Veraxis, your illustration is the best ever for explaining the misunderstood relationship between rim width and tire profile/volume. Full props for that! I've referred people to it before, mainly to help them understand that the tire will always be round, and to break up fights where one guy's saying "wider rim = taller tire" and another is saying "wider rim = shorter tire", and name calling starts because they've both "seen it with their own eyes".
I don't agree with all of the subjective conclusions you draw however, such as....
"Low pressure" actually feels pretty hard on a wider rim to me......18/20 feels closer to 30 psi on a wide rim
18-20 on a wide rim does feel like higher pressure on a narrow rim in that the tire remains stable and doesn't fold sideways under high cornering loads. However, when the tire impacts objects on the trail, the low pressure/wide setup is much more absorbent, filtering out more of the impact from the rider. I consider that "smoother". Any higher pressure setup will transfer more of that impact, or "ping" off the offending rock. I'd call THAT "harder feeling". This is the crux of my ride quality statement....the wide rim gives you the traction and bump absorption benefits of low pressure without having to pay the cost of cornering instability (and potential unseating/air loss) that comes with low pressure on a narrow rim.
Going from 30 to 40mm doesn't seem to offer as much benefit as going from 20-30mm did.
What are you basing that on?... Scientific law, practical ride experience, or eyeballing your diagram? Not a challenge, I'm genuinely asking. Technically it may be true that going from 30mm to 40mm produces a smaller percentage increase in volume than going 20 to 30 (it's also a smaller percentage increase in rim width!), how significant is that jump in practical terms? Should we discount something that has a tangible benefit in the real world simply because that benefit isn't compounding at the same rate across a theoretical range? It seems like you've made a fairly arbitrary decision about the point at which increased width goes from being good to bad. The OP's not considering any rims wider than 30mm internal anyway.
I personally believe it's better to get a larger tire (2.5 vs 2.35, or 27.5 vs 26) than to get a wider rim.
That's probably true in some cases, but in others it's clearly not. Does a 2.4" tire on a 19mm internal rim perform better than a 2.3 on a 30mm internal rim? No way, Jose. And a wide rim is often a do-able upgrade on an existing bike, where a larger tire or larger diameter wheel is not.
I think Ibis with their 741 is basically gambling on shock factor... people will get impressed by it, but perhaps would want lighter, and that will lead to more purchases down the road.
Disagree... 40mm is hardly shocking in the +size / fatbike world we're living in, and you're implying a cynical and misleading intent on Ibis' part...that's never been their jam....maybe you're confusing them with Specialized??? If anything, we're seeing widths continue to grow beyond 35mm internal, getting wider still, for trail riding. But I'm sure the pendulum will swing for some riders, as you say...it usually does.
As far as torsional stiffness goes, good torsional stiffness likely comes from the how minimal the wheel's dish is, how wide the spoke bracing is, how even tension is between the DS and NDS, how elastic the spokes are, axle width, width of the spacing between the hub's bearings, among other things, more than the rim. The rim's at the mercy of what keeps it attached to the axle.
All those things influence wheel performance. I'm not advocating extra wide rims to bandaid a poor wheel build. But the rim's inherent stiffness has a huge influence on on the finished wheel....it's interacting with what it's attached to, but it's not simply at the mercy of the other parts. Rim stiffness matters. Don't take that to mean that I'm saying narrower carbon rims are inadequately stiff, because I'm not.
The OP should also bear in mind that he's nearly 100 lbs heavier than you are, so he may be well served by bumping everything up...rim width, tire size, PSI, etc. You can get away with a lot of things at 135 lbs that he may not be able to.
Your diagram (which, again, is awesome) would be even more useful if you did it for various sizes of tires and labeled it accordingly...would greatly increase it's practical usefulness rather than illustrating a general point. What is the tire used (or the casing width at least) in that image? Inquiring minds want to know.
As far as magic carpets go, clearly you need to upgrade, bro. Mine rides almost as smooth as the 2.4 tires on my 34mm Derby rims.
