An important part of a rear derailleur's job is keeping the chain taut as you change gears. Chain-wrap capacity is a number that tells whether or not a derailleur will be able to do this on the drivetrain you plan to use it on. Calculate chain-wrap capacity with this formula: Chain-wrap capacity = (large chainring - small chainring) + (large rear cog - small rear cog). Example: 46/34/24 chainrings and a 12-34 cassette: (46 - 24 = 22) + (34 - 12 = 22) = 44-tooth chain-wrap capacity.