I purchased a used Sun Classic 20" unicycle recently. I noticed that with enough force, the cranks and the axle will move without the wheel moving. I feel it shift while freemounting and braking. How on earth does this happen? Are there any fixes, or do I need a new wheel?
That should not happen. That looks like a broken hub. I imagine that makes it much harder to ride.
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It looks like the flanges of the hub have broken off of the central shaft, preventing most of the torque from the cranks from being transmitted to the wheel… The only thing holding it all together at this point is the tension of the spokes as far as I can tell. Unfortunately this does mean you’ll need a new hub. If it were me I wouldn’t try to ride it as it is.
Your options are either getting a new hub or a new wheel. Getting a new wheel can be a bit of a search itself since it’s hard to source complete unicycle wheels that would fit (Modern Bike has some listings for replacement wheels but unfortunately it looks like they’re not in stock at the moment).
Getting ahold of a new hub by itself is much easier IMO, though you also have to deal with making sure you have the right length spokes, re-lacing, truing, and tensioning the wheel, etc. but you might be able to have a LBS do that for you if you don’t have the tools or the time. Unicycle.com usually has hubs in stock, just make sure you order the right bearing spacing and spoke count. The rim itself can be reused just fine.
EDIT: From what I can find the bearing spacing for your uni might be around 92mm, which might be a hard size to find for a square taper hub. There are hubs available at UDC that are the correct bearing spacing, but they’re ISIS hubs which use differently sized bearings unfortunately. Be sure to measure the bearing spacing just to be sure
Thank you for all the info! I’m thinking I might just buy a new unicycle. This is an older one that I got second hand, and I don’t think it’s worth sinking more money into. I think I’ll try tac welding the hub on, just so I can put off getting a new one for another month or so. Before I attempt it though, does that sound like an exceedingly bad idea?
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Hmm… Would it work? I think so, at least temporarily 
Would I recommend it? Ehhhh, that’s harder for me to say. If anything I would only trust the uni with some easy riding around the block (with a helmet on just in case the welds don’t hold).
Seems like a reasonable idea – I’d try a couple of small tacks on each flange and see how it goes – don’t put too much heat into it though and treat it fairly lightly until you get a new one.
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Sorry about your secondhand uni. I agree with using a helmet. If in the event your welding fix doesn’t last, you’ll have the world’s first bi-directional freewheel. Sounds like a new challenge.
My Schlumpf hubs disagree. They were there before!
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There is no axle on a unicycle. The crank arms are probably square taper and have loosened or worn. Tighten them. The other possibility is that the spokes are loose. TIghten them. If the problem persists, go to unicycle.com and buy a new one. Sun uni’s are old, cheap and not as well built as modern uni’s.
Watch the video in the first post again. The flanges were probably press fitted to the shaft where the cranks mount onto, but for some reason the fit has become loose over time. So the cranks and their shaft may rotate while the flanges, spokes etc. don‘t.
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Welded mine up about 40 yrs ago. Still working
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I’d just weld it! Put a lot of heat on the axle because that’s really thick VS the flanges.
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