From a www.steamlocomotive.com page :
“The sharpest curve the Big Boys could negotiate was a 20 degree curve. In HO scale, this would be a 40 inch radius curve”
So the current models of the Big Boy need that radius to be accurate?
Jacon
Jacon,
What does a 20 degree curve and a 40" radius curve have to do with one another. Is this an accurate quote? The degree of the radius of the curve should NOT make any difference. The size fo the radius should. You can still have a 20 degree, 40" radius curve. One has to do with angle; the other, the size or radius of the angle. Sorry, it just doesn’t read right. Am I misunderstanding something?
Tom
No he posted it right but he just didnt clarify between protoype and model.
Prototypically a real big boy loco could only negotiate a 20 degree curve.
Real railroads lay their curved track sections based on a degree system.
On the website he was quoting they are saying a 20 degre prototypical curve is equal to a 40" radius curve in HO scale.
I have seen two layouts in MR and GMR where the builders used 40 inch curve so the locos looked plausible. I think most bigboy models will negotiate a 22 inch radius turn but of course your boiler swings out almost an inch. Model companies did this so that a broader range of people would buy one cause they could then run it on their tight radius layout. So if you want to run big boys and have them look prototypical you need to run the largest radius you can.
I myself have a minimum of 34" on my layout I do not currently own a big boy but from past experience it didnt look too far off kilter.
Tom, Sean said it better than me. Being new to the hobby I sometimes don’t know exactly how to phrase a question, so I’ll make errors. I ‘think’ I understand now, from Seans reply, that if (for instance) the model Big Boy was constructed as an EXACT scale model of the prototype, which in my newness I don’t know if it is or not, it would need a 40 inch curve. But, if I understand correctly, the model maker ‘fudged’ a little in design so that the loco will negotiate a much smaller radius thus getting a much larger buyer segment.
Do I have that about right?
Jacon
Exactly!!!
Hope this helps
You can fit a Big Boy around an 18" curve if you want. I don’t really care if it “looks horrible doing so”. But I’d still recommend 22" and up. I’ve got 26" curves, so a Big Boy will look OK on my layout.
The difference is, to simplify it, the prototype rated curves in degrees. because of hills, buildings, mountains, etcetera, it was easier for surveyors to set up the centerline 100 feet at a time. the tightness of curvature was based on how many degrees the transit was offset for that 100 feet. A 5 degree curve is pretty good, 10 degrees gets to be too sharp for high speed, anything over 10 degrees is taken at slower speeds. I happen to have a spec for Indiana Harbor Belt Line for Industrial spurs that gives a minimum curvature of 12 1/2 degrees which equals a 459 foot radius. That scales out in HO to 63.3 inches. So you can see that even 48 inch radius would be a pretty tight curve. from my experience that is my minimum and even 72 inches looks really good. Look at photographs of full size passenger equipment (85 ft.) and you will see from 7 to 9 or 10 cars on a good curve. That is why modellers 22 or 24 inch curves continue to look toylike. And don’t forget to use easements!
Sorry, guys. This is still as clear as mud to me. I think there maybe a particular given that I am out of the loop on. (Sorry, no pun intended.)
Alright, from what I can understand from the explanations that have been given so far is that because the Big Boy is articulated, the amount of degrees allowed to the prototype on the curve were limited to limit the amount of swing “outward” (G-force) of the boiler frame.
So, are you saying that in order for a prototype 4-8-8-4 to negotiate a complete 180 degree directional change (e.g from north to south), it could only be accomplished in 20 degree increments or less? Or, and perhaps is the missing link, was there a standard radius on all curves used by the railroad lines for running the Big Boys? If not, then to me it’s still comparing apples to oranges. Radius and degrees of radius are two completely different things.
If the radius of the curve were 1 mile, how much of the curve would the prototype be able to negotiate safely? Obviously, speed would be a consideration. The larger the radius; the faster the prototype could travel through it. A smaller radius; the slower the prototype could travel through it…(re-reading comments)…hmmmmmmm…re-reading…re-reading…
Ya’ know…actually, the more I re-read johncolley’s explanation, the more I think it’s starting to make since to me now.
Okay, forget the above ramblings of a mad man and see if I have this right. As johncolley stated, “the prototype rated curves in degrees”. If a curve had a degree of “0”, it meant that the transit was not offset from the 100’ long centerline and it was a straight line. A curve of 10 degrees meant that the transit was offset 10 degrees from the 100’ long centerline. The next 100’ centerline could easily go back to 0 degrees. So, the curve (or degrees) we are talking about is actually in
So…am I?
YES! Grasshopper, you’ve got it! That’s where easements come in. Ye olde surveyor might start his 100 foot increments with a 1 degree, then 3 degrees, then 6 degrees, then 12 degrees, then his 20 degrees until half of the desired change in direction was met, then he would start reducing the amount of curvature until he was at 0 degrees or tangent track. Are you with me? An easement is a continuously increasing curve until you reach the maximum amount of curvature for the speed you are going to run. In the mountains you might get a 20 degree curve which might be good for 5 to 8 miles an hour. On the prairies a 5 degree curve would be sharp because you expect 45 or 50 miles an hour speed and 75 for passenger.
I need some asprin.
No, really… I think I’ve got it… the centerline and the easement part… I just gotta reread it a few times[:-^]
Jacon
johncolley,
You had me worried. I thought since you didn’t answer immediately that you were looking up the number for Bellvue Hospital. (Not for you, but me.)
Just so I know for sure I’m getting this all straight, you stated:
So, if our surveyor were pointed due N(orth) and he performed or completed the above given exercise you gave as an example, he would now be pointing either due E(ast) or due W(est) - depending on which side (left or right) he intially offset his transit from the centerline. And the gradual increase of the angles would be your easements. Correct?
Jacon, pass me the asprin bottle when you’re done. My head hurts. [xx(]
Tom
Ah! you are confusing the Rate of curvature with the total angle of curve. What you compare to radius is the rate of curvature. the total angle is the difference between the straight track you were on to the straight track you are now on. Big difference. If you only needed a slight bend you might go 1, 3, 6, 3, 1, and only have a change of 14 degrees. If you were wrapping around a river bend or mountain you might go 1, 3, 6, 12, 20, 12, 6, 3, 1, for a total of 64 degrees. If it were a really great turn and 20 degrees was your maximum rate for the speed you wanted (or in the mountains, were able to make because of grades) you might add a few more 100 foot segments of 20 degrees before easing out of it. Remember, none of us were born knowing all this stuff! Take it a little at a time until you understand it, then move forward. The object is Happy Railroading! running your train through large smooth eased curves is a beautiful sight, far different from the toylike effect of sectional track, and worth the effort! After you understand this look into banking of high speed curves…then you WILL need the tylenol!
John,
In the example in your last post, approximately how long (50 feet, 100 feet etc.) would each of the easements be, i.e. the 1, the 3, the 6 etc… talking prototype now. Would it be governed by the speed of the trains, the amount of room they have to do the curve. My guess would be some of all. I’ll bet on a layout it does look good.
Be right back, I’m out of tylenol…
Jacon
Jacon, take it in small doses. Actually in modelling we only approximate. What I was saying before as an example was the surveyor doing 100 foot increments. There is a MUCH easier way to approximate easements! get a copy of John Armstrong’s “Track planning for realistic operation.” Using offsets and a batten (thin bendable strip of wood or plastic) it is very easy to layout your centerline. I was just explaining an example of how the surveyors had to do it, not the actual amounts of curvature. But once you understand the what and why it is easier to approximate and the results on your layout are worth the effort!
Oh, another point of information: it was not the ‘G’ forces that were a concern for articulateds but the fact that the boiler is very long and the front engine was pivoted (articulated) so it would swing with the curve of the track. On sharp curves this resulted in a lot of boiler overhang in front requiring wider track center spacing on curves and clearances on the outer curve to signal posts, buildings, etc.
Thanks John, I have Armstrongs book but haven’t gotten that far in it yet. I’ll go back and look it up. I appreciate your time and I do understand you’re giving an example of what the surveyors had to do.
Jacon
OK, I think I can explain this “degree of curvature” thing. Since I’m currently studying for the Civil Engineering Prof. Eng. exam (this Friday!!![xx(]), it’s fresh in my mind.
From my handy PE review manual:
“The railroad degree of curve is the angle subtended by a chord of 100 ft.”
Suppose you have a slice of pizza. The CHORD length is measured in a straight line from one end of the crust to the other end. (The CHORD length is slightly shorter than the actual curved length of the crust, which is called an ARC length.) The degree of curve (D) is the angle of the slice (e.g. - if the slice is 1/8 of a whole pizza, then D=45 degrees).
Railroads hold the chord length constant at 100 ft and express the track curvature in terms of D. If D is large, then the curvature is “tighter.” If D is small, then the curvature is more gradual.
Suppose you’re out railfanning on a long, continuous section of curved RR track. You stand somewhere on the curve between the rails holding the end of a 100-ft rope. Your buddy holds the other end and starts walking down the track (staying between the rails). At some point, the rope stretches tight and your buddy stops walking. This is the 100-ft chord length. Now imagine that you are each standing at opposite ends of the crust on a giant slice of pizza (mmm…giant slice of pizza…) - the RR tracks represent the crust. The angle made by the pointy end of the giant pizza slice is the degree of curve (D). Remember that the point of the pizza slice will always be at the center of the curve, so if the curve is tight, the center will be closer to you. If it’s more gradual, it will be further away.
The degree of curve (D) has nothing to do with the “number of degrees” in a curve. If you had track in a complete circle, that doesn’t mean that D=360. The degree of curve is purely a measure of the “tightness” of the curve.
For those of you who are technically inclined, you can calculate the actual radius (R)
Hi everyone, lifetime hobbyist, I have a Rivarossi big Boy and I have been able to successfully run 4001 on a 22 inch radius, I am currently designing a helix for my home that will encompass from basement to 2nd story, I only have about a 4 ft dead space to accomplish this… anyone have plans for a helix, ? I checked the calculator, and it is putting me at a 2.8% grade but only gives me a 96 inch plan
Many recent articulated models ‘cheat’ a little, by having both sets of drivers able to pivot under the boiler. On a prototype articulated, only the front drivers can pivot.
Degrees or radius are just two different ways of expressing the same thing, how sharp or broad a curve of railroad track is. According to steamlocomotive.com, an 18-degree prototype curve would be 319.6’ radius, which would be 44.1" radius in HO. A 24-degree curve would work out to 33.2"R in HO, so 20-degree being about a 40"R curve seems about right.
Keep in mind, on a real railroad, a 10-degree mainline curve is considered sharp. (The higher the degree, the sharper the curve). A real train would be restricted to maybe 20 mph around that sharp a curve…which would translate to 79.1"R in HO. So a 20-degree curve would be very sharp.