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Trouble Understanding Curve Radius

Brian asks this question:

“Hi everybody. Hope someone will be able to help. I am 77 years old and disabled I decided that i should have a Hobby so started a train set OO scale,  my board is 11ft X 38 inches.  I am having trouble understanding Radius. I want to have three lines running around can I get away with this what does radius mean.  I  have laid track but feel the curves are to tight for some coaches to go round. Any advice would be Appreciated.”

19 Responses to Trouble Understanding Curve Radius

  • Matt Jacksonsays:

    Curve radius in a two-rail context is typically measured to the track center line (the nail holes in the ties are a good guide). As I recall, H.O. and OO passenger coaches (85-foot North American prototypes) usually require about a 20-22″ radius to operate smoothly with body-mounted couplers. I’m not sure about European coaches as they’re smaller, but use buffers and link/hook coupling.

    Hope this helps.

  • Will F.says:

    Sorry to disappoint you, but the smallest of the Hornby radius track, called First Radius, is the only one that will fit as a loop on a baseboard only 38″ wide. This radius will take only the smallest of carriages and goods wagons, and possibly 0-4-0 wheelbase locos.

  • J E Wilsonsays:

    For your board dimension of 38″ wide and holding in 2″ from the edge on each side (which is usually recommended) for the curve. The curve would be 36″ wide which would be a radius of 18″. As I model in HO, HOn3, HOn2 1/2 (N scale track) and Z scale and do not know what is recommended for OO scale, hopefully someone that models in that scale can help. I hope this is some help to you in understanding Radius = 1/2 the circumference (width) of a circle

    • S Travssays:

      Radius is defined as from the very center of a circle to the outside edge. Radius is also 1/2 the diameter. If your train reguires a 22″ radius for the curve, at the very minimum, your board would have to be 44″ wide.

    • Bill Haywardsays:

      Radius is not 1/2 the circumference; it is 1/2 of the diameter of a circle. The radius is the length of a line drawn from the center of a circle to its edge (circumference). The diameter is the length of a line drawn through the center of the circle to opposite edges of the circle.

      Brian may I suggest you consider the following:

      I am also 77 and just started in this hobby last year. You mentioned a disability. Most modern scale train setup’s require a large amount of wiring to connect power/controllers to the various items that will be in your setup. Will you be able to crawl around under a table to do these activities? I am in pretty good shape (for the shape I’m in) and still find it difficult to get this old body to contort itself under a table.

      I would suggest that you first consider what is the maximum size you have available for a table top. Then select a model scale that will work within those limits. Your scale size is 4mm to the foot or 1 to 76.2. In other words, a 76.2 foot car will be scaled down to 1 foot in OO scale. I am working in HO scale which is 1 : 87. My max radius is 22″ or 44″ in diameter. Add about 4 more inches for clearances on the edges of the table and that put me at 48″, if I were using a continuous table top.

  • Peter Whitlocksays:

    “HO” is 1/87 scale trains and accessories running on HO gauge tracks.
    “OO” (double O) is 1/76 scale trains and accessories running on HO gauge tracks.
    As the minimum “optimal” HO radius is 18 inches, you have space for only 1 line around your space.
    Suggestion: consider running a point-to-point switching operation. You will have a number of options fro track layout, industries, etc.

  • Newman Atkinsonsays:

    Brian, Radius is the curve around a center point if you were circling it. You said 3 tracks well then you need to find a center point that will be your fixed point to measure out each track. From that point I mount a string with a pencil and the radius that you want is the length of that string. So if your inner track is 18 inches, then you are going to make an arc with your pencil at the end of your string and that would be your center line of your inner track. Your next track out will need the clearance to clear a coach swing on each track safely usually about 2 and 7/8 inch farther out to it’s center line so add 2 and 7/8 to the 18 inches from your first track center line and your second center line should be 20 and 7/8 inches from your center point you are measuring from Then draw that arc as before. The third track should be the same way add another 2 and 7.8 inch and tour third track radius should be 23 and 1/4 inch.
    that is how to arc each radius. You have to set the radius that you want that will clear the overhang of coaches on each track so that if both pass that same piont at the same time they would clear each other. But my experience is that you may not have room to turn your trains in a 38 inch board One track would take at an 18 inch radius would take 36 inch from centerline to center line to get turned. To give you an example if you had a 19 inch radius from a center point of your turn then your center line will fall right on the edge of your layont with one rail hanging over. Now some trains can make a tighter turn but most likely coaches will not make that turn very well. If you make them too tight then you are going to have problems the bigger the cars or the bigger the engine. My Tree Stump Railroad is the newest running at the Danville Train Show. The outside track (the largest radius is is way to tight for most of my good equipment and the inside track is even worse and is limited to short cars and small engines. My Main Layout I have the smallest radius of 22 inches but most are up in the 28 to 40 inch radius because pulling trains around a tight curve is not a good thing. My Tree Stump Railroad used to be the board I used under the Christmas Tree. I installed the first track at the boards most outer limits with very tight 18 inch radius turns. The like you wanted to add a second track. The only way I could add a second track was to add it on the inside of the first track and I can only get a little more that 15 inch radius on it. This is my take to shows layout and it works fine using small engines and cars. Anything big cars will hit each other between the tracks. Although it is working fine for me there is very little space to run anything good. But that is what I do is take it to shows and I use my inexpensive tight turning equipment. My Main layout will often have 20 plus feet of train and I have had up to 40 feet of train which both of those trains would pull off the curse you are talking about. The shallower the turns the better. So decide what you intend to do with your layout and what kind of equipment you will be running on it. I suggest you find the video of mine called the Heavy Freight on the Shrinehill Railroad while you checkout the Tree Stump Railroad video. That heavy freight was done on level tracks with 36 inch radius turns and that 40 foot train needed 3 engines in front and a pusher in the rear. So look at what you are planning to do on your layout and adjust the curves as you need and just maybe that 38 inch board my not be wide enough to make those turns. Hope this helps from Newman Atkinson

  • Randall Styxsays:

    First one needs to distinguish between gauge and scale. Gauge refers to the distance between the rails. The scale refers to the proportional relationship between the model and the prototype being modeled. One can model in 1:48 scale (commonly associated with O Gauge track) and use HO or OO gauge track to model narrow gauge railways.

    The scale most associated with HO gauge track is 1:87. Multiply the actual distance between the rails of HO gauge track by 87 and you get almost exactly the 4′ 8.5″ used on standard railways in the USA. When OO gauge track was developed, the scale most associated with it was 1:76. That means that a scale model of a specific prototype will be slightly larger in 1:76 than in 1:87. But since they’re so close, track manufactures went to using the same gauge track for both scales. That makes the track out of scale for 1:76 (or “OO”, if you will), but at that size few people can actually see the error.

    Now for minimum radius. First, radius is one of the three main dimensions of a circle. It is the distance from the center to the edge. Diameter is the distance from one edge to the opposite going directly through the center. The diameter is always twice the length of the radius. The circumference is the distance around the edge. It is pi times the diameter (pi = approximately 3.14) [Although model railroaders seldom need to figure the area inside a circle, it can be computed as pi times the radius squared, or 3.14 x rad x rad.]

    Most models in what is loosely called “HO scale” (1:87 scale) have a minimum track radius (measured at the center of the track) of 18″. That yields a circle of 36″ diameter and a track outside diameter of about 37″. If the total width of the layout is 38″, that leaves only a half inch of layout between the edges of the track on either side and the abyss to oblivion (at least for the scale engineer). That half inch is not enough for a transition curve to straight track parallel to the edge of the layout. Flex track will bend to tighter radii, but only very small rolling stock will still work without derailing. (0-4-0 steamers, old time freight and passenger cars, trolleys, etc.)

    Since “OO scale” (remember, that’s a misnomer) is 1:76, to get the same relationship between the model and the severity of the curve, it would have to be a larger radius.

    Just what the relationship of model to the curve is remains a matter of personal choice, so long as it’s not so tight that it forces derailments of your preferred stock. Some large models in 1:87 scale require a minimum of a 22 inch radius. Many modelers prefer a 30 inch minimum radius in 1:87 scale because of the look, especially with long stock. In reality, however, all those “minimums” are much sharper curves than found on real standard gauge railroads. We’re willing to make that compromise because we don’t have gymnasiums to house our model layouts. Some modelers go to narrow gauge railways for the same reason they went to them in the real world: narrow gauge equipment can handle sharper curves (as was often necessary the first time through the mountains).

    While referencing the real railroad, just by way of information. Prototype railroads did not plot out their curves using radius. (They didn’t have the option of planning the rails before building the mountains, nor could they measure into the mountain or out into space to the center of the curve.) They plotted curves by degree. Field engineering guides had tables for curves from one half to 30 degrees. If they were plotting, say, a 10 degree curve, they would deviate from straight ahead half of that to the side, measure out 100 feet in that direction and mark a point. Then they would lay the track so it would be centered on that point. If one could plot out the new direction then lay out lines perpendicular to both the old and the new direction at the start and end of the curve, those perpendicular lines would intersect at 10 degrees. (In actual practice, they probably often laid out the road bed location and then measured to make sure they were not making the curve too sharp.)

    The recommended maximums for general practice were 14 to 16 degree curves. Road engines had a maximum sharpness of 18 degrees, steam switchers 23 degrees, and diesel switchers just under 31 degrees. Box cars had a preferred maximum of 30 degrees and an absolute of 60 degrees, while passenger cars had a preferred maximum of 12 degrees and an absolute maximum of 18 degrees. A 12 degree curve has a radius of 478.34 feet (or 65.98 inches in 1:87 scale); and a 30 degree curve has a radius of 193.18 feet (or 26.64 inches in 1:87 scale). Note that these maximums were for yards and wyes where train speed would be at a minimum. Where trains were at speed curves had to be much gentler.

    So, it looks like you can have only one loop on a 38 inch wide layout, unless you stack them one directly above the other, or have separate loops in different portions of the layout, perhaps overlapping. One loop might reach the full length but be hidden in a tunnel under a smaller second or third loop at a higher elevation. Or you could limit your rolling stock to very short equipment, both loco’s and cars, and have tighter curves.

    • Randall Styxsays:

      Reference: The book from which these statistics were taken is “Design – Data Book for Civil Engineers” by Elwyn E. Seelye, Copyright 1945 USA, John Wiley & Sons, Inc, New York and Chapman and Hall, Ltd, London, 417 pages. Box cars of that year did not include modern styles. This book also lists the maximum curve for 70 mph track at 3 degrees with 10.125 inches of super-elevation. The maximum curve for 35 mph is 10 degrees with 8.875 inches of super-elevation. For mixed speed trackage super-elevation is to be reduced by 3 inches.

  • JBHsays:

  • Riff251says:

    First, you can fit a nice sized railroad on a baseboard of 11ft. x 38in. You probably can’t fit concentric ovals in this space but there’s room for a lot of railroad.

    Since you specify OO gauge I assume you are a British modeler. Track radii in Europe are slightly different than in the U.S. Click on the link that JBH provided. It shows the geometry of Hornby track.

    On the baseboard you have, you can fit a loop using 2nd radius curves. These have a radius of 438mm (about 17.25 inches). This creates a semi-circle of 34.5 inches diameter (on track centers,) leaving space on each side of your layout of about 1.25 inches. While this is a little too close to the edge, it is workable. You can protect your trains from falling off the table by attaching a facia to the sides of the baseboard. Raise the facia 1 – 2 inches above the table to “catch” the trains if they should fall off the track.

    This arrangement will create a very long oval with enough space in the center for additional track and scenery. You can add turn-outs from the straight sides of the oval and create several spurs and a switching yard. You could even add a turntable to turn trains around if you so desire.

    It seems to me that you have enough space available to keep your railroad hobby hopping along for quite a while.

  • David Broadsays:

    As above, 38″ is too small for sensible H0 double track curves you need 48″ and even that looks tight.

    The curve radius is normally taken from the center of the track so a 2nd radius track is about 18″ radius to the outer rail 36″ Diameter and can just squeeze round in 38″. That might be challenging if you need an upstand to stop stock hitting the floor. Most H0 stock will run on 2nd radius. Most will go round on 1st Radius as long as it is laid absolutely perfectly.

    However it comes down to rolling stock. If you run sub 60 foot cars, you can squeeze the track spacing down from the traditional 1st radius /2nd Radius by simply easing the 1st radius out to 16″
    However if you run 75ft 00 cars and 4-6-2 locos this wont work.
    I cut the sleeper base and ease set track out to larger radius, it works well. I find Flexi track is hopeless for long curves below 24″ radius , it kinks on the joints and wont stay in place

    You can still use transition curves with 36 dia on 38 ” by swinging the tracks in towards the middle of the board

  • Davidsays:

    To have 3 tracks using standard hornby curves which come in 4 different sized curves or radius you will need a wider board. Even then not every coach or train will run very well on the inside curve as the smallest isnt really compatible with that sized curve. You’ll need a width of about 48 inches or 4 feet to fit your 3 curves / radius.

  • jim ourslersays:

    An important consideration when designing a layout is access. An accessible reach is 24 inches, so your layout must be accessible on all sides. Coaches tend to be longer than freight cars, and require a larger radius.

    I an finishing up an N scale layout and found that 30 inches is absolutely unworkable from only one side. My next layout will adhere to the 24 inch rule.

    I trust that with all the explanation above on radius, you are well versed. Another consideration is the S curve. That is a pair of curves connected together like the letter S. If there is not a full straight length equal to your longest coach between the two opposing curves, you will have problems with derailment. This becomes a real gotcha when a curve connects with a switch that redirects the curve in the opposite direction.

  • robsays:

    You’re correct that your curves are too tight for O guage on a 38″ table width. My advice is switch to HO or better yet, N. I was collecting HO and switched to N for this exact reason – the broader the curves, the more real it looks and the smoother it runs!

  • Briansays:

    Thanks for all your reply’s it would seem that I will have to extended it from 38″ to 48″ and try to get my family to help as I will find it very difficult to reach across 48″. But once again thank you all for your help.


  • Peter Simmonssays:

    My advice is to buy 3or 4 lengths of set radius track in 2 or 3 different radius and see if the running stock will handle the radius. Then buy or adapt the running stock to suit the radius.

  • David Stokessays:

    I think 3 parallel curves in 38 inches is going to have you looking for another hobby. Have a look at some published track plans – there are heaps out there catering for your foot print, and try one of those proven layouts. Good on you for picking the world’s greatest hobby!

  • Brian Ralphsays:

    Hi Everybody

    thanks for your advice, I have extended it to 50″

    Thanks again

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