Why does my 25″ wheelchair tyre say 26″ on it, when it only measures 24″ across?
Call ISO International Rescue. . .
In the 120 years since the pneumatic tyre was invented, several tyre sizing schemes have developed around the world under both Imperial and Metric systems, different nationalities, sizing methods, fashions and industries have unsurprisingly created a complicated and confusing situation.
Early last century tyre sizing relied on roughly measuring the outside diameter of the tyre and rim combination. With fixed gears or hub brakes, clearance to the frame mattered, not the rim size. However, this took no account of the varying sizes of tyre which could be fitted on a particular rim, and gave no indication of which tyres will fit which rims.
Conditions, fashion and uses of wheels and tyres have now moved so far, that all of these traditional sizing systems are now wholly inaccurate as a guide to the actual size of any tyre/rim combination.
To clarify the situation the European Tyre and Rim Trading Organisation (ETRTO) devised an ISO numbering scheme which should now be written on all tyres and rims. It relies on measuring the nominal internal tyre or rim width, along with the diameter of the bead seat of the tyre or rim in millimetres:
For example, the ETRTO tyre 23-622 from the French system, will usually have 28″x1.25 (711mmx32mm equivalent) written on the side, is generally known as a 700C (yes, there are A and B series too!), but is actually only 7/8″ (23mm) wide. To add to the confusion this common size is actually slightly smaller than the British 27″x1 1/4″ wheel (23-630), which you would have thought was bigger…
However, by using the ETRTO number (22-622), its true diameter can be roughly calculated by adding together its ETRTO rim diameter and twice the tyre size:
622mm+(2x23mm) = 668mm/26.3″
This is clearly nowhere near its nominal 700mm/27.6″ diameter, derived as it was from the days when road surfaces were worse and tyres were much fatter. However, within reason, any tyre ETRTO marked **-622 will fit on this rim, irrespective of the tyre width.
Why then is a 25″ wheelchair tyre marked 26″ when its only 24″ across?
Similarly, the American rim size ETRTO**-559 has existed over there for generations, but has only recently become globally available through its use on most adult mountainbikes. These can still have fat tyres over 2″ wide, similar to the fat balloon tyres fitted to this rim size 50 years ago. This gave an overall wheel and tyre diameter close to their nominal 26″(559+(2×50)=659mm/25.9″), so was marked 26″.
However, when fitted with a typical 25mm wheelchair tyre the 559mm wheel size comes out a lot smaller than either of the names by which it is known: 559+(2×25)=609mm/23.9″.
This 559mm US rim size falls very conveniently between two British rim sizes which have been widely used on wheelchairs but are now almost extinct in the cycling world: 24 x 1 3/8 (**-540) and 26 x 1 3/8 (**-590). Fitted with a smaller section tyre for wheelchair use, they are sensibly known as 25″ as they fall roughly halfway between the existing 24″ and 26″ sizes, but you’ll get strange looks if you ask for a 25″ inner tube down at the bike shop.
Download this technical document from Schwalbe for a lot more information than you probably need.