If you are going to have the space for a 30" radius curve, use a #8 turnout for sidings and #10 turnouts for crossovers
in the yards use #6
Keep in mind - unless as noted, you're fitting a turnout into a curve - there really isn't a connection between minimum radius and your turnout sizes.
Say your space allows for a 24" wide shelf layout along a long wall of the basement, but at each end there's just enough room to widen it out to 48" to allow for 22" radius curves that would allow you to do a dogbone (or water wings) continous run mainline. Just because you have 22" min. radius doesn't mean you'd have to just use no.4 or no.5 turnouts. You could use no.8 or no.10 turnouts on mainline crossovers and sidetracks, and maybe no. 6s for spur tracks. Your layout would look much more prototypical, especially if you can disguise / hide the tight reverse loops.
Conversely, of course, you could have a layout with very large mainline curves, but with an urban industrial switching area using no. 4 turnouts.
sectional track (set track) turnouts are designed so that the turnout can be substituted for a curved section. The diverging path of those turnouts has a constant radius throughout the turnout. For example, 18" to subsitute for an 18" radius set track piece. You'll want to stay away from those.
Most other turnouts are more prototypical. Their diverging routes do not substitute for any specific radius curve, and are made up of multiple short sections of track, some straight, some curved. Different commercial manufacturers use different designs for their turnouts, or as pointed out, even multiple different designs.
So the frog number is absolutely not a perfect proxy for 'what locomotives and rolling stock will operate through this turnout reliably'. It depends on the manufacturer's total design, not just the frog number they claim (which might not be exactly accurate either.
All that said, here is a table from NMRA RP 12.3 (I deleted some columns not needed for this discussion):
Turnout Number / Closure Radius
4 / 15”
5 / 26”
6 / 43”
7 / 49”
8 / 67”
10 / 117”
So if the manufacture follows the NMRA RP exactly then the above is what you'd end up with. Since they don't, you have to use it as an approximation.
But given that #5 is too small, and #6 is much larger than your minimum, going with #6 is your answer, and you won't have to worry about manufacturer's variations because you've got a lot of leeway between 30 and 43 inches.
(And as mentioned, the Peco S-M-L turnouts are a different animal, one that doesn't correspond to north american prototype turnout geometry, so just avoid those)
Thanks all that's very informative.
Be aware that the turnouts by PECO that are labeled "small, medium, or large" radius are exactly that. The diverging route is curved througout the turnout. If you make crossovers with these PECO turnouts, you will be abutting two opposing curves against each other, making an S curve with no intervening straight track.
Most modelers recommend having a section of straight track nearly as long as you longest car in order to prevent derailments from two cars curving in the opposite directions. PECO does not list these turnouts with frog#s, because the frog angle is not the important dimension in these trunouts. Its the radius of the diverging route that's relevant.
All other turnouts mentioned have straight sections immediately past the frog, so abutting two of these together makes for better crossovers. These kinds of turnouts are referred to by their frog#
Atlas Custom Line #6 turnouts have the gentlest diverging route of all other #6 turnouts, even gentler than their competing #6 Super Track turnout. (Code 83)
You will probably end up using different turnouts from different manufacturers if you have a lot of track and things going on with your layout plan.
- Douglas
Most #5 curved path turnouts, as opposed to those whose points are tangent, or close to tangent, will suffice for a minimum radius near 30". I don't believe a #4, even a true #4.5, would be sufficient. The NMRA publishes a compliance chart that shows the 'substitution' radius and the points radius of the various numbers of turnouts suitable for most rolling stock and wheelsets in the hobby.
"...Turnout dimensions The nominal size of a turnout is represented by a single number (#6, #8, etc), which is determined by the angle at which the rails cross at the frog. Figure 5 shows how that number relates to the frog angle. The largest possible closure radius for a given frog angle is a matter of geometry, as determined by the frog angle, switchpoint length, and frog length. Those dimensions in turn establish the turnout length as represented by the lead (Fig. 1). A smaller angle (larger number) means a larger closure radius and a longer turnout as shown in the following table. For a given frog angle, the lead length of prefabricated turnouts varies with the manufacturers and it is often shorter than necessary with a correspondingly smaller closure radius. The dimensions in the following table are for HO scale from NMRA RP 12.3. The lead length and closure radius were specifically determined for model railroad purposes and may not be directly scaled from prototype standard dimensions. Turnout Number, Lead length to Frog, Frog Angle, and Closure Radius:
#4-5.06” 14° 15’ 15”
#5-5.69” 11° 25’ 26”
#6-6 6.25” 9° 32’ 43”
..."
from: Kolm-TurnoutsWhatYouNeedtoKnow-PCR2008-handout.pdf (pcrnmra.org)
Ray, if you use Atlas #6 turnouts and 30" radius flex track, you can run just about anything on such track work. Go for it!
Rich
Alton Junction
Assuming the turnout is not a direct part of the large radius curves, the size of the turnout is not a factor (re the curves).
What is a factor is the usage of said turnouts. For the mains, I preferred #8s, for sub-mains and where longer (50ft plus) a #6 would work. And for tight industrial sidings where a 40 ft car was the max, a #4 would work just fine.
ENJOY !
Mobilman44
Living in southeast Texas, formerly modeling the "postwar" Santa Fe and Illinois Central
Hi All,
I'm designing a layout with 30' minimum curves.
Am I right in presuming that if I'm aiming for 30"minimum curves then there is no point using turnouts with smaller radius curves? SO I should not use any turnouts smaller than an atlas #6 turnout on mainlines and maybe #5 or #4 in yards?
or in PECO terms for the mainlines :
SL - 95 and 96 Medium 914mm radius and PECO SL86 curved turnout Radius: Outer: 1524mm Inner: 762mm
Ray