Hmmm…this seems to have posted before I could finish changing from feet to inches. Anyway, you get the idea. I am curious as to what experiences others have when they shift back and forth between these unicycle wheel sizes. When I try to ride the 20" after riding the 36" I have all kinds of trouble. But going back to the 36" after the 20" seems to be less of a problem. Just thought I would try to get some feedback about something I have been dealing with lately.
No takers so far… sorry, you’re stuck with me until the experts arrive. 
I guess this one fits under the universal law of unicycling. It takes practice and time.
I rode a trials sized wheel for the first time long after I had become well versed in 29er riding. At first the smaller wheel felt very awkward, it felt very squirrely. With time that feeling faded.
I can now hop from size to size and only the first several revolutions or so feel strange, after that I get locked in.
One thing, and I don’t know how much difference this actually makes (hopefully others will comment) is that you have one variable that you can make a constant - the crank size. I run 150s on the 29er muni and the 24 muni, so there’s one less difference. All my unis currently have either 150s or 125s, even on purpose. There for a while I thought that maybe practice on one size would translate better to other wheel sizes if the crank length was the same. One less thing to adjust to when hopping from one uni to another. Maybe having similar crank lengths can help with muscle memory for all unis with similar crank lengths.
These days I think the #1 down side from hopping from a 36er to a 20" would be how frustrated I would be with the hit in speed. Heck, after long 36er rides the 29er felt unbearably slow to me.
It’s naturally easy for some, and others not. For the not- practice.
I know of near beginners who have no problems switching sizes, and people who have ridden thousands of miles who still aren’t good at it.
It’s anecdotal, but the biggest problem I had was after Ride the Lobster. After racing a 36" at maximum speed for a week, I REALLY struggled with riding a 24" that was there for a demo. Several others struggled too.
As far as crank length goes, I don’t find the difference in lengths to be much of a factor.
I do know when changing Muni sizes, the difficulty is more in judging the rollout through technical situations than it is while just riding along on relatively smooth terrain.
sifting from 36" (inch) to 20" (inch) and back!!!
It is good to hear other comments. I have noticed when I am on the 36" wheel for over a week then go back to the 20" unicycle I am very herky jerky, stop and go kind of motion. This can go on for five to ten minutes. When I am on the larger wheel for a shorter period of time and move to the smaller wheel it takes much less time to adjust.
Interestingly enough, when I go from the larger wheel to riding the smaller wheel backwards I don’t notice as much of a change. Perhaps this is because I do not ride the 36 inch backwards and my body only knows going backwards on the smaller wheel?
Anyway, I will keep practicing because I need the use of both wheels. The large wheel is fun and stimulates me and really moves but I can’t always find places to ride it (especially in the winter). The small wheel offers me the opportunity to work on more tricks and backwards riding and I can usually ride it about anywhere.
No real problems here. Oddly I sometimes get into trouble going from my 36 to 29, when freemounting. I push to hard on the 29 after getting used to the 36 and come right off the front. I just have to remember to have a light touch with it after getting used to freemounting the 36 with shortish cranks.
When I first got my 36 and 29 it was a bit awkward at first. But after a while my legs seem to just go into a different mode depending on what uni I grab. It might help to ride something in between. Ride your 36, ride a 24, then ride a 20.
I mount my 20 and 24" with pedal down and basically a stationary mount. On my 29 I do a small lunge forward to mount and on my 36 I do a rolling jump mount. Maybe I have less issues because of the mounts I do. But I think it’s really just a time thing.
A few questions to maybe help more:
How long have you been riding?
What size uni did you learn on?
What size uni’s do you own currently?
How long have you been riding your 36er for?
I had a very similar experience. Once I started learning to ride my 26 I suddenly couldn’t ride my 20. It just seemed to squirm about everywhere. Weird.