Chainring question....

If your chain is long enough to wrap around the Big-Big (that is, the 50t Chainring + 32t Cog ... ) AND if your particular Di2 rear derailleur can accommodate the 32t Cog & the extra couple of inches of chain then you should be good-to-go ...

[INDENT]If you are sure that you will never shift the chain onto both the 50t Chainring AND 32t Cog at the same time, then you can probably use your existing chain as-is.

[/INDENT]

alfeng said:

If your chain is long enough to wrap around the Big-Big (that is, the 50t Chainring + 32t Cog ... ) AND if your particular Di2 rear derailleur can accommodate the 32t Cog & the extra couple of inches of chain then you should be good-to-go ...

[INDENT]If you are sure that you will never shift the chain onto both the 50t Chainring AND 32t Cog at the same time, then you can probably use your existing chain as-is.

[/INDENT]

Thanks however, I don't have the 50T chainring yet as I don't want to buy it and find out its useless.

Is your cassette Di2 (never heard of a Di2 specific cassette) or is your bike Di2 system equipped?

You'll need a chain that's 4 links longer.

An Ultegra Di2 GS rear derailleur will throw an 11-28 to a 12-32 cassette. The operative two letter part in that sentence being GS. The cage will wrap 16T chainring difference and a total of 37T so a 50-32 (usually an unused combination due to cross chaining) is 18T...good to go on the next cassette gear

A standard Ultegra Di2 SS rear derailleur will throw 11-23 to 12-28, so a no-go if you have that rear derailleur. The SS will wrap the same 16T chainring difference and a total of 33T.

A mechanical 105/Ultegra SS that is rated at 28T can be forced to shift a 32T cassette gear with some monkeying around, but I wouldn't try that with a Di2 rig without doing some careful experiments to see if it's going to work or wreck the derailleur. Maybe Google can point you to someone that's already tried it.