TextTextText[:(] What is the minimum radius for a “N” scale helix. ?
Depends on the trains you have. 24" min radius would be safe for all but the largest of N scale brass engines. 12" min radius would work with many smaller locomotives pulling shorter trains.
I think it’s more of a question of what radius, at an acceptable grade, provides enough vertical clearance between successive turns.
For example, 12"radius, equates to a 24"diameter circle, equals 75"circumference. If we assume you don’t want to have a grade more than 2%, then 2% of 75 gives us a rise of 1-1/2" between loops.
That rise is measured from top of rail in one loop to top of rail in the next loop. This won’t be enough. You need to allow at least 1-1/2" clear from top of rail to bottom of the subroadbed for the next level. If we assume 3/4" for thickness of track, cork roadbed, and plywood base, then you need to have a total rise of around 2-1/4" (1-1/2" clear plus 3/4").
Now an 18" radius curve gives 36"diameter and 113"circumference. A 2% grade here dives total rise of 2-1/4" which might just be enough for you.
This is just to give you an idea of the thinking you need to do. You’ll have to decide what grade you are willing to accept and what your track/roadbed thickness is, and what clearance you want to provide.
Best regards
Ed