Is there way to determine the minimum radius of a given train? Perhaps based on number of wheels, length etc.?
I’m thinking of adding a new line to my ‘OO’ layout which will be roughly oval in shape and I’ll have enough room for about 15" or maybe 18" radius curves.
I have a couple passenger trains that I want to run on the line (Budd RDC and a few British DMU units) but I don’t know what their min radius is?
I know I could lay some track on a board and see how small I could go but thought I’d see if anyone knew a more ‘scientific’ method [:D]
As the push for more and more fine scale detail on models and true scale lengths and wheels has happened in the last few years; so has the push away from making cars capable of handling such sharp curves. 15" is really only the realm of light rail and trolley systems. 18" is still going to work with most HO scale freight equipment, I can push an SD40-2 model onto an 18" and it will still work somewhat… but just be aware you’ll need to run it slow and easy. Most passenger cars and long steam engines only agree with 22" curves if not only 24" curves.
If you honestly want to pick a realistic curve set, the 24"-40" range is what you’d want to look at were you can run most trains fine at slow to medium speeds and not worry to much about the curve being an issue. You’ll see some modular layouts like Free-Mo suggest a 42"+ curve with 48" being recommended for mainline runs.
Your Budd RDC might travel an 18" curve but it depends on the brand. An older LifeLike RDC is more likely to take that curve just fine, but the Rapido one with its heavy detail on the undercarriage would likely not. You might just have to test it individually via building your own loop of temporary track and just seeing if the car you want will run at the speed you want on the curves you plan for.
The Layout Design SIG posted a minimum radius rule-of-thumb based on rolling stock length. But specific models may need more or less because of underbody detail, number of axles, etc.
That page may not be currently available due to ongoing updates of the website, but here are the values:
Here are some curve radius guidelines based on the lengths of your longest pieces of rolling stock.
2X - Some model equipment may be able to track reliably on 2X their length, but this is generally considered pushing it.
3X - Making your curve radius at least 3X the length of your longest cars gets reliable tracking around curves, but looks toylike.
4X - If you make your curve radius at least 4X, your longest cars will look much better on curves.
5X - If you make your curve radius at least 5X, your longest cars will couple easily with minimal manual fiddling of the couplers.
This measurement is based on the length of your longest car (coupler to coupler).
A Budd RDC is slightly under 74’ long, which is pretty much the length of a passenger car. Minimum radii for those is R24" in HO. So, if R15" or R18" is your maximum, you better figure on small…SMALL 4-axle switchers and short 40’ rolling stock in OO (1:76.2).
Thaks everyone, in my scenario I’m not really too bothered about how it looks as the cuves will actually be hidden behind a wall, so I guess I’ll lay some temporary track and see what I can get away with
For curves on hidden track, tunnels and the like, you also will need to consider the overhang for planning clearance for longer cars and locomotives.
When you make your test track, hold a pencil against the center of your longest car and run it a bit so you will get a line next to the inside of the track. Do the same for the end of the car hol
In your case it really has more do to with whether the locomotive or car will actually track on a curve than how it looks doing it. My MTH 20th Century Limited (84’) passengers cars won’t even track around a 90-degree curve of R22"; they derail.
Also, if two concentric curved sections of track are going to be laid side-by-side, you need to be concerned about ample clearances; should trains pass one another on the curves going in either direction.
Very respectfully, I looked at it this way when I was planning my layout. I decided to think outside the box.
Instead of trying to plan, how narrow can I get away with planning my curve radius? I decide to think, how wide can I get away with planning my curve radius?
TF
I kept going back to the full-sized drawing board for over a year until I was happy with all my wide curves.
Track planning is going to be an iterative process if it is going to produce as a minimum satisfactory results. There is the space to consider, the type of track plan, the curvature that will permit that track plan in the space (as it is configured), and then how well your various combinations of rolling stock and locomotives will do on that arrangement…trailing the cars and shoving them, clockwise and counterclockwise, through facing points and through the frog ends of turnouts.
Bottom line, encourage your rolling stock and locomotives to participate in the decision-making; do practical testing with temporary tangent and curved tracks to see ‘how low they’ll go’. Armed with that bottom line detail, and bearing in mind that laying track curves is rarely a perfect process, maybe add a couple of fudge-factor inches to the minimum radius figure and go from there…but wider when you can. Always, wider when you can.
In the US, all makers of HO rolling stock do everything they can to make their stuff go around 18" radius curves. 18 inches is the train set radius, and all the makers want to sell rolling stock that works on trainset layouts. Forty foot freight cars, 4 axles diesels no problem. Wise makers, selling stuff that won’t make an 18 inch curve will mark the box with some thing like “requires 22 inch radius” Unwise makers who omit that marking suffer thru a 100% return rate which cuts into margin and leaves a lot of unhappy customers. The only passenger cars that I know of that run on 18 inch radius right out of the box are the Athearn cars. They are “selectively compressed” i.e. short, but they look pretty good and come with truck mounted couplers that really help on tight curves. I have passenger cars from IHC (long gone, sob) and Walthers that have trouble on 22 inch curves.
I guess one thing I didn’t mention in my original post that I might as well add… if space constraints are part of why you want a 15" or 18" radius to fit an oval, you might be best to just kill the oval idea entirely. Build a switching layout instead. One of the funnest ops sessions I had was on the Hudson Terminal N-scale layout (RMC May 2019 had an article on it). That was all switching in something not much larger than an average cabinet, with plenty of realistic features and a sense of challenge that was a blast to operate on. An HO switching layout when well designed would look great on a wall spanning shelf, and it can be placed high enough the space under it can be used for storage or other purposes. If you want to do the British OO stuff, you could probably pull off a great looking train station scene or switching scene of some small branchline pre-Beeching on a single wall shelf layout.
Or go modular. Make it a layout you can disasemmble when you need to store it, but it can be put back together when you want to operate it. Benifit of a modular system is you can disassemble it and move it if you ever decide to change houses.
The minimum radius that any loco can operate over depends on the flexibility of the wheel base. As Travis stated, a Lifelike RDC can run on 18" and indeed I have one.
However, being full length the overhang of that car on my layout is ridiculous even on 20" plus and it is not terribly pretty on sections where I have 24" and 32" radius curves. I use Athearn Budds which have been selectively shortened for that reason and that works well for me.
I don’t know which DMU’s you have but some of the later Hornby offerings also look a bit strange on the smaller curves.
If you check out http://xdford.freeasphost.net/stag19c.html, there is a picture of a modern BR DMU on a third radius curve (just over 20") while the inner curve (just under 18"), to me looked just plain silly. Also two such trains would have a lot of trouble passing each other despite being on Hornby set Track curves as we tested at the time.
However, you may well have Sprinter sets or shorter DMU’s that would easily handle that, so perhaps we need a little more input about that.
My advance is simple and without the foofaraw… Use the largest curve you can in your allotted space.
A word about switching layouts. A well planned Industrial Switching Layout(ISL) is a joy to operate but,a poorly plan ISL is a grief to operate. In the case of a well planned ISL avoid industries on a switchback where you need to move cars in short simple is better.
I re-read the thread having been distracted while replying so I missed the parts where you said the curve was hidden.
However the next page on my web site shows my Life Like Budd on a Hornby 3rd Radius (about 20 inch radius) and the relative overhang… I have not got access to two full length cars but I would surmise that they would be fairly close which could be awkward if it is hidden and out of reach.
Many years ago, I saw a Hornby layout at an exhibition which was quite a reasonable layout but the curves were close as mine with the radius (I think R1 and R2 rather than R2 and 3 as the layout in my web site) and subsequently you could hear the carriages side swipe on curves. If they would side swipe they could also derail. As said earlier, go for the largest radius you can fit,
The thing is, most commercial model trains are designed to handle a given minimum radius – with necessary alterations and compromises of various sorts to achieve the goal, and with different manufacturers having different goals. High cost, prototype-accurate model manufacturers are likely not all that worried if someone with trainset quality track can run their trains or not.
The more finely detailed and prototypical a given model is, the less the designers are going to meet a goal of 18" or 22" minimum and I suspect, the less they care if they do or not. This extends to rolling stock as well as locomotives.
A modest example of what I am talking about, is if you like to add complete air brake system details and accurate coupler draft gear to a model freight car, let’s say a 40’ boxcar; you often find you have increased the minimum radius by doing so. So you can’t just say “a 40’ boxcar will take an 18” minimum radius curve." It depends on the model.
My point is, knowing what the prototype is, is not always enough information.
If the model manufacturer does not publish a minimum radius for a given piece of equipment, I would search for and seek out a review in a model magazine. MR trade reviews almost always give that info (and they generally test it out on one or another of their in-house layouts); I assume most other magazines including British and European, do the same. I would just caution that knowing the minimum radius a locomotive can take still does not tell you whether a car can be coupled behind it and still handle that radius, even if both the locomotive and the car alone could take the radius. Coupling particularly with body mounted couplers creates its own issues.
My sense is that in recent years the “assumed” minimum radius has been getting larger, and that time-honored
Eighteen inches is sharp and fifteen is insanely sharp in HO. Rapido’s RDC stated minimum radius is 18" but it’s generally pushing your luck to run anything at it’s minimum.
I had a rough sketch of my planned layout, but when I lost more than half of the original space, I simply built the around-the-room benchwork based on the desired aisle space.
The resultant room was a very odd shape (10 corners - 7 inside, 3 outside)…
…so I cut plywood in test radii starting at 30" and increasing by 2" up to 38", then layed each curve at each corner, in-turn, with the largest that would fit.
There may be a 30" curve in staging, but all on the visible layout are larger, with a couple around 48".
If you’re going to use small radii, you need to suit the cars and locos appropriately. If you want to use big locomotives and long cars, there are a couple of options:
As suggested previously, eliminate any major curves and build an industrial switching layout or a point-to-point layout.
The latter could be along one wall of a room, but if you have more than one wall available, wide curves can be had at the corners of almost any room.
My layout is an around-the-room type, partially double-decked (the area in grey in the picture), with five points between which trains can travel.
