The NMRA RP 12.x series is just that - Recommended Practice. Any given manufacturer will have their own adjustments and design geometry. The NMRA charts will lead to a very close approximation, but not likely an absolute value.
One can measure a given cord length and offset to determine the ACTUAL radius that's employed (at least through the cord measure points). Some turnouts are much more complex without a fixed curvature radius - but one can examine and determine the minimum radius. The 'formula' for the calculation is:
L = Length between curve points (base)
Z = Max Offset ( ~1/2 base to intersect)
r = effective radius of curve: r = (L^2)/(8Z) + Z/2
I don't have a ME On3 number 5 to make the measurements.
That's the way I read it. I used #4 and #6 Lambert turnouts on my first "serious" HO layout (about 35 years ago) and it was a big mistake. We all started out in my gang with the Atlas snap switches but they are actually about #4 and a half with an 18" minimum radius! Thus, my rolling stock that would not negotiate a 15" radius would not work. I was working with a graduate student budget and it was really tough to take as the Lamberts (Shinoharra) took my spending money for a long time. Anyway, all these years later I'm seriously looking at On30 for all the arthritus, eyesite and space reasons familiar to all of us older ones.
Thanks to all of you guys for the inspiration. You have a really good thing going on this forum and if I can help even a little I'll try.
i have a ME on30 #5. The "L" from the point to the frog I measured to be 5-7/8" and the curvature offset of the rail at mid-point to be 9/64" which calulates an R=29.005"
also, this web site calculator gave me a different number of R=30.88
The point of the frog is likely on a straight section. Curve calculations need to be on the actual fixed curve. The straight part at the toe of the frog will lead to skewed results. You would also likely have to measure with a caliper to get the degree of precision for accuracy necessary.
Turnout geometry is not a pure curve from frog point to the switch blade heal. There are straight sections at the closure rail just beyond the switch blades and also at the entry to the frog. Most prototype trunouts will have straight switch blades - but many model configurations use curved (or bending) switch blades where the curve is less than the closure rail radius. Pick a convenient base (L) area in the closure rail curve, and precisely measure the offset at the mid point.
i definitely see what you mean. I will pick up a caliper today.
The cast frog rails are straight. The closure rails appears straight. The fixed rail from the frog to the closure rail is curved and seems to have the same radius as the fixed "inside" rail, which seems to have a uniform curve from a point beyond it's closure rail to a point tangent to the frog. i will scan the turnout and post it later today. Also, note in the scan that the "main line" straight rail in the "un-installed" turnout is NOT straight. So...based on that, any calculations from this turnout will be off.
The closure rail is the curved rail. It goes from the frog toe (or throat) to the switch blades (which are or should be straight).
If the tangent (main rails) are not straight - then you have effectively a wye turnout - albeit one side may be very slight. The radius calculations are still effective on the curved portions - provided it's measured and calculated from points that are on the continuious curve.
The ME On30 turnouts are very well done and very accurate to specifications (for HO). Flangeway clearances and the check gauge are reported to be very accurate. You only really need to ascertain that the minimum radius is adequate for your equipment.
This is a #5 RH turnout. I agree, ME make excellent TOs. I modeled N-gauge for years and used mostly ME turnouts and was very impressed with their QC. Of the 4 On30 turnouts i have, they all have a slight curvature, due to manufacturing, but should straighten out when install.
I just bought a caliper on e-bay, so will measure the Turnout when it comes. Should i take measurement off the inside curved rail or the curved rail coming off the frog toe.
Jim
PS I tryed to attached and/or copy in a jpg scan of the turnout, but it doesn't show up. Can you help me in how i can attach a photo.
Measure the curved closure rail from the frog toe (somewhat inside and ahead to be on the curve) to the switch blade point heal (again a little more towards the curve part).
The base (L) should be sighted and made at a nominal fixed even length on the curve (makes the calculation a little easier).
Mark the points - use a straight edge and measure the widest offset (Z) point from the straight edge to the rail curvature (should be approximately at the mid-point of the (L) base line).
Run the calculation and SUBTRACT 0.325" to arrive at the center line - this is the MINIMUM radius of the turnout diverging route. (Radius specifications are for center line of the track).
For the number 5 on 0.649" gauge - expect this to be somewhere more than 24" and less than 29". Note: the gauge will likely be somewhat larger than the nominal 0.649" - full accuracy would measure then subtract one-half the actual measured gauge (instead of using the 0.325") from the closure radius.
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