# helix minimums

Discussion in 'N / Z Scale Model Trains' started by medicking, May 10, 2006.

1. ### medickingNew Member

what do you recommend for a minimum radius for a helix to accept some of the larger rolling stock?
2. ### pgandwActive Member

In an N scale circular helix, the radius is pretty much determined by the grade and separation between levels, and will usually be much bigger than the minimum radius of your equipment. I don't think a radius smaller than 16" is practical - a 16" radius helix has about a 4% grade with 4" separation (rail to rail) between levels.

In planning an N scale helix, 1st thing to determine is your separation between levels. You need to allow for the subroadbed, roadbed, and tie and rail thickness, as well as the height of your rolling stock. Plus some if you want to reach your hand in to rerail the equipment (unless you are a master craftsman and your trains never derail, you will). I suggest setting some rolling stock, track, and roadbed inside a small cardboard box with a "ceiling" at the planned height, and see if that is sufficient for your hand toreach in and rerail your tallest rolling stock without tipping the box. The chosen separation distance between levels is the rise for the 100 times rise divided by run equals percent grade.

Over a 2% grade in N means seriously shortened trains or double-headed locomotives to pull a train up the helix. On a 4% curving grade, your locomotive will pull less than 1/5 the number of cars it will pull on level track. So an N locomotive that will pull 20 cars on level track will likely only pull 4 cars on a curving 4% grade.

Using 4" as our separation (depending on rolling stock height, construction technique, and finger thickness - that might not be enough!), and a 2% grade, we need a run of 200" each circle. Dividing by 6.28 (2*pi) gives us the radius of the circle - 32 inches!

Hope this helps
3. ### MeiriongwrilMember

Hi - Fred gave me a similar answer some time back, and I'm sure he's got some important points here.
Just to note, however, that Noch of Germany manufacture helices for various gauges, and one recommended for N has a radius of around 11 inches (they also have a somewhat wider one for N). As I recall, you need two packs (to get 3 turns) to obtain about 3 inches separation (but you'd better check the Noch catalog to be sure about that). They're not cheap, but presumably are designed to be usable; so I guess the grade and 'finger' width for rerailing must be okay.
I'm going to Germany this summer and intend to check them out!