How to Read Mountain Bike Rim Size

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--Howard Sutherland

• Diameter
• Circumference
• Tools
• About Math

Diameter

The rim'southward diameter will generally be well-nigh 10 mm larger than the ISO/E.T.R.T.O. bead-seat diameter, depending on how high the rim flanges stick upwards in a higher place the bottom of the rim channel.

Lay your measuring tape or ruler across the rim from ane side to the other. Get the largest measurement, between ii points direct opposite one some other. Slide one stop of the tape dorsum and forth along the rim until the measurement is largest. Mensurate across a few different diameters and have the boilerplate, in case the rim isn't quite round. (If information technology's fashion out of round, don't use it. If it's a millimeter or two out of round, the spokes volition pull a shallow-section aluminum or steel rim into shape -- not so much for a deep-section carbon-fiber rim..)

Next you leave your pocket calculator or smartphone app, or sometime-school newspaper and pencil. As an case, the measured diameter of the rim shown in the photo is 20 7/viii inches, or 20.875 inches. Multiplying by 25.four 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/Eastward.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 out the diameter, but if you lot have a built-up cycle, the hub will get in the mode of the tape mensurate, making it difficult to get an accurate measurement. For a built-upwardly wheel, information technology is easier to measure out 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 beneath the table.

Traditional Designation	Applications/Notes	ISO Bead Seat Diameter	Dewdrop Seat Circumference (Rim measurement)
36 inch	More often than not unicycles	787 mm	2472 mm	97.3 inches
32 inch	Mostly unicycles	686 mm	2155 mm	84.8 inches
28 ten ane 1/2, 700 B	English, Dutch, Chinese, Indian Rod-restriction roadsters (Likewise marked F10, F25, 700 B)	635 mm	1995 mm	78.5 inches
27 x anything except "27 five" and rare Danish 27 x i 1/2	Older road bikes, went out of fashion in the early 1980s	630 mm	1979 mm	77.nine inches
700 C, 28 x 1 5/8, 29 inch (28 x 1 i/2 F.xiii Canada)	Most newer developed bikes for route employ apply this size, including most route bikes and hybrids. 29 inch are fatty tires, aforementioned rim diameter. Also 700C tubular.	622 mm	1954 mm	76.nine inches
27 x 1 1/2	Rare Danish size	609 mm	1913 mm	75.3 inches
26 ten 1.25, x 1.375	Very Rare U.S. size, 1940s and older. Not bachelor.	599 mm	1881 mm	74.1 inches
26 x 1 1/4 EA.1, 26 ten 1 three/eight (S-6), 650	Schwinn "lightweights", older English "club" bikes	597 mm	1875 mm	73.8 inches
26 ten 1 3/8 (E.A.3), 650 A	Virtually 3-speeds except Schwinn; department-shop or juvenile 10 speeds	590 mm	1853 mm	73.0 inches
Mavic 26" tubular	Older Mavic 26" tubulars	587 mm	1844 mm	72.six inches
650B, 26 x 1 1/2, 26" tubular, "27 five"	French utility bikes, tandems, and loaded-touring bikes; some mountain bikes.This size is currently undergoing something of a renaissance. "27 v" are fat MTB tires. Some 26" tubulars.	584 mm	1834 mm	72.ii inches
700 D	Oddball size formerly used on some GT models. Not available. 584 mm tires may work. Annotation: 700D is ofttimes incorrectly identified equally 587mm bead seat.	583mm	1832 mm	72.1 inches
26 x ane, 650 C, 26" tubular	Triathlon, time-trial, high-operation road bikes for smaller riders. Some 26" tubulars.	571 mm	1793 mm	70.6 inches
26 x one 3/iv	Schwinn cruisers
26 x i.00- x 2.125, and wider on fatbikes	Well-nigh mountain bikes, cruisers, fatbikes	559 mm	1756 mm	69.1 inches
24 10 1 i/iv, 24 ten 1 3/viii (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 10 i 1/8, 24 ten 1 iii/8 (E.5)	British juvenile, most wheelchairs. French 600A is 541 mm, close enough. Saavedra 25" tubular	540 mm	1696 mm	66.8 inches
24 x i, 24 10 one 1/8, 24" tubular	High-operation wheels for smaller riders; Terry forepart, most 24" tubulars.	520 mm	1633 mm	64.3 inches
24 x 1.five- x two.125	Juvenile mount bikes, BMX cruisers	507 mm	1593 mm	62.7 inches
22 x one 3/eight	Wheelchair	501 mm	1573 mm	62.0 inches
550 A, 22 x 1 iii/8	European juvenile, folding bicycles	490 mm	1539 mm	60.6 inches
550C, 22 x one 1/4", 22" tubular	European juvenile and racing bicycles (rare); 22" tubular	470 mm	1477 mm	58.one inches
22 x 1.75; 10 2.125	Rare juvenile size...Schwinn	457 mm	1436 mm	56.5 inches
twenty x one i/8; x i 1/4; x one three/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 one iii/4, 20" tubular	Rare Schwinn juvenile, specialty racing bicycles, older Like shooting fish in a barrel Racers recumbents	419 mm	1316 mm	51.viii inches
20 x 1.5- 10 2.125	Most BMX, juvenile, folders, trailers, some recumbents	406 mm	1275 mm	50.2 inches
eighteen 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 ten 1 1/4, 18" tubular	Alex Moulton AM serial, 18" tubular for specialty racing bicycles.	369 mm	1159 mm	45.6 inches
xviii x 1.5- x 2.125	Birdy folding bikes	355 mm	1115 mm	43.nine 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 10 one 3/viii	Very rare mystery tire	337 mm	1059 mm	41.seven inches
xvi x 1 three/8	Very rare Polish juvenile	335 mm	1052 mm	41.4 inches
sixteen x 1 iii/iv	Rare Schwinn juvenile. Probably the same rim diameter as sixteen" tubulars.	317 mm	996 mm	39.two inches
xvi x 1.75- x 2.125	Juvenile, folders, trailers, some recumbents	305 mm	958 mm	37.7 inches
12 1/2 X anything, deicmal or fraction.	Juvenile, scooters, trailers.	203 mm	638 mm	25.i inches
ten 10 2	Wheelchair casters	152 mm	478 mm	xviii.eight inches
8 x 1 1/4	Wheelchair casters	137 mm	431 mm	16.nine inches

Circumference

You lot may measure the circumference of a rim by wrapping a measuring tape all the way around the rim. Y'all derive the bore from the circumference.

A narrow, metallic record measure -- 1/4 inch or half-dozen mm wide -- will fit into the well of the rim. (A wide metal tape measure out won't fit into the well of the rim and and won't bend smoothly around the rim.)

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

Use the metal tape measure out as shown in the image beneath.

Here are the steps to measure using the circumference:

1. The tape has a tab at the terminate. Hook the tab into the valve hole and wrap the tape all the way around the rim, measuring the full circumference at the bottom of the well.
2. Separate the circumference past pi (iii.142) to get the bore of the well.
   
3. If the record measure is divided in inches, besides 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 lot don't have a narrow tape measure, you could wrap a length of thin, flexible electrical wire or wheel cable inner wire around the rim, mark 2 places on the wire which line upward with one another, lay the piece out flat and measure the distance between the two marks.

Our example rim is a claw-border rim without clearly-defined dewdrop seats, and so we'll measure out from the well (merely not the lesser of the recessed spoke holes) to the outside of the rim and and so subtract twice the typical flange height. Our highly-sophisticated tool for this task is a bicycle spoke. We are likewise using a small ruler as a bridge across the rim flanges. Holding the spoke with a thumbnail confronting the ruler gives us a expert enough measurement for our purposes.

The measurement can be transferred to the ruler:

Now, computing, the circumference of the well measured as 64 one/8 inches, (64.125 inches). Multiplying by 25.iv gives 1629 mm; then dividing past 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 exist near 10 mm smaller, and this is a 520 mm rim.

More sophisticated tools

This article has featured mutual, inexpensive and improvised tools. More sophisticated tools such every bit a caliper with a depth estimate can brand the piece of work become faster.

Nigh mathematics

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

Anyone who cannot cope with mathematics is non fully man. At all-time 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. OK, then Heinlein (actually, a character in one of his books) said that, and Sheldon quoted that, simply anyone who cannot take that other people have different abilities and educational opportunities shows an unfortunate lack of compassion! -- John Allen, whose wife teaches immigrants who never got to attend schoolhouse in their countries of origin.

Simply as well at that place'southward this:

"Do not worry about your difficulties in Mathematics. I tin can assure you mine are even so greater."

--Albert Einstein

I'yard hoping that this commodity provides an like shooting fish in a barrel guide to some practical math, and helps to dispel math anxiety. As well, a longer discussion of the Einstein quote is online, in example you're interested...

Sutherland's Rim Measuring System

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Reports of the demise of this Web site are greatly exaggerated! We at sheldonbrown.com give thanks Harris Cyclery for its support over the years. Harris Cyclery has closed, but nosotros go along going. Keep visiting the site for new and updated articles, and news almost possible new affilations.

Copyright © 2006, 2008 Sheldon Brown

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