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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Sorry for the confusion. The GS (medium/long-cage) RD you have will work perfectly.
The length of the cage determines its chain wrap capacity. Basically, when you're in the big chainring/big cog combo, the chain is just long enough to handle that. But, when you're in the small/small combo, there's way more chain than is needed, so the RD has to take up all that slack. The longer the cage, the greater amount of slack it can take up; therefore, the greater difference in number of teeth between your big/big and small/small combos that the RD can handle.
Chainwrap = ([big chainring - big cog] + [small chainring - small cog])
Your chainwrap = ([50 - 34] + [27-12]) = 31
The SS RD can handle up to only 29, whereas the GS (your) RD can handle up to 37.