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Bar Diameter 25.4, 31.8

The diameter of the handlebars at the stem clamp. Alias: Bar Clamp Diameter, Handlebar Bulge diameter

Common Diameters:
 - 25.4 The old ISO standard handlebar diam. was 25.4mm (1 inch). All four of my bikes have this size.
 - 31.8 Almost all new bikes now have this diameter

Most handlebars have 2 diameters, one at the stem clamp and often a bit thinner at the grip area. The grip area diameter is so standard that new shifters, brake levers do not specify this. Flat bars are 22.2 and drop bars are 23.8.