Hi, Kikenji! I’m glad you made it to this forum which is an awesome resource for unicyclists (although Japanese unicyclists are underrepresented). Another great resource you should check out if you haven’t is the Facebook group “Unicycle Chat”. I’m not a member but there’s a large community and a lot of information there. For my current freewheel unicycle I’m using the hub of a Huffy Green Machine which is a type of tricycle with a 20 inch wheel on the front. The hub was donated to me by a generous member of the forum (Mark aka lobbybopster). I used the rims and spokes of another unicycle to build the wheel and then replaced the bearings to fit a unicycle frame. The process is similar to the one I recommended on this thread but that dispenses with the need to build a wheel. The biggest problem for me was getting the old bearings off. What a pain! I’ve done it on two wheels and each time one bearing came off easy and the other broke and I had to use extreme measures to get the inner race off. Another problem is that the bearing spacing of the freewheel hubs I’ve used is wider than a regular unicycle frame which is 100mm. To get around this I just forced the frame on but it’s far from ideal. Other companies make or at least use freewheel hubs for drift trikes and I’ve been contacting some of these to get details about the hubs they use to see if I can find one more suitable for unicycle frames. I’d also like to add brakes to my freewheel unicycle at some point, possible by drilling the frame and putting on caliper brakes.
If you’re going to practice something before making your freewheel I highly recommend one foot coasting where one foot is on the frame and the other one is not. This is probably the most directly transferable skill to freewheel unicycling although it also helps to be able to coast with both feet on the frame. Right now I’m limited to making strong clockwise turns on the freewheel unicycle and have more trouble turning counterclockwise. This is due to the fact that I freewheel with my right foot down and cannot do it with my left foot down yet. To get better at this I plan to practice one foot coasting with my left foot on the frame which is a scary proposition. Let me know if you need any other advice or help! I hope you enjoy freewheel unicycling as much as I do!