From N scale supply .com,
"Front and rear engines (cylinders and coupled drive wheel sets), both pivot for 9-3/4" recommended min. radius."
To be safe I wouldn't go below 11" to work well, and at least 18" to look decent, but thats just me. It all depends on what your definition of "looks good".
Does anyone know the minimum radius curve that the n scale athearn challenger would 1) work well on, and 2) look good on.
I'm thinking of using a 12 inch curve and wanted to make sure it would take it well.