Minimum radius for a real main line in HO-scale

[gregc]

i tried to show in the chart for Reading locomotives that prototypes have restrictions on their locomotives.   Presumably they use this chart to determine which locomotives can run over various trackage.   My impression is that they also design locomotives to meet the restrictions of specific trackage

one example I've read about is the choice of the Reading T1 for the Freedom train on the east cost.   It could handle the relatively sharper curves on the trackage.

when we design our layouts, we make choices, this includes recognizing the limitations on what locomotives we can run, just like the prototypes

[riogrande5761]

slammin

In HO a 30" radius is not consider "big".

"Big" is a relative term.  John Armstrongs Track Plannining for Realistic Operation book lists 18" as sharp, 24" as conventional and 30" as broad.  For some, 30" would be considered a big radius, if they are very cramped for space. 

But times have changed and standards with it.   If I were to revise John Armstrongs terminology for today, I'd list 18" as very sharp, 24" as sharp, 30" as conventional and 42"+ as broad.  Very broad would be limited to what John Armstrong coined as scenic curves, a feature he recommened layout builders include to have a least one scene with a very broad curve.

While some have disputed my recollection, I did remember reading lots of MR magazines in the late 1970's and through out the 1980's and noticed a pervasive trend; that moderate to large layouts had standardized at 30" for their minimum.

Large home and club layouts have 48" and larger radii when modeling class 1 railroads. 

That may be true, but I've read a lot of articles showing large home layouts and the minimum was not 48" radius that I've seen.  That is unless you are talking about home layouts the size of a small barn or aircraft hanger.  Large layout minimums still seem to be less than 48" - again, I'm talking minimums which are often the lions share of the curves, baring scenic curves.  On my 10x18' layout, I did include a John Armstrong feature, a scenic curve of 56":

Most us don't have that kind of space and tend to model short lines and branchlines that look reasonable running 30" and smaller radii.

Yes, but even if we don't have a large layout space, it definitely does not limit us to branchlines.  My small layout was a railfan type layout with mainline running in the 10x18' room and minimum curves were 32" except in the yeard where a few spurs were 28 or 24 inches.

[rrinker]

Richard Hammond (you might know him from the original British Top Gear show) did another show on engineering marvels, and in one episode demonstrated very clearly that it's the wheel taper adn not the flange that keeps the train on the track. He laid out a J shaped section of track and rolled a cylinder down it, starting on the straight vertical leg of the J. Of course, it hit the curve and apart from a slight deflection, ran right off the table. Next he took a piece made of two cones glued together, fat part in the middle, tapers outward. An exaggerated set of railroad wheels. At any speed, until inertia overcame friction, it faithfully followed the curve in the track.

 Of course, no one compares to Fenyman when giving a lecture. There's a series of talks he gave at Cornell on various aspects of partical physics that are on YouTube and quite understandable. Had he been lecturing, I never would have had to drop the quantum physics class I took in college. And yet this incredibly brilliant guy sounds like a brawler from the heart of Brooklyn.

 The singing is the flanges momentarily hitting the rail - if the flanges actually rode continuously against the rail it would be more of one long continuous screech. The flange does help guide things back - if the curve is too sharp, the wheel will want to shift even more on the conic section which would push the wheelset completely off the rails, the inside wheel inside the rail, as the wheel tried to move to even a smaller diameter area past the face of the wheel, and the outside wheel outside the rail as it tried to climb to a larger diameter past the inside edge. But they don't usually rub continuously except on very sharp curves, and it's very obvious as an annoyingly loud squeel. These spots also require very slow speeds - the flange won't really be able to hold the wheels on the rail if enough force can be generated between the flange and the rail, it will lift right up. Poor trackwork makes it even more likely. We used to listen on the scanner some times at the train club, one of the members was an engineer on the PBNE when the Bethlehem plant of Bethlehem Steel was still operating, at least in some capacity. Seemed like every day something would go on the ground.

                             --Randy

[gregc]

base on Dr Feynman's explanation, the minimum curve radius is related to ratio of the circumferences of the rails and the ratio of the wheel diameters due to taper.   The following drawing indicates that there's only about 0.2" of taper per wheel.   This corresponds to curve radius of 742'.   Anything less and one of the wheels has to turn more than the other.

[NittanyLion]

The tracks outside my office window are in the low 20s and long equipment looks like it is hanging over like an 89' flat on a 22" curve. Flanges scream on the garbage flats.

It would be a 139" curve in HO. 

[gregc]

i think there's a common misconception that it is the flanges that keep a train on the track.   They probably do for models, but not for the prototypes which have smaller flanges

see Dr Feynman's explanation 

https://www.youtube.com/watch?v=y7h4OtFDnYE

[wjstix]

I've often thought that the comparatively sharp curves we have to use is another reason to run trains slower than the prototype - running a model train at 70 scale MPH on a curve that a real train would have to slow down to say 30 MPH doesn't seem to me to be as realistic as running the model train at 30 scale MPH is.

[Doughless]

cuyama

 

A 10-degree curve in real life is over 570' in radius and thus about an 80" radius in HO. 

 

Bumping this for its importance.  

With seemingly more vocal angst over prototype fidelity these days, its important to know that we suspend realism once we decide to build curves (among other things).  But we do the best we can.

[rrinker]

 Welp, can't even run a D5 on my layout, and I can totally forget about the I10.

Good news, Boxcab 50 and 51 (or 98 and 99, depending on what year you are talking about - they have to be 98 and 99 for me, as I have S-1 50 and 51) can take a tighter radius than the Roundhouse models can manage. I don't think the trucks on mine turn enough to do under 7" radius.

                           --Randy

 

[gregc]

Here's a table for maximum-curvature/minimum-radius for Reading steam locomotives.

i've converted some to HO and listed the loco types

    6.9"   50'  OE1 boxcab

   10.3"   75'  A4 0-4-0
   13.8"  100'  B8 0-6-0
   17.2"  125'  B7 0-6-0

   32.8"  238'  D5 4-4-0
   35.9"  260'  E5 0-8-0
   37.7"  273'  I6 2-8-0

   39.4"  286'  I8 2-8-0
   39.4"  286'  M1 2-8-2

   41.4"  300'  D11 4-4-0
   41.4"  300'  I10 2-8-0
   41.4"  300'  Q1 2-6-4

   45.9"  333'  P3 4-4-2

   49.4"  358'  G1 4-6-0
   49.4"  358'  L10 4-6-0
   49.4"  358'  N1 2-8-8-2
   49.4"  358'  P7 4-4-2

[Overmod]

Real railroad curves are impossibly long for indoor layouts.  I wanted to simulate 100-mph mainline traffic and I did not have room on a 2-acre property, outdoors, in HO scale, to model the necessary 'prototype' radius.  As noted, you could fit something 45-55mph track speed into something that limited 

[cuyama]

wjstix

I recall in one of John Armstrong's books he said that in HO I think a 30-something inch radius curve (forget exactly what) worked out to be about a 10-degree curve.

A 10-degree curve in real life is over 570' in radius and thus about an 80" radius in HO. 

[cx500]

The magic formula is radius (in feet) = 50 divided by the sine of D/2 where D is the degree of curve.  A 12-degree curve (rare, but sometimes unavoidable) works out to be 5'-6" in HO scale.   Remember to run your trains very slowly through that sharp a curve though.

John

[wjstix]

Randy's statistics sound good. I recall in one of John Armstrong's books he said that in HO I think a 30-something inch radius curve (forget exactly what) worked out to be about a 10-degree curve. 10-degrees would be about the tightest curve for a prototype mainline you'd find, and would limit trains to like 15-20 MPH.

BTW same goes for turnouts, our model passenger equipment will run nicely through No.6 turnouts, but the prototype would use No.20 or larger.

[7j43k]

I just measured the curve radius of the UP (formerly WP) mainline to Oakland at Sunol, CA.  I picked a near 180 degree turn just a bit east.  It's 1000'.  That's 11 1/2 feet in HO.

It is not, by the way, a high speed section of the railroad.

Ed

[selector]

CP Modeller

I see there is a thread going on here regarding the minimum radius for model train equipment but I was wondering what the minimum radius would be if I was to model an actual minimum radius of a main line. I have heard in the past that it is. somewhere approaching 30' radius but that would seem to be an almost impossible curve to model. Is this correct or am I way off the target???

Regards  Colin  

 

I believe some early roads used 100' chains to run a cord to the next intersection point along the circumference of a section of a circle, that section comprising the needed curve to maintain grade.  They'd specify a degree of curvature, meaning the cord line had to angle from the tangent at its first reference point at so many degrees.  Degrees in the 6+ range meant the trains would be restricted to slower mainline speeds there.  Less than about 3 deg meant higher speeds, and less than 1.5 deg or so meant high limited speeds one would expect from passenger express trains.

http://trn.trains.com/railroads/ask-trains/2011/01/measuring-track-curvature

For a more robust answer:

https://mysite.du.edu/~jcalvert/railway/degcurv.htm

 

 

[rrinker]

 Real railroad use degrees, because unlike a model, you can't just sit above the earth adnd use a giant hand to draw out a radius. You have to use surveying instruments and measure how far you offset over a given distance. Common distance is 100 feet, so a 1 degree curve changes 1 foot for every 100 feet. This gets you the chord of the circle (in this case 100 feet) The radius of such a curve can be computed as Chord Length/2Sin(Degrees/2). Say you have a 4 degree curve, that's 100/2Sin(4/2), 100/2(.0349), 100/.0698, 1432 feet. In HO that's 16 feet. A 2 degree curve is about 32 feet. A 10 degree curve is about 6.6 feet in HO. You can work backwards and figure what degree curve a typical HO radius is, but the real world equivalent of 18" is pushing it even for trolleys. Luckily physics does not scale and we can get away with things in the model world that would never work in the real world, or we would be forced to work in Z scale or smaller.

                                     --Randy

[slammin]

Welcome to the forum Colin. I remember a thread that discussed how much room it would take to model a real curve. Prototype road measure their curves in degrees. In HO a 30" radius is not consider "big". Large home and club layouts have 48" and larger radii when modeling class 1 railroads. Most us don't have that kind of space and tend to model short lines and branchlines that look reasonable running 30" and smaller radii.

[CP Modeller]

I see there is a thread going on here regarding the minimum radius for model train equipment but I was wondering what the minimum radius would be if I was to model an actual minimum radius of a main line. I have heard in the past that it is. somewhere approaching 30' radius but that would seem to be an almost impossible curve to model. Is this correct or am I way off the target???

Regards  Colin