The curved leg of a standard railroad turnout is not of uniform radius. A short section through the frog is made straight to improve the tracking of the wheels at that critical point and so that the same frogs can be use in right and left hand turnouts.
Only "toy trains" have a curved frog. The switchpoints are not in a perfect curve because that would make them impractically long, slender and fragile. The straight section at the frog plus a short section in front of the frog is part of the "radius of substitution". There are two radii that are relevant in the construction of a turnout. One is the "radius of closure", that is, the curved rail between the switch points and the frog. This is a concern only for long rigid wheelbases. But even with a #4 frog, this radius is about 20 1/2 inches. For a #6 frog it is 58 inches and for a #8 frog it is 91 inches in S gauge. The "radius of substitution" includes the two straight sections and is of concern to cars with long distances between truck centres, and hence, the offset of couplers as these cars enter and leave the turnout. AS LONG as these two sections are STRAIGHT, the radius of substitution is relatively astronomical. Even for a #4 frog it is about 39 inches, for a #6 frog it is about 76 inches and for a #8 frog it is 150 inches. That is why I say a turnout is, in effect, an easement curve for model railroad radii. Even a #6 turnout joined to a 30 or 36 radius curve constitutes an easement. The point of intersection with the diverging route falls at a point between the switch point and the frog, just where depends on the frog number. AbBaird (Canadiannorthernhistorian) --- In [email protected], "Michael" <meldridge@...> wrote: > > It looks to me like a turnout has two easements: straight to curve to > straight. In any case, I don't think they help the problem at hand, because > unless it is a curved turnout, every leg enters it straight. If you need an > easement coming out of the turnout, I think you should draw a straight line > about a foot past the frog in both directions, and work as if that is where > the track is. > > -Michael Eldridge > > --- In [email protected], "canadiannorthernhistorian" <bairds_nest@> > wrote: > > > As an addition to my comments, a turnout constitutes an easement curve. > > However, the track for 2-3 inches beyond the points must be straight, as > > this is part of the easement. > > > ------------------------------------ Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/S-Scale/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/S-Scale/join (Yahoo! ID required) <*> To change settings via email: [email protected] [email protected] <*> To unsubscribe from this group, send an email to: [email protected] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
