At what radius does the "S" curve not matter??

The bigger the radius, the better, obviously.  

HOwever, the best rule of thumb is to put a straight section in between the two curves equivalent to the longest car you use.    Or get some sort of straight in there based on what will fit.    You can probably get by with a much smaller radius if you can get the straight section in.

Really depends on the type of equipment you are running. Long passenger or freight cars, large steam engines like a 4-12-2, scale couplers, 6 axle diesels in a multiple unit consist etc etc. Even a crossover can be a problem for certain equipment.   

The best answer would be to ask what you are running. If you are using tinplate and non scale, they were made with carpet layouts in mind and have no problem with S curves.

But since you are asking about large diameter curves, I assume you are working with scale trains, and as a result, large cars. Surprise, they will all negotiate S curves with no problem because they are engineered to do that.

BUT

If you want them to look right, which was the first reason to switch to scale size, you have to consider why the straight in an S curve is used.

If you have two long passenger cars with vestibules and connecting curtains, as they negotiate an S curve you will notice that the two openings swing Wildly past each other. If you have close coupling, they can actually connect and derail the cars.

So the answer basically is that the only purpose of the straight track in an S curve is to make the whole thing look right.

If you were running SCALE cars with end mounted couplers, story changes, but by then you would be using much wider curves anyway.

So the answer comes out that for what we do, the straight is for looks. The best you could do is at least as long as your longest car, If you are running passengers. If freight only, 10 inches keeps things looking good.

Thanks for the answers.

Jon

no matter the radius , there should be a straight piece between the opposite curves greater then the length of your longest piece of rolling stock or engine for best results..found this out on my s curve (on grade) when I got the acela set. Smallest curve piece was 081. After change it worked fine

With very long spiral easements, you might get there, but it would take up so much space there wouldn't be much point to it.

If you want to take out a boring length of straight or elevate and traverse across the layout, something like an  Atlas O-108 serpentine should be ok.

What say ye, straight in-between proponents?

I run on ACG T top track and have a couple s curves, even ACG switches together as crossovers, run through them at reduced speed and you will have little to no problems, if wheels are properly gauged.

Ray