Must have:
-horizontal rear dropouts
-freewheel rear hub (unless you are buying a new wheelset)
Really should have:
-forged dropouts
-a decent 3 piece crank with bolt-on rings, so you can mess with chainline.
Nice to have:
-decent lugged, cro-mo, butted frame
-decent aluminum wheels (unless you are going to buy a new wheelset)
-decent alumimum components (post, stem, bars, etc)
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Cross Check Nexus7, IRO Mark V, Trek 620 Nexus7, Karate Monkey half fat, IRO Model 19 fixed, Amp Research B3, Surly 1x1 half fat fixed, and more...
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SB forever