XT cassette (12-34) with 105 derailleur (long)



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Jason Hodges

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Has anyone had any luck with this? I looking for some lower gears on my commuter (Specialized
Sirrus). Appreciate any feedback.
 
T

The Pomeranian

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Jason Hodges wrote:
>
> Has anyone had any luck with this? I looking for some lower gears on my commuter (Specialized
> Sirrus). Appreciate any feedback.

The "road triples" are not specified for such a large cog. As has been pointed out here, many people
do go beyond the 27t spec with varying measures of success. In short, you should use a "MTB"
derailleur for any rear clusters well beyond the spec of the manufacturer. "Well beyond" is a
subjective term and open to interpretation, but it is fair to say 34t is too much.

The prime difference between the MTB and Road derailleurs (in cog handling capability) is the slant
angle of the parallelogram. The MTB derailleurs have a steeper slant angle than road derailleurs
and this helps keep the guide pulley from riding right upon the cog when pedaling in those big
cogs. The Road derailleurs have a shallower slant angle and this helps maintain the chain gap when
using more tightly spaced cassettes. (This means essentially means better shifting performance for
tightly spaced cassettes.) The MTB derailleur has a longer parallelogram to compensate for the
steeper slant angle.

The two types of derailleurs are optimized for the intended usage. The "right thing" to do is
use a system which is matched. That means: use a MTB derailleur if you intend to use such a
large rear cog.
 
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Rich Clark

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"Jason Hodges" <[email protected]> wrote in message
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> Has anyone had any luck with this? I looking for some lower gears on my commuter (Specialized
> Sirrus). Appreciate any feedback.

The official maximum cog is 27 teeth. On my commuter with a Tiagra RD, which seems to have the same
dimensions as a 105, I use a 30 with no problems. I think 34 might be pushing your luck, and using a
MTB RD like an LX would be indicated. This will work well with the 12-34.

RichC
 
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