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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 (ETRTO). It is designed to make tire sizing consistent and clear. It replaces overlapping informal systems that ambiguously distinguished between sizes. For example, at least 6 different "26 inch" sizes exist (just by American notation), and "27 inch" wheels have a larger diameter than American "28 inch" (French "700C") wheels.
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Wired-edge tires
Wired-edge tires can be mounted on straight-side or crochet-type (aka hook bead) rims. Such tires are designated with their nominal 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:
32-597 inflate to 400 kPaThe standard notes that the minimum inflation pressure recommended is 300 kPa for narrow tires (25 mm section width or less), 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 (for tires less than 28 mm, 2.5 mm have to be added to the width to get the height). 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
The standard defines a set of nominal section widths for wired-edge tires and lists recommended rim widths for each:
Older markings
Older markings of wired-edge tires 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
20×1.375Rims
ISO 5775-2 defines designations for bicycle rims. It distinguishes between
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:
SS 400×20, HB 422×25, 620×13CThe nominal width of a rim is the inner width between the straight sides or beads as one can easily measure it with a caliper (see the standard for drawings and exact measurement procedures).
The standard widths of straight-side rims are:
18, 20, 22, 24, 27, 30.5The standard widths of crochet-type rims are:
13C, 15C, 16C, 17C, 19C, 21C, 23C, 25CThe standard widths of hooked-bead rims are:
20, 25, 27The standard diameters of straight-side and crochet-type rims are:
194, 203, 222, 239, 248, 251, 279, 288, 298, 305, 317, 330, 337, 340, 349, 355, 357, 369, 381, 387, 390, 400, 406, 419, 428, 432, 438, 440, 451, 484, 489, 490, 498, 501, 507, 520, 531, 534, 540, 541, 547, 559, 565, 571, 584, 590, 597, 609, 622, 630, 635, 642The standard diameters of hooked-bead rims are:
270, 321, 372, 422, 459, 473, 510, 524, 560, 575, 611