Rim and Tire StandardsJan Heine
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 3 mm if it’s a cap head). It doesn’t matter whether you’re touring in the U.S., Japan, Europe, or almost anywhere else in the world, you’ll be able to replace that bolt. (There are also standards for the bolt’s strength, which are 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 ERTRO standards. 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. The rim makers actually have the easy part of the job: Their rims just need to match the standards. Then it’s up to the tire makers to design tires that fit on all rims that meet the standards. It makes sense: The rim is made from metal or carbon and has dimensions that are easy to specify. The tires are flexible, and it would be difficult to make standards for all different types of tires.
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 with tubeless-compatible rims are:
Bead Seat Diameter (BSD; diameter of the rim at the shelf where the tire bead sits):
- 26″ : 559.0 mm
- 650B: 584.1 mm
- 700C: 621.95 mm
- Tolerance: ±0.5 mm
- Rims for tires with tubes: standards for 26″ and 650B are ~0.5 mm smaller. (700C is the same for tubed and tubeless.)
G Height (sidewall height on the inside of the rim):
- Tubeless rims with hooks: 5.5 mm
- Tubeless rims (hookless): 6.0 mm
- Tolerance: ±0.5 mm
These are the latest 2021 standards — much simplified from the past. 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. (The standards are presumably for bare rims.)
What this means in practice:
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 marked in yellow are fine for use with tubes, but not tubeless.
Values that fall clearly outside the standards are marked in orange. 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. This is especially important when mounting tires tubeless. (Tubes provide a reinforcement of the tire/rim interface, which gives you a little more leeway with rim/tire fit.)
Note: If the values are ≤0.1 mm outside the specs, we didn’t flag them.