Freehub
A freehub is a type of bicycle hub that incorporates a ratcheting mechanism. A set of sprockets (called a "cassette") is mounted onto a splined shaft of the freehub to engage the chain. The ratcheting mechanism is a part of the hub, in contrast to a freewheel, an older technology, which contains both the sprockets and a ratcheting mechanism in a single unit separate from the hub. In many high-end and midrange bicycles, freehubs have replaced freewheel systems. A freewheel mechanism allows a rider to stop pedalling whilst the cycle is still in forward motion. On a cycle  without a freewheel mechanism, the rider has to keep pedalling whenever the cycle is moving. Comparison to freewheels. The freehub concept answers several drawbacks encountered with the freewheel design: Beyond removal from the hub and of the cassette, there is limited, if any, access for cleaning and lubrication. The part can be fabricated relatively inexpensively and is not intended to be serviced or disassembled with hand tools. The latter is only possible by means of specialized or shop equipment. The outer cup covering the ratchet pawls and bearings is pressed into place at the factory, secured by interference fit, leveraging  the same inner threads of the shell into which the cassette lockring normally screws. History. The concept of a freehub was devised and manufactured by British company Bayliss-Wiley in 1938 and won the Cyclists Touring Club (CTC) award for that year. On the Bayliss-Wiley design the freewheel unit was threaded to accept the sprockets. A different four speed design was manufactured by BSA Cycles Ltd in 1949 to accompany their BSA 4 Star derailleur gear. The BSA design had a splined freewheel unit (BSA part No.8-1913) which attached to the hub shell (BSA part No.8-701) and  carried four sprockets. Shimano made their first freehub in 1978 in both the Dura-Ace, and 600 (later known as Ultegra) models. It was a significant improvement. It proved to be the first widely used commercially successful freehub. Freehubs, manufactured by various companies, are now common on mid- to high-end bicycles today. Nevertheless, freewheels continue to be fitted on some new bikes, especially single speed, and cheaper models of derailleur bicycles.