Rim and Tire Standards

Rim and Tire Standards

Standards exist mostly to ensure interchangeability of parts. If you need replace one of the bolts that attach the bottle cages on your bike, you can go to any hardware store in the world and buy an M5 bolt. Unless it’s specified otherwise, the threads will have a diameter of 5 mm and a pitch of 0.8 mm. The head will match an 8 mm hex or a 4 mm Allen wrench (or a 4 mm if it’s a cap head). It doesn’t matter whether you’re touring in the U.S., in Japan or in Europe, you’ll be able to replace that bolt. (There are also standards for the bolt’s strength, which is important if you replace a highly-stressed part, like a handlebar clamp bolt on your stem.)

These standards are defined by different organizations. One of the most important is the International Standards Organization (ISO). There’s also the European Tire and Rim Technical Organization (ETRTO) that has defined standards for tires and rims, mostly for cars, but also for bicycles. Today, the ETRTO standards have been adopted by the ISO, so they are international. If you need a new tire for your car, you know that a 205/60 R 15 tire can be replaced with any tire that has the same designation.

The same applies to bicycles. A 38-622 mm tire – what many of us still call a 700C x 38 mm – will fit on any 622 mm (700C) rim that is made to the same ERTRO/ISO standards as the tire. The actual standards are voluminous documents that specify everything from how wide a tire you can fit on a given rim, how tall the hook needs to be (G height), all the way to the radius at the transition from hook to well. These standards allow us to develop tires that fit on all rims. For rim makers, the standards provide measurements that the rim needs to match – otherwise the standards won’t work.

If a component doesn’t match the standards, all bets are off. If the deviation is small, it may work, but there’s no guarantee. For high-performance parts, meeting the standards is especially important. A steel bolt can stretch a bit if the thread pitch is slightly off, but a bolt made from titanium or aluminum may break. The same applies to supple high-performance tires. A stiff tire may stay on an undersized rim, because it holds its shape even without proper support from the rim. A supple tire can lift off the rim in just one short section and blow off.

The most relevant parts of the current standards for performance bikes are:

Bead Seat Diameter (BSD; diameter of the rim at the shelf where the tire bead sits):

  • 26″ with hooks: 558.4 mm
  • 26″ Mtb tubeless: 559.0 mm
  • 650B with hooks: 583.55 mm
  • 650B Mtb tubeless: 584.1 mm
  • 700C Road with hooks: 621.95 mm
  • 700C Mtb tubeless: 621.5 mm
  • Tolerance: ±0.5 mm

G Height (sidewall height on the inside of the rim):

  • Rim width 13-15 mm (inside): 5.5 mm
  • Rim width 16-21 mm (inside): 6.0 mm
  • Rim width 22-29 mm (inside): 6.5 mm
  • Tubeless rims 19-23 mm (inside): 5.85 mm
  • Tubeless rims 24-30 mm (inside): 6.0 mm
  • Tolerance: ±0.5 mm

That looks quite complicated, probably because these standards evolved over time. (Rene Herse Cycles was not involved when the original ETRTO standards were developed.) The larger BSD for road rims may have been specified for the very high pressures many road cyclists used in the past. At high pressures, the tire stretches more, and its diameter increases. It’s unclear why 700C tubeless has a 0.5 mm smaller BSD than 26″ and 650B. There’s also no Road Tubeless standard yet. (As part of ASTM, the American standards organization, Rene Herse Cycles is working with other tire, rim and bicycle makers on Road Tubeless standards.) One thing to remember, too, is that adding tubeless rim tape reduces the G height by 0.2 mm and adds 0.4 mm to the BSD.

In essence, these standards might be boiled to to this:

  • Bead Seat Diameter (BSD): no more than 0.5 mm larger or 1.0 mm smaller than nominal wheel size. For tubeless, no more than 0.5 mm smaller than nominal wheel size. (26″: 559 mm; 650B: 584 mm; 700C: 622 mm)
  • G Height: somewhere between 5.35 and 6.35 mm if you want to run tubeless. Wide rims should have slightly taller sidewalls (5.5 – 6.5 mm).

If the BSD is too large, the tire will be hard or impossible to seat. If the BSD is too small, the tire will fit loosely and can blow off the rim. If the G height is too small, there is not enough support for the tire bead, and the tire can blow off the rim.

Manufacturers rarely list the Bead Seat Diameter and G height of their rims. The assumption is that they are made to the relevant standards. At Rene Herse Cycles, we test a lot of rims to check compatibility of our tires with different rim shapes and models. Below are the measurements of rims we’ve measured recently.

Tire fit depends on both BSD and G height. If the BSD is larger, the tire will fit tightly on the rim, and the G height can be a little smaller. We see this with some carbon rims: The outer diameter of carbon rims is determined by the mold and doesn’t vary much, but both G height and BSD are determined by inserts in the molds. If the inserts sit a bit higher, BSD will be larger and G height a bit smaller. These cases are marked in light red. They usually don’t cause problems.

Values that fall clearly outside the standards are marked in orange. Please remember that these standards are for use with tubes. For tubeless, based on the mtb standards, the lower boundary for acceptable BSD measurements is about 0.5 mm larger (at least for 26″ and 650B, although it makes sense for 700C as well). As a tire maker, Rene Herse Cycles cannot recommend running rims that don’t meet the standards to which we and everybody else designs their tires. Rene Herse Cycles can’t offer warranty support for tires mounted on rims that don’t meet these standards.

These measurements are part of a large overview of rims that will be published in the Spring or Summer 2022 Bicycle Quarterly. We’re looking at tire fit, ease of mounting, and a number of other factors. Since some of the measurements are relevant for safety, we are publishing them online now. We will update this list as we measure more rims.

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