Low car diet
Join Date: Aug 2007
Location: Corvallis, OR, USA
Bikes: 2006 Windsor Dover w/105, 2007 GT Avalanche w/XT, 1995 Trek 820 setup for touring, 201? Yeah single-speed folder, 199? Huffy tandem.
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I have the Shimano 105 ST-5600 levers. The left/front shifter has 6 positions: Chainring/wheel 1, trim 1, chainring 2, trim 2, chainring 3, trim 3.
For a double crank, you're supposed to use the first 4 positions. But I think the chainrings are all spaced evenly, so I don't see how using the last 4 positions wouldn't work. Though it may be more difficult to set up and, as you mentioned, you'll have a slack cable if you ever over-downshift.
However, if you used the first 4 positions, I imagine it would take so much force to break the lever when overshifting--much more than what you'd expect when shifting--that you'd realize you're already in the outer chainring before it broke.