To make a fine adjustment to the position of the front derailer cage. This is sometimes necessary as the rear derailer is shifted because changes in chain angle can cause the chain to rub on the front derailer cage.
Some indexed front shift controls provide a «trim» function, usually an extra click or two, to let the rider compensate for this.
In the early 1980s, Sun Tour used to make a special shift lever that automatically adjusted the position of the front lever as the rear was shifted, to automate the trimming process. This system turned out to be very troublesome, and was abandoned after a single season.
Modern front derailers are designed around specific chainwheel combinations so that trimming
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