OK, we all know that bigger is better from looks of large engines and pulling capacity. I all so know the NMRA standards.
But I have never seen a rule of thumb of what % at bigger radius will give against string lining of the cars.
Lets say a 18 inch radius turn can handle say a 30 car train, if all the rolling stock is of same caliber would a 22 inch radius equal say 40 car train? That would be a 33% increases. Is there such a rule of thumb?
I am not a newbie and I know bigger turns will help the engines pull more rolling stock. All my cars are at or above NMRA standers and are free wheeling.
What spawned this question is I have 1 set of hidden 18 inch turns left. I just wondering if it would be worth the effort to replaces them. Around 35 fifty foot cars is about my max with reliability. 22 inches would be the biggest I would be able to install.
There are so many variables in any conceivable equasion, I really don’t think it could “standardized”. The weight of the car is obviously a critical factor, but so is any given car’s center of gravity. A string of properly weighted tank cars is going to stringline long before a string of flatcars weighing the same.
Start mixing in auto racks, high cube boxcars, etc. and the equasion would quickly change. The only way a distinct formula could be contrived, would need to be based on a complete train of identical type cars. For example - how many tank cars can you reliably pull around your 18 inch radius curve before they start to pull inward. Then do the same test on a 20 inch radius. Would the percentage be the same for a continuous string of flatcars ? Possibly with the percentage increase, even though you will probably run more cars in each of the two situations.
The trouble is with any mathematical equasion is the variables. Add a single hi cube boxcar in the middle of all those flats and your calculations have gone right out the window. Even more so depending on whether that hi cube is at the front or rear of the train. And we haven’t even begun to factor in grades ! …
So, in a nutshell - the bigger the better is the “rule”.
You not only have to consider radius, but superelevation. I have also heard of reverse banking to prevent stringlining.
I would think that speed would be a factor as well.
Consider coupler size and how far the knuckles rotate inside of each other. Also are all the couplers the same? if different is that a factor?
If all cars aren’t exactly the same, where are the heavy ones? long ones?
Are all the locomotives at the front? Some in mid-train? At the back?
I have never heard of a rule-of-thumb based on radius (or any other factor).
The biggest problem I see is that any such rule of thumb if it can be devised probably depends on using identical cars which most of us don’t do.
I would say if you like long trains, use the biggest curves you can fit in and have your sharpest curve easily accessible so you can get at it if something goes wrong
Yes, superelevation is a big consideration. You’ll have a string-line on a superelevated curve before you’d have one on a flat curve. Gee, I wonder how I know this?[B)]
I learned the hard way years ago that unless you have a toy train set, or you’re modeling narrow gauge, traction or early steam era, you have no business using 18" radius curves in HO scale.
Ken, there’s no magic or formula that I know of, but an engineer could help…I mean a mechanical engineer.
If your cars are weighted well, if the gauges are good, if they are free-wheeling (they’ll want to roll down a grade around 0.7% on your layout), and the couplers have enough swing for their boxes on those curves, that’s about all you can do. If you keep adding cars and keep contributing to the moment that the first car jumps up and out of the track, it’ll happen. Adding more grade or curvature are two examples that will have an impact as they get steeper and sharper respectively.
The LDSIG (Layout Design Special Interest Group) developed a rule of thumb relating radius to length of longest cars. The rule is fairly conservative. But it does allow pretty quick evaluation of the practicality of a given curve radius in light of planned operations (the intent of the rule of thumb). All the empirical and anecdotal evidence I have seen suggests that the 3X minimum is right on for long trains and operations with a helix.
Here are some curve radius guidelines based on the lengths of your longest pieces of rolling stock.
2X - Some model equipment may be able to track reliably on 2X their length, but this is generally considered pushing it.
3X - Making your curve radius at least 3X the length of your longest cars gets reliable tracking around curves, but looks toylike.
4X - If you make your curve radius at least 4X, your longest cars will look much better on curves.
5X - If you make your curve radius at least 5X, your longest cars will couple easily with minimal manual fiddling of the couplers.
To get a rule of thumb one would have to do the math and get the real answer first. Seems like the main variables w ould be the force through draw bar (couplers), generating a reverse tanget/cord force (the string line), vs the force of gravity holding the flanges into the rails. Those variables are dependent on the rail curvature, depth of the flanges, slop of the axles in the bearings, tilt factor on the, bolster, mass of the car(s), center of gravity on the car(s), length of the car. I believe speed only becomes a factor as it is a component of the force through the couplers. Then one gets into the complex variables as all the cars not being the same length/weight/center of gravity, etc. Hmm have to stew on that one for a while.
A similar post was asked about a year ago, last December; or, so. The poster was in the process of designing a first layout. I simply suggested that he use the largest radius he could and the largest turnout size he could on the mainline. For this I was taken to task by the so called layout design experts! I was at a loss to understand where these people were coming from and chalked it up to my being a relative new comer to the Model Railroader Forums and supposedly not having paid my dues, yet.
I probably still haven’t paid my dues yet, as far as those people are concerned. However, I’m going to stand by my original suggestion of the largest radius you can fit and biggest turnout size on the mainline that you can use as this make absolute sense, (at least to me).
IMHO any design decision is about finding a balance. A smaller radius might be your choice when you want e.g. a yard outside the customary oval most people want on their tabletop. The added length of the yard tracks might outweight the slightly smaller radius. However if you really want to run long cars and engines you might go for a larger radius.
The very same applies to turnout size. As usual every thing has its prize.
Thank for the answers. So there is no rule of thumb to go by.
More than likely not worth the effort to replaces the turns. It was only when I went to 35 cars that I started having problems with the 18 inch turns and not all of the time. 20 inch would be about the biggest I can install by the car shop where the turn is located.