Turn Radius Measurement?

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.
Rob

Normal model railway practice is to measure at the center-line of the track gauge.

Thank you. I am drawing a layout plan and that little bit of info will help keep it on track.
John

In article snipped-for-privacy@ihug.co.nz, Gregory Procter at snipped-for-privacy@ihug.co.nz 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.
Dieter Zakas

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 achieve. 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.
Regards, Greg.P.

determining the 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.
Dave