How are tire sizes measured?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.