I was thinking of converting my nimbus II 20" for trials. I think the tire might be too thin as is. anyone know which is the fattest rim and tire I can get onto this frame?
Do mean one like this one http://www.unicycle.com/nimbus-20-inch-trials-cycle-10.html/ ??? If so then it’s a nice beginner trials unicycle 
I believe the widest 20" rim is the 47mm KH Flat rim. There are 3" cruiser tires (really only ~2.5") but usually have really thin sidewalls and may not fit in your frame. If on a 20", I’d use the 2.4" Onza Sticky Fingers.
Why not get the Nimbus Trials? The parts are more suited to trials riding and stronger. It would be less $ to get the proper uni than upgrading a cheaper one.
Edit: I mis read the original post. I’d just get the Sticky fingers tire and some stronger, longer cranks.
http://www.unicycle.com/unicycles/freestyle/nimbus-ii-20-inch-unicycle-with-isis-hub-red.html/
thats the one that I have. Do you think its worth upgrading or should I just get a new one?
you’d need new spokes, a new rim, and you’d definitely want kh cranks and better pedals.
tube ~$10
tire ~$40
rim ~$50-70
spokes&nipples ~$80
kh cranks ~$80
pedals ~$25
wheelbuild ~$30
shipping ~$10
a brand new nimbus trials with kh cranks is about $360, and shipping is free for orders over 300. you’re better off getting a brand new one. then you’ll have a spare.
Thanks for the cost break down. Looks like the new decision will be between getting another 20" uni or maybe getting 26" muni. Thank you again for all the help