Hello all,

I am reworking a design I made trying to use wider curves. I am very new to this and have no idea how to design a layout by hand, so I used SCARM to create this layout plan (the blue is the dimensions of the house) using stock Atlas track. However, I found that 49.5 in radius curves are about as small as I can do, and I'm looking for 54" to be the new smallest radius I use for the branch line track.


The layout below is what I am going to try and build. There is a double track mainline loop that I need to redesign with much wider curves (60" and 64"?) and the yard will need to have wider curves as well. 


My trouble is in figuring out what the next largest curve radius is. the 54" radius curve is 21.21 inches long in sectional track. How wide should the next radius be? 58" if I keep 4 inch rail centers? How long would the sectional track be? It might be "newbie thinking" but I'm trying to keep the geometry to a minimum with the flex track and maintain simplicity.


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Original Post

My recollection is that 54" radius is the largest Atlas makes in sectional. It's flex from there on, but the bright side is that curves of 54" and above are easier to form. I use RR-Track Version 5 and I can create curves of any size I want provided the length doesn't exceed that of a single piece of flex track. Using a 30-degree arc, the largest radius you could do with a single section of Atlas flex is about 72", well above what you're trying to do. Best bet would be:

  • Make a curve template of the sizes you want to use
  • Lay out the track center lines.
  • Glue down your ballast board
  • Lay out the flex, cutting off the excess on the inside rail. Avoid having track joints on top of ballast board and/or sub-roadbed joints except at lift-outs.

By the way, the X-Y mapping for a circle is "Y" = square root("R" squared - "X" squared) where "R" is the desired curve radius. Mapping this way, you can connect the dots to lay out your curves from a given point on the circle.


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Assuming you're measuring from centerline to centerline and you're talking about concentric loops, the centerline to centerline diameter of a 54" radius track is 108". In order to maintain a constant 4" gap from the centerline of the inside track to the centerline of the next largest track, that next largest track would have to have a minimum centerline to centerline distance of 108 + 4 + 4 = 116".  Therefore the minimum radius of the next largest track would be 58" (116/2).

However, that does not take into account the distance from the center rail of your particular track to its inside and outside edges which, depending on how large that is, may or may not be enough to allow trains to pass w/o hitting each other or may not even allow one loop to fit within the other. For instance, if that distance were just 1", the inside loop would have an outside dimension of 110" and the outside loop would also have an inside dimension of 110" - literally on top of one another.

It's the same reason an O-31 loop of Fastrack will not fit within an O-36 loop of Fastrack (unless modified).

Not sure why the sectional length matters if the diameter of the new track plus 2 X the center to edge distance fits w/in your layout dimensions.

If you use the same reference point on both curves, 4 inch wider radius will clear just about anything  you run with the exception maybe (I don't have any) of some big articulateds.      I use 52 inch radius and my Max Grey 2-10-4 on the inner curve easily clears 80 ft passenger cars on the 56 inch radiius wider curve.

If measure from the middle on one line, measure from the middle on the next.     Or a simple fix is to lay the tightest one, the 54 inch in this case, and then mark 4 inches out from (for example) the inside of the ties, and lay the inside of the ties on the next curve at those marks.    You could make a mark every 6-12 inches around the curve to guide laying the flex track.

I don't understand what the purpose of the cross reference to sectional track 21.21 inches is for.



A way to make the first curve is to use a piece of cord/string as a compass.     Tie a loop in one end.   And then put a pin/nail/something on a base at the center of the curve.    This is hard part, finding a place to anchor the string.    Then put a loop in the other end so the distance between the loops is the radius you want.    Hold a pencil in the second loop and keeping the string taut and the pencil vertical, draw a line on your benchwork to locate the curve.

I would suggest using a 1X2 piece of lumber the approximate length of the radius you plan to use.  Drill a hole at both ends, one to hold the pencil/marker the other for a bolt to secure to something stationary.  Scribe your curve with this, repeat as necessary.  4” centers work well in O scale.  The wood bar will not stretch and will give you a more accurate curve than string.  

Since the largest sectional curve Atlas makes is O-108, you'll need to use the 40" flex track to make O-116 curves (58" radius). SCARM doesn't make it as easy as RR-Track to make flex curves, but in the status bar at the bottom of the screen, it shows the radius of the curve as you move the end of the flex track around.

I created a 116" diameter circle using the Circle tool in the Figures library.

I added a 40" flex track and moved the end around until the status bar showed 58".

I used copy/paste to duplicate the track 8 times to give me that 9 sections that formed the circle of track you see in the photo.

You have to zoom in to get enough control to land exactly at 58", but it wasn't very hard to do.

As you can see, 9 sections doesn't quite complete the circle. I expect that if Atlas made O-116 curves, they'd make each section around 22.77" and it would take 16 sections to make the circle.

I actually used the Split option to whittle away a section to get a 22.77" sectional track and joint 16 of them to make a complete circle, but I doubt you'll be making a circle, so just lay track in SCARM and then Split the last one to the length you need to finish your design. I did attach a SCARM file with the full 40" track and a 22.77" track in case you want to play with them.



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I do use the beam to scribe the circle, but use the inside rail as the radius.  Four inches is enough down to 60" radius.

I simply figure out how much space I have, and make the largest radii I can.  The rest is just letting the chips fall.

DoubleDAZ posted:

Keith, that’s a tool I haven’t noticed, thanks for mentioning it. I haven’t had a need to create a curve like that, but I’ll have to check it out.


It's also good for creating exact straight lengths. Much easier than the snip method.


Roo posted:

Just draw or get someone to do it for you an accurate plan, transfer to baseboard, bend track around your marks and finished. You do need a drawing anyway this one is for the feeders. if you draw a plan first you know that everything will fit. Roo.

It is complicated because it can be! This layout will be deadrail only since my current locomotive, along with any future engines, will be battery powered.



Also thank you to everyone who has replied! I have been working this out and think I have an idea of how to redesign this. As a music major, I haven't had to do this kind of math in a while. To answer someone's question from earlier, I want to keep this sectional approach so I can work with very simple geometry for the lower mainline loop (as many 90* and 180* angles as possible). Also, my idea is that if I need to adjust a curve, I can reuse the cut section of a curve to cancel out the adjustment and continue in a straight line (i.e. in a yard). 

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