The most important component in a good frame, be it bike or unicycle, is good design. The secondmost important component is good build; in other words it was assembled properly with good welds, brazes or joints. It doesn’t matter what it’s made out of if those two requirements aren’t met.
Aluminum is stiff and basically brittle. This means frams generally have to be fatter to retain stiffness. And they have to be stiff, because if they flex, they will fail that much faster. That’s what worries me about aluminum unicycle frames, because all the twisting force that goes into them has to be completely absorbed or the frame is in danger of stress fractures over time. On the other hand, that stiffness is just what you want for most types of unicycling, so it’s fine as long as the legs or crown don’t have to be too fat.
Titanium flexes, which is generally a plus on a bike frame, but is not desirable on a unicycle frame. So you have to design to minimize the flex, which kind of eliminates one of the nice advantages of using Ti. However, if a Ti frame flexes a little bit, it can do that forever without leading to cracks or eventual failure. If the flex is minimal enough it will not be noticeable by the rider while even that amount of flex is probably not acceptable for an aluminum frame.
I love the look of anodized aluminum, but unfinished titanium looks pretty cool too. Nobody wants to paint it, because then you have to tell people what it’s made out of, taking away from that aspect of the coolness factor.
All of that said, the Koxx frames look great. I borrowed one for the coasting and gliding competitions at Unicon and enjoyed riding them. They have very tight clearance on the tire though, so it could limit your tire choices. Also they’re new, so they don’t have an established track record for reliability. I think they’ll hold up fine though, unless you really abuse them. The construction looks very stiff and beefy.
The Miyata frames have been available in Ti for several years, but I don’t know much about them. Most owners of those frames live in Japan, and use them for either track or Freestyle, which generally is a lot less abusive to frames than stuff like Trials or Street. I’ve never heard of a failure (with either frame), and I expect the Miyata ones to hold up as well as, or better than, anything aluminum that’s currently out there.
To me the Koxx frame looks like a better design for stand-up skills, though Japanese riders make it obvious that Miyata frames can work just fine. The Koxx frame may be more of a knee-banger for other skills though, but at least nothing sticks out–it just has the corners at the top.
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