We have a uni with a twisted axle - the pedals are no longer 180 degrees apart. How to fix this? Can I get a new axle somewhere? Or do I need a whole new hub (which means rebuilding the wheel around it)?
Thanks
Gavin
You need a new hub. If you have a good rim, you can rebuilt the wheel. If it’s not, I’d consider just buying a wheelset, since once you factor in the cost of new spokes, the difference usually isn’t big.
A twisted axle is still better than a twisted ankle 
What kind of axle is it?
If it is an older axle maybe the axle to crank joint is starting to give out. On a newer splined axle, maybe just off a tooth.
Hi. It’s a square taper axle, unicycle.com. 36 spokes. I’ve never rebuilt a wheel from scratch; maybe time to learn.
I once got interested in learning about building, maintaining and repairing spoked wheels, not to go into business but just for my own needs and interest. But that turns out to need knowledge and skills along with some tools and expensive equipment, and possibly even more expensive equipment. I gave up on that idea and have always gone to somebody who knows what to do, how to get the job done and has the equipment to do it. And the benefit of doing it this way is because wheel building and especially wheel truing is partly an art which usually is best done by somebody who does a lot of it, not just occasionally.
For my practical needs, all I need is a screwdriver, a spoke wrench and a ziptie. No truing stands or tensiometers needed for a typical unicycle wheel. We tend run way overly strong rims, so comparing the spoke tension by hand to a wheel that you know is good is enough, and for the slow speeds, the roundness you can achieve with a ziptie on a unicycle frame as a visual guide is enough. Just take your time and go in steps. It’s only really difficult if your wheel is bent to start with.
If I invested in a 200$ carbon rim, I’d have a professional build it. But for the trials wheels I’ve build so far, my accuracy is enough.
Before you get too far along make sure that your axle is twisted and the misalignment is not just in a crank that has started to fail. Normally axles are harder then the crank and the crank will deform before the axle.
All that is essential is a spoke wrench, though a flat blade screwdriver can be used instead to turn the nipples using the slot in their head. The unicycle itself makes a fine truing stand.
A gauge can be used to measure the run out but I have never used anything more sophisticated than holding a screwdriver against the fork with the point near the edge of the rim. Sub-millimetre deviations can be seen with this technique which is more than accurate enough. I just use my thumb and gauge the rim from the nail until the wheel is getting close to true.
Truing isn’t as difficult as it might look. A person with a lot of experience will be faster but a novice can do it by being logical and adjusting the nipples in small increments.
Think about the rim in terms of it being centred around the hub, circular and running true. If the hub isn’t in the centre then pull it towards the centre by adjusting the spokes accordingly. If it is oval then pull on the high points and relax the tension of the low points.
Each spoke is pulling the rim towards the hub and to one side or the other. Tighten the nipples that pull in the direction you want the rim to move. If necessary, loosen the opposing spokes and work towards equalising the tension in all spokes. If a spoke pulling in the direction you want to move the rim already seems tighter than the others then you should be loosening the opposing spokes to get the same movement.
Adjacent spokes are affected by every adjustment so generally apply a little of the same adjustment to the nearby spokes that pull in the same direction as dictated by their tension.
Towards the end of the process the spokes need to be set by stretching them. Plenty of explanations of this process online.
Correct tension can be judged by squeezing the spokes and comparing the force and displacement with a known good wheel. There is a considerable acceptable range. Getting the tension even is more important so long as the tension is appropriate for the wheel loading.
The tension in a spoke is cyclically unloaded when it is at the bottom of the rotation. The spoke must always remain in tension even at this point or it will fail from fatigue or unwind the nipple making the problem worse. The tension in a unicycle wheel should be closer to the high end of the usual range than a bicycle wheel because it is loaded a lot more heavily.
@ finnspin and OneTrackMind
Your keep-it-simple ways of maintaining a wheel could probably do everything I could need, thank you. My experience goes back to 27x1-1/4" and then 700c bicycle wheels having narrower and more flexible rims.
Presumably the twist is due to your riding. It sounds like you would be better putting the money for a hub towards a more suitable unicycle.
I have seen twisted spindles, but not for a very long time. It is more likely to bend a crank; however, not in a way that would make the arms go off in the way you describe. If the square taper isn’t fitting right you would have a loose crank, and so you would know. In other words, yeah you probably have a twisted spindle.
Rebuilding a wheel isn’t terribly difficult if you take your time, and are methodical. There are quite a few people on this forum who have successfully built a wheel using Sheldon Browns instructions, and a spoke wrench.
I have coached non-mechanics through lacing, and tensioning wheels without putting a hand on their work. So, if you have the mind to do it swapping the hub is not unreasonable.
It may be more cost effective to look at a new uni if you need to have the wheel built by a pro. For reference only, I charge $35 to build a wheel with all new parts, and $50 for a wheel built with any combination of new and used parts. So you can see that the cost will go up significantly.
So, why did it twist? It could be that this hub missed the hardening, and was going to twist on anyone. You may want to call UDC to see what they say. It could be that you are riding beyond the limits of what your unicycle was built for. In the first case a new UDC CrMo hub would fix it, in the second a more robust uni is the solution.
About used spokes:
I have no problem building with used spokes on my own wheels. If I pulled them from a wheel that was tensioned well there is no reason to think that they have fatigued. If I pull spokes from a wheel with poor, or uneven tension I clip them so they will not be confused with “good” used spokes. I have ridden thousands of miles on wheels built with used spokes. When I build wheels for customers I use new spokes. Mostly because I guarantee my wheels, and if for some reason a customer got a random bad spoke it would lower their confidence in the wheels, and lead to more time for me to correct the problem.
Hehehe! That is why I bought a beginners 20" uni with a hardened CrMo cotterless hub, plenty strong enough for ordinary use but weak enough to remind me to not attempt any of the wild and crazy unicycle stunts as seen on YouTube.
The CrMo hub is pretty bombproof. I don’t know that I can recall anyone doing much damage to them. The interface will eat cranks if you ride hard though.
The Club unicycles used to come with a UDC hub that looked like the CrMo, but didn’t say so. I always thought that they had a run of cheaper hubs made to keep the entry level cost down. I suspected that this might have been the hub, but the OP said it was “hardened.” Maybe it was hardened plain steel instead of CrMo. I don’t know.
I am new to unicycles and unicycling but I can draw on extensive experience as a bicycle commuter along with some minor touring and, yes, hardened CrMo is tough stuff. Of course unicycles power both frontwards and backwards and so there is a rotational power exchange at the crankset not found on most bicycles, except fixies, but even then I think the hardened CrMo should be just fine for what a beginner should be doing on a unicycle.
The first thing I did was to study all I could find about unicycles. The 20" size gets the nod for beginners and then I studied the few UDC had to offer. In the end I settled on a new, red, Schwinn Retro 20" model which, as a selling point, includes a hardened CrMo hub. BTW, the entire Schwinn uni gets high marks for quality of design and construction, considering its price point.
On square taper axles, cranks out of line usually means the axle is starting to fail, or one of the tapers is getting ready to break. But you could check this by first taking off the cranks and repositioning them in different positions to see if they line up, or are always off by the same amount.
If this happens on an old, cotter pin hub, it usually means you just have to take out one of the pins and put it in facing the other way. 
It’s funny you brought up cottered cranks. The only truly twisted spindles I’ve seen were cottered because of the low grade steel they used sometimes. The nice thing about those is that you can file the taper of the pin until the cranks line up again.
My first couple of bicycles had cottered cranks. But then along came tapered square, cotterless cranks. Woo-hoo! Way to go!
My theory about beginning with a low cost uni having cotterless cranks is that if my “ordinary” day-to-day riding starts to break the cranks then I could congratulate myself for having graduated beginner school and then get the splined ISIS cranks I will have earned. But until then I have not invested too much money.