Need a lesson on Radius

Discussion in 'HO Scale Model Trains' started by PeteMorich, Apr 17, 2007.

  1. PeteMorich

    PeteMorich New Member


    To make a long story short, I swapped out my oval track with Kato Unitrack (it was a gift and BTW I love the stuff!). I have a first time layout with just loops. 4x (about) 7 oval and with an over/under figure 8 inside. I would now like to replace the figure 8 track, but I don't think I can use Unitrack as it only comes in 22.5" curves. If I am wrong on this, please advise.

    Since I only want to use sectional track, from all I've learned here (which is much I can assure you) the next best option is EzTrack.

    So here is my question:

    If the diameter of my figure 8 "circles" are 36" (outside rail-to-outside rail) is that 18 degress radius?



    I don't understand half of it, but I read all of it!
  2. Gary S.

    Gary S. Senior Member

    Hi Pete, I can't say much about the EZ track and such, I have experience only with flex track.

    As for the radius, I was thinking that radius was measuered to the center of the track, but could be wrong. Or, perhaps your sections are not in perfect alignment resulting in a slightly smaller radius. But Yes, I would guess that the 36" outside rail to outside rail is actually 18" radius curves.
  3. jeffrey-wimberl

    jeffrey-wimberl Active Member

    Figure your curve radius by measuring from centerline (between the rails) to centerline and divide by 2=radius (36 / 2 = 18)

    I use EZ Track exclusively and I have no problems with it. Like any other track, care must be taken when assembling it because rail joiner can slip under the rails and some rails (on curves) don't always line up precisely. The best way I've found to make it trouble free is to make sure it's flat and properly aligned then solder the rail joints. Once the joints are all soldered I cut gaps for my blocks/districts and/or expansion/contraction gaps. The only time I've had a problem with it was when hurricane Rita ripped part of my roof off and soaked part of the layout, causing a kink in an area that wasn't gapped. It's gapped now.
  4. MasonJar

    MasonJar It's not rocket surgery

    Radius in HO is measured to the centre-line of the track. So 18" radius needs about 40 inches for a loop by the time you figure enough room for roadbed, ballast, and a bit of "oops" room.

    Degrees of a curve in model railroading refer to how much of a circle the track section covers. For example, 30° curves need 12 sections to make a circle (30° x 12 = 360°). Similarly, 22.5° curves require 16 sections to make a circle (22.5° x 16 = 360°).

    The confusion arises because sometimes prototype curves are described in degrees. This is a different measure however. It is the angle between the tangent of a given point on the track, and another point along the arc that springs from the tangent, usually 100 feet away.

    In the diagram below, the tangent point is blue, the point on the arc is green, and the "degree of curvature" is the red angle. The reason the prototype did it like this is because it wasn't often practical for a guy with a tape measure to actually directly measure the radius, which was often on the order of several thousand feet...


    Attached Files:

  5. PeteMorich

    PeteMorich New Member

    Thanks guys, but I am a little confused. EzTrack comes in both 15 and 18 degree curves. Which one do I need to ensure I can fit in onto the inclines/risers that are already installed and (due to scenery, etc.) cannot be moved. The current circle of track (Power Loc) is 36" apart center-center.

    Appreciate all the help!
  6. MasonJar

    MasonJar It's not rocket surgery

    In MR, the degree of the curve does not have anything to do with the radius. You are looking for somthing that will fit inside a 36 diameter (18 inch radius) curve. It must therefore be 15" RADIUS track. Whether it covers 15 or 18 DEGREES of a circle (ie. takes 24 or 20 pieces to make a circle) is irrelevant.

    It appears that the Bachmann EZ track is available in 15" RADIUS, so that is what you would need in order to fit inside the existing trackwork.

  7. PeteMorich

    PeteMorich New Member

    I may be parsing words with you here Andrew but I am not looking for something to fit INSIDE the 36 diameter. I need something that fits 36 EXACTLY. So I am thinking the EZ Track 18 would do the trick?
  8. MasonJar

    MasonJar It's not rocket surgery

    Sorry... my misunderstanding...!

    So yes, if the measured diameter of the track you wish to replace is 36" (thirty-six inches) measured centreline to centreline as a true diameter, then you need 18" radius track to replace it.

    Atlas (TrueTrack), Bachmann (EZ Track), and Life-Like (Power-Loc) all have "roadbed" track in 18" radius.

    EDIT - In re-reading your posts above, it seems you are replacing the Life-like track with something else... Why?

    Hope that helps.

  9. PeteMorich

    PeteMorich New Member

    Thanks Andrew!

    I am replacing the Power Loc for a few reasons --it's loud, the fit between pieces isn't that snug and causes additional clacking / occassional derails, the electrical connections short out at times, a few of the clips have broken over time. Plus it's black and I would prefer grey...which I know is available in PL, but (like a wife) if you're gonna change, you might as well upgrade!

    As I posted, I received the Kato Oval as a gift. I considered returning it, then my curiousity got the better of me and I figured hell I'll set it up and check it out. Man was I suprised. The positive comments I had read about it were well deserved.

    My layout is overly simple (as am I) since it was created for my 2 boys (2/6) and I never (and I mean never) was into model railroading (at 40). But I'm hooked and really enjoy the hobby, However the track problems (most probably self-created by my inexperience) seemed to be taking up too much of my time and are so frustrating. I just want to move on -- of course this means to complete, redo, complete again, then completely change to something else. But it beats watching TV!

    This forum (and you as I recognize your screenname) have been a great resource. Keep it up and I'll keep reading (and even possibly understand it all one day!)
  10. MasonJar

    MasonJar It's not rocket surgery


    Glad we got it worked out! And thanks for the kind comments.

    Hope to see some pictures at some point of your work, even if it is "just for the kids" ;) :D

    I have two kids about the same age, and they enjoy seeing the trains run too.

  11. jetrock

    jetrock Member

    One thing you might consider, instead of 15 or 18 inch radius EZ-track, is getting some flextrack and using that for your curves. Flextrack allows for smoother curves, can be fit to whatever radius you need, and has far fewer joints between pieces of track, which means fewer places for electrical problems or misalignments to sneak in. It's a little more work, but not much more (it means laying the curve will take an hour or two instead of ten minutes) and you'll have a better long-term result.

    If you really must use sectional track, I'd suggest using the Atlas True-Track brand--it uses sectional track pieces that can be snapped into roadbed or taken out and used with cork or foam roadbed (essentially, the same as their Snap-Track no-roadbed track.) I hear lots of horror stories about Bachmann EZ-Track. Less than with Power-Loc, but only just.

    And yes, track radius on model railroads is measured in INCHES, not degrees...Atlas track is sold in 15-inch and 18-inch (as well as 22-inch) radius. A circle has 360 degrees. Half a circle is 180 degrees, a quarter-circle is 90 degrees. 15 degrees would be 1/24 of a circle, 18 degrees 1/20th--whether the radius was 15, 18, 22 or 200 inches.
  12. jeffrey-wimberl

    jeffrey-wimberl Active Member

    The Bachmann EZ Track comes in both steel (black roadbed) and nickel-silver (gray roadbed) 15, 18, 22 and 36 inch radius. Don't let the so-called horror stories scare you off. I've heard them all, yet I use EZ track exclusively. Problems usually arise from the track being put in place too quickly, joiners not aligning properly (happens sometimes), track isn't flat at joiners (because it was laid incorrectly), mixed track types (some people try to join in other types of track), while this is possible, I don't recommend it. If everything is done properly, you'll have very few problems.

    1GUNRUNNER New Member

    I see there is limitations on what engines can be used with a smaller radius curves. I am going to build basically a square layout to run around the edge of a room. The tighter the radius for me the better. Am I going to really limit my options on engine and cars by going with a 15 deg curve? I am wanting to go with a steam engine setup.
  14. MasonJar

    MasonJar It's not rocket surgery


    As mentioned above, curves are generally measured in inches, not degrees...

    But the answer is yes, you will limit the engines you can use on such a tight radius. Four-axel diesels, and small steam (4-4-0, 2-6-0, 4-4-2, etc) are the biggest you will be able to use.

    You will also limit the length of rolling stock to 40 scale feet or less. This is due in part to looks (excessive overhang), and also operations - the ends often swing out far enough that they pull the trailing car off the track.

    But if you are running on a shelf around a room (without the need for a turn-back loop), you can actually have quite broad curves. E.g. on a two foot wide shelf at a 90° corner, it is possible to achieve a 40" radius curve.

    Hope that helps.

  15. 60103

    60103 Pooh Bah

    Pete: ifyour curve measures 36" over the outside rail you may have a problem. A true 18" radius curve in HO should measure 36 5/8" over the outside rail; 36" either center to center or inside rail to outside rail (easier to see). (However, if you have some track that is not quite 18" radius it could happen.) If you have squished the track together to have the 36" measurement, there are some track joints that aren't perfect and will cause problems.

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