35C vs measured width: 37mm? 39mm? 40mm?

[Serge_K]

Conundrum time for us big dudes.

We used to ride narrow tyres where a 28C wouldnt measure more than 30mm. We've had lots of reports of 32C measuring 34mm, up to 35mm, afaik. It hasn't been too much of a problem, 1mm here or there.

Now that we're dealing with 35C, i expect measured width to be close to 40mm. I think Pogi's front tyre at Roubaix was 39 or 40mm, on that unreleased Enve rim? I think Jesse Coyle from the Nero Show measured his 35C at 38mm?
We can safely assume that 35C on a modern (wide) rim will measure more than 35mm. I'd say 37mm at a minimum. But if it measures 39mm, or 39 point something, then, and that's the question, does your frame clear 35, or 40?
Put another way,
- do frames need to start communicating as Xmm+4mm+4mm, and that way it's clear (with the caveat that front derailleurs can be the limiting factor, to complicate things further)?
- if the frame says 35C, should you expect / assume / require 39mm+4+4? 40mm+4+4?
- where do we draw the line?

Personally i think i can live with 39.5+3-3.5x2, but if it's less, i would be worrying that my entire build may be jeopardized by a tyre that rubs on a chainstay or a front derailleur. And that's not something i'd want to gamble on.

Are there any standards?

[Sakizashi]

35C is literally just shorthand for 700x35c which is just a way to say that this is a nominal 35mm tire that fits on a 700c rim (the actual sizing convention would be 622-35). The sizing convention is only relevant to ISO/ETRTO references.

The actual WAM depends on the IW of the reference rim the manufacturer uses for development. Some manufacturers disclose it, others don't, but it is usually somewhere in the middle of their ETRTO-listed compliant IW. It varies by manufacturer and this discrepancy is why a lot of 28mm and 30mm tires actually have the same size casing, but different size treads. For some brands, the jump is significant, going from a 21mm IW for 28mm tires to a 25mm IW for 30mm tires.

Frame manufacturers usually communicate clearances as some number in mm based on WAM, but this also varies. For example: WAM w. > 6 mm of clearance (most Western brands like Trek or Specialized following general ISO), WAM w. >4mm clearance (Usually TT bikes or 3T using ISO carve-out for "racing" bikes), or WAM w. >5.5mm (EN standard used by many Chinese factories). Rarely, they communicate a specific tire size as the max to be more consumer-friendly. Given the variance in how tire sizes measure out in real life, it's not that useful as a measurement, except to say that as a consumer you should expect a tire of that size to fit on a rim of "normal width." Given this, my current assumption would be that I would have at least 4mm of clearance with a 25mm IW rim for marked tire widths above 30mm.

Asking a factory repeatedly for their clearance in "C" probably just leads to them guessing at what size tire they think will fit.

[jfcb]

If a frame states clearance for 35C, I expect clearance of 35mm +4mm clearance on both sides cfr iso standards. For road bikes I expect this takes a FD into account.
For gravel bikes (once we go above 45mm and need to accomodate to mud clearance), I would use the standard of 6mm clearance on both sides and personally I don't care about FD clearance since it's 2026. However, I would still expect brands to report on it.

To figure out how wide a tire realy is, I beleive there is somewhere on the WeightWeenies forum a database with references per tire/rim dimension.