a 3x9 setup would mean you don't get the crazy chain lines you would get with a 1x9 setup - providing you use the correct gear positions anyway! - (i had this on one of my MTBs) you could play around with the position of the chain ring though to get a happy medium (could use chainring spacers too).
if going for the 1x9 setup you will definitely need a chain guide, otherwise you'll spend all day putting your chain back on!
i've been recommended the truvativ box guide for a 1x9 setup.
or you could setup your front mech to act as a chain guide.....which ever you trust will do the job.
otherwise, fit a full set of 3x9 or 2x9 and put up with the extra weight
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