Licensed Bike Geek
Join Date: Oct 2002
Location: Los Barriles, Baja Sur, Mexico
Posts: 1,360
Bikes: Look 585, Kirk Terraplane, Serotta Ottrott, Spectrum Super Custom, Hampsten Carbon Leger Tournesol
Mentioned: 4 Post(s)
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Quoted: 92 Post(s)
The middle chainring of a triple has ramps and pins on the inside to help the shift up from the small ring. The rings on the double don't have them in the correct places. Also, I believe the triple front der can't lift a chain to a ring that is more than 12 teeth larger. Or it can't without a great deal of effort and/or crappy shifting.