Well, part of it is a mathmatical calculation based on things like space available, grade, separation between levels, and so on.
So what are those numbers? A 6x6 space would allow a circle of maximum 36" radius (72 inch diameter = 6 feet). But because we need to measure to the track centre line, a radius of 34 1/2 is probably the biggest you want to go for the outside track.
Each loop of outside track would require C=pi*d or 216.66 inches (just over 18 feet) of track.
The inner track would have a radius of 32 1/4, allowing for 2 1/4" between tracks. 215 inches or just under 18 feet of track per loop.
To meet the clearance standard, three inches of separation is required from the top of the rail to the overhead structure. If your helix structure is 3/4" thick, you will need to go up 3 3/4" per loop.
Grade is therefore 3 3/4 divided by 216 x 100% or about 1.75%, which is good.
Going up 3.75" at a time, you will need something like 4 to 5 turns minimum to get any useful separation between levels.
So 5 turns @ 18 feet per track per turn, times two helixes is 360 feet of track required for the helices.
Hope that helps...
Andrew
Hello, has anyone built a multilevel helix in HO scale? I could use any help in planning, ie. need Angles to cut the Circle or Octagon pieces, I hope to build 2 helix using double wide track 6 ft X 6 ft square, with 16 ft of track between. If anyone has ever built one and has the necessary dimmensions, I sure would be greatfull for your help. 