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Measuring Bicycle Rims for Tire Fit
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by Sheldon "ISO/E.T.R.T.O." Brown
and John "Measure Twice" Allen
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One measurement is worth 50 expert opinions.

--Howard Sutherland

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What size tire do you need to buy for your bicycle? There are so many different tire sizes and different systems for marking tire sizes that have been used over the years that this is often a serious problem, especially for older bicycles.

rim diagram I have a major article on this web site explaining the different tire sizing systems. Whenever possible, you should try to match your new tire to the old tire, using the ISO/E.T.R.T.O. size number, if it is marked on the tire or rim.

If the ISO/E.T.R.T.O. size number matches, it means the tire will fit on the rim. Tires are also designated for different widths, but you can interchange tires of different widths as long as you match the ISO/E.T.R.T.O. size number and the widths are not wildly out of line.

But what do you do if you don't have the original tire, or if the markings are not legible? All is not lost! If you have a tape measure, you can measure the rim to determine the ISO/E.T.R.T.O. Bead Seat Diameter. You can measure either the diameter or the circumference.

Diameter

The rim's diameter will generally be 5-10 mm larger than the bead-seat diameter, depending on how high the rim flanges stick up above the bottom of the rim channel.

Lay your measuring tape or ruler across the rim from one side to the other. Get the largest measurement, between two points directly opposite one another. Slide one end of the tape back and forth along the rim until the measurement is largest.

measuring rim diameter directly

Next you get out your pocket calculator or smartphone app, or if you would like to show that you attended elementary school before such things existed, paper and pencil. The measured diameter of the rim shown in the photo is 20 7/8 inches, or 20.875 inches. Multiplying by 25.4 gives the diameter in millimeters, 530 mm.

The table below gives common (and uncommon) rim sizes with the corresponding bead seat circumference dimensions. 530 mm is the outside diameter, and the ISO/E.T.R.T.O. diameter is a few millimeters smaller, so this is a 520 mm rim.

If you are working with an empty rim, it is easiest to measure the diameter, but if you have a built-up wheel, the hub will get in the way of the tape measure, making it difficult to get an accurate measurement. For a built-up wheel, it is easier to measure the rim's circumference. It may also be helpful to take a circumference measurement to confirm that the diameter measurement was correct.

Instructions on measuring using the circumference are below the table.

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Traditional Designation Applications/Notes ISO Bead Seat Diameter Bead Seat Circumference
(Rim measurement)
28 x 1 1/2, 700 B English, Dutch, Chinese, Indian Rod-brake roadsters
(Also marked F10, F25, 700 B)
635 mm 1995 mm 78.5 inches
27 x anything Older road bikes, went out of fashion in the early 1980s 630 mm 1979 mm 77.9 inches
700 C, 28 x 1 5/8, 29 inch
(28 x 1 1/2 F.13 Canada)
Most newer adult bikes for road use use this size, including most road bikes and hybrids. 29 inch are fat tires, same rim diameter. Also 700C tubular. 622 mm 1954 mm 76.9 inches
26 x 1.25, x 1.375 Very Rare U.S. size, 1940s and older. Not available. 599 mm 1881 mm 74.1 inches
26 x 1 1/4 EA.1, 26 x 1 3/8 (S-6), 650 Schwinn "lightweights", older English "club" bikes 597 mm 1875 mm 73.8 inches
26 x 1 3/8 (E.A.3), 650 A Most 3-speeds except Schwinn, department-store or juvenile 10 speeds 590 mm 1853 mm 73.0 inches
700 D, 26" tubular Oddball size formerly used on some GT models. Not available. 590 mm or 584 mm tires may work. Also, older Mavic 26" tubulars 587 mm 1844 mm 72.6 inches
650B, 26 x 1 1/2, 26" tubular French utility bikes, tandems, and loaded-touring bikes; some older Raleigh and Schwinn mountain bikes.This size is currently undergoing something of a renaissance. Some 26" tubulars. 584 mm 1834 mm 72.2 inches
26 x 1, 650 C, 26" tubular Triathlon, time trial, high performance road bikes for smaller riders. Some 26" tubulars. 571 mm 1793 mm 70.6 inches
26 x 1 3/4 Schwinn cruisers
26 x 1.00- x 2.125 Most mountain bikes, cruisers 559 mm 1756 mm 69.1 inches
24 x 1 1/4, 24 x 1 3/8 (S-5) Rare British or Schwinn juvenile 547 mm 1718 mm 67.7 inches
600A French juvenile, very close to the nearest British size. 541 mm 1699 mm 66.9 inches
24 x 1 1/8, 24 x 1 3/8 (E.5) British juvenile, most wheelchairs. French 600A is 541 mm, close enough. Saavdra 25" tubular 540 mm 1696 mm 66.8 inches
24 x 1, 24 x 1 1/8, 24" tubulars High performance wheels for smaller riders; Terry front, most 24" tubulars. 520 mm 1633 mm 64.3 inches
24 x 1.5- x 2.125 Juvenile mountain bikes, BMX cruisers 507 mm 1593 mm 62.7 inches
22 x 1 3/8 Wheelchair 501 mm 1573 mm 62.0 inches
550 A, 22 x 1 3/8 European juvenile, folding bicycles 490 mm 1539 mm 60.6 inches
550C, 22 x 1 1/4", 22"tubular European juvenile and racing bicycles (rare); 22" tubular 470 mm 1477 mm 58.1 inches
22 x 1.75; x 2.125 Rare juvenile size...Schwinn 457 mm 1436 mm 56.5 inches
20 x 1 1/8; x 1 1/4; x 1 3/8 Juvenile lightweights, BMX for light riders, some recumbents, Bike Friday Pocket Rocket 451 mm 1417 mm 55.8 inches
500 A European juvenile, folding 440 mm 1382 mm 54.4 inches
20 x 1 3/4, 20"tubular Rare Schwinn juvenile, specialty racing bicycles, older Easy Racers recumbents 419 mm 1316 mm 51.8 inches
20 x 1.5- x 2.125 Most BMX, juvenile, folders, trailers, some recumbents 406 mm 1275 mm 50.2 inches
18 x 1, 18 x 1 3/8 Wheelchair 400 mm 1257 mm 49.5 inches
450 A European juvenile 390 mm 1225 mm 48.2 inches
17 x 1 1/4, 18" tubular Alex Moulton AM series, 18" tubular for specialty racing bicycles. 369 mm 1159 mm 45.6 inches
18 x 1.5- x 2.125 Birdy folding bikes 355 mm 1115 mm 43.9 inches
16 x 1 3/8 Older Moulton, Brompton & other folders, recumbent front, Greenspeed trikes, juvenile 349 mm 1096 mm 43.2 inches
400 A European juvenile 340 mm 1068 mm 42.1 inches
16 x 1 3/8 Very rare mystery tire 337 mm 1059 mm 41.7 inches
16 x 1 3/8 Very rare Polish juvenile 335 mm 1052 mm 41.4 inches
16 x 1 3/4 Rare Schwinn juvenile. Probably the same rim diameter as 16" tubulars. 317 mm 996 mm 39.2 inches
16 x 1.75- x 2.125 Juvenile, folders, trailers, some recumbents 305 mm 958 mm 37.7 inches
12 1/2 X anything. Juvenile, scooters, trailers. 203 mm 638 mm 25.1 inches
10 x 2 Wheelchair casters 152 mm 478 mm 18.8 inches
8 x 1 1/4 Wheelchair casters 137 mm 431 mm 16.9 inches

Circumference

You may measure the circumference of a rim by wrapping a measuring tape all the way around the rim. You derive the diameter from the circumference.

A narrow, metal tape measure -- 1/4 inch or 6 mm wide -- will fit into the well of the rim. (A wide metal tape measure won't fit into the well of the rim and and won't curve smoothly around the rim.)

Don't trust a fabric measuring tape as used in fitting clothing. This kind is usually inaccurate, because the fabric stretches.

Use the metal tape measure as shown in the image below.

Measuring the circumference of a rim

Here are the steps to measure using the circumference:

  1. The tape has a tab at the end. Hook the tab into the valve hole and wrap the tape all the way around the rim, measuring the total circumference at the bottom of the well.
  2. Divide the circumference by pi (3.142) to get the diameter of the well.
  3. If the tape measure is divided in inches, also multiply by 25.4 to get the diameter of the well in millimeters.
  4. Add twice the height from the well to the bead seats (see instructions below).

If you don't have a narrow tape measure, you could wrap a length of thin, flexible electrical wire or bicycle cable inner wire around the rim, mark two places on the wire which line up with one another, lay the piece out flat and measure the distance between the two marks.

Our example rim is a hook-edge rim without clearly-defined bead seats, so we'll measure from the well (but not the bottom of the recessed spoke holes) to the outside of the rim and then subtract twice the typical flange height. Our highly-sophisticated tool for this task is a bicycle spoke. We are also using a small ruler as a bridge across the rim flanges. Holding the spoke with a thumbnail against the ruler gives us a good enough measurement for our purposes.

Measuring rim depth

The measurement can be transferred to the ruler:

Transferring measurement to ruler

Now, calculating, the circumference of the well measured as 64 1/8 inches, (64.125 inches). Multiplying by 25.4 gives 1629 mm; then dividing by pi (3.142) the diameter is 518.5mm. 16mm additional (twice the depth of the well) gives 534 mm, but the bead seat diameter is about 10 mm smaller, and this is a 520 mm rim.

More sophisticated tools

This article has featured common, inexpensive and improvised tools. More sophisticated tools such as a caliper with a depth gauge can make the work go faster.

Sutherland's sells a handy measuring tape which automatically calculates the diameter of a rim from the circumference. It is intended to measure spoking diameter but may also be used to measure the bead seat diameter.

Howard Sutherland demonstrates the Rim Diameter System in the video below:

About mathematics

Measuring rims involves some elementary-school or pocket-calculator math. Sheldon quoted Robert Heinlein at the end of the article:

Anyone who cannot cope with mathematics is not fully human. At best he is a tolerable subhuman who has learned to wear shoes, bathe, and not make messes in the house.

--Robert A. Heinlein

More about that quote is online. OK, so Heinlein (actually, a character in one of his books) said that, and Sheldon quoted that, but anyone accepting that other people have different abilities and educational opportunities shows compassion! -- John Allen, whose wife teaches immigrants whose educational opportunities were very limited in their countries of origin.

But also there's this:

“Do not worry about your difficulties in Mathematics. I can assure you mine are still greater.”

--Albert Einstein

I'm hoping that this article provides an easy guide to some practical math, and helps to dispel math anxiety. Also, a longer discussion of the Einstein quote is online, in case you're interested...

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Sutherland's Rim Measuring System

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