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The modern range of Shimano shifters (e.g. FD4700 and related: https://si.shimano.com/pdfs/dm/DM-FD0002-05-ENG.pdf) have a top position for big-small, and a top-trim position for big-big. The difference in derailleur position is tiny from what I can tell, maybe 1mm on my shifters. Would there have been any drawback in performance/setup if they'd made this difference larger, which would mean it was easier to set up the extremes without rub, especially if the frame flexes.

FWIW, the difference between low and low-trim is much larger, maybe 5mm, and it's much more usable because of it.

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  • Maybe they wanted to avoid accidental down-shifts when you engage the trim position? From the small chainring it’s much harder to accidentally shift up.
    – Michael
    Feb 10, 2022 at 17:21
  • So the physical movement of the switch it related to the distance of the derailleur movement? I thought it was like a release mechanism, i.e. you click once and it releases a predetermined amount of cable independent of how far you had to move the lever.
    – Wilskt
    Feb 10, 2022 at 17:29
  • A derailleur is designed to move a distance based on mm of cable pulled or released (so called, actuation ratio), so yes movement of the switch if it is pulling or releasing cable, moves the derailleur. The trim positions in the shifter are actual detents (stops) where cable is is pulled or released to, causing derailleur movement. It's just not enough cable pull and derailleur movement to effect a shift to the other chainring.
    – Jeff
    Feb 10, 2022 at 22:44

1 Answer 1

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The drawback is that the bigger you make the difference, the sooner/likelier you have to use the trim again as you shift back down the cassette. Or, another way of saying it is that the closer you can make them while still doing what you need them to do, the more license you give users to forget whether they're in the trim position or to not understand anything about the trim function (which is very common).

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  • Ha, that's a good point, it took me a few days to 'discover' it when I got my first road bike! Yeah that makes sense, almost a minimum tolerance thing, makes it more of a pain to set up though.
    – Wilskt
    Feb 10, 2022 at 19:49

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