frustrated mechanic

Understanding Tyre Sizes

  • 27” wheels are bigger than 29” wheels.
  • 28” wheels come in three sizes. (There was a 4th, but it is now obsolete).
  • 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.
  • 29” wheels, 700c wheels, and most 28” wheels are the same diameter.
  • 26 x 1.75 tyres are not interchangeable with 26 x 1-3/4 tyres.
  • 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.