Planning a helix can be a daunting task. You must think three dimensionally.
- How much climb do you need?
- What is the minimum height that can be gained in one complete turn?
- How much run can you provide to get it there?
- What if you can’t divide the rise evenly between turns?
Begin by determining the total climb. This is simply the distance between the starting level at the bottom and the track level at the top. This may be only a few inches, it may be several feet.
Next, determine the amount of climb that can be accomplished in one complete 360 degree turn. This climb needs to be enough for the bottom of one level to clear a train on the level below. This needs to include the height of your tallest train, the track, roadbed, benchwork, and if possible, a little extra room for you fingers should you ever need to re-rail a train. For the HO scale helix you see here, I chose a 4 inch clearance between levels. This allows a little extra headroom while maintaining a reasonable grade. With benchwork included, the climb between turns is actually 4.5 inches.
These two numbers will help determine the number of turns needed to make the helix. If you have a 37 inch climb, and a 4.5 inch climb / turn, it will take 8.2 turns to complete. The 8 turns are easy, but what do you do with the remainder?
First, your track arrangement may not require the train to exit the helix exactly above the place it enters. This may mean using less than a full turn at the top. Any remainders can often be accommodated here. Alternately, it may be possible to use a small conventional grade at the entrance or exit to even grades, or eliminate a turn altogether. A third option is to simply increase the climb of each turn slightly to accommodate the difference.
With the rise determined, the grade itself depends on the length of the run. In a helix, this means the radius of the curve. The greater the radius you can provide, the easier the grade will be.
Determining the radius of a helix is a balancing act between making the grade as long as possible and conserving floor space and limiting the travel time for trains in the spiral. To determine the grade for a specific radius, multiply the radius by 6.28 (2xπ) and divide by the rise.
You can chose a radius based on either criteria. If you desire a maximum grade of 2% or less, and space is not an issue, you can make the helix as wide as necessary. If however you can only fit an 18 inch radius curve, then your only variable can be the height of climb or you are at the mercy of the grade and will have to plan operations accordingly.
There is also no rule that a helix has to be a circle. Oval-shaped helices offer longer runs and potential for using conventional turnouts for crossovers inside the helix. The trade-off of course is an increased footprint.