Early on tried steel, easy to work with, braise, weld, machine, but heavy. So for the last 30+ years have been making all my Uni’s from Aluminum and Titanium, also easy to work with, machine and weld, lite, doesn’t corrode or rust (rust never sleeps)
I will say this is my biggest disappointment with my Oracle 27.5, my tire rubs my frame slightly under load. I’m not good enough to drive uphill yet this is just like when mounting or otherwise putting more than normal riding pressure. I think it’s more that the Duro Crux 3.25 Tire has very little clearance from the frame, only about 2 mm on each side. I was hoping the problem will go away if I get maybe a bit smaller Tire, maybe a 3.0 nstead of 3.25. Otherwise I love it
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It’s also because the fork flexes. I’ve also had this experience with various models, although you can read on various pages that aluminum is stiffer than steel, then it should also flex less. However, I have had exactly the opposite experience. That’s why I’m back to steel / CrMo. The 400g extra weight doesn’t bother me as much, I save the weight on the wheel. I think my 20kg overweight bother me more 
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As a material, steel is about 3x stiffer than aluminium (Youngs modulus of ~210 GPa for steel, ~70 for GPa for aluminium).
In the bike world, aluminium frames tend to be stiffer, because they use larger diameter tubes than typical steel frames. On unicycles, the frames are far less optimized, many steel and aluminium frames are very similar in shape, so the steel frames end up usually being equally stiff or stiffer (depends on the exact models you are comparing).
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There are a few reasons why Al is “considered” to be stiffer than steel. A really bad quality of Al is the limited fatigue life, compounded by rapid crack propagation. The more Al bends the less this fatigue life is. So bike designers either added wall thickness and avoided strength challenging welding (bonded Alans and Vitus for bike frames which rarely crack but do become unbonded)) or increased the diameters to reduce the amount of bending (and the number of cracks increased, hence the “crack and fail” nick name for Cannondale). Both ways tended to weigh less than a same geometry steel frame, one was more flexible in use, the other stiffer. IMO the early 1980s focus on racing and the easy marketing manipulation of statistics meant the stiffer path won out.
IME wheel rub when under pedaling power is more about tire/frame clearances than frame materials. Andy (who is going to make a steel uni)
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I understand all (ok, not all 
) of the technical arguments. I understand why steel is theoretically “better”.
But I really like my aluminium muni, and would buy another one in a heartbeat if anything happened to it .
OK not specific to unicycles but this guy explains the differences excellently for all you engineers out there 
- YouTube
My main road bikes are both steel and 20+ years old, but they are made of really nice tubing - Reynolds 853, and Columbus SLX. Those are high quality steels, with variable thickness (butting), sold as a set from the tubing factory. So the downtube is the right length for, say, a 56cm frame, and has butting at the bottom end that is appropriate for the bottom bracket area, and at the top that is appropriate to join the head tube. It’s all very optimized and specific.
I don’t think anything like that is available for unicycles. “Nice” steel unicycles seem to be generic CroMo straight gauge tubing. You can make a sturdy unicycle from that, but not a high end unicycle.
I think the ride quality claims are mostly imaginary for a unicycle. Your weight is bearing straight down on a vertical tube, there isn’t much opportunity for it to flex and absorb anything. Tire, rim, spokes, saddle, all of those would be much more important.
So I would get an aluminum frame. It’s much easier to work with and optimize. The Impact hydroformed frames, for example, are awesome.
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As far as longevity, I’ve broken two steel bike frames, two titanium bike frames, and an aluminum bike frame, all from metal fatigue. And a carbon frame where various non-carbon pieces came unbonded. I wouldn’t expect a steel frame to last longer in real life unless it weighs a ton (my steel Torker DX is probably indestructible, but weighs so much that I only use it to walk the dog).
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At UDC USA, the steel nimbus 27.5 is 17lbs, and $490, but has no brake or brake mount under the saddle.
The aluminium Nimbus Oracle 27.5 is 13.9 lbs with the hydraulic disc brake included for $685.
I figured if I got the steel one and add a brake, rotor, mount, that eats up a lot of the ~$200 price difference and you did more work to get there, and are stuck with a heavier uni in the end. I also thought The Oracle was a lot nicer looking.
So, I just got the Aluminium Oracle and called it a day!
This is an old thread, but FYI, I weighed my 27.5” steel Nimbus muni. It weighs 14.1lb.
John
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Yeah the UDC USA still says “approx 17lbs”, but if you go to UDC UK, it says 6.6kg or ~14.55lbs, much closer to your measurement.
I might have more seriously considered the steel one if I knew they were that close. Oh well I am happy with my Oracle other than I had to change the tire to a smaller one than 3.25 to eliminate the tire rubbing the frame under even slight loads. Maybe the steel one would not have done that.