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Most of the curves on my layout-under-construction are eased.  The two under-table horseshoe curves are continuously variable radius, a bit above 112/104 at their ends, 96/88 at the 45° point, and 80/72 at their apexes, with 5" spacing at the apex and 4" at entry/exit.  I took a deep dive into the math of this two years ago and have a PDF discussion available that summarized my research.  The continuously-variable horseshoes were the key to fitting the desired track plan into the available space while keeping all visible mainline curves at 112/104.  Here's a pic of one of them, before the upper layer was added.20220427_181902[1]


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Real railroads use complex tables and math to compute the best spiral easement. Because we don't need optimal but functional (they really do help trains get around curves) model railoaders have develped a large number of tools to make things easier--ranging from the templates published in MR to the "classic" bent yard stick. Any of them are probably good enough for our purposes. Just remember to allow for the additional width in planning.

I am new to the forum but not to o-gauge railroads.  I am designing a new layout and am considering using spiral-curve-spirals on my main lines.  I have never known of these being used and am wondering if there are any disadvantages.

May I ask  what you envision for your layout ?  What shape , size and track system . This will determine how and to what extent using an easement's  will work.    ( and yes they do work)      Think of all the video's and layouts , where the train rambles down the strait  track , and slams into the curve!  Easements help with that!

My main line will go around the edge of my 22’ by 12’ layout.  I deal with spirals on a fairly regular basis and have CAD software so I have the ability to print the curve along the inside end of the ties at a 1:1 scale.  Then I could use that to cut a template out of 3/8 inch material.  I can envision this as being a very smooth running line.

There are ample sources of info about model railroad easements on You Tube and by just Googling it.  You will find charts and diagrams including those from Mr. Armstrong's book.   It's not hard to do and the results are noticeable.  Add a little super elevation and  your trains will look and run better. Most charts stop at HO scale, but you can just double the numbers, It's not rocket science.  My favorite tools are an aluminum yard (or meter) stick, a hand full small finishing nails and whatever compass tool you choose (create) to draw a radius.  Go for it!               

I use a very similar approach in simulating curve easements in Fastrack as what ADCX Rob does with tubular track. I have parallel main lines on a 6" center. The difference in size is shown in my diagram. Two simple curves are shown overlaying two of my simulated eased curves. The difference in size is shown by the blue box which is about 2½" high and 7½" long.

The full ovals look like the following illustration where the upper ovals are simulated eased curves and the lower ovals are constant curves of O48 and O60. The lower ovals are 4½" narrower and there are 15½" more straight track.

The difference in operation and appearance is beautiful.


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That's what I thought, but by using flex track I can have a true spiral which by  definition the radius at any point of the spiral varies inversely with the distance measured along the spiral.  I appreciate the input and look forward to actually seeing the operation.  If you have used a spiral in CAD and picked the spiral you see that it is a constant line of boxes regardless of the zoom extent.

I used atlas O flex track which is by its nature difficult to bend.  I just put in enough bend to make the curves work in the space I had so it pleased the eye.  No formula, just get the broadest curve with the space.  I was also able to incorporate a couple of small deviations from straights along the front of the layout where it is visible. This slight change in direction from straight running I find pleasing to the eye especially with the longer equipment. The Atlas O bends smooth without a worry of a line.

Last edited by wb47

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