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Dura Ace RD7900 with 11-28T (7900)
#1
To measure the proper chain length i have to pass the chain from the biggest front chain ring and the biggest rear cassette ring (28), without passing through the derailleur pulleys. So, after the chain front and rear meets, and if i am correct, i have to add two more links.
Ok?
But....the problem is that shimano in the service instructions of rd 7900 says that i have to add , especially for the 11-28t sprocket, two more links. Is it correct? Pls help. Thanks

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#2
It should just be 2 links no matter which derailleur you use.
Big chainring to biggest rear cog + 2 links.
See :- http://bicycletutor.com/calculate-chain-length/
If you are unsure then add the extra links (4 links?) but I think you'll find they are not needed. You can always take them out after. If the chain is too long then the derailleur will rotate right back (small ring to small cog) and allow the chain to rub on the bottom of the cage. If the chain is to short then you won't get the big to big without straining something.
There is another way shown in this tech' document. It involves big chainring to small cog and getting the derailleur 90 degrees to the ground.
http://techdocs.shimano.com/media/techdocs/content/cycle/SI/DA_Di2/RD7970/SI-5X20A-002-ENG_v1_m56577569830673791.pdf

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