Campagnolo officially recommends various rear derailleur cage lengths for use with specific cassette and double or triple chain ring combinations. However, it is important to understand that any 1999 to current, Campagnolo 10 speed rear derailleur, will shift up to a 29 tooth cog and down to an 11 tooth cog. The problem you will run into is that the derailleur cage may not be long enough to take up all the slack in the chain for a wide range. Thus Campagnolo recommends using a short cage derailleur with a maximum cassette cog size of 26 teeth even though it will shift to a 29 tooth cog. Chain wrap or CW is a helpful concept to understand when you need to determine proper derailleur cage length.

Current Record, Chorus, Centaur and Veloce 9 and 10 speed rear derailleurs use the same three cage lengths; 55, 72.5 and 89 mm measured from the center of the upper jockey wheel to the center of the lower jockey wheel. 55 mm cages have a chain wrap value of 27; 72.5 cages have a CW of 36 while 89mm long cages have a chain wrap value of 39.

Chain Wrap is a simple calculation that will help you determine whether the rear derailleur you choose has a long enough cage to take up the chain slack in your drive train. The shorter the cage, the lighter and stiffer the derailleur and the quicker it will shift. Calculate Chain Wrap to get the greatest shifting performance.
To calculate Chain Wrap Capacity determine the difference between your largest and smallest chain ring. Then determine the difference between your largest and smallest cog. Add the differences and you have Chain Wrap Capacity.

Example: 42/52 chain rings and a 12/21 cassette yields a CW of 19. 52 - 42 = 10. 21 - 12 = 9. 10 + 9 = 19. So, with a 42/52 chain ring and a 12-21 cassette you would need a rear derailleur that has a Chain Wrap Capacity of at least 19.
For a 39/53 and a 13/26 combo you will need a minimum CW of 27. 53-39 = 14. 26 - 13 = 13. 14 + 13 = 27. In practice the CW numbers are flexible by one or two points. You could use a 39/53 with a 12-26 cassette with a CW of 27 even though the calculation calls for 28.

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