Max cog size for DA 7900 rear deraiileur

[shawmn]

I am thinking about trying a 12-30 cassette with my 7900 53-39, 7900 RD. Shimano says the chain wrap capacity for 7900 RD is 33, and when I do the math for my setup it comes to 32, so that seems within the limits. However, Shimano also says that the cog size limit for 7900 RD is 28. Anybody have any experience trying a 30-tooth with this rear derailleur and a 53-39?

I am thinking of doing this because I will be riding in the Santa Monica mountains for a few weeks. I don't want to change to a compact right now, and think that a 28 tooth may not be quite big enough.

[mattr]

Depends on your frame geometry (specifically the drop out/gear hanger) and how you set your chain length up

Though shimano max cog sizes and rear mech capacities are notoriously pessimistic.
I've had a 32 tooth cassette on a short cage road mech, with a stated max of 28. (It only worked on the one frame, had to get an mtb mech when i replaced the frame.)

[shawmn]

Thanks. I think I will try the 30. With the 27 that is on there now, there is quite a lot of room, and the b screw is loosened off most of the way.

[eric]

I've used 30t cogs on 7800 and 7900 (and Sram Red 10sp) derailleurs with no problem or even a B screw adjustment, on Cervelo R3 and R3SL frames. Many people have used 30t cogs on Shimano 10sp derailleurs with no problems. 32t does not often work though.

Of course make sure your chain is long enough to handle the large cog+large chain ring in case you shift there by mistake.

[SixThirteen]

Long time lurker, infrequent poster... yes 30T works fine on a DA7900 short cage RD, I have one myself.

The one I have is 12-30, and I switched to a 52/36 from 53/39 with 11-28, so I didn't need to change the chain length since the big/big combo is only 1 tooth greater.