I haven't been burned at the stake in a while, so let's jump right in!
In customer conversations, a very often expressed preference is "I like nice, stiff wheels." And when I say it's phrased like that, it's always phrased like that. I'm going to do a post sometime about things that are always phrased like that (which will focus mostly on the tired tropes of equipment reviews, like how wide rims help tires "plump up," how wheels "spin up fast," and "hold speed well"). The next time after this that I feel like getting burned at the stake...
This was a great kit design. Also, the day this picture was taken I did the last k of Smuggler's Notch either 4 or 5 times, twice with 40# of kettle bells in a back pack. These days, I need a rest day after having typed that sentence.
We've done a whole lot of testing and writing about this in the past. And for the purposes of today's discussion we're limiting to wheels built for the uses that we primarily build for - road through light bondage XC MTB. Downhill and enduro wheels are still bike wheels and more similar than different to our focus, but excluding them might alleviate a bunch of noise.
Simply, my top line premise is that there is a line dividing "not stiff enough" from "stiff enough," and once that line is crossed, there is no further benefit to be had. The big "on the bike" tell for me is that an under built front wheel will steer badly. Put it into a hard corner and it doesn't want to turn. With all of the thinking about wheels while I ride that I've done in 10 years and lord knows how many miles, that's where I've gotten.
It's long since settled that brake pad rub isn't caused by flexy wheels. That link is just one of a ton I could put up, but it's conclusive and representative of all of them. In general, Slowtwitch does great tech work.
The list of factors that affect wheel stiffness is also settled, but the world is still full of belief that spoke tension is one of them. It isn't. Simply, it is not. Once spokes are under tension and not slack, nope.
From a wheel building standpoint, I prefer to replace "stiffness" entirely with "stability." As I learn about video recording, we'll put more of this into video form, but we torture wheels as we build them. The wheel is done when torturing it no longer changes it. It becomes stable. Some wheels can't reach stability. The AForce C25 disc rims could not be made stable at a spoke tension that would reasonably ensure against off side spokes going slack. Try building a 24h Stan's Crest (which they thankfully don't make), bring it up to tension, and then squeeze the spoke pairs hard. The rim flexes so much that when you take that squeeze pressure off, the wheel "re-settles" into a new shape.
If you have a wheel that does that "re-settling" bit, then you wind up with some degree of what's happened to the wheel above. That wheel is not stable, which is within my concept of not being stiff enough.
That trick of squeezing spoke pairs is something we do many times on each wheel that gets built, and there's a range of how much they move when you do it. An All Road 50 rear is clearly the most resistant to movement (what you would call "stiffest") wheel we build. They simply don't move when you do this. Cafe Racers are also in the same league. It's no coincidence that these are both carbon rims, and the two deepest rims we use. There are wheels that move quite a bit more than All Road 50s or Cafe Racers when you do this, but they return to stable. A Boyd Altamont Lite build, built to the same hub/spoke/spke count spec as a Boyd Altamont build, will exhibit quite a bit more movement. That proves the concept of rim dimensions having a big effect on wheel stiffness. That extra movement will be fine for some riders (my squeeze is certainly more load than a wheel sees in anything like normal use) but not for others, and that's the basis of our alchemy for determining appropriate spoke count.
This simple trick is my quickest and probably best gauge on what works and doesn't. If they can't return to stable, they're no good. If they move a lot but return to stable, I'm not wild about it. And then there's a range of movement that makes me totally comfortable. And within that range, I am devoutly certain that those wheels are "stiff enough" and that more stiffness would be of no benefit.
Spoke for spoke, a deep rim will be inherently stiffer and more stable than an otherwise similar but shallower rim. A Cafe Racer is slightly more stiff/stable than an RCG built the same way.
The same rim built with more spokes will be more stiff and stable than if it is built with fewer spokes.
The same rim built with thicker spokes will be more stiff and stable than if it is built with thinner spokes.
Hubs have a small impact on this, as well, but as we explored in withering detail recently, it ain't that big once you're using good hubs.
Clear as soup, right?