27-inch bicycle tyres

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Sizes & dimensions of bicycle tyres

3 different types of size specification are standard in Europe:

1. ETRTO

2. In inches

3. French abbreviation

ETRTO

This is the European tyre and rim standard (European Tire and Rim Technical Organization). The size specifications feature a two-digit number block and a three-digit number block, whereby the blocks are separated by a hyphen.

Example

Source: Wikipedia (Moebiusuibeom-en, Bicycle tire marks 02, CC BY-SA 3.0)

The first number block represents the tyre width in millimetres, therefore, in our example the tyre is 47 mm wide. The second number block represents the internal tyre diameter in millimetres, and therefore, in our example, the internal tyre diameter is 622 mm.

Size specification in inches

The classic tyre specification for bicycle tyres is in inches, with one inch corresponding to 25.4 mm. This system has the disadvantage that tyre manufacturers only state approximate values. Nonetheless, this tyre specification is still used to this day, as it has become deeply engrained into the mind of the layperson.

Here there are two ways the size specification in inches may be structured:

1. The size is specified by means of a two-digit number block and a decimal number with two decimal places. The number block is separated from the decimal number by an x (multiplication symbol)

   Example

   

   Source: Wikipedia (Moebiusuibeom-en, Bicycle tire marks 02, CC BY-SA 3.0)

   The two-digit number block states the approximate external tyre diameter, i.e. 28 inches in our example. The tyre width is specified by the decimal number. In this example, this is 1.75 inches.

   Schwalbe explains why the use of inches to specify the size can prove problematic. Here it states:

   "The specifications in inches are neither precise nor clear. For example, diameters of 559 mm (MTB), 571 mm (Triathlon) and 590 mm (Dutch touring bikes) are all designated as 26-inch tyres. Tyres with diameters of 622 and 635 mm are both referred to as 28-inch tyres. Curiously, tyres with an internal diameter of 630 mm are referred to as 27-inch tyres. These designations find their origin - and their meaning - in the era of the plunger brake. At that time, the exact external diameter of the wheel was determined by the brake. Depending on the tyre width, there were then different standards for the internal diameter."

2. Here the size is again specified by means of a two-digit number block, however, the tyre width is not stated using a decimal number, but rather a fraction.

French abbreviation

Here the tyre size is specified by means of a three-digit number block, followed by an x (multiplication symbol) and a two-digit number block, which is combined with a letter.

Example

Source: Wikipedia (Moebiusuibeom-en, Bicycle tire marks 02, CC BY-SA 3.0)

The external tyre diameter in millimetres is stated via the three-digit number block, i.e. 700 mm in our example, while the two-digit number block defines the tyre width in millimetres, i.e. 45 mm in the example. What is unique about the French short designation is the letter - this actually states the tyre height, whereby an A stands for a tyre height of approx. 30 mm, a B for a tyre height of 33 mm and a C for a tyre height of approx. 39 mm.

How do you find the right tyres?

The simplest way is probably to read the tyre height on the side of the existing tyre, and to buy a tyre in exactly the same size. If the size specification is no longer legible, you can search for the most suitable version using the ETRTO, as the internal tyre diameter does not change. On the other hand, the external tyre diameter - the size from the size specification in inches and as found in the French size specification, very likely may change on account of differing tyre heights.