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I tried getting a good measurement with my caliper, but the flexibility of the tire doesn't help much. Let's say I have a 26"x2.125" tire. I know this fits on an ISO 559mm bead seat diameter rim, and 26" = 660.4mm.

26":

  • The OD of the tire including the tread?
  • The OD of the tire excluding the tread?
  • When aired up or deflated?
  • Means it fits a 559mm ISO BSD
  • None of the above?

2.125":

  • Width of tread?
  • Width of sidewall?
  • Inflated or deflated?

Essentially, I'm converting the tire sizes to "height off rim in mm" and drawing them with a wheel in a web application, so I need to know how "tire size" translates to this measurement. I'm somewhat assuming that the fractional sizes can be converted to the same measurement, but I'm aware that they have certain differences - so any help there would be of value as well.

Sheldon Brown's Website has been helpful, but still leaves me with the questions above.

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I've never seen anything more complete than the Sheldon Brown website. If you read through it you'll see that there IS NO "SYSTEM" for the inch-dimensioned tires, but rather manufacturers kind of made it up as they went along. And, although "tire size is determined by the diameter of the tire", the real reference point is the diameter of the rim, since that determines what tires will fit. A "slick" and a heavily lugged off-road tire can fit the same rim, even though their actual outer diameters are significantly different. –  Daniel R Hicks May 2 '12 at 1:14
    
Got it. In this case, I don't much care for fitment, just how to draw it. Essentially I'm using if (tireSize > 70) tireOD = tireSize; else tireOD = tireSize * 25.4; to cover what I needed. For some bizarre reason I got to thinking the second number was height off the rim, and I was making it much harder for myself. –  Ehryk May 2 '12 at 1:45
    
Follow-up question: What is the 'C' in 700C? Is there also '700B' or '700D'? –  Popup Jun 12 at 11:59
    
Answering my own question - it looks like the 'C' is for 'clincher', as distinguished from earlier rim shapes. –  Popup Jun 12 at 12:09
    
The C isn't for clincher. See my answer here. –  Batman Nov 27 at 0:22

3 Answers 3

up vote 4 down vote accepted

First Number = Nominal Outer Diameter of tire, including tread.

Second Number / Letter = Nominal width of tire at the start of the tread.

Tire size is determined by the outside diameter of the tire, as regards the 26 inch or 700c number. The 2.125 inch number is the nominal width of the tire at the point where the tread starts.

These are nominal measurements, though, and there is no universal standard applied consistently other than bead diameter.

700c x 23 tires from Schwalbe will measure 28mm in width, and about 5 mm extra in diameter. From Conti, its about 24mm and 1-2mm extra. There is no stadard for point of measurement. It is left to the manufacturer to decide, even before the marketing department gets involved.

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But the second number is width though? That explains some things. So I think the number I'm looking for is just the first one converted to MM - 660.4 for any 26" tire, 700 for any 700C tire. The 'height above rim' is thus variable based on the rim ISO bead seat diameter + 2 * (rim height above BSD (3-4mm or so)). –  Ehryk May 1 '12 at 18:43
    
At least as a nominal measurement, yes. A particular tire's measurement may vary. –  zenbike May 1 '12 at 18:53
    
If I was to guess based on your answer, you're saying: - 26" = OD including tread, inflated to recommended PSI - 2.125" = Width at tread, inflated to recommended PSI On a side note, how do you know where tread begins on a road tire? –  Ehryk May 1 '12 at 19:00
    
Road tire is measured to the highest point. But once again, these are marketing numbers, not physical measurements printed on the tire. They may or may not be more than passingly related. The bead diameter is the only one you can count on fro real accuracy. –  zenbike May 1 '12 at 21:54
    
The OD / Height is to the highest point, yes. How do you determine the start/end of the tread for the width though? Or is it just at the widest point, regardless of tread? Or expected width at rim? This would also apply for MTB / BMX slicks as well, where there isn't a clear transition from sidewall to tread. –  Ehryk May 2 '12 at 3:32

The only reliable tire number is the E.T.R.T.O number that is required by EU law. It's on every tire made in the last 10 years or so. Anything else on the tire is often just marketing, in particular you cannot trust the widths that tire manufacturers provide in English units. One manufacturers 2.4 is a 2.125 from a different manufacturer. However, they will have the same ETRTO code since that is based on the actual physical measurements of the tire.

As a reasonable first approximation, you can take the width part of the ETRTO code as the height of the tire above the diameter part of the ETRTO code when inflated. It does not take tread height into account, but that's lost in the noise. It's not perfect, but there's no better guess you can make w/o measuring each tire on each rim.

For example: 700C originally meant a wheel/tire with a 700mm diameter when installed and inflated. Since the rim is 622mm, that meant the tires added 78mm to the wheel or were roughly 39mm in height above the rim. Makes sense since C was the "fat tire" version of the various 700mm wheels.

The old A, B, C model is making a comeback in the MTB world. You can get 26,27.5 and 29'er wheels that are nearly the same diameter by choosing the right mix and match of tires.

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The "26" is for historical reasons. Ignore it as a measurement; it is just a name.

Early on, tires were designated by overall diameter not by rim size. Perhaps rims were destroyed as often as tires, and people tended to replace whole wheels. With this measurement system you would have a good idea about frame clearance and stand-over height, at the expense of clarity about the tire-rim compatibility.

It's not just the inches. Metric wheels did this too. If you look at the 700 series there used to be 700A, 700B, 700C, and 700D. A was the narrowest, D the widest. As you would expect, if they all had the same outer diameter, A is the rim largest rim with a diameter of 642mm, 700C is near the middle with 622mm and 700D were originally balloony tires with a rim size of 587mm.

29ers are the new name for 700C. 700mm works out to a little over 27.5 inches, yet the "27.5" inch tires are the same rim diameter as the 650B.

Any diameter numbers other than the bead seat diameter are just names, not measurements.

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"Long ago"?? What is a "29er" then? –  Daniel R Hicks Nov 27 at 3:41

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