Designing Easements

Discussion in 'Track Planning' started by billk, Jun 21, 2003.

  1. billk

    billk Active Member

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    Since I just finished designing and am in the process of laying out a track plan incorporating easements, I'd thought I'd describe how I did them. Briefly, an easement provides a transition between straight and curved tracks. Without them, the track curvature changes abruptly from an infinite radius (the straight part) to the radius of the curve. This can cause problems with reliable operation and with appearance.

    John Armstong's book Track Planning for Realistic Operation has an excellent description of easements, why they are good to use, and how to draw them. I implemented his method, for the most part, in the following.

    Easements have to be allowed for, to some extent, when you are designing your track plan. Here's what you have to do:

    1. Pick values for the curve radius and the length and offset of the easement. These are called R, L and X, respectively, in the Armstrong book (Fig 9.8, page 116 in my copy.) He has provided values of L and X for several curve radii R. It appears that L is somewhere between 3/5 to 2/3 of R, and X is somewhere between 1% and 3% of R. Both L and X are be picked to be easy to measure. For my plan, I had 11" curves, so I used 3" for L and 1/4" for X.

    2. When you draw a curve of radius R on your plan, also draw a curve of radius R+X using the same center.

    3. When you draw your straights connecting the curves, connect to the larger curves (the R+X ones).

    4. Centered at the points where your curves and straights connect, draw circles of radius L/2. These easements will go inside these circles, and you should not put anything else inside them.

    The result should be as shown in Figure 1. Note, I didn't have to draw the actual curve of the easement! Actually, I go a step further (I use a CAD program) and trim off the unneeded portions of the R+X and the L/2 circles, and make the rest a different color (Figure 2). This cleans up the drawing quite a bit, and hightlights the easement area nicely.

    To get the easements onto the layout surface, I used the method described in the Armstrong book. First I drew onto the layout surface the R and R+X curves, connected the R+X curves with the straights, and drew the L/2 circles, all as described above. Then I used a length of 1/8" dowel to draw the easement curve. You could use something else, but thats what I had laying around,

    I used T-pins (my layout surface was extruded foam) to fasten the dowel along the straight leading up the the L/2 circle (see Fig 3), then bent it and fastened it along the R curve. Don't use any pins inside the L/2 circle, instead let the dowel bend naturally. Make sure that there are pins on the outside of the dowel at the two places it crosses the L/2 circle (the red pins in the figure). Then just connect your curve to your straight by drawing along the dowel in side the L/2 circle.
  2. csxnscale

    csxnscale Member

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    Thanks Billk,
    Now I understand how to design this easements.

    Paul
  3. volnation

    volnation New Member

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    I appreciate the help, however, for the geometrically challenged, surely there is an easier way to work this out...I've read your post for several days, I've tried to work it out in my head and on paper...It's just not coming to me...I'm wanting to use a 24" radius and 20, and 22" and I just can't seem to figure out how you get the larger radius drawn to fit to the tangent and still use the original radius drawn...Again, the whole thing is GREEK to me...Thanks


    Since I just finished designing and am in the process of laying out a track plan incorporating easements, I'd thought I'd describe how I did them. Briefly, an easement provides a transition between straight and curved tracks. Without them, the track curvature changes abruptly from an infinite radius (the straight part) to the radius of the curve. This can cause problems with reliable operation and with appearance.

    John Armstong's book Track Planning for Realistic Operation has an excellent description of easements, why they are good to use, and how to draw them. I implemented his method, for the most part, in the following.

    Easements have to be allowed for, to some extent, when you are designing your track plan. Here's what you have to do:

    1. Pick values for the curve radius and the length and offset of the easement. These are called R, L and X, respectively, in the Armstrong book (Fig 9.8, page 116 in my copy.) He has provided values of L and X for several curve radii R. It appears that L is somewhere between 3/5 to 2/3 of R, and X is somewhere between 1% and 3% of R. Both L and X are be picked to be easy to measure. For my plan, I had 11" curves, so I used 3" for L and 1/4" for X.

    2. When you draw a curve of radius R on your plan, also draw a curve of radius R+X using the same center.

    3. When you draw your straights connecting the curves, connect to the larger curves (the R+X ones).

    4. Centered at the points where your curves and straights connect, draw circles of radius L/2. These easements will go inside these circles, and you should not put anything else inside them.

    The result should be as shown in Figure 1. Note, I didn't have to draw the actual curve of the easement! Actually, I go a step further (I use a CAD program) and trim off the unneeded portions of the R+X and the L/2 circles, and make the rest a different color (Figure 2). This cleans up the drawing quite a bit, and hightlights the easement area nicely.

    To get the easements onto the layout surface, I used the method described in the Armstrong book. First I drew onto the layout surface the R and R+X curves, connected the R+X curves with the straights, and drew the L/2 circles, all as described above. Then I used a length of 1/8" dowel to draw the easement curve. You could use something else, but thats what I had laying around,

    I used T-pins (my layout surface was extruded foam) to fasten the dowel along the straight leading up the the L/2 circle (see Fig 3), then bent it and fastened it along the R curve. Don't use any pins inside the L/2 circle, instead let the dowel bend naturally. Make sure that there are pins on the outside of the dowel at the two places it crosses the L/2 circle (the red pins in the figure). Then just connect your curve to your straight by drawing along the dowel in side the L/2 circle.
    [/QUOTE]
  4. billk

    billk Active Member

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    Volnation - You don't fit the larger (R+X) curves to your tangents. It's the other way around - you fit your tangents to the larger radius curves. The easement then connects these tangents to your original curves. Does that help?
  5. volnation

    volnation New Member

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    That would mean that the tangent (Straight) is not straight...In other words the straight track is not parallel with the edge of the board...It would have to come in at an angle...I know you are experienced in this stuff, and I appreciate you taking the extra time to help me, but it's just not sinking in...I've been toiling with this for over a month...I guess I'm frustrated with the whole thing...


    Thanks


    Volnation - You don't fit the larger (R+X) curves to your tangents. It's the other way around - you fit your tangents to the larger radius curves. The easement then connects these tangents to your original curves. Does that help?
    [/QUOTE]
  6. Paul Davis

    Paul Davis Member

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    I just do the layout design using 1" larger radius curves than I'm actually planning to use. Then I have plenty of room to fit the easements in when I actually lay it. When I draw it out for real I always draw the straight track 1/2" out from the curve. and make a mark between the two. I then go 9" from that point in both directions and make another mark. I then join the three marks to make a smooth looking curve.

    Attached Files:

  7. MasonJar

    MasonJar It's not rocket surgery

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    Well, I was working on an explanation, but Paul beat me to it with his pictures.

    volnation - you are right though, in that part of the "straight" track will no longer be straight when you are finished. See Paul's pictures.

    (By the way Paul, the Gauge box car should be on its way to you this weekend!)


    Andrew
  8. volnation

    volnation New Member

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    Thanks

    Thanks guys this really helps...Guess a picture is worth a thousand words...Thanks again

  9. Woodie

    Woodie Active Member

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    I've bookmarked this. Transitional curves are a must, for realism, and operation. None of the jerky "wooooooooh.... round the corner we go" especialy for long wheelbase rollingstock. :) :cool: