Originally Posted by
Grim
Spacing was due to more gears. The actual space between the gears stayed close to the same the freewheel just got wider.
I think 7 speed speed is when it went 126 then with the introduction of 8 speed it went to 130. Mountain bikes went to 135 because as the gear cluster got wider some of the hubs were more narrow to fit the gears. The more narrow the hub the weaker the wheel becomes to side loading. Thats when Shimano started a Mountain bike line of parts different from the road bike line.
4- and 5-speed freewheels used 120mm spacing. 6-speed arrived around the early '70s and used 126 mm (actually 126.5). Then around the late '70s 7-speed freewheels that used 126 mm spacing started coming out. The cogs were the same thickness, but spacing between the cogs (and between the smallest cog and the dropout) had decreased. This required a narrower chain that used a "bent" or chamfered inner link.
8-speed freewheels in 130 mm spacing, a technical disaster, followed. Problems were the extreme dishing of the rear wheel and the distance between the drive side bearing and the dropout, leading to bent axles. The cassette hub solved the axle problem by moving the drive side bearing to the outside and the left flange to the inside.