Question on minimum HO radius

Hello all,

I'm thinking on trying a layout using an O Gauge train on HO track (or On30 as I guess it's commonly called). From what I understand so far (haven't been into this hobby for a real long time) the minimum HO radius is about 18" which I assume is formulated to accept any type of HO engine and cars, however I want to compress the overall layout size plus have a lot of curves. Now the only engine that will be running on this track is a On30 0-4-0 Porter and maybe 2 or 3 short flatbed cars and a small caboose (it's not representing any historical setup). Theoretically I'm hoping this will allow for tighter curves and I'll use flex track to set the eventual radius.

So the question to the group is, has anyone experimented with such small engines on a tighter a radius, or is there a formula to go by? If not I guess the best way to test it is to compass out some curves on a board, tack some flex track down and see what happens? Any advice will be appreciated. Haven't purchased the engine yet (Bachmann has what I'm looking for) but trying to work out as many problems as I can ahead of time.


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There's no set formula because the minimum radius is normally set by the design of locomotive. Larger steam locos are normally designed for 18" radius, but some of them look ridiculous on that radius.

With the Porter alone, the minimum radius will be little more than it's wheelbase, but your problem will be one of keeping the couplers of loco and rolling stock together. I guess you're going to have to go with trial and error.

A problem with flexi-track is that as you bend it into tightish curves you tend to get parabolas rather than perfect curves, so you will need to bend it around a solid former like a plate or dish and then pin it at very close intervals. (the pins don't need to go through the sleepers, they can be beside the rails)

Roco (Austria) makes a variety of curved sections down to 14 5/8" radius plus a 10" radius. I would recommend you use those in preference to flex track.

A transition curve, even/particularly into very small radius curves will look much better than a straight section linked to a sharp curve.

Good luck!

Regards, Greg.P.

Reply to
Greg Procter

18" counts as "sharp" radius: good for locos to about 4-6-0, and cars to 50ft. You need 36" radius to accommodate any type of HO car and loco (with easements, and a certain obsessiveness about trackwork, you can shave this about 30".)

You'll be able to use 6" radius curves for the Porter alone, or the cars alone, but when coupled up you'll have to settle for 10" to 15" radius. You'll also have to modify coupler mountings so that the couplers can swing wide enough, otherwise you'll get side pressure that will encourage derailments. Keep cars to 20ft max. overall length (== 5" over coupler faces.) Avoid railjoints on curves, but if you can't, solder them before curving the track. Also bear in mind that at such small radii, the gauge should be widened, so that handlaid track may be a better choice. And add easements to the each end of every curve. Finally, do not build S curves anywhere, period. Allow for a straight stretch between curves 1-1/2 times as long as your longest car, or longer if possible.


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Wolf Kirchmeir wrote: > Hello all, Any advice will be appreciated. Haven't purchased the > engine yet (Bachmann has what I'm looking for) but trying to work out > as many problems as I can ahead of time. >

I have the 0-4-2 Porter by Bachmann, don't intend to use it for anything but decoration, but it's pretty nice. Curiosity lead me to put it on an oval of 16" radius, no cars behind it, but no problem with the loco at any speed on that radius, including going through a turnout which lead to the floor. (Oval temporary on the floor, of course.) As most cars don't have any problem with 16" radius, I wouldn't expect the HOn30 smaller cars to have any.


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[ ... ]

It was done as a trick, but I remember reading of a guy who laid a rail around a silver dollar and ran an 0-4-0 around it.

I remember a description of a layout built on an 18" x 24" drafting board, with grades and lots of curves.

I think 4 wheel cars would be easier to use than 8 wheel cars. Couplers would be a problem, and you might have to devise your own system. Free rolling cars, steep grades - you could just push them up and let them roll down against the engine and you wouldn't need couplers.

You're on the right track. Experiment.

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<wkaiser responded:

Since I don't have my train yet to test I'm not sure what you mean: Will the problem be with the couplers unhooking due to the sharper angle they will be pulled on (sort of a binding effect) or would it be more in the area of cars wanting to tip or be pulled sideways off the track?

Yeah, had definitely planned to either gather or make some forms. However the curves end up, I want them to be smooth. Nailing beside the rail sounds scary but will do if needed.

Wow; 10" would be great if I could get everything to work on it. I noticed in a Bachmann catalog yesterday that that the tightest sectional tracks they have in HO is 15". Am waiting for a Walthers HO catalog to come in and hope they list those Roco tracks in it. Sections would probably be O.K. for the bulk of the layout but because of the various curves I'm thinking of, the odds of them meeting up correctly at the end isn't good. But then I could use a short piece of flex to make the final connection I suppose and save myself all the cut & fitting (which I was sort of dreading). Wonder what the common use is for that 10" radius track?

I agree there too. And the faster the train goes the worse it looks when it hits that curve.

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Sorry: forgot to put Greg's name in the title on his response. responded:

6" would be so nice but need a couple short flatbeds at least. Hope the 10" might work. Greg mentioned the coupler problem too. Will have to check that closely. If the situation is just the angled pull rather than binding up, would some weights under the flatbed cars help stabilize them?

Will note that car length. I had not thought of not using railjoints. Assumed they were always used but if the track is nailed down I guess they really aren't needed. Is this common to not use them? When you say a wider gauge is better, does that mean that the thickness of sectional track is different than flextrack, and if so, does that mean there would be a problem if I had (or wanted) to connect one to the other? And by "easements" do you mean to tilt the track into the curve a bit? I assume that would help at higher speeds (this particular layout will be low speed only) but if this also has benefits at lower speeds with tight curves I'll work that in as well.

Now that could be another problem. I had envisioned this track snaking throughout the layout with hardly any straight sections at all. Not sharp S curves but a meandering at least across the mid section. What happens with these types of curves?

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""greybeard\"@mcleodusa responded:

I like the 0-4-2 as well. It's only about $10 more than the 0-4-0 but thought it safer to go with the shorter model for what I'm doing. Another aspect of this planned layout is it being mostly a night scene and the 0-4-0 has not only the headlight but an interior cab light, so that's going to work out well. Later on I may have questions about adding lights to cars if that's possible.

Glad to hear the 0-4-2 worked smooth on the 16" radius. But only using it for decoration? Maybe when nobody's looking you could let him take a spin around the track. :)

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flyingdragon64 responded:

Lol -the owner at my local hobby shop mentioned that same story when I was asking him similar questions about the tighter radius curves. He's really into HO scale trains running on N Gauge track, and pointed me to the On30 to help getting sharper curves out of an O scale train. He had some basic advice that related to this kind of hybrid train modeling but never actually experimented on anything exactly like I'm planning. I'm thinking that silver dollar track was N gauge too. Would like to see that.

Think I've seen pictures of such in Z scale. I was going to say N gauge would be pushing it but if they can get an engine to run on the edge of a silver dollar then 18" x 24" would be open range.

Definitely going to have to take a closer look at the coupling situation. Putting in grades crossed my mind but thought I'd really be pushing my luck with all the curves creating issues already. It's a thought though.

Will do. Thanks everyone for the advice so far!

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One could do it in a much larger scale than Z or N. See

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look at Ron Ham's G gauge layout. His layout "with grades and lots of curves" fit on a 36" x 48" table. Cutting everything in half would suggest that one could build the same layout with S gauge track on an 18" x 24" drafting board. That should work with an S scale Dockside, or an O scale bastard gauge (On42) 0-4-0T. Geezer

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Geezer wrote: [...]

Oh yes, you need rail joints. That is, you need railjoiners to keep the ends of the rails in line with other. They also help with electrical continuity, but you shouldn't rely on them for that (solder them.) For electrical gaps, use plastic railjoiners and/or rail spikes either side of the joint. I also fill the gap with epoxy, and file it so shape after it hardens.

And BTW do not nail down the track. Track support is whole 'nother subject, especially for tight curves. But the general rules still apply: you need a well built substructure (subroadbed) on which you mount the track itself. The track structure should be:

a) subroadbed about 1" wider than the ballast (1/2" ply or nominal 1" lumber). This should be wide enough to provides a ledge for connecting the right of way to the scenery. Screwed and glue to the framework. Be obsessive about its alignment and grade (elevations). b) roadbed (foam board or 1/4" ply) - optional - should be about 1" wider than the ballast former. c) a ballast former (cork strip, usually) d) the track NB that some sectional track combines b and c. If you use such sectional track, put a layer of foam between subroadbed and track, to reduce unwanted noise transmission.

Use glue to hold it all together. Pin the cork in place until the glue dries. Pin the track in place until you've laid and glued the ballast (it will hold the track nicely). The tighter the curve, the more important it is that the flextrack (or rail, if handlaying) be formed close to the final curve before installation.

The gauge is the distance between the inside edges of the rails.

Tight curves are a problem for real railroads, too, and several compromises are used to ease running on tight curves, including wider track gauge, narrower wheel gauge together with wider tires, axles that slide sideways, check rails through the curve, grease applicators at each end of the curves, etc.

Sectional track come in two basic forms: a) rigidly molded tie strips on which the rail is mounted -- this is completely compatible with flex track. All brands of the same rail code (height) are compatible with each other. b) a plastic base representing ballast and ties on which the rail is mounted. These differ in the way they join, so that different brands are not compatible.

Tilting the track is called "superelevation", and it's used to allow higher speeds through the curve.

The easement is the transition from straight track to constant curve. On real railroads it is a spiral. (Highways also spiral into curves; if they didn't you'd have a high rate of skids and worse at the beginnings and ends of curves.)

Flex track automatically forms such an easement when you curve it. By contrast, with sectional track you go directly from straight to curve at the joint between the two sections. One of the effects is that the coupler on the leading car swings too wide, which increases side pressure, and hence derailments at the beginning of the curve. This bad effect increases with the length of the cars, and with the difference in their lengths. Another effect is that the train lurches into the curve - looks bad.

The end of the lead car swings one way, the end of the following car swings the other. If the couplers have insufficient side swing the cars will derail on account of side pressure. Real railroads put as much straight track between opposing sense curves as possible, and if they get anything like an S curve, speed is severely restricted -- and I mean severely. I rode the train from Vancouver to Squamish BC one summer, it's a very curvey line. The train rarely exceeded 20mph, and on some sections, it ran at a fast walking pace.


Reply to
Wolf Kirchmeir


Oh, I see, you thought "avoid railjoints" meant "don't use rail joiners." Sorry, that's not what I meant.

I mean that you should try to cut the flex track pieces so that rail joints fall on straight track.


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Wolf Kirchmeir posted:

Thanks for the info -and also the link! That Cavorite Mine layout is very similar in appearance to what I'm thinking of (except I'm going for more than a circle); A Porter, couple cars and a caboose (plus looks like they have a tender). And how close the train runs by objects and structures with very little clearance, and the overall look, very inspiring. Thanks again!

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Thanks for all the info Wolf. I'm making notes of the info from yours and all the other posts and will put them to good use when the experiments begin. I sincerely appreciate the input and will try to pop back in when I get some test results.

By chance anyone is wondering what this layout I'm dreaming up is about, let me explain:

It started with getting a few small pieces in one of those seasonal Halloween villages. Hallmark has them in the Fall in their Dept.56 line but my budget only allows for the Spooky Town collection that the Michaels craft store chain carries. Not into the big fancy buildings but like the older looking structures and graveyard elements. A lot of this stuff is lighted and some even have animated effects and sound and there's lots of little accessories, some of which light up too. Anyway, I thought about doing a diorama scene with landscaping and such but only if enough people could see it to be worth the work. Since we get about

200 trick or treaters, I decided to fix something up on the porch in a protective case for viewing.

Didn't have time before last Halloween to do that project so rescheduled for 2006 but since then had a thought that running a train through such a scene would be interesting. So the little project got bigger. I like an old fashion, vintage look for Halloween rather than the gore and hollywood commercialized stuff so prevalent today and even the train itself needed to reflect that. That's why I'm going with an old Porter rather than a short diesel variety. Something that looks like it came out of the forgotten backwoods somewhere. The curves in the design go with the unorthodox theme too.

I hope to have part of it winding through dense woods to where you almost only see the lights moving through it at first. It can then pass close to haunted houses and structures, through a graveyard, maybe a small cornfield or pumpkin patch and even into a cave. All with very little lighting to keep it spooky. Each year I'll modify and add to it to keep it interesting. Hopefully it'll be something the kids won't soon forget and who knows, maybe even get them interested in model railroading. :)

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Cool concept!


Carl Arendt's website features "micro layouts". You may not find a plan to suit you, but you'll certainly find inspiration and design ideas!

Take pix of your progress and post them to from time to time. The regulars don't mind the occasional model rr picture there. :-)


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Wolf Kirchmeir (Wolf=A0Kirchmeir) posted:

Thank you! And thanks for the web site (yes; lots of inspiration there) and the suggestion to post pictures at I'd like to do that eventually here too. Not actual pics of course but the urls to click on (I've got an account at PictureTrail where I can store photos).

There's so many possibilities with a haunted railroad layout I'm now thinking of making 2 versions: something simple that will actually be ready by Halloween and then another; the REAL layout, I can work on without having to rush it. When I've accomplished something I'll get some photos. :)


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