So my question is mainly out of curiousity, but it may have bearing on future layout plans…
What is generally the minimum curve radii for high-speed mainline track (1940’s era), and how does that radius translate to HO scale? I would imagine that a 24" radius in HO would represent an unacceptably tight curve in real life, correct?
Just wondering what the minimum HO radius would be if one wants to at least “approach” the prototype. Thanks
Prototype curves are measured in ‘degrees’ - Here is a NMRA table that relates prototype to model radius curves:
http://www.nmra.org/standards/sandrp/s-8.html
Note that a 10 degree curve on the prototype(tight) is something like 53" radius in HO! A 24" radius curve in HO is about 34 degrees on the prototype.
Jim
Unless you have a lot of room, or are in a smaller scale (N, Z) you’re not going to be able to do prototype radius curves. In HO, generally 18"R is considered “sharp”, 24" Ris "conventional and 30"R is “broad”. Some folks designate 36"R or greater as “super-broad”. For using full-length passenger cars with body-mounted couplers and working diaphragms, 30"R or higher is probably the best.
Otherwise, it becomes a matter of preference re the appearance. Most HO equipment will take pretty sharp curves. A BLI 2-10-4 will do a 24"R curve, Walthers full-length passenger cars do 24-26" radius curves. But both will look better (in most people’s opinions) on larger curves.
Going to the FRA track safety web pages and looking at 213.57
E = elevation of the outside rail in inches.
D = degree of curvature in degrees
maximum velocity of a curve = square root((E+4)/(.0007*D))
Maximum super elevation is 8 inches, so on a 10 degree curve the max speed possible would be 41 mph.
At a 3 inch superelevation on a 10 degree curve the max speed would be 31 mph.
A 24 in radius curve is a 33 degree curve. Figuring max super elevation of 8 scale inches, the max speed would be about 23 mph. At 3 in super elevation, that would be just 17 mph.
I wouldn’t be too concerned about wide radius curves. In fact, in track planning I follow what I read about in an NMRA publication many years ago. The concept was “costmetic” versus “functional” curves. A costmetic curve is a curve that looks good with your equipment and is used out in the open part of the layout. A functional curve is the minimum radius that your equpment can reliably operate on. The functional curves are saved for curves that are not out in the open like hiddent track. By balancing the two, you can get more operation per a given space and have curves that look great. For example, your HO steam locos may be able to operate fine on 24" R (maybe even 20" R but not reliably) but need a 42" R to look good. Use the 24 in hidden tracks and the 42 out in the open. However, there is a caveat to functional. A 24" R is not advised in an HO helix as the grade will be way too steep for most conventional equipment and conventional length trains. I have found 30" R works fine in my helixes with a 2% grade, but that is the minimum of “functional” track in helixes for my layout.
Well, after playing with these little trains for about 40 years now, and being mainly interested large Class I systems of the 1950’s, I don’t use anything less than 36" on mailines, visable, hidden or otherwise.
I like my pasenger cars close coupled with working diaphragms that touch and say touching. I try for near prototype couping distances between ALL equipment including using short shake Kadee couplers on some freight cars to make them more correct. When my back to back E units couple to a passenger car, the diaphragm on the car rubs the buffer of the loco, just like it should.
And even at 36" radius, I minimize the use of 80’ or 85’ passenger cars, prefering most passenger cars on the layout to be 70’ to 75’. I limit steam locomotives to those with rigid wheelbases less than 20’ - so no X-10-X anythings for the most apart.
I use easments on all curves and many curves are parabloic curves just like the prototype.
My helix minimum is also 36" creating a 1.8% grade, not factoring in the curve and 2% is my max grade anywhere.
These standards are based on the need and desire to run trains in the 30-50 car range, and sometimes even longer.
Sheldon
As others have indicated, in model form go for the best appearance achievable within the space available since matching the prototype is completely unfeasible. Some sample hard numbers provided below:
10D curve (used in CP’s spiral tunnels) has a radius of 574’, and is considered a tight curve with a severe speed restriction. In HO that is 6’-7", almost possible.
6D curve has a radius of 955’. In HO that is 11’-0. This was used as the maximum when CP built the new track through Rogers Pass, and permits a speed of 40mph. Spiral easements were 240 feet each end, or 2’-9" long in HO.
3D curve will allow speeds up to 60mph, which is starting to get more desirable for a mainline on the prototype. Radius is 1910’, or 22’'-0 in HO. Better have a warehouse to accommodate that model empire!
Some old industrial areas had curves as extreme as 30D, radius 193’, 26" in HO terms. 40’ cars and switchers only, please. A lot of equipment can negotiate a 20D curve with caution, which translates into 288’ radius or 39" in HO. But for larger locomotives that might be only if they were not coupled to any cars.
John
To follow up what Wabash2800 said, some years back John Armstrong wrote that a cosmetic curve was a very large radius curve that you put in for appearance sake, to break up a long straight section for example. A functional curve was a curve you had to make lest your train run into the basement wall.
[;)]
It comes down to a decision on what you want, balancing appearance and operation. You could build a mainline along one long wall of a basement for example, using large turnouts (No. 6 to No. 10) and a few gradual one or two percent curves, but then have reverse loops at either end that were fairly sharp (say 26"R) but hidden by scenery. That way the train would have gradual curves in the visible area, but would still have room to turn around at the ends. A yard made up of No.10 turnouts is going to look great, but take up a lot of room. There you could use No. 5 or 6 turnouts and still operate your equipment satisfactorily, and have a lot longer yard tracks.
Railway curves are “chord definition”. This definition states that the degree of curve is the central angle formed by two radii drawn from the center of the circle to the ends of a chord 100 feet (or 100 units) long.
Most highway curves are “arc definition”. This definition states that the degree of curve is the central angle formed by two radii drawn from the center of the circle to the ends of an arc 100 feet (or 100 units) long.
The radius of a 1 degree chord definition curve is 5729.65 feet (or units).
This corresponds to a 65.793 foot (789.517") radius in HO scale assuming the unit of measure used is the US survey foot.
A 10 degree railroad curve is considered very tight. This corresponds to a 6.579 foot (78.952") radius in HO scale…So you see what we as modelers are up against. Given the limited space most (or all) of us have for a layout, modeling curves that even approach a realistic prototype radius is impossible. The best we can hope for is to use the largest minimum radius that available space permits. And I too recommend the use of spiral curves (easements) at each end of your curves.
Engineer, machinist, physicist, chemist, or accountant? [(-D]
And I’ve been threatened when I use 1/87.1 in my calulations rather than 1/87…
Busted…engineer. My degree was actually in geodesy, then came the harsh reality of there being no jobs for a geodesist. [banghead]
Note that one successful modular group back east uses 88" radius for its’ corners and they are a beauty to behold! My preference is to go at least 60" on visible mainlines.