# Question about "S" curves



## Mattsn (Nov 22, 2015)

Basically do they work? I'm thinking of using one that would be a 23 inch radius going straight into an 18 inch radius with a straight section of about 6 inches separating the two curves. Before committing to this I want to make sure that this isn't a potential derailing problem.


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## wvgca (Jan 21, 2013)

a longer straight section would be better if possible ... ideally train length ... the tighter the radius and the longer the train, the greater tendency for cars to be pulled off and derailed


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## mesenteria (Oct 29, 2015)

There are three principal factors that govern whether an S-curve of a certain geometry will work: the length of the curve as a whole (depends on radius of curves and any tangent between them), the length of the cars' wheelbase, and the mounting and type of couplers.

S curves look good, and needn't be avoided if they are designed for current rolling stock AND possible new longer additions, including locomotives. In a perfect world, you would have a tangent segment between the two curves, and in a perfect world that tangent would be at least as long as the length of the cars between couplers. In fact, you can get by with less, provided the couplers are NOT body-mounted. When they are part of the swiveling trucks, they do better. Unfortunately, such arrangements usually mean a lot more derailments when shoving strings of cars, such as when backing, and it's usually on tighter radius curves.

If you will ever run longer passenger cars, auto-racks, and such, and if you might run a longer steamer that is not articulated, and has a centipede-type tender, your curves had best be about four inches wider than the minimum listed for the engine, and you should have at least 6" of tangent between the curves. Or, simply ensure that the inboard ends of the curves are heavily eased if they can't be truly tangential. In any event, your proof of concept will always be determined by the rolling stock, itself, in a carefully constructed and faithful mock-up. When the rolling stock works, so does the geometry.


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## Mattsn (Nov 22, 2015)

mesenteria said:


> There are three principal factors that govern whether an S-curve of a certain geometry will work: the length of the curve as a whole (depends on radius of curves and any tangent between them), the length of the cars' wheelbase, and the mounting and type of couplers.
> 
> S curves look good, and needn't be avoided if they are designed for current rolling stock AND possible new longer additions, including locomotives. In a perfect world, you would have a tangent segment between the two curves, and in a perfect world that tangent would be at least as long as the length of the cars between couplers. In fact, you can get by with less, provided the couplers are NOT body-mounted. When they are part of the swiveling trucks, they do better. Unfortunately, such arrangements usually mean a lot more derailments when shoving strings of cars, such as when backing, and it's usually on tighter radius curves.
> 
> If you will ever run longer passenger cars, auto-racks, and such, and if you might run a longer steamer that is not articulated, and has a centipede-type tender, your curves had best be about four inches wider than the minimum listed for the engine, and you should have at least 6" of tangent between the curves. Or, simply ensure that the inboard ends of the curves are heavily eased if they can't be truly tangential. In any event, your proof of concept will always be determined by the rolling stock, itself, in a carefully constructed and faithful mock-up. When the rolling stock works, so does the geometry.



OK that makes sense. As I said it would be about a 6" maybe more tangent between the curves. It would be a midsized or smaller diesel and it would older box cars, not long passenger cars


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