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What spread does the audax have now?
If you are not going to be fast chainganging / road racing then you can get a usable and reasonably close gear range with 44t 11-36.
2x won't exactly hinder you on gravel tho and is probably better on a 1 bike solution.
Depending on budget that new Ultegra clutch mech could be a good fix for chain slap on gravel 2x.
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Mine's 1×10 with 40T ring and 11-36 cassette. It's too high for loaded bikepacking and too low for club runs, but OK for everything else, as long as you can handle the slightly gappy gears. If you're used to riding fixed then you'll probably be just fine, as you're used to being in the wrong gear all the time anyway.
For a gravelpacking bike that’ll also be a audax/club run road bike, would the hive mind go 1x with the long rear mech, or 2x with a long rear mech? I don’t have enough room for a third bike, so I’m looking to build something that’ll replace my audax bike but that can handle reasonably bad terrain. The alternative is to lose my fixie but I like it too much for commuting/training.