Concur with riogrande. I’ve designed 200+ layouts for individuals, clubs, and commercial organizations. The largest HO multideck Class 1 mainline designs range from 1400 square feet to 3800 square feet. None of those large layouts has a greater minimum radius than 40”, and most use smaller radii. A handful of other layouts I’ve designed have used 48” minimum radius, but they are railfan layouts that made many compromises to attain those curves. And one is an HOn3 backwoods rambler.
The massive (5500+ square feet) Colorado Model Railroad Museum layout is one that I think uses 48” minimum radius – but it’s an extreme outlier, in my experience. So I think that it’s incorrect and quite misleading to say that mainline modeling in HO demands 48” radius. Thousands of successful layouts over the decades prove otherwise.
For the “slip point” radius* in model railroading, as opposed the the real world (above), I calculated the radii for HO scale 36" wheels with the 3 degree taper:
The NMRA standards include tolerances for track gage and wheel gage.
For track gaged narrow, and wheels gaged wide, the “slip point” radius is 191".
Fro track gaged wide, and wheels gaged narrow, the “slip point” radius is 60".
So it’s pretty much impossible to not have wheel slip in typical HO curves.
Ed
*Slip point being where the curve is tight enough that the wonderful wheel radius differential for railroad wheels fails.
Minimum radius for a prototype NKP Berkshire works out to about 39-40" radius in HO, and that means the tightest curve allowable. Any Berk negotiating that curve would do so at restricted speed. Typical HO passenger cars don’t feel comfortable on anything less than about 30" radius. But look at the misalignment of the vestibules when they do it, and you can see that something in excess of 40" is more reasonable for the safety and comfort of the passenger.
In reality, few of us have the space to build our layouts with curves anything like those of the prototype, so we do what we can and what we must. In practice, bigger is always better when it comes to operation on curves.
Yes, it’s not unusual for moderate to large layouts to use minimum curves under40 inches.
I have a primary space of about 500 sq ft with an additional room 144 sq ft and just roughed in one possible layout sketch which uses 32-inch minimum radii but have been able to fit in larger curves in part of the track plan. It it is fully walk-in but walkways narrow to 24" in some places. I’m going to work up a 2nd track plan which makes use of a lift out gate and will allow more generous walkways.
In my experiance, 36" radius seems to be a very common minimum for those with larger layouts, 30" being the next “step down” for those with less space.
Club layouts and large home layouts in this region seem to trend around the 36" radius number, even 3-4 decades ago.
Paul Mallery was a big supporter of larger curves, more so than Armstrong, and lobbied hard in all his books for 48" radius for modeling class I lines in HO.
I have managed to stay comfortably at and above 36" for all my mainlines for many years now. My next layout will push that minimum to more like 40".
And yes, I too have been explaining to people for years that the flanges don’t hold the train on the track, wheel tapper does…
Since space is limited in my new basement, and I can’t manage a 40" minimum, and the next step down is 30", I like to try to boost things a little because even at 30 inches, a couple of extra inches makes a difference in better operation, even if you don’t see any real visible improvement. So I’m going with 32" as my minimum but whereever possible, bumping it up to 36" in visible area’s where I can manage it. I’ve already drawn up a scale plan to rough in benchwork, walkways and main line. In my case, I’m debating using a lift out for two tracks near the bottom of the stairs to be able to fit in wider walkways - leaning in that direction.
does a fine job of having the sharp curves concealed, while having wide curves exposed. If the concealed curves were 30" radius, I estimate the “showing” curves at, very roughly, 90". I recommend considering this approach in track planning.
It’s Michael Rose’s Georgetown and Allen Mountain, which shows up from time to time in the Layout section of this forum.
Further to Sheldon’s comment, it is not just the model side that has no understanding of how real trains work. It can be found fairly easily in the railfan hobby too. In my experience even railroaders may know comparatively little beyond the narrow confines of their job or desk.
As RioGrande mentioned, IMO, the important part is minimum VISIBLE radius.
Hidden radius simply has to be operational.
When planning, its sometimes possible to make the minimum visible radius a bit broader if nonvisible radius is cheated towards the minimum needed to operate.
Hidden curves have to pass the most severe tests of reliability if you want to enjoy essentially derail-free, or derail-proof, tracks where it’s hard to fix things or even to reach in and drag them back out…hopefully still coupled. You’d want the probability that any one axle will derail in a given consist less than 1/1000, or close to that. As you tighten those hidden curves, and use sloppy joining techniques or smaller codes of rails, your overall probability rises, and not linearly. I have to admit that I am one of a long list of modelers who thought they could beat the odds. First time I did it with tangent track and Code 100 in a long tunnel 5’ in length, very tough to reach up tight against the wall and retrieve something which I think I only had to do one time. My helix in the last build had a tricky turnout in it, the worst setup on my layout. I achieved about 1/99 out of it eventually, but that was it. Once a week I had to slide under the helix on my back and fix or retrieve a piece of rolling stock.
Take great pains to ensure reliability on hidden trackage, but double that up if it’s on a curve near the minimums, especially for long strings or long cars coupled in numbers.
I have to agree with Selector. I have never been able to get my head around using a smaller radius for hidden track.
For me, close coupled passenger cars with working, touching diaphragms are a must, so my 36" in minimum is just that, hidden or visable.
Also, I have found that once you get into the 36" and above zone, it is not necessary to increase the spacing of parallel track by any noticable amount. In fact on my last several layouts all parallel tracks were spaced at 2" even on curves with clearance issues.
On the new layout, 36" will be the minimum radius on secondary trackage, and 40" minimum on the mainline.
Except for staging, I am also working hard to eliminate hidden trackage, it was one of the problems with the old layout, too much hidden track.
If my retirenent location works out, once moved to its new home, the staging tracks will be accessable from service isles behind the layout. In its current location they are reached from both above and below in different spots.
What I was advocating was not using a smaller radius for hidden track. It was using a LARGER radius for showing track.
Yeah, it looks like I’m saying the same thing. Nope.
Taking your stated minimum of 36", I think it would be really nice to have minimum mainline radius of showing curves to be up around 100".
Easy to say, hard to do. I know.
But if you look at the track plan I showed, it CAN be done. If you have the room. It will, of course, affect the amount of track you can fit in. But that’s a discussion that is ALWAYS happening with layout design, so it’s not a surprise here.
Ed, I was not refering specificly to your post but rather was considering the wisdom of using the bare minimum functional radius for hidden track, and then having some larger cosmetic minimum for visable track.
But even in your example I do not consider 30" radius an acceptable minimum for the equipment I run.
The layout I am currently dismantling and replacing has a 36" minimum radius. Being a double track mainline, that puts a great many curves at 38" and 40" right off the bat.
That layout included a number of “cosmetic” curves in the 50" and above range.
Not the best photo, but this curve is 54"/56" respectively:
Easements are essential on the prototype and should be considered so with our models, large radius or small…
In fact, on the prototype, your typical curve that changes the vector of the route 45 degrees or less typically has no fixed radius at all, but is rather two long easements back to back.
There are several easy formulas for figuring this out and I have build a number of curves this way.
And, when doing this, the theoretical point of minimum radius can actually be somewhat smaller than your minimum constant curve radius, because that radius on exists for about an inch of track…
Armstrong suggested very long flowing easements, but easily gave in to sharper curves.
Paul Mallery suggested that with large curves, 36" and above, easements could be much shorter and be just as effective.
Armstrong shows, in Track Planning for Realistic Operation, that 18" radius with easements causes less coupler offset (his “coefficient of lurch”) than 22" radius without easements, and still takes up less space. Once the radius gets so large that it might be something a prototype might be able to negotiate, the need for easements goes down.
You are correct, Randy and Sheldon, as far as I can tell. After so many inches, the greater value for an eased curve on a layout would have to be visual fidelity and appeal in order to more closely approximate the prototype. I don’t think any curves greater than about 32-34" should need real easements unless we’re talking highly prototypical brass or scale rolling stock. It’s just that if they have the appearance of snap or set track curves, with sharp and immediate onset into curvature, it just doesn’t look right, not for a main line with higher speeds on which passenger trains will run.
On the other hand, if space is at a premium, you can certainly get away with those immediate curve onsets with radii above about 24-30", depending on what you’re running behind the power. For example, if you want that tunnel with curved right of way in it, and a sharp curve to save room out in the visual world, one needn’t ease the curve as long as the rolling stock can handle it. But as I said earlier, the closer you creep up on those real engineered limits, the higher the probability of ‘failure’. Seems to me it’s a big enough bummer dealing with the failures out in the open without having to stoop, crawl, bend, contort, and reach behind scenery or into a tunnel to retrieve something intended for broader curves and finer joints.
I disagree that 18" in HO is tight even for streetcars. I belive that the tightest curve on the Boston system is 35 actual feet. I just measured, on Google Maps, the curve at Ninth Avenue and Irving Street in San Francisco, and it came out to just under 9" in HO scale. Very sharp curves can be found on “steam” railroads on industrial trackage, especially in streets. Only 40 foot freight cars and short wheelbase locomotives can use the tightest curves, and then only at speeds well under 5 MPH. A modern example of sharp cuves is the area at the east end of the Steel Bridge in Portland, OR. Passenger trains turn south from the bridge at walking speeds on a curve that scales to about 40 inches in HO scale. The grain elevator trackage just north of the bridge includes a curve of about 20" in HO.
A train traversing a 10 degree (79" HO) curve will run at slow speed, probably 10 MPH, maybe as much as 25 MPH.
Using larger radius transition at each end of a curve will help operation. For realism, avoid placing sharp curves where the viewer sees the outside of the curve, unless you are modeling street trackage.
