It sometimes seems like topics are coincidental. I had a big interaction with today's topic yesterday having planned to post this post a few days ago, but the Stan's and RaceFace carbon post took priority. Then you realize that some topics aren't really coincidental, they are just always active. Alloy nipples is one, and a post on those will follow soon. Tire and rim width is a subject that's never more than an hour away from happening, so let's discuss it.
People most often want to know if an AForce Al33 build can be used with a 28mm tire. That's the number one question on the topic and in any given week will be the question we get the most. Yes is the answer. The internal width of the AForce or Boyd, Easton, HED, FSW3 or Mavic road builds we sell would have been considered crazy wide for mountain biking not that long ago. These are all also quite appropriate for gravel tires, and for the most part they are all excellent for cross. Cross is the most demanding of a tubeless interface, by far, so we tend to get pretty specific about that but that's getting away from the topic.
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For aerodynamic optimization, you want the rim's external width to be just about the same as or very slightly wider than the tire. But that's optimization, and we're talking about differences of hundredths of a mile per hour there. When you're talking about using a 28 for rolling resistance versus a 25 for aerodynamic optimization, rolling resistance overcomes aerodynamics in almost every instance (and if "aero trumps weight" we need a new cliche about the relationship between rolling resistance and aerodynamics). Plus most people are looking at 28s for either comfort benefit or to be able to use them on "not quite road" roads, which are totally orthogonal to the "aero vs rolling resistance" discussion. And then the question of will a 28 fit your bike, and what is a "28" anyway, as most of the 25mm size of the tires that people want to use measure 28 or more when inflated on a current rim. For road, I use Vittoria 23s that measure 26 and change. So yes, you can use a 28 or 33 or 42 on any "road" build we offer and it will be just ducky.
But then yesterday's interaction was talking on a forum about hookless rims, and I advocated for them (within their domain, they are great), and a guy thought that we were doing our own product (we totally aren't - topic for another day) and he hoped that it would be very wide. And we've known for a long time that there's this belief out there that if some wide is good, more wide must be better, but we believe quite strongly that that's a fallacy. Rather, there is a range of tire width to rim width ratios that will work best.
For road, aerodynamics optimizes at about equality between tire width and rim outer width, like in this sketch (these are concept sketches, not dimensioned drawings, btw):
This is also a good handling match. With a tire that's a whole bunch wider than the inner width of a rim, you get the dreaded lightbulb. That is known to be aerodynamically lame, and more important to me it handles like crap. The tire is unsupported in hard turns and it's not good. Here's a sketch of that:
See the lightbulb shape? As a general rule, the wider tires are going to tolerate and even need a bit more overhang (lightbulb shape) than narrower tires, for several reasons. One, wider tires are generally used in "less paved" situations, and a bit of tire overhang protects the rim from damage. It's much more normal to hear about people damaging rims without having a flat tire now.
For another, the tire can actually roll if the rim's inner width is too wide relative to the tire. This should intuitively make sense, but if the tire's "pressing out against the rim" component gets too small, well, you see what happens.
For a third, again on the "less paved" deal, if the tire sidewalls are vertical, the tire's contribution to suspension recedes. Gravel and cross, you definitely want that suspension. Need it, in fact.
For a fourth, the tire's grip in turns really starts to suck. Like in this crappy sketch:
You've got the near vertical sidewalls, but note also that the turning knobs come off of the side of the tire and the knob profile gets very flat. When you lean into a turn on this, it creates a very "on, then off" grip scenario. The side knobs are unsupported and then you lean enough that they are not in ground contact. No bueno.
So a better tire/rim combo for a knobbed tire looks more like this crappy sketch:
You have a little overhang to protect the tire and maintain suspension, and the turning knobs are well supported and in a place where they'll stay in ground contact. All good. And this is why we think that a 30mm inner width rim is a bad match for a 35mm gravel tire. There really aren't any hard and fast rules, and different companies will vary in their recommendations.
Last, this GCN video is pretty good and hits some of the topics here.