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Half-step Gearing
In the days of 4- and 5-speed freewheels, 8- and 10-speed bikes were commonly set up with chainwheels that were very close in size, for instance, 46/49, or 47/50. When used with typical freewheels of the era, the difference between the two front gears was about half as large as the difference between adjacent gears on the freewheel. (One reason for this was that early front derailers couldn't handle much more than a 3-tooth difference reliably!)
With half-step gearing, the larger shifts are made with the rear derailer, and the front is for fine tuning. This allows an 8- or 10-speed set up to have a reasonable range with fairly close spacing of the gears. One downside of half-step is that it uses all possible combinations, including those that run the chain at a fairly severe angle. This was not a big deal in an 8-speed rig, but waskind of marginal for 10-speeds. Another serious disadvantage is that every other shift in the normal sequence is a double shift (front and rear derailers simultaneously). These issues are largely overcome with modern handlebar-end index shifters and narrow-spaced cassettes, but half-step requires mixing and matching sprockets these days -- no manufacturer offers an appropriate ready-made cassette.
Half-step gearing is most suitable for riding in flat terrain, where shifting is rare. It allows finer gradations to get as close to the "ideal" gear for the particular wind conditions as possible, and a wide range with even spacing.
Modern multi-range shift patterns use larger jumps on the chainwheels to select general ranges of gears, and fairly closely-spaced 7-or-more-speed clusters for the fine tuning. This greatly simplifies the shifting pattern, allowing constant adjustment to different grades in rolling terrain, with only occasional need for a double shift.
See my Article on Gear Theory.
