or do you have to order another?
(why are bicycles more popular than unicycles again0
Depends on the commute. Distance? Hills? terrain? And your personal taste. How fast do you like to go?
It’s your first uni but you didn’t make it clear whether or not you can ride yet. If you’re going to be learning on it, I’d start with 140 or 150, but go no longer than 125 for commuting. Shorter will be faster, but may be less fun on hills.
Why are bikes more popular? Really? Short answer is because people can ride them. Also they can go faster, especially down hills, and they are easier to load up with gear. And they usually have multiple gears, parts are easier to get, and they generally cost less per wheel.
asking for “knowing how to ride already”, mostly on the sidewalk because no bike lanes but I suppose that there are a few steeper slopes around, no bumps though. 125 seems a bit low for a 24 but I’ll take your word on it
well these aren’t really reasons they’re not popular, these are BECAUSE they’re popular, imagine an alternate universe where unicycles and bikes are reversed and the same is true in reverse
They’re not popular not by some mystery of the universe, but because they are just hard to ride. And it takes a long time to really master them.
Which is not compatible with our “instant gratification” society.
Back on the subject. Commuting on a uni… it depends on the distance. If it’s just a mile, then I agree with John. 125mm cranks are the max you can use. 110mm on a 24 is a nice combo. That will still be slow and make you sweat after a mile though.
Again on the basis of “knowing how to ride” it’s not that short at all, not for commuting - I have 125s on a 29er I use for transport (also 125s on my Schlumpf 29er!) and 125 on my 20 I use for messing around. For covering distance on a 24 I’d probably go shorter, though TBH a 24 isn’t the ideal machine for commuting - if you can ride I’d suggest going for a bigger wheel.
125mm sounds like a good compromise for commuting with only mild hills. OP: What crank length did you have on your previous unicycle? If you make a big change in crank length, it’s going to feel weird. Cranks that are too long or too short can wear you out, but in different ways. Too long and you feel like you’re slogging, not enough resistance in the pedal stroke, less comfortable on the seat, slower. Too short and your legs wear out from the added leverage you need to provide for every little imbalance; you’ll go faster but feel closer to performing a face-plant. Search the forum and you’ll find riders who use much shorter cranks, for example, 110mm on a 36er. These are experienced riders with a lot of hours in the saddle, and they may have learned unicycling on a more middle-of-the-road setup, then gradually moved toward shorter cranks. IHMO, when in doubt, pick a crank length slightly longer.
Welcome to the forum. Nice screen name!
110 if you feel confident on them. 125 if you don’t.
My one way commute is 8.5 miles through the city on a 29er with 110mm crank arms. The route has 350 feet of elevation which inludes a one mile hill at 3-4% grade.
You’ll eventually figure out what works well for your commute.
UDC offers Kris Holm crank arms with two holes. I have two pairs that I used as I moved from 150mm to 110mm. I currently ride with the 110mm/125mm pair and have not moved the pedals off of 110mm hole for a couple of years. These cranks are ISIS and will not fit on a Sun. The Sun only comes with 150mm cranks.
Bicycling isn’t as popular as it should be. Just look at all the snide comments on local news websites concerning bike lanes. There’s a war on bicycles by some car drivers who don’t want to share the road.
125mm is plenty long enough on a 24", that’s what my learner 24 had 
Because you barely have to move your legs to ride a bike.
If you observe people riding bikes, you will see that most of them are coasting most of the time.
(Only exception is hardcore road-bikers.)
My Sun unicycle has a big 24x2.4" CST Cyclops tire and 114 mm cranks on it and that’s been a fun combo for casual riding around the neighborhood and nearby streets. It’s easy to control and I can keep up a pretty decent pace on it. Hills were fine after a little bit of practice.
I agree with the pros…shorter is better for a 24.
It depends a lot on terrain but 125 is a good all-round crank on the road. Despite being able to pedal faster, shorter cranks are harder to extract the full leg power potential on hills. However I find 24 a bit small for most road use.
My road unis have 100s on the 20 inch and 137s on the 36 while all the in-betweens have 125s and I ride them up to 18 percent grades.
I like 150s off-road (26 and 29 inch wheels).
I’d never use anything longer than 125mm for commuting on any wheel I think, but I don’t commute.
Unicycles suck at commuting. They attract unwanted attention, they require way more concentration to ride than bikes, you can’t have a bike rack to put your groceries, they are slow. I can hang with bikes speedwise on a 36", but: I can ride my bike onehanded, drunk and text on my phone in the dark if I want to, (on empty bike paths, not the city of course). That I wouldn’t be able to do on a uni.
As a student I could take my bike 5 km to school at 8 am, do school and 2 different sport trainings, and go home completely tired on my bike at 9 Pm. If I had a uni instead of my bike, that wouldn’t have been possible.
I’ve been unicycling for more than 2/3 of my life, but giving up my bicycle completely never seemed attractive to me.
To me, unicycling is a fun sport in all of it’s disciplines, but it’s a hobby, not a kind of transportation for me.
I worked for 35 years and for the last few months I commuted a short distance from where I parked my car. It was mainly cycle path with some short sections of road. I tried 20, 24, 24 muni, 26, 28, 29 and 36, and found the KH29 with 125s was the most versatile. Much smaller than 29 and you get sweaty and you get too many clown comments and jeers, but on the 36, you are vulnerable to every idiot on the path who isn’t looking where he’s going.
Clearly it’s not the most efficient way to cover ground, but then for most of my journeys a car is also faster than a bike! It depends if that’s all you want to do -I quite often use a unicycle for transportation, but then I’m often not all that bothered about being a bit slower. Some advantages over a bike which makes it a better choice for some things - for example if I’m going for a night out in town I tend to put a unicycle in the boot of my car as I can just throw it in rather than having to take wheels off a bike, I ride the uni home when I can’t drive. Also if going to the shops a uni is easier as I can carry it around the shops.
I’m also surprised about your comments about texting - I don’t very often text when riding a uni, but I can - wouldn’t want to try on a bike.
No argument that bikes are faster and generally more efficient, but I disagree that unicycles are not good (and fun) for commuting. For about 20 years I’ve commuted primarily by human-powered wheeled transport (bikes and unicycles) for distances of 2km, 4km, 5km, 6km, 9km, 10km, 11km, 14km, 15km, and 19km and in heavy urban settings as well as nice cycle paths in 5 different cities in 2 different continents, and depending on the setting, the unicycle can be a great choice. For close to 3 years I commuted 16kmx2 on a 36 unicycle and that was great!
I agree with aracer that (even though I don’t do it frequently), texting on a unicycle is both easier and safer than on a bike, as are other things that require 2 hands. On the other hand, something as simple a tucking in a loose shoelace is relatively easy on a bike (coast, balance and lean over) and almost impossible on a uni.
While for many typical situations a bike is faster and more versatile, especially for transporting lots of stuff, there are some distinct situations in which a uni can be better: going into/through a mixed-pedestrian area is generally less problematic on a unicycle (in Germany you will eventually get a ticket for cycling through pedestrian zones), combining with public transit or parking somewhere with little space (a uni can be leaned against a wall inside a college classroom or in your office).
With the advances in electric bikes, pretty soon you could reasonably argue that it would be inefficient and slow to ride a non-powered bike, but I and I think many others will still frequently prefer a human-powered bike as it’s more fun (and simpler with less to break) and the same logic applies to unicycles. For my current 2km commute the time I would save bicycling is so negligible (actually only in that I could sprint to 40km/h to make the traffic light) but the unicycle is way more fun 
Oh, as to the original question: like so many things, it totally depends (on the rider, the distance, etc.)
As someone else already said, for me anything smaller than a 29 is too small for commuting, and even with a larger wheel size, any crank longer than 127 too long for commuting (I used 100s, 114s and 127s on my 36 and 125s on my 29). That being said, 125/127s are generally decent for commuting, but on a 24 I would ideally want 110 or even shorter. But again, are you looking for the optimum (in which case I’d say get a 29") or something that works, in which case 125 is probably workable although 110s would be better.
On a 36", I think even 150’s are ok for commuting, it’s only a matter of taste and technique.
As long as you use the gravity in a certain way to make the pedal stroke super easy, cruising a 12-13mph is ok, and in case you have to do a lot of dismounts/freemount/uphills/slalom between walkers/quick accelerations or decelerations, it’s a lot easier with 150’s (unless you want to go at 14-15 or faster).
The only downside I can see is at the crotch area, long cranks are said to be more problematic for long rides than short ones.