Brompton 13T rear sprocket 3.1mm for BWR 6 speed, 3/32" chain, Shimano splines. This sprocket is used as standard on the Sturmey Archer BWR (Brompton Wide Ratio) six speed rear wheel It can also be used on the single / two speed Brompton rear wheel (though as standard the single speed uses a 12T sprocket and the two speed uses a 12T / 16T combination)