Easements vs. increased radius

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SPC_1909

New Member
Hello,

I am in the final stages of designing a HOn3 trackplan for a 8' X 12' spare bedroom. Space is tight and the curves will be too. From an operational and aesthetic point of view, would I be better off using 18" radii curves with 12" easements or using 24" radii curves and no easements? Both options occupy the same amount of space for a 90 degree curve.

I'm modeling the SPC in the early 1900's and don't envisage running anything larger than a 2-6-0. Passenger service was an important part of operations so I am on the lookout for short passenger cars.

Suggestions?

Thanks.
 

jbaakko

Diesel Detail Freak
HOn3 SHOULD be fine around 18-22, with maybe a 6" easement, I think. But what do I know, I'm not a narrow gauge modeler!
 

RCH

Been Nothin' Since Frisco
I don't want to jump in with too much technical jargon, but I deal with this sort of geometry for a living, so I have a tendency to assume people know what I'm talking about. So, get ready for the world's longest answer...

I wouldn't bother with easements, well, not in the grand sense that most people think of easements. The way model railroaders have been taught to think of easements (or spiral curves) is really how they're used in conjunction with superelevated curves, but the superelevation part usually gets left out of the discussion. Most people tend to think of easements in these large, slowly sweeping transition terms, when the truth is all real-world curves have easements to one degree or another.

Just to clarify what I'm talking about, the spiral curve is used to as a "bridge" or a transition between a straight (or "tangent") section of track (or highway) and a radial (or "circular" or "fixed radius") curve section. The length of this parabolic curve is determined by the circular radius and the design speed of the curve. The design speed and radius defines how much the outer rail (or outer lane of a highway) is elevated or banked above the inner rail. Throughout the circular curve section, the difference in elevation (superelevation) between inner and outer rails is constant. On the tangent, both before and after the curve, the inner and outer rails share a common elevation. It is throughout the spiral curve section that both the trasition from flat to superelevation and the transition from straight to curved occurs.

Now, back to the question at hand. If you think of a track gang setting down a rail at the end of a section of tangent (straight) track, fishplating and spiking the end in place, then having to wrestle the rail to bend it into place while it's being spiked as the curve begins, you might imagine the little difference between joining a straight piece of sectional track and a piece of circular radius sectional track. In that 39 ft. length of rail is the complete transition from straight to curve, which is unlike the two pieces of sectional track, where one piece is straight as an arrow and the other piece has a common radius point that can be measured from any point on the rail. The prototype's rails have to slowly ease into that curve because they are bent into shape - not by machine, but by lateral force. The transition from straight to curve might only occur in that one 39 ft. section of rail, or it could be stretched out over a couple, but it's not abrupt like on model railroads built with sectional track. And this transition curve is defined by the tendency of the steel to stay straight, not by a complex curve formula.

Another thing I can tell you is that in my experience, narrow gauge railroads didn't design spiral curves into their alignments. Many of the roads and even some of the highways in southwestern Colorado are actually old RGS or D&RG railroad grades, so when you do boundary retracements out there, the survey data defining these rights-of-way is the old railroad alignment. All the ones I ever dealt with, including the tight curves - were all radial curves. Now, you can ride on some of these railroads and feel the smooth transition from straight to curve while in the back of your mind you know that "this is the point of curvature of a 5 Degree curve to the right." And I think this smooth transition is where the reality of bending steel meets the academics of tangents and radial curves.

Not only that, these railroads were in the business of getting the rails laid cheap and quick. The surveyors and track gang needed to be able to glance at the plans, turn the angle and shout to his head chainman "drive it!" and be done with it, not sitting on the dirt doing long division.

So, assuming you're not using sectional track (does anybody even make it in HOn3?), you can just go with the largest radius possible. But, when you're laying your track, try to imagine how the track gang laying it would have done it, the resistance the steel would have put against their backs, and use your eye to judge how smooth that transition between straight and curved can be. In the field, even a well-equipped surveyor with a crew of two, a decent transit and a steel chain couldn't have told the track gang anything more than whether they were "on line" at 100 feet or not. Once the points of curvature were staked, it was the steel that defined the curve, not the surveyor there on the ground and not the engineer back in the office.
 

Brakie

Member
Ryan,Nice read and very informative..:D
Back to the question..I would go with 22" curves and fore go the easement in order to have the larger curve...;)
 

cuyama

Member
Well, I wouldn't argue with Ryan on some of the technical aspects, but even narrow gauge prototype railroads used immensely broader curves than nearly all model railroads.

In the model, one of the important things we are trying to accomplish is to minimize the "lurch" that results from the abrupt transition of curved to straight track. That lurch will be there even with a 24" R curve. Personally, if all of your existing and planned equipment will negotiate an 18" R curve, I think you'lll have smoother operation and a better looking model with the easements leading into the curves.

Again, I am not arguing with most of what Ryan said, but rather talking about what seems to work best in a model environment. And I'm certainly not saying that any fancy math is needed. The offset-and-bent-stick method seems to work out fine in the model to determine the easement.

Regards,

Byron
 
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RCH

Been Nothin' Since Frisco
cuyama said:
Well, I wouldn't argue with Ryan on some of the technical aspects, but even narrow gauge prototype railroads used immensely broader curves than nearly all model railroads.
I definitely agree with you there.

In the model, one of the important things we are trying to accomplish is to minimize the "lurch" that results from the abrupt transition of curved to straight track. That lurch will be there even with a 24" R curve. Personally, if all of your existing and planned equipment will negotiate an 18" R curve, I think you'lll have smoother operation and a better looking model with the easements leading into the curves.
I think if anything is key here, the smooth operation of your railroad is it. This is the most important point to be made in this discussion.

Again, I am not arguing with most of what Ryan said, but rather talking about what seems to work best in a model environment. And I'm certainly not saying that any fancy math is needed. The offset-and-bent-stick method seems to work out fine in the model to determine the easement.

Regards,

Byron
Argue all you want! I just wanted to amplify on the gray area between easements and circular curves. I have the experience of actually staking out track, so I can give insight on this matter from a surveyor's perspective. In the construction world, you have the expression "field fit" which is the way the plans translate to the real world. Sometimes engineers even specify to "field fit" certain things because of unknown conditions. Ever the antagonist, I wanted to point out that in the real world, this is one of those things that is "field fit" whether engineers like it or not, so there is a certain justification for doing the same on your layout. :)
 

Brakie

Member
From my railroad knowledge the real railroads was also face space limitions especially the poorly finance narrow gauge railroads that was doom to fail from the start because of the limited customer base and route miles.This would also apply to some standard gauge railroads that went from nowhere to nowhere simply because two towns wanted a railroad or in some cases for hauling coal,crush stone and lumber to a bigger railroad.Either way these types of railroads suffered from the lack of construction financing and was doom to fail from the start so,no fancy track work...
 

SPC_1909

New Member
Thanks

Thanks for the input, both philosophical and practical. One of my main goals is smooth operation.

It is my understanding that the SPC branch running through the Santa Cruz Mtns. was, mile for mile, both the most expensive and the most profitable narrow gauge railroad ever built. The line carried a lot of passengers and was better engineered than most narrow gauge lines.

I am modeling that branch before it was switched to standard gauge. All rules are tossed aside on the line that extended north from Boulder Creek where the grades increase and the curves sharpen.
 




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