For you rail road surveyors. What's the minimum radius (in feet please)?
This is for open road main line - not spurs and yards.
I do model railroading in HO scale as a hobby. That's 1:87.1
Reason I ask is I have a Union Pacific DDA40X (the biggest diesel-electric ever built) that will NOT take my 18" radius curves.
That's just shy of 131' in real-life feet.
I see why the model won't handle it on the layout but am curious if it would be un-realistic of me to even try to get it to run on my curves.
Some technical specs I have on the real deal says min. radius of 57å¡.
I don't believe I've seen radius described in degrees so how does that relate?
My own answer probably is right in there.
E.
Hmmm... a 131-foot radius calculates to somewhere around a 44-degree curve, and a 57å¡ curve is only about 104.79 feet by chord definition... something doesn't sound right...
The formula for the radius of a chord definition of curve is
50 / SIN (0.5 * degree of curve)
so in this case it would be 50 divided by the sine of 28.5 degrees which produces 104.7869 feet. The train should work in my opinion.
EDIT: I'm not a RR aficionado but I think I got the formula correct, for what it's worth 🙂 I have an old RR surveying book somewhere in the bowels of my house, maybe I can find some more info that may help...
I would guess the UP Main Line is 1,000'+ minimum radius curves.
18" radius curves are for toy trains not models. To run that DD40 I suggest 36" minimum radius model curves.
The radius is not all you want to be concerned with, track center to center distance also becomes a problem. That long model overhangs the rail a great deal on the inside and outside of the curve, especially the shaper ones. Overhangs can interfere with trains on adjacent tracks and any fixed structure such as poles, signals, bridge and tunnel portals along the track.
By the way, that DD refers to the number of powered axles on the trucks, B=2, C=3, D=4. Models have the axles pretty much locked into the rigid trucks, no side motion. Real live locomotives have allowances for some of the axle bearings to move side to side. 2 powered axles per truck are the most common with 6 powered axles common on higher horsepower units. 4 powered axle trucks are rather long and only railroads in the wide open spaces used them, In the past there were 3 axle locomotive trucks that had a non powered center axle to aid in weight distribution, they were called A-1-A.
Paul in PA
Paul in PA, post: 350618, member: 236 wrote: I would guess the UP Main Line is 1,000'+ minimum radius curves.
18" radius curves are for toy trains not models. To run that DD40 I suggest 36" minimum radius model curves.The radius is not all you want to be concerned with, track center to center distance also becomes a problem. That long model overhangs the rail a great deal on the inside and outside of the curve, especially the shaper ones. Overhangs can interfere with trains on adjacent tracks and any fixed structure such as poles, signals, bridge and tunnel portals along the track.
By the way, that DD refers to the number of powered axles on the trucks, B=2, C=3, D=4. Models have the axles pretty much locked into the rigid trucks, no side motion. Real live locomotives have allowances for some of the axle bearings to move side to side. 2 powered axles per truck are the most common with 6 powered axles common on higher horsepower units. 4 powered axle trucks are rather long and only railroads in the wide open spaces used them, In the past there were 3 axle locomotive trucks that had a non powered center axle to aid in weight distribution, they were called A-1-A.
Paul in PA
Yes Paul, I know all about that stuff.
Some engines also have articulated drive trains/trucks. The only models that do that cost as much as a small used car.
My real curiosity was if my model is close to the real thing.
I don't know if I'd say 18" curves are for toys. I'm running an SD40-2, FP45 and an ES44AC with zero problems - ever. Neither do they have a problem with my #6 turnouts and my cross-over.
I do completely understand all about overhang as well. I took SERIOUS care when putting down my 4 tunnel portals to be sure the FP and ES units could handle it. Most of my long intermodal cars don't like the curves either. Surprisingly a few are ok. I did have to raise the portals to be sure a double stack would pass through.
In honesty, I never really expected for my DD to run on the my layout. I figured it'd just look cool sitting on a spur near the engine house. But in the same vein, many of my mrr buddies poop-pooed the thought of a SD40-2 being ok - much less the FP or ES units.
BTW: if you are saavy on model rr'ing, I'm running all DCC.
E.
In the end, you have to use whatever works.
The Railroad (chord) definition is that the degree of curve is how much the forward direction changes in moving to a point 100 ft chord from the start. Your 18 inches * 87 scale = 130.5 ft radius which is railroad degree 45 d 3.5'.
As opposed to the highway arc definition where the forward movement is 100 ft of arc. The difference isn't much for typical railroad usage and the chord made calculation and layout easier with the tools of the 19th century.
Highway curves can be calculated and laid out to any precision you can work with. However, there are approximations built into the chord formulas and layout methods that end up giving you maybe a tenth of a foot slop on typical rr curves depending on the order things are done.
Of course most model RR layouts do not really scale the real-world curves but make them much sharper to fit more into a layout. Even so, 18" radius is sharper than a friend who does a lot of model work told me he ever uses - 24" is his usual minimum and 26 or 28 more common. Those scale to 174, 188, and 203 ft real-world.
My brother who worked track inspection told me that 3 degrees or 1910 ft radius is considered a moderate curve and 6 degrees or 955 ft a pretty sharp one for cross-country. That's in line with Paul's advice. The limit for a 40-ft car on a slow-speed siding is about 21 degrees or 275 ft. Scaled to HO that would be about 38 inches radius. So if your model can turn sharper than that you are ahead of the real world.
The famous Horseshoe Curve near Altoona, PA has portions around 9.4 degrees or 609 ft; depending on who you ask you might get a little different number.
I wonder if it would be possible to build model railroads with transition spirals? Do they make track where you can possibly do so?
SellmanA, post: 350623, member: 8564 wrote: I wonder if it would be possible to build model railroads with transition spirals? Do they make track where you can possibly do so?
Some of my buddies hand-lay their own track so I imagine they could build any sort of track you want - whether an engine or cars could take it or not would be another matter.
Then again, I don't know what a transition spiral might be used for on a model rail road.
E
If I ever built my own model RR layout, I would spirallize it for authenticity, increased running speed, and because I just plain like spirals.
SellmanA, post: 350628, member: 8564 wrote: If I ever built my own model RR layout, I would spirallize it for authenticity, increased running speed, and because I just plain like spirals.
But what exactly is that? I don't know.
BigE, post: 350629, member: 435 wrote: what exactly is that?
A spiral is a curve of constantly increasing radius. Railroads and high speed highways use them as a way of transitioning an alignment from a tangent (ie/the "straightaway") to the circular curve, and back to the outbound tangent.
BigE, post: 350629, member: 435 wrote: But what exactly is that? I don't know.
Try this link.
Spiral on a model railroad? The easiest way to spiral model track is an approximate method and I do not recall the exact proportions. Been 30 years since I designed a layout or laid any model RR track.
Lay out your tangent center line in pencil, at your PC measure over 1/4" into the curve, mark it, measure over another 1/4" and mark it, from that 1/2" mark your layout a 36" radius., measure back on the tangent 1' and forward along the curve 1'. Using flex track you lay a long piece, securing it along the tangent until you get a foot from the PC, then you just secure the middle of the tie at the midpoint of the offset then fit the track to the curve securing from 1 foot into the curve for at least a foot of fixed radius. Because the track has some stiffness it should assume a rather reasonable looking spiral, now secure the spiral. For a larger radius curve reduce the offset. If my memory serves me for a 24" curve use 3/8" and 3/8" and for 18" radius use 1/2" and 1/2". Almost all Ho spirals can be of 2' length.
Using spirals on 18" track greatly improves the smoothness into and out of the curves. As the curves get larger it is more for visual effect than actual performance. A # 6 turnout is close to a 12" long spiral, OK for slow speeds as in yards. Us a # 8 for continuously used passing sidings and double track crossovers. Use 24" or larger on mainlines, 18" is OK in yard and minor sidetrack areas.
Paul in PA
It is not simply the spiral curves.
The rails are also built with a transitioning super elevation like NASCAR raceways that allows vehicles to enter and leave curves at higher speeds.
They widths between the rails in tight curves are also designed to reduce friction and overloading and creating excessive stress points that would cause problems like you have with that engine not being able to navigate the curve.
I don't know if the tract width is adjustable or if the width of wheels on the engine are adjustable. A combination of both may resolve your problem.
The differences on the actual rails are a matter of an inch or so and that would be reduced to a hair and a smidgen at HO scaling.
Yes, you do need both horizontal transitions and superelevation ("banking") to handle the acting centrifugal and centripetal forces properly. Going from my ancient memory the maximum amount of real-life superelevation permitted on a railroad, was done by raising the outer rail by 8" and no more (need to find that RR surveying book of mine!!). This translates to approximately 0.092" maximum at HO scale if I did my math correctly.
Adding spiral transitions and superelevation to model railroad curves might be overkill for some, but I would build them in if it were mine- they make a substantial difference in the real world and as Mr. Dorfmann in the movie "The Flight of the Phoenix" would say, "The principles are the same."
We have an unspiraled curve on the street railway in Dallas and if you hit it too fast it will throw you out of your seat.
The modern switches that we have purchased in the recent past all seem to have tangent sections in them which makes for a very poor ride. Same with a section of track that crosses around some AT&T manholes. Curves and spirals are better than curves and tangent track. Seems like you can't get good street switches anymore.
Our minimum standard centerline radius is 50 feet.
The tightest curve on the line is in the carbarn and I think it is 43 feet centerline radius. The cars complain when passing through that one and the concrete has been chewed up significantly from when it was first new, but it works.
If I recall correctly, DART light rail broadens the gage of the tracks on curves by a fraction of an inch.
BigE, post: 350615, member: 435 wrote: For you rail road surveyors. What's the minimum radius (in feet please)?
This is for open road main line - not spurs and yards.
I do model railroading in HO scale as a hobby. That's 1:87.1
Reason I ask is I have a Union Pacific DDA40X (the biggest diesel-electric ever built) that will NOT take my 18" radius curves.
That's just shy of 131' in real-life feet.
I see why the model won't handle it on the layout but am curious if it would be un-realistic of me to even try to get it to run on my curves.Some technical specs I have on the real deal says min. radius of 57å¡.
I don't believe I've seen radius described in degrees so how does that relate?
My own answer probably is right in there.
E.
On the internet (YouTube) Athens says to run their train on a 28 inch radius.
JOHN NOLTON
sorry for being so late to post.
