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Minimum Radius for HO

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  • Member since
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Minimum Radius for HO
Posted by Precast on Saturday, October 10, 2009 7:16 PM

Hey Guys,

Just getting started on a shelf layout.  What's a good minimum number to use on a radius?  I want to make a complete 180 degree U turn.  Thanks in advance. 

  • Member since
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Posted by nfmisso on Saturday, October 10, 2009 7:23 PM

For what kind of equipment?

Many years ago in MR, there was a picture of an 0-4-0T steam locotive going around a circular layout where the inner rail was a silver dollar.

Close to the other extreme, is the MTH 4-12-2 with the chassis locking plate installed (see MR 11/09 pg 78) has a minimun radius of 42 inches.  WIthout the locking plate, it will do 22".

If you stick to 40' and shorter cars, smaller steam locomotives like Bachmann's 2-8-0 or smaller four axle diesels like GP7 and RS3; 18" radius is do-able. 


Nigel N&W in HO scale, 1950 - 1955 (..and some a bit newer too) Now in San Jose, California
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Posted by jecorbett on Saturday, October 10, 2009 8:25 PM

What should your minimum radius be. My answer would be the largest radius your available space can handle. There are two issues when it comes to radius. What is the minimum radius your equipment can negotiate and what is the minimum raidus it will look good running on? The latter requires a much larger radius than the former. Sectional track is widely available in 22" and 18" radius but 15" is still around as well. In order to make their equipment marketable to a large segment of the hobby market, manufacturers try to develop equipment that can at least negotiate 22" radius track. While large locos and full length passenger cars might be able to handle sharp curves like that, they are not going to look good doing so. Model railroad curves, even on large layouts with very broad curves are going to be much sharper than the prototype curves so some compromise is going to be in required. Regardless of the size of your layout, the broader you can make your curves the better. Your railroad will look and operate much better. I have never heard of anyone complaining that they made their curves with too large a radius.

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Posted by IVRW on Saturday, October 10, 2009 8:42 PM
As nfmisso said, it depends on what you run. If your running 85 foot Passanger cars, a good minimum Radius is 32. However, if your just running a small beginners layout, 22 is good as well. In a case of dire needs, 18 will do. But before you kick the bucket, I would say 15 is good for REALLY small locomotives, no bigger than 4-6-0, and VERY TINY cars, no bigger than 36 feet. But if it is Modern or 1950s, 32 is kept as a guideline for realisticness.


19 Years old, modeling the Cowlitz, Chehalis, and Cascade Railroad of Western Washington in 1927 in 6X6 feet.

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Posted by cuyama on Saturday, October 10, 2009 9:15 PM


32 is kept as a guideline for realisticness.


32" is much too broad for many smaller model railroads in HO. If one chooses too large a minimum radius, it can be just as problematic as too small, since it so severely limits what can be accomplished in a given space.

This guideline is based on some experienced modelers' findings from the Layout Design SIG site:

Curve radius rules-of-thumb

Fortunately the rules-of-thumb work for all scales.

Tags: HO , Curve radius
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Posted by tomikawaTT on Sunday, October 11, 2009 12:01 AM

Real-world curve radii on a railroad I am actually operating:

  • Main line - shorty (about 10 inches long) passenger cars, 2-8-2 steam, a few long rigid-frame diesel-hydraulics and long freight cars - 610mm radius (24 inches) with easements.
  • Secondary trackage - no passenger cars, medium freight cars, electrics and diesel-hydraulics with no more than 8 drivers - 500mm radius (19.8 inches) with easements.
  • Mineral hauling shortline - short and articulated cars and short-wheelbase steam - 350mm radius (13.8 inches) with easements.


Leaving out the easements would increase the radii required by at least 100mm.

I have proved by experimentation that the Mantua 2-6-6-2T will take a 305mm radius (12 inches) - which is just about the same as the 1:1 scale 68 degree curve the prototype was designed for.

Incidentally, the 0-4-0t that was photographed on the Silver Dollar Central was the original Varney B&O "Little Joe," AKA, "Dockside," not to be confused with the MILW electric of the same name.

Chuck (Modeling Central Japan in September, 1964 - on some pretty tight radii)

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Posted by selector on Sunday, October 11, 2009 12:19 AM

Byron, I am pretty sure he meant "realisticity".

The minimum radius in HO is a synthesis of what you have to push around the curves and the shape and dimensions of your overall track plan.  As stated earlier, some larger steamers in HO will go around 18" curves.   We all learn to live with compromises.  We all learn that there is a better way to do things after we accept what "better" means to us.  Often, for most of us, better usually is a combination of more/larger.  We get more engines, and usually not the same kind.  We develop a hankering for a larger steam or diesel monster, and there is where we realize that we shot ourselves in the foot...we didn't plan ahead, or we bought the wrong monster.

As a general guideline, subject to individual bias, the smallest practical curves in HO are accepted widely to be in the order of 18".  Not a hard and fast rule, but a guideline.  Mountain logging operations had tighter curves in scale and nasty track of a temporary kind.

For those with the room, and who know not to fill all their space with every conceivable track apparatus and length, but to keep tracks to an operational and effective minimum, curves between 24 and 30 inches are highly desirable, and would be called medium curves.  The lower limit in that range, 24" is what Walther's states is the minimum to operate their heavyweight passenger cars.  I'll bet 100 people who post here regularly would say "baloney!"  They'll tell you they had to alter their cars or widen their curves before the derailments went away looking for another layout to haunt.   (I have it on good authority that they are busy to this day...)

Curves in excess of 30" radius are generally considered broad curves, and if laid properly are essentially trouble free.  They look the best, too, for longer rolling stock and large engines.

But, to close the loop, and this post, it is all a synthesis between what you have to run that you like, and the space you have to fashion minimum effective radius curves.  It's almost a chicken and egg debate...what you buy because you gotta have it, and what curves you can squeeze into the room you have to play with.



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Posted by jwhitten on Sunday, October 11, 2009 12:32 AM



32 is kept as a guideline for realisticness.




It's an industry term.

Modeling the South Pennsylvania Railroad ("The Hilltop Route") in the late 50's
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Posted by wabash2800 on Sunday, October 11, 2009 12:42 AM


I agree with jecorbett. IIRC, some years ago someone wrote an article on the subject in an NMRA magazine. They called the two types of curves, "functional" and "cosmetic". Functional curves were the smallest radius curves that your trains could run effectively on and cosmetic were the curves that your equipment looked more realistic on.

So, of course, this is variable depending on what type of equipment you are using. And if you would like to use the concept to its best advantage, you can use the smaller functional radius curves in hidden areas and the cosmetic curves where they will be seen to get more bang for your buck in layout design. But when I say functional I'm not necessarily talking about the absolute smallest radii your trains can run on as that might be asking for problems in hidden areas. I'm taking about the smallest radii that will be reliable.

So if you are running old time trains with small steam locos and short cars you can get by with tighter curves both functional and costmetic, say, 15 and 18 inch radius in HO.  But obviously, with larger locos and longer cars you need much larger radii. For my layout in the early 50's with medium steam locos and long heavyweight passenger cars, my functional is 30 inch and my my minimum cosmetic is 42 with spiral easments. Even with 42, I have some overhang btw the long cars. Most medium size diesels will run fine on 18 inch curves and No. 4 turnouts though some need 22 and 22 might be a better choice anyway for reliability. But if you are modeling long autoracks and doublestacks it those smaller radii might not cut it.

I agree with what someone else here said about wasting space with too large of curves. But the other fellow that had  a point about wide radius curves was not that far off. It depends on your priorites. For example, I use No. 8 or larger turnouts on my mainline because I think they look a lot better than No. 6's, but No 6's would work fine.

But having said all of the above, using a smaller functional radius curve on a helix, for example, will give you a steeper grade which will limit train length. Personally in HO, I would never build a helix with a smaller radius than 30 inches (give me about a 2% grade) but I know of some that have built them down to 24 with very short trains. Another trap to note is that because a smaller radius curve will cause more overhang with longer equipment (even if you push the enevelope with truck mounted couplers) this can be problematic on multiple track unless the track spacing is widened. The wider spacing can also look very unrealistic. Another thing to consider on sharp curves is that sometimes coupling and uncoupling on a sharp curve can be difficult because of couplers not lining up.

I would recomend experimenting with sectional curved or flex track or possibly visiting other layouts and asking questions. 



Tags: radii
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Posted by Anonymous on Sunday, October 11, 2009 1:18 AM

 Linn H. Westcott classified radii as follows (HO scale)

  • Sharp: 18"
  • Conventional: 24"
  • Broad: 30"

 With sharp curves, you´ll limit your operation to short wheel-base locos and rolling stock, with conventional curves, you should be OK with most of the R-T-R equipment currently in the market, but if you intend to run those 85" or 89" - cars, broad curves seem to be the minimum.

If you intend to operate those Big Boys or UP 9000´s - well, then you´d better go for 42" and above, if you want them to look OK on the track...Big Smile

The type of operation and equipment you like to see on your "road" finally determines your minimum radius!

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Posted by wedudler on Sunday, October 11, 2009 2:24 AM

 You minimum radius depends heavy on the equipment. For this shay and short cars I can run it on my Pizza layout.

At my mainline at Westport Terminal I have 47''.


I've widened the gauge for this 8.7'' radius ! My 44-ton will run there too.


Pueblo & Salt Lake RR

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