A 26" wheel has more volume than a 24" wheel for any given cross section due to the diameter. It would follow, I think, that a 3" cross section on a 26" tire would effectively behave like a bigger tire than a 3" cross section on a 24. So, would it be fair to say that a 2.5" cross section would be appropriate for a 26" wheel, where a 24 would need 3"?
I’m trying to figure out if a slightly narrower, and lighter tire would make sense on my 26" wheel when I build it.
It really depends on the terrain you’re primarily going to be riding. I’ve been riding a 2.5 on my 26 for a few years now and it works pretty well for me. Most of my local riding is on reasonably smooth terrain; dirt, fist size or smaller rocks, smoothish log piles, 2 foot drops at the largest.
On larger rock sections in Moab I could tell I wanted more volume, though. I’m also running a dx-32, so maybe I would have been fine with a wider rim. I didn’t have that feeling last year when I was using a Large Marge.
Your general reasoning seems to be sound but the proportions are off. All things being equal a 26" tire is only going to have about 8% more volume than a 24", while going from a 2.5" tire to a 3" tire will change the volume by 17%.
So following your logic a 26" uni with about a 2.76" tire should have comparable volume to a 24" with 3" tire.
Now the question is does tire volume equate with tire “behavior.” For impact absorbance on smooth ground it probably does. For rough surfaces one has to think about the shape of the contact patch. Assuming the average bump height is a small fraction of the wheel’s radius (i.e. not trying to roll over logs) I suspect that a more even ratio of contact patch width and length would be preferable so as to avoid lateral instability. Also it is generally harder to balance a unicycle laterally than fore-aft so I suspect tire width is independently important to rough terrain riding.
The preference for narrower tires on large wheels is likely due to their being preferred for less technical riding.
This is the kind of discussion I was looking for. I didn’t actually do any math to make my point I just threw a number out there, but by your approximation a 2.7 should be similar volume.
The idea of contact patch is interesting. This is where my question about the “sweet spot” comes in. I wonder if there is a narrower tire that will give the best performance as compared to a 24x3, yet without as much weight.