How is minimum radius determined?

[dbaker48]

I'm sure this seems like a dumb question, but what I am trying to figure out is how to determine what is the minimum radius a engine would require.
For example: I have a Virginian FM, and what is the minimum radius track I can run it on without causing a short or derail. 27" - NO, been their done that,
31, 40, 49, 54, 60, 72 ?

Don't have all those tracks to try to find the minimum. Also I don't have the marketing information, catalog, or operating guides which sometime will say.

Is it based upon the wheel base of the truck? If so any idea where can find those dimensions.

Of course their are other factors involved also, physical constraints how and where the truck is mounted, maybe even the number of axles,

Any ideas?

[Big_Boy_4005]

There are only a few postwar pieces that can't do 027. The main issue is the switches. Things like the trainmaster have enough overhang to hit the motor housing.

Modern pieces are a little trickier. Most will do 031, unless they are considered "scale" and are more than about 16" in length. Those cars should have their minimum curves noted on their boxes. An example would be the Atlas articulated auto rack. Minimum curve 054, if I'm not mistaken.

[dbaker48]

Elliot,
You are getting directly to the issue I am trying to resolve. I put my FM on 31" fast track oval, no switch and would derail very easy and short.

Now, I really admire your layout but don't have the space, I am putting in two 72' radius reversing loops, but trying to determine if I can use 31" on the balance of the layout or should plan on 54 or 60. Unfortunatly I do not have the information on some of the modern stuff, (Fundamensions). Or know where to find it. So thir must be a formula or critical dimension. Wheelbase, # of axles, length of engine or car, somethin?

[Big_Boy_4005]

Actually Don, isn't the minimum diameter (not radius technically, but I know what you mean) for Fastrack 36". Seems odd that it would derail. Have you inspected the trucks, making sure the wheels and rollers are OK. The trainmaster has those blind wheels on the inner axle, if I remember correctly.

As for Fundimensions, all of the pieces made during era should be 100% 031 compatable. They didn't introduce many new designs during their 15 year run, and the ones they did, were engineered to work on the standard tight curves. That was long before Fastrack though.

Are you going to be using Fastrack on the whole layout or are you going with tubular, or a mix? It is possible that it's the Fastrack design itself and not the curve that is giving you trouble, but I haven't heard that complaint before.

[lionelsoni]

The Trainmaster illustrates all three of the kinds of obstacles that keep larger pieces from running on O27:

The body hits the O27 switch machines. This can be fixed by moving the truck center bearings inboard; but that is not practical for the Trainmaster. My solution was to remove the fuel tank, so that the middle of the body passes over the switch machines. I have thought about putting a narrower tank on, perhaps just a sort of black keel to close the open space, but have not done it.

The wheels or pickups touch the wrong rail, causing a short circuit. The Trainmaster's pickup is located between the inboard blind drivers, which maximizes its sideward motion on curves. I simply reversed it, using the same mounting hole, to put it between the flanged drivers.

The trucks won't turn far enough. The Trainmaster's trucks are mounted through large holes in the floor. A key on the motor casting keeps them from turning completely around. I simply enlarged the part of the hole that limits the key travel. I also trimmed a little off the motor field-coil bobbins that seemed to be hitting the inside of the shell.

[fwright]

Without modification, virtually everything (except the "scale" Hudson) post war or Fundimensions (MPC) era will run on O31. Most can be modified, as Bob explains, to run on O27. Atlas advertises O36 for all their rolling stock, but most of the shorter items (16" or less as Bob stated) will actually go smaller. MTH Rail King is all supposed to run on O31, and again, the smaller items will go to O27. MTH Premier varies widely, but is normally stated in their catalog/advertising. Same with the expensive, modern Lionel stuff - minimum radius is in the catalog. Williams also states their minimum radius in the catalogs, older advertising, and web site.

So the answer for your purposes (same as other scales/gauges) depends on the equipment you plan to run or acquire. If you stick with Lionel postwar, MPC, Williams, K-Line, and Rail King - an O31 turnaround should be safe. O36 is a little better, and O42 will take all but the big scale locomotives and full scale passenger cars.

Most of the manufacturers aimed their production that would/could be in train sets(except for high end full scale such as MTH Premier) at their favored track system. Since Williams didn't make track, they went with the prevailing O27 and O31 of the time. Atlas, although making O27 curves, standardized on their O36 track. MTH Realtrax (Rail King sets) is built around O31. K-Line built around O31 and O27, except for their heavyweight passenger cars (O42). Lionel, until the advent of Fastrack, was built around O31 and O27. So while there is no magic formula, knowing what era and manufacturer the tooling is from can determine what track the model was designed for.

Hope this helps

[lionelsoni]

The prewar 700E needed O72 scale rail. The prewar 763 also needed O72 but would tolerate tubular rail. But the postwar 773 Hudsons and their descendants will run on O31 tubular--and can be modified for O27.

[brianel027]

Let's add one more little factor into this thinking. It's the little dreaded phrase "027" which sends either chills of fear or mostly anger down the spines of many scale operators, and even officers the various train companies. I had one CEO personally tell me that their company was intentionally shying away from the use of the 027 phrase for fear of angering the scale audience.

Some of the train companies have tried to steer clear of the "027" phrase for no other reason than trying to please the higher end of their customer base. There are guys who just consider anything that runs on 027 track a "toy" whether it actually is or isn't.

You can see this in some of the newer Lionel catalogs. Although the standard sheet metal framed 14 inch GP-9, GP-20, GP-38-2 and the U36B will ALL negotiate and clear 027 curves and switch housing, there are catalogs that list these locos with a minimum curve as 031. It's even more of a hoot in the catalogs that show the same loco in an 027 track included train set, and then as a separate sale item it's listed with a minimum 031 curve. Either way it's a case of poor communication, a lack of direction (and maximizing sales) or just sheer idiots proofing the final catalog copy.

Now for more confusion, MTH makes no 027 track. So before something is listed as clearing 027 curves and switches it has to be tested first. No doubt the sales department at MTH would like to see everything in line with their own product line, meaning that everything would be listed as an 031 minimum. Although some MTH Railking cars are listed with 027 minimums, some are not. There are cars, such as the Railking operating coal dump car and the Railking PS-2 hopper that absolutely will clear MPC-era Lionel 027 switch housing and make the tight curves, yet are listed as 031 minimum.

In the past when I was able to buy more trains, I found the best solution was to find a dealer who was willing to allow me to test the car or loco on his layout before purchasing. Of course, this means you have to be serious and committed to purchasing the car from that dealer. He'll wise up pretty fast if you are only using him as a test lab so you can go mail order the item elsewhere.

It's so ironic that there are far more 027 operators out there than anyone wants to acknowledge. They may not buy as many trains, but they still want to buy trains. So it makes no sense to me that any company would have a business plan that aims their product line at the smallest, narrowest segment of their buying audience. Which is why companies like Williams and RMT may not get all the raving reviews and attention, but will survive because that make products for the clear majority of train operators.

[dbaker48]

Thanks for your comments and recommendations. Again the input here is outstanding. (Another post to print and keep a hardcopy of)

Thanks again

Don