=>Suggest you check the NMRA Standards (on line at their site). IIRC, =>~17" is about the largest minimum radius they spec, for modern =>mainline equipment (6-axle diesels, 80' cars etc.). I'm not sure =>whether their standards are based exclusively on mechanical =>reliability, or include esthetic considerations as well. =>
The recommended practices (not standards) relating equipment and minimum radii are based primarily on mechanical considerations. The assumption is that the couplers are body mounted, which limits coupler swing. So, longer cars will need larger radii so that the side thrust on the couplers won't derail the cars. Larger locos, with longer wheel bases, need larger radii so that the wheels won't bind. And so on. BTW, six-axle diesels need wider radii than four axle diesels. The min. rec. radii also assume easements, BTW.
From an aesthetic p.o.v, none of these minimum radii are satisfactory. They are a good guide to what you can get away with on hidden track. There are a few coniderations besides min. radius when planning a layout. The general rule is, use the minimum radiius only where you absolutely must. Otherwise, use the largest radius that will fit. Here are a few guidelines.
a) Viewed from inside the curve, the curve doesn't look as sharp as viewed form the outside..
b) Curves viewed from a high angle look sharper.
c) If you hide the beginning of a curve, it will look less sharp.
d) Easements make a curve look smoother, and make a train moving into the curve look smoother.
e) When faced with S curves, you should have a straight between the curves that is at least 1-1/2 times the longest car that will run through it -- and it's worth shifting track around to get it.
f) Readymade turnouts should have a straight leading into the points that's at least as long as the longest fixed wheelbase of any of your equipment, but
2x as long is better.. EG, if that N scale Pacific has a driver wheelbase of about 1", then you should try for a 2" straight. You can get away with less, especially if it's low-speed track, but I wouldn't. How do I know? Don't ask.
g) If you build your own turnouts, easements into a curved turnout are essential if you want the sharper side to be the min. radius.