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Join Date: Aug 2010
Location: Madison, WI
Posts: 11,182
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
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Yes, 24 is the smallest you can put on a 74mm BCD crank.
On my derailleur touring bikes, I run a 11-32 rear cassette and a 24T granny gear (smallest chainring) on the crankset. Difference between 30 and 32T sprocket is small, so your lowest gear will not be that much higher than on my derailleur touring bikes. Since you already ordered the 30T, use it when it comes. But for steep uphills if you can go with a 32t or larger, you will be happier.
By increasing the size of your biggest chainring and biggest sprocket in back, your chain will be a few links too short. Assuming you are putting a new chain on it, do not cut the chain until you have the new cassette and new big ring installed. Keep the extra links you cut off, someday you might want a couple more links.