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I have been running 8 speed components for years. I was under the impression that 9 speed cranksets had different ring gears and crank arms. I read somewhere recently that the 8 and 9 speed cranksets have the same spacing the ring gears are the only difference. So I said why don't I use a set of 2001 9 spd Shimano LX crank arms w/ my 8 speed gears. Well I compared my 8spd. Truvativ 5D's to the Shimano LX's and the spacing is off by .5mm. Are crank arms spaced differenty for 8 and 9spd?
 

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Nope. The chainring spacing has remained constant. The difference is in the pickup ramps on the insides of the chainrings, because since the 9 speed chain is narrower, they must "reach out" a tiny amount farther to engage it.

But you'll notice, for instance, that SRAM, who sells front individual front shifters, doesn't distinguish between 8 and 9 speed models. They're just "front shifters". And having use "9 speed" cranksets with 7 and 8 speed drivetrains, I can tell you the older front shifters have always worked fine with the newer 9 speed cranks.

You may want to switch to a 9 speed chain if you upgrade cranksets, but even that isn't a given.
 
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