Re: Whats the deal with tube sizes?
- Author: mike (---.dyn.aci.on.ca)
- Verified User: northernmike
- Date Posted: 03-25-08 21:45
thanks to rick who measured for me i know my moby wheels are 434mm BSD (ETRTO)
from the late sheldon brown:
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 26×2.125 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 26×1.75 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 25 mm 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. (26×1.75, 27×1 1/4…650B, 700C…)
Does Point Seven Five Equal Three Quarters?
Note that the inch-based designations sometimes express the width in a decimal (26×1.75) and sometimes as a common fraction (26×1 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.
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 700×25 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.
B.S.D.
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 Organisation.)
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 the inner width measured between the flanges as shown in the diagram.)
The second ISO number is the critical one, it is the diameter of the bead seat of the rim, in mm (“B.S.D.”). 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 700×20 C road tire would be a 20-622; a 700×38 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.
A general guideline is that the tire width should be between 1.45/2.0 x the inner rim width.
If you flatten out a tire and measure the total width from bead to bead, it should be approximately 2.5 x the ISO width.
If your tire is too narrow for the rim there’s an increased risk of tire/rim damage from road hazards.
If its too wide for the rim, there’s an increase risk of sidewall wear, and a greater risk of loss of control in the event of a sudden flat.
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
29 inch622 mm This is a marketing term for wide 622 mm (“700c”) tires.
28×1 1/2635 mmEnglish, Dutch, Chinese, Indian Rod-brake roadsters
(Also marked F10, F25, 700 B)
622 mm(F.13)Rare Canadian designation for the (F.13)
28×1 5/8×1 1/4 Northern European designation for the 622 mm (700 C) size
27 x anything630 mm Older road bikes
26×1 (650 C)571 mm Triathlon, time trial, small road bikes
26×1 1/4597 mm Older British sport & club bikes
26×1 3/8 (S-6)597 mmSchwinn “lightweights”
26×1 3/8 (E.A.3)590 mm Most 3-speeds, department-store or juvenile 10 speeds
26×1 1/2 (650B)584 mmFrench utility, tandem and loaded-touring bikes,
a very few Raleigh (U.S.) & Schwinn mountain bikes.
26×1 3/4 (S-7)571 mmSchwinn cruisers
24×1520 mm High performance wheels for smaller riders; Terry front
24×1 1/8520 mm or
540 mm!Caveat emptor!
24×1 1/4547 mm British or Schwinn Juvenile
24×1 3/8 (S-5)547 mmSchwinn Juvenile lightweights
24×1 3/8 (E-5)540 mm British Juvenile, most wheelchairs
20×1 1/8
20×1 1/4
20×1 3/8451 mm Juvenile lightweights, BMX for light riders, some recumbents
20×1 3/4419 mmSchwinn juvenile
17×1 1/4369 mm Alex Moulton
16×1 3/8349 mm Older Moulton, Brompton & other folders, Recumbent front, juvenile
16×1 3/8337 mm Mystery tire
16×1 3/8335 mm Polish juvenile
16×1 3/4317 mmSchwinn Juvenile
12 1/2 x anything203 mm Juvenile, scooters
10×2152 mm Wheelchair
8×1 1/4137 mm Wheelchair
Traditionally, fractional sizes are made for straight-sided rims.
High-performance sizes (571 mm /26×1 & 630 mm /27”) have evolved toward hook-edged rims.
Decimal sizes:
DecimalISOApplications
29 inch622 mm This is a marketing term for wide 622 mm (“700c”) tires.
28 x decimal622 mm Some German tire companies use this non-standard designation for 622 mm (“700c”) tires.
26×1.00 through 2.3559 mm Most Mountain bikes, cruisers, etc. except:
26×1.25 (rare)599 mm Very old U.S. lightweights
26×1.375599 mm Very old U.S. lightweights
24×1.5-24×2.125507 mm Juvenile mountain bikes, cruisers
22×1.75, 22×2.125457 mm Juvenile
20×1.5-20×2.125406 mm Most BMX, juvenile, folders, trailers, some recumbents
18×1.5355 mmBirdy folding bikes
18×1.75-18×2.125355 mm Juvenile
16×1.75-16×2.125305 mm Juvenile, folders, trailers, some recumbents
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 A642 mmObsolete
700 B635 mm Rod-brake roadsters.
700 C622 mm Road bikes, hybrids, “29 inch” MTBs.
(28×1 1/2 F.13 Canada)
700 D587 mm Oddball size formerly used on some GT models.
650 A590 mm French version of 26×1 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 mm28×1 1/2, 700 B
630 mm27 x anything
622 mm700 C, 28 x (two fractions), 29 inch
(28×1 1/2 F.13 Canada)
599 mm26×1.25, x 1.375
597 mm26×1 1/4, 26×1 3/8 (S-6)
590 mm26×1 3/8 (E.A.3), 650 A
587 mm700 D
584 mm650B, 26×1 1/2
571 mm26×1, 26×1 3/4, 650 C
559 mm26×1.00- x 2.125
547 mm24×1 1/4, 24×1 3/8 (S-5)
540 mm24×1 1/8, 24×1 3/8 (E.5), 600 A
520 mm24×1, 24×1 1/8
507 mm24×1.5- x 2.125
490 mm550 A
457 mm22×1.75; x 2.125
451 mm20×1 1/8; x 1 1/4; x 1 3/8
440 mm500 A
419 mm20×1 3/4
406 mm20×1.5- x 2.125
390 mm450 A
369 mm17×1 1/4
355 mm18×1.5- x 2.125
349 mm16×1 3/8
340 mm400 A
337 mm16×1 3/8
317 mm16×1 3/4
305 mm16×1.75- x 2.125
203 mm12 1/2 X anything.
152 mm10×2
137 mm8×1 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
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