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How can I design and build a helix?

Many layout designs call for a helix to change from one level to another in a small area. They can be a challenge to build! A helix needs to have a large enough radius to minimize forces that could pull a train to the center of the helix. In the worst case, this could result in the whole train cascading to the floor in the middle of the helix.


The author of 3rd PlanIt has seen this. It happens so quickly that there is little time to react - and not a lot of room to do anything about it. The best way to make a reliable helix is to design and build it perfectly every step of the way.



Perfectly? Well, yes, that is the goal. And with the tools now available to the model railroader, the goal can be met by almost any dedicated hobbyist.


The helix on the right was designed and built by John Hall two years ago. He used 1/2” plywood and “all-thread” rods inside and outside the roadbed joined by steel plates. It took a lot of work and runs reliably.

A helix used for model railroading need not be a circular spiral. Many years ago, layout designer Don Mitchell of San Diego asked if there was a way to design a “Bathtub Helix” in 3rd PlanIt. Don’s non-circular helix was a new concept to me that came up again recently while working with Alastair Brown of Scotland and a track plan originally designed by Bob Sprague.


He had established the elevations of various levels in his room-filling layout design. There wasn’t enough space to build a circular helix with a 2% grade, so he designed the Bathtub Helix shown at the right. It is a double-track helix from top to bottom. Bob Sprague cleverly used a “third track” around the primary helix to allow trains to move between intermediate levels as well as using the primary spiral to reach the staging track level.


This was a wonderful opportunity to use 3rd PlanIt’s Roadbed Splines. They are created by the program from existing track and take into account the extra length required when track is on a grade.


We also wanted to minimize any extra hardware, allowing the most possible room between successive turns of the helix. It’s important to be able to get your hands on the train while it’s inside the helix. Alastair will line the inside with window screen or guardrails to prevent a dreadful train cascade.

These are three Roadbed Splines for the first turn of the helix. A reversing wye allows traffic from either direction to enter any of the three tracks on this turn. The outside track continues into the second turn, exiting on the lower left as shown in the 3D view above.


The helix has a flat “Turn 0” that serves as a mounting base that supports the structure and fixes the location of the all-thread rods that will support the roadbed. The image farthest right shows detail for the all-thread holes in Turn 1.


The all-thread holes are at a specific spacing, with two rods “missing” because they would have pierced the top-left track of the reversing wye. The all-thread holes for Turns 1-6 have the same spacing in 2D - which, when projected on the roadbed spline at a 2% grade, are at a slightly longer spacing. It’s not much - perhaps 1/8” at the most - but it allows the all-thread rods to be exactly vertical.


Alastair is looking forward to building this when he receives the parts from the CNC vendor near Glasgow. Track Splines will ensure smooth curves throughout for high reliability. While you can purchase Roadbed and Track Splines from El Dorado Software, it is also possible for you or a nearby vendor to make them.


3rd PlanIt continues to lead the way in features that let you plan your layout quickly and build it with incredible accuracy. For details on circular helix design, click this link.

Click here to learn more about 3rd PlanIt!

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