Your 30 isn’t a 30.
Now the UCI cares.
By Dan Bigham
The UCI Equipment Team have just reiterated in their latest Clarification Guide of the UCI Technical Regulation that tyre and internal rim width are regulated, and it now includes a compatibility table showing which internal rim widths are recommended for each nominal tyre width. It covers all rim types, hooked or hookless, and is taken from the ISO 5775 series of standards for bicycle tyres and rims. Teams have been asked to "check that your tyre and rim combinations fall within the recommended ranges" as 2027 preparations get going.
It reads like housekeeping, but it isn't. The rule itself isn't new, but this is the first time the peloton has been handed in writing the table that says which sidewall numbers suit which rims. That drags a question much of the industry has been happy to leave alone into the commissaires' truck: what does "30mm" actually mean?
There's been a lot of discussion in the media lately about what size a tyre actually is or should be. The answer is uncomfortable: a tyre can't be a true-to-size 30mm on a 25mm rim and a true-to-size 30mm by the standard that defines the number. Only one of those can be true, and it isn't the marketing one. On paper it's been a racing issue ever since the UCI tied tyres and rims to ISO. Now nobody can pretend it's just a consumer one.
What the UCI has actually said
The ISO link isn't new in itself. Article 1.3.018 of the UCI regulations already says that, "in order to comply with the requirements and ensure compatibility between the components, rims must comply with the standard ISO 5775-2 and tyres with the standard ISO 5775-1." The UCI last pointed teams at it in March 2024, after Thomas De Gendt's tyre came off a hookless rim at the UAE Tour, but it never spelled out what compliance looked like in millimetres. Beyond that, the line has mostly mattered to wheel brands getting onto the approved wheels list. What's new is that the UCI has put ISO's compatibility table in front of every team and asked them to check their equipment against it.
Redrawn so the gaps are easier to see:
Three things jump out straight away:
- The bins have cliffs. A 28mm tyre is recommended up to a 23mm internal rim. A 29mm tyre is recommended up to 25mm. One millimetre on the label buys two millimetres of rim.
- Wide road rims run out of tyres. At 26 to 27mm internal, nothing narrower than a 35mm tyre is recommended.
- "Recommended" is doing a lot of work. The regulation says rims and tyres must comply with ISO 5775. The table says recommended. The UCI doesn't say which of those a commissaire will apply, or whether a combination outside the table can be refused at the start.
And the table is only as good as the numbers you feed it. Which brings us back to the sidewall.
What "30" means on the sidewall
In ISO 5775-1, the 30 in "30-622" is the nominal section width: the inflated width the tyre is designed to measure on its design rim at its maximum pressure (clause 4.2.2.2).
The manufacturer doesn't choose the design rim. Table 4 sets it from the nominal width, and every tyre from 29 to 34mm shares the same one:
| NOMINAL TYRE WIDTH (MM) | ISO DESIGN RIM, INTERNAL WIDTH (MM) |
|---|---|
| 25 to 28 | 19 |
| 29 to 34 | 21 |
| 35 to 46 | 23 |
The test method (4.3.3) is explicit that the measuring rim must have the Table 4 width "for verification of compliance". The tyre goes to maximum sidewall pressure, sits for 24 hours, and is measured with callipers at four or more points.
So a 30 is a tyre that measures 30mm on a 21mm rim. Nothing in the standard lets a brand nominate a 25mm "modern" rim instead. And because the UCI table is lifted straight from that standard, nothing in it does either.
Why a 30 grows on a wider rim
Once a tyre is made, its width on any rim is set by a single number: the casing length from bead to bead. Inflation tensions the casing into a circular arc between the rim hooks. That arc can’t stretch (at least very much at all!), so moving the beads further apart forces it to bulge outwards.
Brace yourself for some algebra. Modelling the casing as an inextensible circular arc, installed width W on internal rim width c follows from:
s = W (π - arcsin c/W)
where s is the bead-to-bead casing length. For a properly sized 30, s is about 71 mm. Each extra millimetre of rim adds about 0.42 mm of width: 30.9mm on a 23mm rim, 31.8mm on 25mm, 32.7mm on 27mm. ISO’s own Formula 2 uses 0.4 mm per mm, so the model and the standard agree to within about 0.02mm.
The only way to get a tyre that reads 30 on a 25mm rim is to shorten the casing. That makes a different, smaller tyre, and it shows up the moment you measure it where ISO says to.
The tyre that measures 30 on a 25
The 30mm tyre has become the default road size, and with it has come a familiar claim: a new 30 "designed for modern rims", sized to measure 30mm on a 25mm internal rim instead of growing to 32. It sounds helpful. Buy a 30, fit it to your wide wheels, get 30.
Here is the same label with the casing sized for four different rims:
| Casing sized to read 30 on | Bead-to-bead (mm) | Width on 21 mm ISO rim (mm) | Width on 25 mm rim (mm) | Passes ISO check (min 28)? |
|---|---|---|---|---|
| 21 mm (ISO design rim) | 71.0 | 30 | 31.8 | Yes, full tolerance available |
| 23 mm | 68.0 | 29.1 | 30.9 | Yes, half the tolerance gone |
| 25 mm | 64.7 | 28.1 | 30.0 | Only just, no tolerance left |
| 27 mm | 60.7 | 27.0 | 28.9 | No |
Table 6 allows a 30 to measure down to 28mm on section width. That 2mm exists to absorb manufacturing scatter, not to redefine the tyre. Size the casing for 25mm and the whole allowance is gone before the first tyre leaves the mould, so ordinary production variation takes real tyres out of compliance. Size it for 27mm and it fails outright: on the rim that defines a 30, it's a 27.
Now put that tyre through the UCI table. Read by its label, it's a 30 on a 25mm rim: recommended, no problem. Read by what it measures, it's a 28 on a 25mm rim, and a 28 stops at 23. Same tyre, same rim, opposite answer, depending on whether you believe the sidewall or the callipers.
It gets worse below the design rim. That 64.7mm casing measures about 27.3mm on a 19mm rim, so by the 25 to 28 definition it's a 27, and the table stops a 25 to 27 at 22mm internal. However you slice it, a tyre that measures 30 on 25mm is, under the standard the UCI table is built on, a tyre on a rim it wasn't meant for.
Where the tolerances stack up
ISO checks width tolerance on the design rim and nowhere else, so on any other permitted rim the forced growth and the tolerance simply add. Table 6 is also less symmetric than "±2mm" suggests. The lower limit applies to section width, the casing itself. The upper limit applies to overall width, which includes lettering, ribs and rim protectors.
| RIM (INTERNAL) | INSTALLED WIDTH, NO TOLERANCE (MM) | WITH TABLE 6 TOLERANCE (MM) |
|---|---|---|
| 16 mm (narrowest recommended) | 28.0 | 26.0 to 30.0 |
| 21 mm (ISO design rim) | 30.0 | 28.0 to 32.0 |
| 25 mm (widest recommended) | 31.8 | 29.8 to 33.8 |
That's a spread of almost 8mm for one sidewall number without breaking a single rule, and every combination in it sits inside the UCI's new table. Nothing in ISO caps the combination of a permitted rim and a permitted tolerance, which is why a frame clearance check built on the sidewall number alone is optimistic. Rims carry their own manufacturing tolerance on top so the real world spread is wider still.
The headaches
For teams
- 28s on wide rims are out. A 28 on a 24 or 25mm internal rim falls outside the table. Move up to a 30 and, on a 25 mm rim, the installed width goes from about 30.7 to 31.8 mm. That changes the aero, the clearance and the pressure you'd choose.
- TT front setups get squeezed. The aero logic for years has been a narrow(er) tyre on a wider rim. A 25 to 27 is only recommended up to 21 to 22mm internal. A 26 on a 23mm front rim, which is exactly the kind of setup that tests well at low yaw, sits outside the table.
- Sponsor tyres may not mean what they say. If your tyre partner's 30 is sized for 25mm rims, the label passes and the callipers don't. Teams will want the ISO design-rim width in writing, not just the sidewall number.
- Clearance has to be built on the worst case. A legal, recommended 30 can reach 33.8mm overall on a 25mm rim. Add frame flex, debris and a wheel that isn't perfectly dished, and the frame clearance margin disappears very quickly.
For manufacturers
- Tyre brands selling a "30 for modern rims" carry a regulatory risk as well as a labelling one, and the table makes it much harder to ignore. The honest fixes are to size the casing to ISO, or to label the tyre for what it measures on its design rim.
- Wheel brands pushing internal width now have a ceiling. Anything over 25mm internal is only recommended with 35mm and wider tyres, which takes it out of most road racing scenarios. Expect a lot more care over how internal width is measured and quoted, because 25.0 and 25.5 mm now give very different outcomes!
- Frame brands need to quote clearance against the overall width a tyre can legally reach on the widest recommended rim, not against the sidewall number.
For the UCI
- Enforcement. ISO measures section width at maximum pressure, on the design rim, after 24 hours. That can't happen in a car park at 9am. In practice a commissaire can check the sidewall and the rim's claimed width, which are exactly the numbers that have drifted.
- The definition contradicts itself. The guide describes nominal width as "the tyre's stated width, measured at its widest point as per ISO specified method". A stated width and a measured width are two different things, and "widest point" is ISO's overall width (lettering, ribs and all), not the section width the size is actually defined by.
- Binned columns, fractional rims. The columns jump from 18–20 to 21–22 to 23. Real rims come in at 22.5 or 23.4 mm, and it isn't clear which column they fall into. Better rule definition is needed
- The bins invite gaming. With a 28 stopping at 23mm and a 29 running to 25, don't be surprised to see a sudden wave of 29s.
- The full table is behind a paywall. The guide says the complete table is in ISO 5775-1:2023, a paid-for standard most mechanics won't have.
- Recommended or required? Until that's answered, teams can't know whether an out-of-table combination is a talking point or a non-start.
What to do
There's an honest way to sell a tyre for wide rims, and the standard already allows it. Clause 4.2.5.6 lets a manufacturer add markings for the design rim and a recommended rim width range. A brand can label its tyre correctly and still tell you what it measures on your wheels:
- Size the tyre properly. A 30 should measure 30mm on a 21mm rim. If it's meant to sit at about 32 on a 25mm rim, that's exactly what a correctly sized 30 does.
- Publish installed widths. A short table of measured width against internal rim width, at a stated pressure, is enough. Better still, publish the bead-to-bead length, because that one number lets anyone predict width on any rim.
- Don't redefine the number. A "30 on modern rims" measures about 28 on the rim that defines a 30, and with the UCI's table now in every team's hands, that gap has (potentially significant) consequences.
None of this is an argument against wide rims or 30mm tyres. Our own testing keeps pointing to wider setups being faster on real roads. It's an argument for honest numbers because width feeds directly into pressure, rolling resistance, aerodynamics and frame clearance, and now into whether your bike is compliant. The UCI putting ISO's table in front of teams is a good thing, it just means the number on the sidewall finally has to mean what it says.