There's little benefit (if any) to using a narrower chain than the drive trains calls for. , as the chart below shows 7 & 8s chains are already slightly narrower than 5 & 6s chains, but the difference is little enough, so there's no real drawback. However going to a 9s chain creates the real risk that the chain may jam or hang on front downshifts, where it can get trapped between the tips of the inner ring's teeth and the face of the outer.
06 speed - 7.8mm (all brands)
07 speed - 7.3mm (all brands)
08 speed - 7.1mm (all brands)
09 speed - 6.6 to 6.8mm (all brands)
10 speed - 6.2mm (Shimano, Campagnolo)
10 speed (Narrow) - 5.88mm (Campagnolo, KMC)
10 speed (Narrow, Direction) - 5.88mm (Shimano CN-5700,CN-6700,CN-7900)
11 speed - 5.5mm (Campagnolo, KMC, Shimano CN-9000)
borrowed from wikipedia, original source unknown
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Last edited by FBinNY; 01-28-15 at 06:14 PM.