650B: the luxury touring tyre
650B is a tyre and rim format pioneered on French randonneur bikes of the 1920s and 30s. It is rare in the UK, but is increasingly popular in the US where fat-tyred 650B road bikes have proven ideal for the extensive network of gravel back roads.
A 650B tyre fits a rim with a 584mm bead seat - between 700C road/29er mtb and 26" mtb sizes. Road 650B tyre width today is generally between 32mm and 42mm; 650B road tyres for the 584mm rim are widely available in France and elsewhere in continental Europe. High-
performance 650B road tyres include the 42mm Hêtre and 32mm Cyprès by Grand Bois.
The growing trend towards the use, for non-competitive cycling, of tyres fatter than the familiar 700x23C and 25C sizes stems from the search for improved ride comfort and lower rolling resistance. 650B tyres are considerably fatter than all but a few 700C models but are smaller in diameter, making the tyre lighter for the same width. For reference, a 650x32B Grand Bois Cyprès is 50g lighter than a 700x30C Cyprès. This makes it possible to
build a smaller, lighter and stiffer frame and fork and, usefully for the touring cyclist, reduces overall gearing while using the same chainring and sprocket sizes.
With a diameter of around 26inches, 650B is big enough to provide a smooth, efficient ride over rough tracks. Rolling resistance, especially with luggage, is low on well-surfaced roads. Spend some time on 650B and it is easy to see why the format is so popular with touring cyclists in Europe and the US.