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To calculate: First, you have to determine the circumference, which is Pi times diameter. So for a 12.5" radius, that would be 25" x 3.14 = 78.5". Then multiply that by the grade, which in this case is 3%, or 0.03 x 78.5 = 2.355 inches of rise for every turn. When you are designing such a helix, the tighter the curve, the more resistance on the train, which adds to how hard engines need to pull up a grade. What that means is that an engine will be able to pull a train easier up a 15" helix with a 3% grade than a 12" helix with a 3% grade. And 3% can be a pretty stiff grade to begin with. It is well worth the "investment in space" to make the helix radius as large as practical. There are many many ways to support each level of a helix. One example would be to build the helix out of 3/8" plywood (which would still give you appox. 2" of clearance between levels in this case), and use 2" blocks of wood between each level of helix on each side of the track(s), spaced in 60 degree increments (which is six around one complete turn). Another example would be to the same 3/8" plywood, and instead of blocks, use threaded rods that run vertically (which will allow for more access to the helix). Drill holes on either side of track on the helix subroadbed, and nuts above and below the subroadbed to get the right height. This method will also allow you to adjust the height of each connection to get get most consistent grade (VERY important). There are many other methods, suited to each person. Please search this site and others for more ideas and experience. Another idea to think about would also be to super-elevate those helix tracks, just a little. ---jps
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