You didn’t buy the uni of ebay did you? I bought mine from some cycle shop on ebay, and it came with 20"/140mm cranks and we never understood why. They used to hit the ground all the time, were strange to balance with, and should have been slow (except I like spinning fast with long cranks!)
After I’d learnt, I started using 100mm cranks, which were much better. The plus side of the 140s was that I was able to ride over a strange obstacle we found in the park, and nobody else has managed it yet
Yes, I got it from St John Street Cycles from ebay. The long crank makes it VERY twitchy, which just gets worse; my lad who can uni realy struggles to do more than 10 metres on it- no wonder I struggle. Can’t wait to get the crank arms- then I can RIDE!
140s and thereabouts are fine for trials and jumping around a bit, but they’re rubbish for actually going anywhere. When I play hockey it’s with my trials uni with 140s; the pedals are always hitting the floor and it’s much slower than other unicycles with shorter cranks on.
For general purpose use of a 20" unicycle I’d say 125s or 127s are the way forward; long enough for good control while learning but short enough to be able to actually go anywhere without your knees flailing all over the place. For £7 you can’t go wrong!