Proper radius for transistion to a grade?

Discussion in 'Track Planning' started by BigJim, Nov 21, 2006.

  1. BigJim

    BigJim Member

    Nov 19, 2006
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    I certainly assume there needs to be a smooth transistion from flat track to a grade.

    If you use a single radius transistion what is a reasonable radius for reliable operation? For a realistic operation? Does this radius change depending on the grade?

    Is it necessary to "ease" (larger radius at the start & end) into and out of the transistion?

    I developed the table below with a wide range of values since I don't know the answers to the above questions. Fairly sure the calculations are correct. For example, a 160" radius transistion to a 3% grade would require 4.8" of track with a 0.072 inch spacer at the end and spacers of 0.003, 0.012, 0.028 and 0.050 at the 1,2,3 & 4 inch points. Do these values seem reasonable?

    I would assume for XtrkCAD (or any other track design program) I would then use a 4.8" length of track and define 0.0 and 0.072 for the height of the two ends.

  2. pgandw

    pgandw Active Member

    Jul 9, 2005
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    There was a quite a bit of discussion about this in the Layout Construction Yahoo Group a couple of months back.

    Based on calculations looking at flanges on rigid wheel bases only, a vertical transition radius of 10 to 14ft in HO (depends on length of steam locomotive driver wheelbase) was deemed to be acceptable. However, this does not take into account possible coupler mismatch and accidental uncouplings caused by the mismatch.

    The single radius transition yields a short, less than car length vertical transition. If the car is longer than the transition, then at some point there is effectively no transition.

    Since coupler mismatches are dependent on coupler size, the best the group could come up with is a transistion length longer than your longest car, and test prior to building.

    Some argued that construction of transitions to better than 1/16" accuracy was impractical. Others pointed out that bending plywood or similar materials caused a sufficient vertical transition to avoid problems naturally, so why bother with calculations.

    Since I am looking at switchback grades in the 6-8% range (depends on transition space), all this was of considerable interest to me. I finally joined the "just bend the plywood and accept the result crowd" as the best and easiest way to get a smooth grade. So I will be using 1.5" foam glued to 1/4" plywood cookie-cutter style, and bending the sub-roadbed to fit the risers attached to my framing grid. I've successfully used this technique with straight 1/2" plywood in the past.

    The other not-so-obvious advantage (besides smooth vertical transitions) of open grid/cookie cutter construction is hollow scenery. With hollow scenery - can be hardshell plaster of some type or even thin foam sheet - you can access the underside of your layout for wiring, uncoupling ramp and switch machine installs, and reach hidden track.

    With stacked solid foam construction, access to hidden track must generally be via a liftout section of foam scenery. Grades must be carved or something like Woodland Scenics risers used.

    my thoughts, your choices
  3. MasonJar

    MasonJar It's not rocket surgery

    Oct 31, 2002
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    You are right in that there should be some sort of gradual shift into and out of a grade. This is sometimes called a "vertical easement", and technically is not a set radius, but a spiral - like the horizontal easements going into and out of curves. However, it can all be approximated with large radius curves as you have done...

    The easiest way to get the right transition is to use "cookie cutter" benchwork - where the 1/2" ply (or other somewhat flexible material) is used for the subroadbed. By cutting and bending this subroadbed, you avoid having to do all the math... ;) :D

    EDIT - I see that Fred has answered this above as I was composing my message :cool:

    Another way, although more expensive, is to use the Woodland Scenics grades and risers. I believe that there is a set that takes you from flat to predetermined grade.

    Your numbers certainly seem reasonable for both relaible and realistic operations. We have modules ( with much more abrupt changes than what you list, and they operate well.