ISO 5775


ISO 5775 is an international standard for labeling the size of bicycle tires and rims. The system used was originally developed by the European Tyre and Rim Technical Organisation, and is now managed by the ISO. It is designed to make tire sizing consistent and clear, through replacing overlapping informal systems that ambiguously distinguished between sizes. For example, at least 6 different "26 inch" sizes exist, and "27 inch" wheels have a larger diameter than American "28 inch" wheels. This standard uses the bead seat diameter as a measurement for rim and tyre widths. The Japanese Industrial Standards Committee also cooperates with ISO 5775. The corresponding Japanese standards are JIS D 9112 for tires and JIS D 9421 for rims.

Part 1, which addresses tyres' designations and dimensions, was last published in August 2023 and Part 2, addressing rims, was last published in March 2021.
Part 1 considers tyres' nomenclature, size designation, marking, dimensions and compatibility with different rims and rim widths.
Part 2 considers both general requirements and specific requirements for 5 sub types of rim: straight-sided, tubeless straight-sided, hooked bead, crochet type, and tubeless crochet type rims.

Tire designations and dimensions

Part 1 considers separately the cases of
  1. clincher tyres mounted on straight sided type or crochet type rims, and
  2. hooked bead rims on which beaded edge tyres are mounted.

    Clincher tyres on straight sided or crochet type rims

Clincher tires can be mounted on straight-side or crochet-type rims. Crochet-type rims are not the same as hooked-bead rims. Such tires are designated with their nominal section width and their nominal rim diameter, separated by a hyphen. Both are measured in millimeters. A typical example of a tire marking according to ISO 5775-1 is:
  • The first number is essentially the width of the inflated tire.
  • The second number is the bead diameter of the tire when it is mounted on the rim, also called the bead seat diameter. The corresponding circumference can be measured with a suitably narrow tape inside the rim.
  • The recommended inflation pressure is marked in kilopascals.
The standard notes that the minimum inflation pressure recommended is 300 kPa for narrow tires, 200 kPa for other sizes in normal highway service, and 150 kPa for off-the-road service.
The inner width of the rim on which the tire is mounted should be about 65% of the tire's nominal section width for tires smaller than 30 mm and 55% for those larger.
The section height of a tire is usually identical to its section width. The overall diameter of the tire is then the rim diameter plus twice the tire's section height. The ISO 5775-1 standard also defines procedures for measuring tires and for calculating from the marking the maximum dimensions of a tire, which are needed by designers to determine clearance distances to other bicycle components.

Nominal section widths (tyres)

The standard defines for each tire size a rim design width based on the tire's nominal section width, and the range of rims widths which are recommended as acceptable to be used in service for a given tire section width.

Older markings

Note how in the below sorted table the "inches" measure is inconsistent with the mm measures, if height varies: Some 28-inch tires then have a measured actual bead seat diameter smaller than a 27-inch tire. 584 mm tires are also marketed as 27.5-inch tires. Older tire markings can be converted to ISO 5775 designations with the help of the following table:
Such older markings can be appended to the ISO 5775 designation in parentheses.

Beaded-edge tires

Beaded-edge tires are mounted on hooked-bead rims. They are marked with an overall diameter code and a nominal section-width code, separated by a cross. An example for such a marking is

Rims

Part 2 defines designations for bicycle rims. It distinguishes between
  • Straight-side type rims
  • Tubeless straight-side type rims
  • Crochet type rims
  • Tubeless crochet type rims
  • Hooked-bead rims
Both crochet and hooked-bead rims have inner profiles that curve inwards near the outside diameter of the rim to provide a hook that helps retain the tire bead under high pressure. On modern bikes crochet rims are most common and hooked bead rims are rare. The distinction is primarily that hooked-bead rims lack the defined bead seat of straight side and Crochet-type rims. The tire is held in position radially by the hook without a bead seat playing a role. Without a bead seat, the primary designation of the diameter in terms of the bead seat is not applicable, and the governing diameter is the OD.
Rims are designated by their nominal rim diameter and their nominal width, separated by a cross. Both are measured in millimeters. The rim type codes SS or HB precede the rim designation, whereas code C is appended to the nominal width. Examples:
The nominal width of a rim is the inner width between the straight sides or beads as one can easily measure it with a caliper.
The standard widths of straight-side rims are:
The standard widths of crochet-type rims are:
The standard widths of hooked-bead rims are:
The diameters of straight-side and crochet-type rims are measured as the bead seat diameter. Standard values are:
The diameters of hooked-bead rims are measured as the outside diameter of the rim. Standard values are: