Cassette fit
Ive a Shimano RD 5800 gs rear derailleur running 11-32 but would like to go up to 11- 34. Shimano tech states it only goes to 11-32
Im aware that Shimano are conservative in their figures sometimes so anyone fitted a 11-34 on a gs derailleur
Comments
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Depends a bit on your front chainring(s) if more than one. You need to take the difference between largest and smallest there , and add it to the difference in the cassette. This is “total capacity” number for the RD, which for GS is 39, and 35 for SS.
so with 11-34, you have 23 teeth gap on the back. If you run 50/34 front rings, you have 16 teeth. Added together you are right on the limit capacity of 39 for a GS.
This is where you begin to have chain tension issues, and derailleur life impact. At either extreme of the combinations, the derailleur may struggle. If you shift to the 34/11 combo - bad idea anyway but still possible - you might have way too much chain slop and even dropped chain from lack of tension. If you try the 50/34 front/back combo, the derailleur will likely be over-stretched, chain too tight.
Sometimes you get lucky with the specific bike geometry for chain stay length and you can set an *exactly perfect* chain length that will still work, but this is rare. This is one reason Shimano are “conservative” in their numbers, as on most bike frames, you end up having to choose between a chain length that is either slightly too long or short by 1 or 2 links either way, causing the chain tension issues, even if the capacity of the derailleur is fine.
The maximum teeth on the cassette should be ok in isolation, with careful adjustment of the B screw, but be wary of the chain tension.
Open One+ BMC TE29 Seven 622SL On One Scandal Cervelo RS2 -
Just make sure your chain is long enough to go in big front and big sprocket. You do not need to use small front and small chainring so a droopy chain in that combination does not matter.
I have used an 11/34 with a 50/34 chainset .No problem
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