Food for thought for those of you who are planning a layout. When I was checking the standards for #6 turnouts a couple of weeks ago I and liking to keep to around 36" as a minimum radius a guy on the board pointed me to the link ot the NMRA turnout standards - http://www.nmra.org/standards/sandrp/rp12_3.html Noticing that #6was 43" I opted for those, but thinking that it seemed a big drop to go from that to 26" for the #5s given that they are about an inch shorter, I got a couple of them too. Below is a photo of a #5 with a 36" Tracksetta on top. I realise that they are not a constant radius and I can see that it may be tighter than 36" - slightly, but it doesnt seem like 26" to me. Any thoughts?
That ‘replacement radius’ figure has to taken with a ‘grain of salt’. As you noted, there is no ‘continuous’ radius through the turnout. To accurately compare the tightest radius in that #5 turnout you need measure from the points to the end of the curve just before the frog area. From your photo, one can see that the curve starts later on the turnout(note the actual turnout is sharper as you get closer to the frog area).
Operation of trains with long wheelbase equipment at ‘track speed’(especially through a ‘cross-over’) will be limited with a #5 turnout. Another point is that the actual radius of the ‘curve’ in various #5 turnouts will vary as the lead from the points to the frog area may be different between different brands.
The radius numbers that you were referring to in the recommended practice are listed under the “CLOSURE DISTANCE” section of the standard. This is the radius of the closure rail only. Supposedly that is the fixed curve portion of the turnout. Again, supposedly, the radius of the moveable point rails (item 6 - Switch Radius) should be 43" in both #5 and #6 turnouts.
Neither of these numbers is the supposed overall “Substitution Radius” for the complete turnout including frog, etc. This radius is list somewhere else as a 56" radius for a #6 turnout and a 44" radius for a number 5 turnout.
So using all these numbers as a “grain of salt” as Jim said, what effect do these numbers have in application?
The substitution radius affects the behavior of the whole train - coupler swings, etc. If you replaced the turnout with a curve of the same substitution radius, this is the effect it would cause.
The length of the closure rail itself is only 3 to 4 inches long. This means that the effects of this radius come into play only for a single truck on a car or the flanges on a rigid wheelbase loco. If an individual truck has a limited swivel capability, for example, then this radius can be the limiting factor.
Really the big issue that gets overlooked or gets confused with turnout angle is the issue of “S” curves - and they’re everwhere. A crossover between tracks is a tight S curve. This is where bigger is definately better. Not really because of the specific radius of the turnout but because the straight track between the two frogs is longer. This relationship is shown on page 80 of John Armstrong’s Track Planning for Realistic Operation.
The bottom line is that you should use a larger frog number anywhere you have the space or if you have an S curve situation. In areas where you may need to conserve
