Join Date: May 2004
Location: Wilmington, DE
Bikes: 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i (RIP), 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
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Two easy methods:
1. Assuming your current set up works and you still have the chain, lay them both out on the floor and make the new one the same length as the old.
2. If you don't have the old chain or if it was the wrong length, wrap the chain around the largest chainring and largest rear cog then make the chain 2 links (1") longer than the length required for that distance.
In that big/big gear combination, the derailler will be pulled almost straight (remember you added an inch) and will not add any distance to length required for the big/big combo. Just don't go changing out your cassette for something bigger and not adding in extra links
One more thing, to keep your chain from being too loose in the little/little combo, you just need to make sure your derailler has enough wrap capacity to handle it.