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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! A so-called "26 inch" wheel/tire could have an ISO rim size of:
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!
26 x (decimal)
26 x 1 (race)
26 x 1 3/4 Schwinn
26 x 1 1/2
26 x 1 3/8 E.A.3
26 x 1 1/4 E.A.1
26 x 1 3/8 Schwinn
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.).
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!
A second number or letter code would indicate the width of the tire. (26 x 1.75, 27 x 1 1/4...650B, 700C...)
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 modern ISO/E.T.R.T.O. system indicated the size of the rim, not the outer diameter of the tire. This makes the ISO/E.T.R.T.O. system the most reliable guide to which tire will fit which rim.
The key ISO/E.T.R.T.O. dimension is a three digit number known as the "Bead Seat Diameter. "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.
ISO 571 mm - 650C, 26 x 1", Schwinn 26 x 1 3/4" S-7, Canadian 26 x 1 1/2 F.12There 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.
- Narrow 571 mm tires, commonly called 650C (or incorrectly called "650" without a letter) or 26 x 1 are used mainly on racing type bikes for smaller riders, and also for some triathlon bikes. This is a size mainly intended for competition.
- The same 571 mm Bead Seat Diameter was formerly used by Schwinn as a proprietary alternative to the 559 mm size. Tires in this size will generally be marked as being Schwinn specific on the sidewalls, and will be marked 26 x 1 3/4" (note, that is not the same as 26 x 1.75"!)
The Canadian 26 x 1 1/2 F.12 size is generally interchangeable with the Schwinn version.
ISO 584 mm - 650B, 26 x 1 1/2"This size is mainly a French size, and was the standard size for French utility bikes, heavy duty touring bikes and tandems for many years.
Various attempts have been 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.
See also my Article: 650B (584 mm) Conversions for Road Bikes
ISO 590 mm - 650A, English 26 x 1 3/8" E.A.3This 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-6The 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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