Join Date: Sep 2008
Location: Loveland, CO
Bikes: Two LOOK 585s, one KG461
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Although it's the shifters that create the indexing by pulling or releasing a specific amount of cable for each shift, that amount is not the same for all shifter brands, even if the cassette spacing is the same, like Shimano and SRAM. The difference is due to the RD's actuation ratio. Shimano 10 shifters only pull and average of 2.3mm of cable per shift and the pulls are not uniform. SRAM shifters pull 3mm per shift and the pulls are identical. Campy shifters are similar to Shimano, in that they use nonuniform pulls, but the average pull is 2.8mm to shift a slightly larger 4.12mm spacing, compared to Shimano's 3.95mm.
FWIW, a Campy 10 shifter pulls 2.5mm five times, 3mm twice and 3.5mm twice. If you have the equipment to make more accurate measurements, you might detect even more small differences in the pulls.