though allowing this to happen through slow shifting can cause excessive clutch wear and render the hub inoperable. In Low gear, this isn't a problem as the low gear pawls can be overrun by a higher gear, while Normal and High gears use the same pawls, requiring an alternative solution. The K hub spawned a Tricoaster version with back pedal brake, designated KC, in 1921 and both close ratio (KS), and medium ratio (KSW) variants in 1932 and 1933 respectively. With the introduction of 90mm drum brakes LBF/BF/BFC and LBR/BR/BRC in 1931, the first Sturmey-Archer 3-speed drum brake was introduced, the KB. The KB was joined in 1934 by the KT 111mm tandem drum brake, part of a family of 111mm drum brakes alongside the BFT and BRT front and rear drum brakes (BR type drum brakes are not gear hubs and are instead threaded for fixed sprockets or freewheels). The KT lasted only four years, being replaced by the AT (AW based 111mm drum brake) in 1938 when the K was replaced by the AW. The AT, BFT and BRT all went out of production because of hostilities in 1941 and never re-entered production. 'T' family hub gears Despite the primary selling point of Sturmey-Archer hubs being their offer of three gears when rivals managed only two, Sturmey-Archer produced a range of two speed hubs in the 1930s. This began with the T and TF (identical hubs, F denoting a fixed sprocket) in 1933, alongside a version with a 90mm drum brake; with a rod brake operation and freewheel, this was TB, with a rod operation and fixed wheel, this was TBF, while a C was added to either designation to denote cable operated brakes instead of rods. In reality, an end user can easily switch between fixed and free, and rod and cable by swapping out common parts, though the hub shell will continue to show the designation for a given hub's original configuration. The T, TF, TB, TBF, TBFC, and TBC all had a 25% reduction below direct drive In 1936, a close ratio 2 speed was introduced, this was called TC and offered a 13.46% reduction below direct drive. Hubs manufactured in the 1930s have a date stamp given as a number at the end of the model designation, this can be seen on examples of all of the hubs mentioned in this section, as well