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here's the situation: bike is Specialized Roubaix Elite (UK/Canada spec). comes with a 50/36 compact crankset, 9 speed 12-26T cassette (SRAM) and 10-speed chain (KMC S-10 Super Narrow). rear derailleur is an ultragra short cage.

i thought the 12-26T cassette was a bit of a joke for a compact crankset so i got the shop to swap it with a 9 speed 11-23T cassette. once that was completed and they were spinning/testing the gears on the workstand, i noticed that there seemed to be quite a bit of slack with the chain when running the gears at 36 front, 11 rear.

being somewhat of a newbie on bike components/mechanics, should i get the shop to change the chain to a 9 speed?

boon
 

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chain slack??

Chain "slack" implies that the chain might be too long and has nothing to do with it's width (the difference between a 9 and 10 speed chain).

Changing from a 12-26 to an 11-23 would not affect the required chain length. Here's the way to evaluate 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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