For most track it's the radius at the centerline. However, for Lionel
O-scale stuff, I think it's the inside rail and it may be the diameter,
rather than the radius, which is indicated. I hope an O-scaler replies
and clarifies that.
-- Bill McC.
Regular Lionel O makes a 31" diameter exterior measured circle... so it
is just under 15" radius as measured by conventional methods(centerline)
taking in to account the 1.25" gauge and some tie overhang.
And then so it goes with O-27, O-42, O-54, O-72, all referring to
outside diameters. Super O and FasTrack is 0-36.
In article firstname.lastname@example.org, Gregory Procter at
email@example.com wrote on 1/11/05 3:44:
In a related vein, is there a mathematical formula one could use to
determine the easement when laying track? Everybody knows the formula to
calculate the area of a circle, for example, but what about determining the
easement for a given curved track? I would want to be be able to plug in the
relevant figures and derive a usable value from that.
Years back I wrote a Basic program for calculating end displacement of coaches on
curves. From this I can calculate set radius curves of one coach cord length
which when strung together (and averaged) create a practical easement into a
specified radius curve.
There are other tables and programs around, but it depends what you want to
The reasons I wrote my one were twofold, to avoid the appearance of passenger
coach corridor connections looking completely unconnected, and to assist in loco
chassis design for my overly sharp layout curves.
Above is a URL for a zip file of a Microsoft Excel program which
calculates coordinates for an easement, given the final radius and the
length of the curve. You can pick apart the formula, but be warned that
it's complicated. It's a repeating series but like most such equations,
you can get an acceptable result with only the first few terms.
The following URL is for a re-hash of the original Oct.1969 Model
Railroader article by Westcott et al. Still the best on the subject
that I have seen. They didn't have a computer on every desk in those
days either. A downloadable pdf file of the easement templates also
seems to be available.
The basic formula is P= L^2/24R
where P is the offset added to the radius at distance L from the Tangent Point.
A copy of the Oct. 1969 MR with the discussion and the sample templates
is the easiest to read, but the info is all available at the URL.
Basically you need to choose the length of spiral curve desired
depending on the longest equipment you want to assume for the
calculations; or you can use one of the sample templates provided.