reroll

Once upon a time there were 5-speed hubs which fit into 120mm dropouts and then efforts were made to fit 6 speeds into 120mm, the narrow Ultra freewheels, for instance. But they met with only limited success and so dropouts were commonly changed to 126mm to fit 6 cogs with "regular" cog spacing. Then it was discovered that freewheels could work successfully with narrower cog spacing, that a 7-speed freewheel could be only slightly wider than the earlier 6-speed freewheel, and because bicycle rear dropout spacing had never been entirely standardized it turned out that 7-speed freewheels could often fit into dropouts designed for 6-speed freewheels although it also often happened that maybe a few millimeters of spacers would be needed to spread the dropouts outward slightly to get the 7-speed freewheels into place. Yet despite such occasional needs to use thin spacers and although 120mm rear dropout spacing became obsolete near the end of the 1970s or so, 126mm dropouts and 7-speed freewheels were in use until the later 1980s along with reported axle troubles with 8-speed freewheels using 130mm dropout spacing, when Shimano introduced its SIS shifting system in 1986 which Shimano made available only on its newer freehub hubs with 6-speed cassettes, followed by 7-speed cassettes in 1987, followed by 8-speed cassettes in 1988 and which soon caused freewheels to become obsolete in the Western bicycle market although even now, in 2022, the massive Eastern bicycle market still includes the widespread use of freewheels.