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"26 Inch" and "27.5" Bicycle Tires
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by Sheldon "ISO/E.T.R.T.O." Brown
Revised by John Allen
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Which Size Tire Fits Which Size Rim?

There is much confusion in bicycle tire sizing, and much of it results from talking in "shorthand." For example, you might think that "26 inches" was a tire size, but it is not!

There are no fewer than 5 different, incompatible "26 inch" sizes which you are likely to encounter. And one of them is even sometimes called a 27-inch size! A so-called "26 inch" wheel/tire could have an ISO rim size of:

559 mm, 571 mm, 584 mm, 590 mm or 597 mm!
559 mm
26 x (decimal)
(Mountain)
571 mm
26 x 1 (race)
26 x 1 3/4 Schwinn
French: 650C
584 mm
26 x 1 1/2
French: 650B

Mountain:"27.5"
590 mm
26 x 1 3/8 E.A.3
French: 650A
597 mm
26 x 1 1/4 E.A.1
26 x 1 3/8 Schwinn
Bicycle tires come in a bewildering variety of sizes. To make matters worse, in the early days of cycling, every country that manufactured bicycles developed its own system of marking the sizes. These different national sizing schemes created a situation in which the same-size tire would be known by different numbers in different countries. Even worse, different-sized tires that were not interchangeable with one another were often marked with the same numbers!

Traditional Sizing Systems

The traditional sizing systems are based on a measurement of the outside diameter of a tire. This would usually be measured in inches (26", 27", etc.) or millimeters (650, 700, etc.). A second number or letter code would indicate the width of the tire. (26 x 2.125, 27 x 1 1/4...650B, 700C...)

Unfortunately, evolution of tires and rims has made these measurements lose contact with reality. Here's how it works: Let's start with the 26 x 2.125 size that became popular on heavyweight "balloon tire" bikes in the late '30's and still remains common on "beach cruiser" bikes. This size tire is very close to 26 inches in actual diameter. Some riders, however were dissatisfied with these tires, and wanted something a bit lighter and faster. The industry responded by making "middleweight" tires marked 26 x 1.75 to fit the same rims. Although they are still called "26 inch", these tires are actually 25 5/8", not 26". This same rim size was adopted by the early pioneers of west-coast "klunkers", and became the standard for mountain bikes. Due to the appetite of the market, you can get tires as narrow as 25 mm to fit these rims, so you wind up with a "26 inch" tire that is more like 24 7/8" in actual diameter -- also "fatbike" tires more than 4 inches wide, with an outside diameter more like 27 inches!

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Does Point Seven Five Equal Three Quarters?

Note that the inch-based designations sometimes express the width in a decimal (26 x 1.75) and sometimes as a common fraction (26 x 1 3/4). This is the most common cause of mismatches. Although these size designations are mathematically equal, they refer to different size tires, which are NOT interchangeable. It is dangerous to generalize when talking about tire sizing, but I would confidently state the following:

Brown's Law Of Tire Sizing:

If two tires are marked with sizes that are mathematically equal,
but one is expressed as a decimal and the other as a fraction,
these two tires will not be interchangeable.

The ISO/E.T.R.T.O. System

In the late 1970s, the European Tyre and Rim Technical Organization (E.T.R.T.O) devised a solution to this mess, with markings which indicate the size of the rim, not the outer diameter of the tire. This makes the E.T.R.T.O. system the most reliable guide to which tire will fit which rim. The International Standards Organization (I.S.O) adopted this system.

The key ISO/E.T.R.T.O. dimension is a three digit number known as the "Bead Seat Diameter". This is the smallest diameter of the tire inside the rim, and the diameter of the little shelf ("bead seat") inside each flange of some rims.. "26 inch" tires will have ISO/E.T.R.T.O Bead Seat Diameters of 559 mm, 571 mm, 584 mm, 590 mm or 597 mm.

If you want more detail on the arcana of Bicycle Tire Sizing, click here.  

ISO 559 mm - 26 x (decimal)"

This is the size used on most mountain bikes. It is based on a traditional American size also used on "cruisers." Generally, any tire where the width dimension is expressed as a decimal inch value will be the 559 mm size, such as 26 x 1.0, 26 x 1.5, 26 x 1.75, 26 x 1.95, etc. -- even on "fatbikes" with tire widths up to 5 inches, though these require extra-wider rims.
 

ISO 571 mm - 650C, 26 x 1", Schwinn 26 x 1 3/4" S-7, Canadian 26 x 1 1/2 F.12

There are actually two ISO 571 sizes: Although these two sizes have the same bead seat diameter, the rim and tire widths are so different that they will not generally be interchangeable in practice.
 

ISO 584 mm - 650B, 26 x 1 1/2", "27.5"

This size was the standard size for French utility bikes, heavy-duty touring bikes and tandems for many years.

Various attempts were made to popularize it in the U.S., by Schwinn and Raleigh in the 1980s, and by Rivendell and other high-end builders in the 2000s. As of 2015, this size has finally caught on and tires are widely available, thanks to the recognition that tires 32mm - 47mm wide (depending on weight load) at moderate air pressure offer a far more comfortable ride than skinnier tires at high pressure -- and without increasing rolling resistance. The slightly smaller diameter than with the traditional, narrower 622 mm (700C) road tire also allows the top tube of the frame to be shorter to accommodate a shorter rider, without toe-clip interference.

See also my Article: 650B (584 mm) Conversions for Road Bikes

Wide, knobby tires are also offered in the 584 mm bead seat diameter, and marketed as "27.5" tires, the same way wide, knobby 622mm tires are marketed as "29ers". "27.5" tires are useful for off-road riders who need a smaller bicycle than is possible with "29ers" but can ride a larger one than with traditional 559 mm off-road tires. The marketing concept is that "27.5" is halfway between 26 and 29. The tire size isn't exactly halfway in between -- do the math if you like -- and the name suggests that this is a new tire size when it isn't!

 

ISO 590 mm - 650A, English 26 x 1 3/8" E.A.3

This size was the norm for most English 3-speed bikes, and used to be very, very common. It was also used on some inexpensive 10-speed bikes in the '70s.

The 590 mm size has fallen out of fashion since the advent of the mountain bike in the late 1970s, but there are still lots of bikes on the road that use it. It remains fairly popular in Japan.

 

ISO 597 mm - English 26 x 1 1/4" E.A.3, Schwinn 26 x 1 3/8" S-6

The 597 mm size is mainly seen in the U.S. on Schwinn 3-speeds. The fact that Schwinn chose to call this proprietary size "26 x 1 3/8" has caused an incalculable amount of confusion and frustration over the years.

A reasonable person would expect that any "26 x 1 3/8" tire would fit any "26 x 1 3/8" rim, but that is not the case, if one of them is Schwinn size and the other is English size.

The 597 mm size was also formerly used on high-end British "club" bicycles, with the marking "26 x 1 1/4 E.A.1" That size was pretty much abandoned in Britain in the late 1950s, when the 630 mm (27 inch) size replaced it.

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Got an unmarked rim but no tire?
Click Here for how to measure Rim Size.

More details? Click Here for Sheldon Brown's Tire Sizing Article

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