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Tire Sizing

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by Sheldon "ISO/E.T.R.T.O." Brown

This article is also available in Russian Russian flag

Tire Sizing Charts:
Inch Based Systems:Metric Based Sizing Systems:
DecimalFractionalFrenchISO/E.T.R.T.O.

Dishonest Sizing | Fractional vs Decimal Sizing | The ISO/E.T.R.T.O. System

Traditional Sizing Systems | Width Compatibility | WTB's Global Measuring System

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Which Size Tire Fits Which Size Rim?

Bicycle tires come in a bewildering variety of sizes. To make matters worse, in the early days of cycling, every country that manufactured bicycles developed its own system of marking the sizes. These different national sizing schemes created a situation in which the same size tire would be known by different numbers in different countries. Even worse, different-sized tires that were not interchangeable with one another were often marked with the same numbers!

Traditional Sizing Systems

The traditional sizing systems are based on a measurement of the outside diameter of a tire. This would usually be measured in inches (26", 27", etc.) or millimeters (650, 700, etc.).

Unfortunately, evolution of tires and rims has made these measurements lose contact with reality. Here's how it works: Let's start with the 26 x 2.125 size that became popular on heavyweight "balloon tire" bikes in the late '30's and still remains common on "beach cruiser" bikes. This size tire is very close to 26 inches in actual diameter. Some riders, however were dissatisfied with these tires, and wanted something a bit lighter and faster. The industry responded by making "middleweight" tires, marked 26 x 1.75 to fit the same rims. Although they are still called "26 inch", these tires are actually 25 5/8", not 26". This same rim size was adopted by the early pioneers of west-coast "klunkers", and became the standard for mountain bikes. Due to the appetite of the market, you can get tires as narrow as 25mm to fit these rims, so you wind up with a "26 inch" tire that is more like 24 7/8" in actual diameter!

A second number or letter code would indicate the width of the tire. (26 x 1.75, 27 x 1 1/4...650B, 700C...)

Does "Point Seven Five" Equal "Three Quarters?"

Note that the inch-based designations sometimes express the width in a decimal (26 x 1.75) and sometimes as a common fraction (26 x 1 3/4). This is the most common cause of mismatches. Although these size designations are mathematically equal, they refer to different size tires, which are NOT interchangeable. It is dangerous to generalize when talking about tire sizing, but I would confidently state the following:

Brown's Law Of Tire Sizing:

If two tires are marked with sizes that are mathematically equal,
but one is expressed as a decimal and the other as a fraction,
these two tires will not be interchangeable.

Note that even some tire manufacturers get confused about this. In particular, some Continental models are mis-marked, using a fractional designation where they should be using a decimal.

Dishonesty in Sizing

Competitive pressures have often led to inaccuracy in width measurement. Here's how it works: Suppose you are in the market for a high performance 700 x 25 tire; you might reasonably investigate catalogues and advertisements to try to find the lightest 700-25 available. If the Pepsi Tire Company and the Coke Tire Company had tires of equal quality and technology, but the Pepsi 700-25 was actually a 700-24 marked as a 25, the Pepsi tire would be lighter than the accurately-marked Coke 700-25. This would put them at a competitive advantage. In self defense, Coke would retaliate by marketing an even lighter 700-23 labeled as a 700-25.

This scenario prevailed throughout the '70's and '80's. The situation got so out-of-hand that cooler heads have prevailed, and there is a strong (but not universal) trend toward accurate width measurements.

The ISO (E.T.R.T.O.) System:

ISO, the International Organization for Standardization has developed a universal tire sizing system that eliminates this confusion. (This system was formerly known as the "E.T.R.T.O." system, developed by the European Tyre and Rim Technical Organization.)

The ISO system uses two numbers; the first is the width of the tire or rim in millimeters (The actual tire width will vary a bit depending on the width of the rim. The rim width is measured between the flanges.)

The second ISO number is the critical one, it is the diameter of the bead seat of the rim, in mm. Generally, if this number matches, the tire involved will fit onto the rim; if it doesn't match, the tire won't fit.

For example, a 700 x 20 C road tire would be a 20-622; a 700 x 38 hybrid tire would be a 38-622. The width difference between these sizes would make them less-than ideal replacements for one another, but any rim that could fit one of them would work after a fashion with the other.

The following is a partial listing of traditional tire sizes that are sometimes seen in the U.S., with their ISO bead seat equivalents.

Fractional sizes:

FractionalISOApplications
28 x 1 1/2635 mm Rod-brake roadsters
27 x anything630 mm Older road bikes
26 x 1 (650C)571 mm Triathlon, time trial, small road bikes
26 x 1 1/4597 mm Older British sport & club bikes
26 x 1 3/8 (S-6)597 mm Schwinn "lightweights"
26 x 1 3/8 (E.A.3)590 mm Most 3-speeds, department-store or juvenile 10 speeds
26 x 1 1/2 (650B)584 mm French utility, tandem and loaded-touring bikes,
a very few Raleigh (U.S.) & Schwinn (S-4) mountain bikes.
26 x 1 (650C) 571 mm Triathlon, time trial, small road bikes
26 x 1 1/2 (F.12)Older Canadian bikes

Schwinn cruisers

26 x 1 3/4 (S-7)
24 x 1520 mm High performance wheels for smaller riders; Terry front
24 x 1 1/8520 mm or
540 mm!
Caveat emptor!
24 x 1 1/4547 mm British or Schwinn Juvenile
24 x 1 3/8 (S-5)547 mm Schwinn Juvenile lightweights
24 x 1 3/8 (E-5)540 mm British Juvenile, most wheelchairs
20 x 1 1/8
20 x 1 1/4
20 x 1 3/8
451 mm Juvenile lightweights, BMX for light riders, some recumbents
20 x 1 3/4419 mm Schwinn juvenile
17 x 1 1/4369 mm Alex Moulton
16 x 1 3/8349 mm Older Moulton, juvenile
16 x 1 3/8337 mm Mystery tire
16 x 1 3/8335 mm Polish juvenile
16 x 1 3/4317 mm Schwinn Juvenile
12 1/2 x anything203 mm Juvenile, scooters
10 x 2152 mm Wheelchair
8 x 1 1/4137 mm Wheelchair

Traditionally, fractional sizes are made for straight-sided rims.
High-performance sizes (571 mm /26 x 1 & 630 mm /27") have evolved toward hook-edged rims.

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Decimal sizes:

DecimalISOApplications
26 x 1.00 through 2.3559 mm Most Mountain bikes, cruisers, etc. except:
26 x 1.25 (rare)599 mm Very old U.S. lightweights
26 x 1.375599 mm Very old U.S. lightweights
24 x 1.5-24 x 2.125507 mm Juvenile mountain bikes, cruisers
22 x 1.75, 22 x 2.125457 mm Juvenile
20 x 1.5-20 x 2.125406 mm Most BMX, juvenile, folders, trailers, some recumbents
18 x 1.5355 mm Birdy folding bikes
18 x 1.75-18 x 2.125355 mm Juvenile
16 x 1.75-16 x 2.125305 mm Juvenile, folders, trailers, some recumbents

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French sizes:

In the French system, the first number is the nominal diameter in mm, followed by a letter code for the width: "A" is narrow, "D" is wide. The letter codes no longer correspond to the tire width, since narrow tires are often made for rim sizes that originally took wide tires; for example, 700 C was originally a wide size, but now is available in very narrow widths, with actual diameters as small as 660 mm.

French SizeISOApplications
700 C622 mm Road bikes, hybrids.
700 D587 mm Oddball size formerly used on some GT models.
650 A590 mm French version of 26 x 1 3/8; Italian high-performance bikes for smaller riders
650 B584 mm French utility bikes, tandems, and loaded-touring bikes; some older Raleigh and Schwinn mountain bikes
650 C571 mm Triathlon, time trial, high performance road bikes for smaller riders
600 A540 mm European Juvenile road bikes, most wheelchairs
550 A490 mm European Juvenile road bikes
500 A440 mm European Juvenile, folding
450 A390 mm European Juvenile
400 A340 mm European Juvenile

ISO Cross Reference:

ISO Bead Seat DiameterTraditional Designations
635 mm28 x 1 1/2
630 mm27 x anything
622 mm700 C
599 mm26 x 1.25, x 1.375
597 mm26 x 1 1/4, 26 x 1 3/8 (S-6)
590 mm26 x 1 3/8 (E.A.3), 650 A
587 mm700 D
584 mm650B, 26 x 1 1/2
571 mm26 x 1, 26 x 1 1/2, 26 x 1 3/4, 650 C
559 mm26 x 1.00- x 2.125
547 mm24 x 1 1/4, 24 x 1 3/8 (S-5)
540 mm24 x 1 1/8, 24 x 1 3/8 (E.5), 600 A
520 mm24 x 1, 24 x 1 1/8
507 mm24 x 1.5- x 2.125
490 mm550 A
457 mm22 x 1.75; x 2.125
451 mm20 x 1 1/8; x 1 1/4; x 1 3/8
440 mm500 A
419 mm20 x 1 3/4
406 mm20 x 1.5- x 2.125
390 mm450 A
369 mm17 x 1 1/4
355 mm18 x 1.5- x 2.125
349 mm16 x 1 3/8
340 mm400 A
337 mm16 x 1 3/8
317 mm16 x 1 3/4
305 mm16 x 1.75- x 2.125
203 mm12 1/2 X anything.
152 mm10 x 2
137 mm8 x 1 1/4

Most of this information was compiled by John Allen for Sutherland's Handbook For Bicycle Mechanics, the bible of bicycle technology. Sutherland's has a more detailed, more thorough version of this chart.

 

Width Considerations

Although you can use practically any tire/rim combination that shares the same bead seat diameter, it is unwise to use widely disparate sizes.

If you use a very narrow tire on a wide rim, you risk pinch flats and rim damage from road hazards.

If you use a very wide tire on a narrow rim, you risk sidewall or rim failure. This combination causes very sloppy handling at low speeds. Unfortunately, current mountain-bike fashion pushes the edge of this. In the interest of weight saving, most current mountain bikes have excessively narrow rims. Such narrow rims work very poorly with wide tires, unless the tires are overinflated...but that defeats the purpose of wide tires, and puts undue stress on the rim sidewalls.

Georg Boeger has kindly provided a chart showing recommended width combinations:

Which tire fits safely on which rim?
[all dimensions in millimeters]
Tire width
Rim width
(interior)
182023252832353740444750 5457
13 XXXX           
15   XXXX          
17    XXXXX       
19     XXXXXX     
21       XXXX XX  
23         X XXX  
25          XXXXX
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Wilderness Trail Bikes' Global Measuring System

From the WTB Website:

imageGMS Global Measuring System The current industry standard for specifying the actual inflated size of a bicycle tire does not account for subtle variation in tread and casing size. To address this problem and provide you with more information for comparing tires, WTB has introduced the Global Measuring System (GMS) for tire measurement.

The GMS uses a two-number system: the first number is the width of the casing, and the second number is the width of the tread, both in millimeters. These measurements are taken on a rim which is 20mm wide at the bead-capturing point, with a tire inflated to 60psi and maintained for 24 hours.

In addition to being able to accurately size a tire, knowing the actual casing size and tread width provides an indication of air volume, tread characteristics and tread contact area; all of which provide you with a more concise idea of what ride characteristics to expect from each of WTB's tires.

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