still possible, but more difficult, and you would definitely benefit from a uni with shorter cranks while learning.
However, once you’ve learned how, you don’t actually have to lean in that much to spin. The trick is shifting your centre of balance. It’s definitely still possible though.
Before i had my freestyle (i just got it) i would practice spins on my onza trials with 140 cranks and they hit the ground alot. I think the 2.5" tire was what made the spins difficult though. So 145s would be alot harder. I started working on them with my semcycle XL 20" with 125’s and spins are much easier, ive even done a few pireouttes.
You can still learn many level 5+ tricks with 145’s except spins, pireouttes, and some others tricks might be quite hard.
When you get better at spins, start to play with and be aware of your arm position. It’s a bit like figure skating. It just dawned on me that I could start spinning when my arms are out wide, and then when I’m comfortable and in balance, I stop pedaling, and pull them in as fast as possible. I’ve done at least 2 rotations of a pirouette like this. Awesome fun.
I have to say though, I believe they’re still possible. I can do a spin on every unicycle I’ve gotten on to date, and I believe it’s thoroughly possible with 145s. That said, learning on something like that is definitely way way harder.
If you’re still having trouble with spins (or with any other skills), requests for help are much easier to answer if you describe the problem. “Can’t spin” doesn’t tell us anything. Try to describe where you think it’s going wrong.
Man, everyone keeps saying the pirouettes are hard, when I got them almost right after I learnt to spin. Is it because I am light with a great freestyle tire and small cranks? Or is it that God loves me more? 
Actually i was putting back this post in order to ask if the trick is possible on my uni. I have nothing that helps for the spin : a 20"x2.5 maxxis trial tire and 145mm cranks. I would like to know if the trick remains possible.
As i had some answer that say it’s possible, i can formulate the problem :
When i do regular circles leaning in with my pedal hitting the ground each turn, i don’t figure out how to make tighter turns without doing bigger hits that throw me off.
Is there a point where the pedals will stop hitting the ground and i would hit the spin ?
Manuel
yeah, there’s a point where you’re turning tigher but your pedals are actually higher off the ground than if you were doing a leaning turn. It’s weird.
and tyler - a ‘spin’ in the Standard Skill list is five rotations around in a circle, I could only just do that recently. A ‘pirouette’ is three rotations on the spot with no pedal movement, which is part of what makes them so hard. Being light with an awesome freestyle tire doesn’t hurt either.
does it help if you don’t try to do them on loose dirt? This might explain why I can’t get them…
Yea, I knew that. Why did it take you so long to learn (no offence)?
well, learning the balance while in the spin took me a lot of falling off, I guess. Getting into it was pretty simple, but once I could do it, I didn’t really put much thought into it. It’s been a slow progression sort of trick, and I usually did it to relax or celebrate when I would pull something new (I’m sure you understand what I’m talking about here ;)). Just recently I realized how big a part my arms played in the spin, and have started toying with their position, to great results.