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Understanding Tyre Sizes
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27” wheels are bigger than 29” wheels.
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28” wheels come in three sizes. (There was a 4th, but it is now obsolete).
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Each 28” wheel size has a 700 wheel size designation. 700a, 700b and 700c are still in use but 700 (with no following letter) is the obsolete size.
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29” wheels, 700c wheels, and most 28” wheels are the same diameter.
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26 x 1.75 tyres are not interchangeable with 26 x 1-3/4 tyres.
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20 x 1.4 tyres fit a much smaller wheel than 20 x 1-3/8 tyres.
Confused? So are many others.
So why are 26 x 1-3/8 wheels larger than 26 x 1.75? Because the 26” (or 20”, 28” etc) refers to the outside diameter of the tyre; a fatter tyre requires a smaller wheel than a thinner tyre of the same outer diameter.
Why did this come about? Because the earliest bicycles used brakes that contacted the outside of the tyre. The outer diameter was the important measurement, and it stuck.
But do not fear. There is also a simple and universal tyre sizing system in use now that was developed by the European Tyre and Rim Technical Organization (ETRTO). ETRTO sizes consist of two numbers; the first referring to the width of the tyre in millimetres and the second referring to the inner diameter of the tyre (known as the bead seat diameter, this number also designates rim diameters).
So a 700 x 23c tyre has an ETRTO size of 23-622. It’s 23mm wide and fits on a rim with a bead seat diameter of 622mm.
A 29 x 2.1 tyre has an ETRTO size of 52-622. It is 52mm wide and also fits on a rim with a bead seat diameter of 622mm. Yes, you can fit it on your road wheel, but it won’t fit into your road frame.
Here is a table of tyre and rim diameters we deal with (most of which are available on our website).
ETRTO BSD
ETRTO Widths
Inch Designation
Common Inch Widths
French
French Name
203
57
12-1/2"
2-1/4"
305
47, 57
16"
1.75, 2.125
349
32
16"
1-1/4
355
47
18"
1.75
406
33 to 57
20"
1,35 to 2.25
451
35
20"
1-1/8, 1-3/8
457
47
22"
1.75
489
37
22"
1-3/8
550a
490
37
22"
1-3/8
550a
501
37
22"
1-3/8
507
47, 54
24"
1.75, 2.0
540
24"
1-3/8
559
28 to 62
26"
1.5 to 2.7
571
23
650c
584
37, to 57
26" or 27.5"
1-1/2 x 1-5/8; 1.9, to 2,25
650b
590
37, 42
26"
1-3/8
622
23 to 57
29"
1.75 to 2.25
23 to 45
630
32
27"
1" to 1-3/8"
635
40
28"
1-1/2
642
37
28"
1-3/8
So, next time you’re at all uncertain which tyre will fit your rim – check the ETRTO size of the old tyre and choose a tyre with the same ETRTO bead seat diameter. Widths can be varied greatly on the rim, but choosing a narrow tyre for a wide rim can expose the sidewalls of the tyre to sidewall damage from rocks etc, and choosing a wider tyre than the bike already has should be done after confirming that the space is available in the frame and fork for the increased width and height of the wider tyre.