I had a Hyperbike with C shafts for my 37-38" mini. I had a difficult time getting it properly balanced for him, the shafts were a bit too long and just sort of "not right" for his size/shape. They were an older, interim version, but not sure the newer ones are going to change the basic issue which for us I felt was due to the combination of his size, my size (small - too light), and the length of the shafts. I had to have the seat ALL the way back and the pony ALL the way forward so there was way too much room between his rear and the seat. The KBike is supposed to be better for a big mini/small pony but I haven't seen one in person. I ended up buying a regular easy-entry which I have set up and appears to fit my pony well - I haven't driven it yet though.