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This allowed them to build the bearing into the inside of the pedal thread, eliminating the need for a pedal axle.The purpose of this was to improve the biomechanics of the pedal by placing the bottom of the foot below the pedal axis.This was to allow shifting while coasting--a solution in search of a problem. A rear hub in which the freewheel mechanism is built into the hub itself, rather than being part of the sprocket cluster. Shimano's patented design places the right cone much closer to the end of the axle than does a conventional hub, greatly reducing the chance of bending or breaking axles.For more detail see my article on Shimano Cassettes.
With the '98 model year, 9-speeds came in and the "600" designation was completely dropped from Ultegra.("ULTimate" "Int EGRity") is the second highest level "road" group.Shimano has many trademarks for different quality levels and technological features/gimmicks.****** Correspondences between older groups and current groups are only approximate. Shimano has gone through many revisions of their groups while maintaining the same name, so comparisions between older versions and newer versions are quite inexact.Became 600EX when indexing was introduced, 600 Ultegra (Gray finish) when 7-speed added.The Ultegra name was added as a suffix to 600 when 7-speeds (and the gray paint finish) came in in the early '90s.For the 1998 model year, Ultegra became a 9-speed group, and the 600 designation was retired.