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I have a compact setup with a 50/36 and from time to time I want to switch to a 34 to get a slightly lower gear for climbing. Generally speaking, does this type of change require the chain to be shortened? Or can the slack be taken up by adjusting the derailler position?

Also, for those that do swittch cassettes and chainrings that requires a different chain length, do you use the same chain and adjust and re-adjust it's length using a couple of master links or do you use a different chain altogether.

Thanks.
 

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no change required...

A 2T change only add or subtracts 1/2 inch from the optimum chain length. The minimum increment of change is 1 inch. You should adjust the length to fit the 34T and you'll be done. There is no safe way to repeatedly adjust the length of chain.

Here's the full story on chain length.

Two simple tests will determine if the chain is the correct length. First, it must not hang loose in the little ring, little cog combination. If there is no tension on the chain in the little ring, little cog combination; remove two links (one inch) at a time, until there is. When the ends of the chain are brought together, some movement of the lower pulley should occur, indicating tension is being applied. Two more links (another inch) may need to be removed, beyond the point of absolute minimum tension, to keep the chain from rubbing on itself or the chain guide tab as it passes under the upper derailleur pulley. If you want to see how much lower pulley movement will occur, without removing the extra inch of chain, shift up four teeth (11 to 15 or 12 to 16). This has the same effect as removing two links. Once this is done, the chain is set to the maximum useable length. Removing additional links will do nothing but reduce the derailleur's capacity.

Second, the chain must be long enough to avoid over-extending the rear derailleur when shifted to the big ring and biggest cog combination. If the chain is set to the maximum length as described, it should always pass this test, unless your setup exceeds the derailleur's stated wrap capacity. If you deliberately exceed the derailleur's capacity and the derailleur is over-extended in the big ring/largest cog combo, then you must either avoid that combo or add another inch and avoid using the little chainring and the smallest 3 or 4 cogs (since the chain will hang loose).
 
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