27.5″ wheels are also known as 650b. Both names refer to the same type of wheel.
“650b” comes from an old French tire sizing that indicates the diameter of the wheel from one tire end to another.
In the case of 27.5″ wheels, the number is 650 because this is the approximate diameter of the wheel.
The letter “b” designates the width of the tire.
For example, a 650x45mm wheel has a rough diameter of 650mm and a 45mm wide tire.
Additional Clarification
Wheel sizing is always an estimate because the circumference and width of a wheel are greatly affected by the tire model and air pressure.
For example, a 27.5″ wheel with a 1″ tire has an approximate diameter of 634mm. The diameter of the wheel increases to 685mm when the tire is bumped to 2″.
Therefore, in some cases, a 26″ wheel can actually be bigger than a 27.5″ one.
For instance, a 26″ wheel with a 2″ tire has an estimated diameter of 660mm and thus has a larger circumference than a 27.5″x1/650×25 wheel.
FAQ: Why Do People Refer To 27.5″ Tires as 584 too?
Sometimes people refer to 27.5″ tires as 584 because the diameter of the rims used by 27.5″ tires is roughly 584mm.
The width of the rim is included in ETRTO listings. ETRTO stands for European Tyre and Rim Technical Organisation. The goal of the organization is to “harmonize” rim and tire sizing across the European Union.
In ISO 5775 (the international standard for labeling the size of bicycle rims and tires developed by ETRTO), wheel sizes are formed by two numbers separated by a hyphen (-). E.g., 54-584
When a tire is labeled as 54-584, the designation refers to a 27.5″ inch wheel with a 54mm or 2.1″ wide tire.
The first number indicates the width of the inflated tire (without the tread) whereas the second is the diameter of the rim.
FAQ: Can I a use 27.5″ tire on 650b rims?
Yes. Ultimately, 27.5″ wheels and 650b rims are built for each other even though the two names are quite different.
FAQ: Are 27.5″ wheels halfway between 26″ and 29″?
In theory, 27.5″ wheels should be 1.5″ bigger than 26″ and 1.5″ smaller than 29″ models.
In practice, however, the calculations aren’t as straightforward.
The rim diameters of 26″, 27.5″ and 29″ wheels are:
26″ – 559mm/22in
27.5″ – 584mm/22.99in
29″– 622mm/24.48in
Conclusions
The rim of a 27.5″ wheel is about 25mm or 1 inch larger than the rim of a 26″ wheel.
The rim of a 27.5″ wheel is about 38mm or 1.5in smaller than the rim of a 29″ wheel.
Hence why people say that a 27.5″ wheel is closer to 26″ than 29″.
As already mentioned, the tire width can greatly increase or decrease the wheel diameter. For example, a beefy 27.5″ tire could make the wheel larger than a 29″ one equipped with a small slick tire.
You can easily observe this effect if you put a 27.5″ MTB wheel next to a 700c road bike wheel.
Just like 29″, 700c wheels are also using 622mm rims. Thus, a 700c road wheel is the equivalent of a 29″ MTB wheel equipped with a narrow tire.
