Join Date: Apr 2003
Location: St Louis, Missouri
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Rear derailleurs have two separate, but related, range issues.
The first is largest cog capability. It is determined by the positioning of the upper pulley relative to the cassette cogs and has nothing to do with arm length. As you install progressively larger cassette cogs, eventually the upper pulley will rub against the largest ones. Mountain bike derailleurs have the capability to handle larger cogs than road bike derailleurs.
The second is chain slack take up. It takes more chain to wrap around the big/big sprocket combination than it does for the little/little combination. The rear derailleur has to account for that difference. Long arm rear derailleurs have the capacity to take up more chain slack than short arm derailleurs.
Shimano mountain bike derailleurs come in different arm lengths. They will all handle a 34 tooth rear cog, but some will wrap up more chain slack than others.