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I'm running a 33:18 and the rear cog is a flip flop hub design which will make changing the rear cog a pain. I assume I can just change the front chainring to compensate. What chainring would give me the same ratio as a 32:20, while still keeping the 18t cog in the back? I hope that makes sense...
 

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Catholic MTBR
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Call me crazy but I think we can just stick with ratios. 32/20 = 1.6 so X/18 = 1.6. Thus, 18x1.6 = 28.8 so you'd be looking at a 29 tooth chainring to maintain the same ratio.
 

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Why is changing the rear cog a pain again? If it is a conventional flip-flop hub: unscrew the lockring, unscrew the cog... install new cog, install new lockring. You need two tools which will cost you less than $20 - a chain whip and a lockring spanner. That said, it is totally possible to do this without those tools if you really have to.

Assuming you have a 4 bolt crank, 30T is as small as you can go up front. Easiest option is probably to keep your current chainring and go to a 20T or 21T in back. The 33:20 (1.65:1) will be just slightly higher than a 32:20, the 33:21 (1.57:1) will be just slightly lower.
 
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