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| Image courtesy of Velocity USA |
I'm not going to get into the wheel recommendation portion of Josh's questions, but I do want to discuss inner rim width and tire width again. This time using external sources to help you understand what tire and rim manufacturers are thinking in regard to this question.
Inner Rim Width:
Before I get there, I think it would be wise to cover what "inner rim width" is and what "tire width" is. I know, it sounds simple, but I want to make sure we are all on the same page here.
As you can see from the cutaway I grabbed off Velocity USA's webpage for their Blunt SS rim, there is a difference between the outer, overall width of the rim and the inner rim width. It appears here that Velocity is using the distance between the "hooks" of the rim to define "inner rim width" and not the distance between the rim wall just above those little shelf looking protrusions we call the hook of the rim.
On a "hookless rim" design, that "inner rim width" is going to be the inner dimension between the faces of the rim where it interfaces with the tire. So, a bit of a difference there. Some will argue that a hook on a rim design is safer, because it prevents tire blow-offs. Some will say the hookless rim is better for tire aerodynamics, support in corners, and that the rim is better suited to resist damage. Some say there is no real benefit to hookless rims. So, I'll leave this alone for now and say only that I am not a real big fan of hookless designs.
Tire Width:
This seems pretty simple. The widest portion of the tire casing is "tire width". However; I feel more folks are thinking "tire tread width" because the tread is the "business end" of a tire design since it is the part that hits the surface you are riding on.
That is a misconception, if you are thinking in this way. What really matters, and what most never talk about, define, or even understand, is the contact patch. That is really what matters most as it helps to define a tires rolling resistance, traction, and over all handling traits. It doesn't natter if a tire is wide at the casing if you air it up too much and only a thin strip of the center of the tread hits the riding surface.
All this to say that "tire width" is probably more important to how a tire might fit into your bicycle first and foremost.. The rider comfort and handling part is also very important, but tire width is only a piece of that equation, not the ultimate defining factor.
Tire width and inner rim width work together to help define rider comfort, handling, and overall performance factors like aerodynamics, rolling resistance, and more.
Tire pressure, rider weight, overall loaded weight, the bicycle's geometry, conditions of the riding surface, and weather all conspire to bring another level of complexity to this equation. The tire construction method, materials used, and quality controls also factor in. One tire from a manufacturer can be very different from another tire from the same manufacturer with similar weights, widths, and mounted to the same rim. Even tires from different manufacturing dates, with model, size, and wheel being the same, can ride noticeably different.
All this to say that there is no "easy button" to make a choice. But you can get yourself into a ballpark of "good" if you follow recommendations given by the manufacturers for rim inner width and tire width match-ups.
Wilderness Trail Bikes offers an excellent chart with tire and rim width recommendations. I like their chart because it also shows combinations to avoid. Light Bicycle, a Chinese manufacturer of carbon frames, carbon accessories, and carbon rims and wheel sets, also has a nice, easier to read chart. The WTB chart is HERE. Light Bicycle's chart is HERE.
As long as you stick to the recommended ranges, you should be okay with the tires and rims of your choice. If you are looking to eek out every drop of performance from your tire and rim choices you will have to do more of your own trial and error research.
I hope this helps clear up any misconceptions about tire and rim widths. Thanks to Josh for the great question.
Is this another way to suck the fun out of bikes? Do we really need to pay attention to this of just know 1.4 to 2:1 ratio for sizing a tire to a rim? AKA a 28mm internal wide rim can handle: 39 to 56mm wide tires (40mm to 2.2)
ReplyDeleteThe old rule of thumb I remember from years ago was simply that tire width should be between 1.5 - 2.0X the inner rim width. More recent aero road wheels the pros are using clearly go well outside of that.
ReplyDeleteMaybe something like "you don't want the rim and tire to look like a mushroom, or a rocket. j/k ha ha
ReplyDeleteI think the simplest rule for a gravel bike (or training or touring bike) is just use 25 mm (inside) rims. ETRTO says those are safe for tires from 29 mm to 71 mm casing width and 25 mm should be close to the design rim width for most gravel tires. If you want better aerodynamics or you want to use MTB tires or you want to use wide (> 45 mm) tires on pavement then slightly wider rims might be slightly better. If the wheels you like have 23 mm or 24 mm rims then don't worry, they'll be fine.
ReplyDeleteThe tires spec'd on my 2025 Ibis Ripley exceed the recommendations on both of these charts. They sell the bike with 35mm internal width Ibis rims and 2.4" Maxxis WT tires. This is just outside optimal, but it is outside optimal. Seems to work great for me, but I know I am not very discerning.
ReplyDeleteI once used 40 mm tires on 30 mm rims because that's what I had. That combination is outside the ETRTO specs but those are designed for safety under all conditions. I used hooked rims, tubes, pressures no more than 60% of max pressure, and an accurate gauge. The combination worked well. Now those rims have 42 mm tires and that combination is also working well.
DeleteTake your tire size and divide by two. Write that number down somewhere where you'll forget it. Go ride your bike.
ReplyDelete