A couple years ago I weighed a few of the common Alu and Steel frames available then (see post linked below) - probably not too much has changed weight-wise. I also had a few thoughts on the differences - also still relevant I think.
The stresses that go through a bike frame are very different. As a longer and more elaborate structure, often ridden hard in gears equivalent to a unicycle wheel of 80 inches or more, a bike frame needs to resist twisting.
A bike frame made of aluminium will tend to need larger diameter tubes in order to resist this twisting.
On a unicycle, a good quality aluminium frame such as a KH is more than adequate.
Remember, it is not just the material used, but other things such as the thickness of the walls of the tubes, the profile (cross section) of the tubes, and the quality of the build.
My old Coker was steel framed and flexed horribly because it was thin round-section steel tube. My KH36 is aluminium/alloy and I seldom notice any flex.
KH kit is good. Some people may think that other brands are better, or not, but the point is not whether KH is “best” but that KH is “very good”. If KH uses aluminium, it is because it is better than steel for that purpose.
A bike frame does gain rigidity by virtue of being more “3D” though (3D in scare quotes because it’s primarily in one plane).
This is slightly tangential, but the handlebar setup on my 36er puts a lot of torque on the seatpost and seatpost/handlebar interface. And the handlebar itself is only 22.2mm in diameter and floating out in the breeze, unsupported.
I recently replaced my Shadow base with the new beefed up version after breaking the old one. Now that the base is more heavily reinforced I wonder where the next weak link is. Probably the handlebar itself. There is already some paint mysteriously missing from the bar about 10mm forward of the adjustment clamp, which is where the bending forces would be concentrated. I expect I’ll see a fatigue failure there at some point.
Probably the real answer for handlebar users is a V frame, at the expense of weight and adjustability.
V-frames need to have fewer legs.
If you want rigidity and long reach handlebars (which is thread drift…) then something combining the principles of the Moulton “spaceframe” coupled with some features of the Hatchet (to keep the frame away from the knees) might be a solution.
To me, the solution is riding within the limits of a simple unicycle, sometimes with a short bar, sometimes without. It is the simplicity of the unicycle that appeals to me: a few simple variables (wheel diameter, crank length, tyre section, overall quality of build) and everything else is down to me as the rider.
Here’s a picture of a Moulton spaceframe, for younger readers to admire.
I’ve been riding a Nimbus 29" road with an aluminum Oracle frame. I recently swapped and put the original steel Nimbus frame on it. Its about a pound heavier with the steel frame. Riding it - I can’t tell the difference. The only time I can tell the difference is when I’m lifting it out of the garage.