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How To Measure Rails For Curve Radius
Jim writes:
“Here is an interesting question to ponder. Where is track measured?? At the outer rail, the outer point of the ties or the center of the two rails? I put together a full circle of 18″ radius track to answer a track planning question. The full circle assembled – I measured the diameter – the outer rail to outer rail was roughly 34 inches. Ergo the question of where track is measured to acquire the radius of a curve.”
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Well most “authoritative sources” on the internet refer to the radius as being measured on the track centreline.
I’m at a loss to explain how you can assemble “18 inch radius curves” and end up with a radius of 17 inches to the outer rail!!! What make of track are you using? Marklin has curves of 16 3/4 inch radius which would probably give you 17 inches to the outer rail.
But as far as your planning is concerned, you are going about it the correct way. Set something up and measure it. And then use the same make on all of your layout. I used trackplanning software, which said it had a track library for the track I was using, but when I got to actually laying it out, I found that the required clearance between tracks in the yard meant that my actual yard was one track less than my plan!
I was using Atlas sectional track and stuff that comes in those cheap sets ( In other words garbage track – I use only Nickel silver and Flex track for most of my layout – except when I needed a short piece ). I saw something in an advertisement and needed to find out what the grade was after one installs the seven risers
( after creating that full circle, and counting the number of sections used the grade came out to over 4% — forget that noise). That is when I found out that if I go outside rail to Outside rail, that diameter was 34 inches. So how in the world these companies can hire kindergarten dropouts to put together track plans using what they call 18″ radius track is beyond me. I do have 22 and 24″ radius sectioned track ( from Atlas ), which when I find a couple of minutes, will assemble in a full circle and measure those to see where they come out — 44 or 48 diameter.
If laid correctly, it should measure out 36″ at centerline
I have always assumed that the radius cited in the various literatures id based on the inner rail,. This is based on the formula used by NRMA planning software in relation to layout building “Add 2″ to your required radius”
Wouldn’t that be for clearance, space to the next track to avoid passing trains colliding ?