--- In [email protected], Bill Roberts <[EMAIL PROTECTED]> wrote:
>
>
> John Armstrong favors the "bent stick" method, which he describes
as a
> fair, practical approximation of the mathematical "cubic spiral"
used
> by railroads.
>
> His book covers the technique for curves of various sizes,
including
> 24, 30, 32, 42 and 54 inches. For example, the transition from
> straight track to a 42-inch radius curve would be 25 inches long
and
> the center of the curve would be offset 5/8-inch. See the book for
> details. (It's also discussed in the NMRA Data Sheets, and the
> technique must be described on the Internet somewhere.)
>
> "Degrees of curvature" has no real application to model
railroading.
> It refers to the angle formed at the center of a curve between two
> points on the curve. It also refers to the procedure used by
railroad
> surveyors to lay out a curve. Railroads favor small degrees of
> curvature, which make for impossibly large radius curves for model
> railroads.
>
> I didn't use transition curves on my small layout, because I
settled
> on a 44-inch minimum radius and didn't figure they would be
necessary
> for operation. However, I do see equipment lurch slightly when
> entering or leaving a curve, so some kind of transition is
desirable,
> if only for looks.
> --
> Bill Roberts
>
Spiral easements are also covered in Paul Mallory's trackwork
handbook, a key reference I have used. In the apppendix he provides
a simple easement template to create spiral easements. I have seen
easements allow a long wheel base locomotive to negotiate curves a
couple inches tighter in radius than the 48" minimum of the
locomotive's wheel base. It may be that "lurch" at the curves entry
is important, even at a wider radius...
FYI.. The 12" to the foot gauge railroad's generally try not to have
curvatures higher than 10 degrees and a 14 degree curve on a mainline
would be considred extremely tight, probably about the max. A 10
degree curve is a little better than 102" in radius in S scale. A
very generous but more "S scale doable" curve of 54" radius relates
to 20 degrees of curvature on the prototype... Some things can
be "scaled", others... probably not.
Jim K.
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