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Clincher sizing standards use the diameter of the bead and bead seat along with rules for where to measure and the dimensional profile of the bead and bead seat.

Tubulars don't have beads or bead seats. They have the contact area between the rim bed and the base tape, and that's it. One can imagine a standard based on the diameter of the the imaginary circles running along the middle of these surfaces, but does that exist in anything other than an informal way?

Tubulars are stretched to fit. If tubular compatibility is in fact based on informal or de facto standards, would changing to a formal standard based on the diameter of the rim bed and the ID of the base tape of the inflated tire eliminate the need for pre-stretching?

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    Good question, upvoted. But be open to the possibility that this may require industry inside knowledge to answer, e.g. the formal ISO specs are majorly paywalled, and posters here probably don’t have that.
    – Weiwen Ng
    Sep 17, 2022 at 19:13

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The most commons ise is 700C/27"/28", but here are smaller sizes that you may come across. As I said in a comment to another answer, I had a bike with tubulars and was sold a tire that would not fit. I presume I had 700C sized wheels and was sold a 26" tire (although the difference was small enough it was not obvious that with a bit of force the smaller tire could not be stretched on.)

Here it says

"Tubular tyres: 27″, 28″ and 700c are the same. 650c and 26″ are the smaller triathlon size and will not fit standard wheels"

Here says the same thing.. "Used primarily for racing, tubular tyres must be fixed to an appropriate rim using glue, cement or adhesive tape. Most tubulars likely to be encountered today will fit a rim with the same brake track diameter as a 700C clincher rim, and are may be referred to as 28”, 27” or even 700C or similar. These will all fit the same size rim. Tubular tyres require skilled fitting; if unsure, consult a suitably trained cycle mechanic. Other sizes including 26” may be encountered; these are rare and it is advisable to make a direct match with a rim before buying."

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I've got one 80's bike with tubulars, and it was a learning curve that ended with buying some clincher rims for everyday rides.

AFAIK all tubular tyres are 622 and 630mm capable. The difference is in the amount of stretching required. This doesn't feel right when coming from the world of hard-wire beaded tyres but it is.

A brand new clincher needs a good 20mm of stretch before it will fit a rim, and some brands require more.

Since this is a faff on the roadside, a "spare tube" should be a complete tyre that has seen service on the bike and has already taken a "set" to suit your rim.

Glue, tape and all that mess are another issue entirely.

The tubular tyre will conform to almost any curve, so this makes the shape of the rim relatively unimportant.

Most tubular tyres seem to range from 22mm to 30mm for road, with some tubular CX tyres in the 30-35mm range. I suspect any road tubular rim would take a width one up/down from the current size without any issue, and probably take all sizes.

Tubulars use a combination of adhesive and pressure to hold themselves to the rim. It is possible to use pressure only, but not just glue, so the tyre conforming to the rim's profile is an integral part of the design.

As for standards? Tubulars have been around since the 1930s (or earlier) so pre-date any kind of standarisations.

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    So, in other words you have tubular wheels and time to speculate but can’t measure the wheels?
    – ojs
    Sep 17, 2022 at 23:16
  • Not sure about 'one size fits all' - years ago I was given a road bike with tubulars (my first road bike, had no idea what I had been given - a old but very high end bike). Purchased a replacement tire and could not get it on the rim (an no amount of stretching was going to work)- turns out there are two sizes and had been given the wrong one.
    – mattnz
    Sep 17, 2022 at 23:48
  • @mattnz was it way off, like a 24" youth bike or something ? My point was the 622/630mm rim size uses the same tubular tyre from new (after its taken a set at 630 I'd avoid fitting it to a 622 rim.)
    – Criggie ♦
    Sep 18, 2022 at 1:01
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    This is the first time I’ve heard of 630mm tubulars and unless you’ve actually measured the rim i don’t believe it. The usual story is that 27” and 28” tubulars are both 622mm. Standardized parts have existed since Victorian times so I don’t believe tires would not have been standardized in 30s.
    – ojs
    Sep 18, 2022 at 1:08
  • The other tubular size is 26” which is coincidentally closer to 622mm so I understand the confusion.
    – ojs
    Sep 18, 2022 at 1:11

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