3 rail curve radius to 2 rail

I dont see the point of these calculations?  What is your ultimate goal or question?

Regards,

Ed

P.S.: 72 degrees corresponds to a 21" radius by my rather crude calculations.

Generally speaking, 2 railers refer to the curves by their radius and 3 railers by their diameter.    The radius is half the diameter.    Hence an O72 curve in 3 rail is 72 inches across.   It would be called a 36 inch radius in 2 rail.

Also, most scale modelers use the prototype designation for switch curvature or angle.    these are numbers referring to some degrees of departure from straight.   I think a #6 is about 9 degrees.    You see #4, #6, and #8 used most often.   #4 are considered very sharp in any scale and most often used for industrial.   #6 is quite common.   For some unknown reason, Atlas in their wisdom, departed from the 4-6-8-10 sizes and came up with #5 and #7.5 I think.    Prototype switches are numbers like 16-18-20 etc.  

Most scale switches are not curves, ie they do not replace a straitch and curved section of track.    The departing line sets an angle from the main, and goes straight through the frog.  

I think you might find an NMRA standards page that lists the angle of departure by the numbers on their website.

Thre you go.  The only other thing you need is measure to centerliine or inside rail.  I do it to inside rail, but do not know what the standard is.

54 inch radius is good.    You should be handle most stuff except big rigid wheelbase steamers.   That is without modifying anything.    You probably get about anything to run on that with some modification.

48 inch radius used to be a very common standard for 2 rail.    But with the new super detailed brass stuff, many modelers prefer much bigger curvers.   Most clubs use 72 inch radius on their main lines, but then most clubs have much more space than the typical home layout.

Any of you 2 rail persons running trains on 36 in rad.( extended oval layout) or do you try and stay away. I am thinking of incorporating 36" curves into the layout as I have a size restriction of wall to wall 132" and don't want to run my trains up tight against the walls to get  a more realistic return at the far ends of the layout. I guess I could try and hide as much as possible the tight turns to solve my problem but then my planned scenery would definitly have to change.  I like the scenery to be behind the rails and not in front and the large radius takes over 3 feet  from the length of the layout.

Phill

Why not build an around the walls layout?   I know that is not so common in 3-rail, but many modelers in all scales do that now instead of an island type.

Then you could use 48 (96 inches diameter) or 52 (104 inch diameter) inch radius and still have lots room behind the rails for scenery.

When you do that you can also use the walls for backdrop and do low relief buildings.   Buildings tend to take up a lot of real estate in O.   But a low relief building rear agains the wall might only stick out 2 inches.

Your decision can be based on the type of rolling stock you plan on and the realism you want.  Freight cars pulled by 0-6-0s can surely run on 30" radius or less, but a full scale Challenger pulling a string of 21" K-Liners will need 70" to look good.

I set up my passenger cars to be coupled fairly closely, and my SP Articulated coach needs 74" to avoid derailment because of end contact.

I refuse to run large steam without tailbeams and undersize or cut cylinders, therefore those too need at least 66" radius.

It is a hobby - you get to choose what is important to you.  What works in 3- rail will work in 2-rail.  That center rail has nothing to do with minimum radius.

As I mentioned above, an around the walls layout generally allows broader curves.   It also allows easiter access than a wide island style.   For example if you want 30 inch wide scenes on an island layout, the island has to be 60 inches wide to get a scene on each side.    Around the walls, each segment can be 30 inches and a much easier reach.

Getting back to equipment on curves, think about body mount couplers.   Body mount couplers much less likely to derail with smooth rolling trucks than talgo mount.    But they do require a broader curve especially with cars longer than 40 scale feet.    A 40 car can probably go around a 36 inch radius with body mount couplers, but a 60 or 80 car mostly like will not without modifcation to the mounting.

the number of a switch comes from the ratio of the track divergence after the frog.  if the siding track diverges one unit (inch, foot, etc)  for every four units after the frog, it is a #4 switch.  the degree of departure is the inverse tangent of the ratio.

inv tan 1/4 = inv tan 0.25 = 14.0° (#4 switch

inv tan 1/6 = inv tan 0.167 = 9.5° (#6 switch)

inv tan 1/8 = inv tan 0.125 = 7.1° (#8 switch)

etc...

Just to give folks an idea of what curves can be put on a layout, here's a simple drawing of a 12x12 room with 24" around the wall framework.  I've added 054 curves in each corner and overlaid them with 090 curves to show what can fit:

I wish I had done this prior to building my layout.  Although simple (done in a hurry just to show a point) a few switches added in and this could be turned into a nice little railroad and shows that larger radius curves CAN be fit into the same space.

Just to add to what has already been said 72" radius will pretty much handle ANY 2 rail loco out there. Sure, there might be an exception or two, but you can use it as a general guide.

The advice for going to an around the walls layout is excellent. Get away from that 4 x 8 sheet mentality!

As far as motive power goes, don't get too hung up on BIGGER is better. There are a lot of really nice smaller locos out there, and they can generally handle tighter radii better.

Good Luck!

Simon

We speak in two different languages.  I submit that on the 2- rail forum using radius to describe a curve is appropriate.  The three rail code is at best confusing.

As an example, Bob's 090 curves above sound like sweeping 90" radius curves, but they are only 45" radius, suitable only for realistic freight trains with small locomotives.

My suggestion - if you have only two rails, or an outside third, use radius.

Now, back to my unanswered question - do we measure radiis to track center or inside rail?

According to the NMRA it's the track center.

 d = 2r? ...confusing?

but to put your question into a better perspective, there is actually a third choice for describing curves which is degree of curvature.

for O gauge, degree of curvature = 2 arcsin (12.5/r).

from this you can see that while for small values of r (radius), the degree of curvature between the inside and outside rails is fairly significant, when you get up to typical 2 rail radii, the difference is negligible.

example...

for O27 track, the difference in degree of curvature between inside and outside rails is more than 24°.  at O54 the difference is down to 2.3°, but when you get up to a 72" radius in O gauge, the pair of rails only differs by 0.25°.

PRRjim, I apologise by not adding more detail to my post. My layout that I am building is an oval at 11 X 36' and I will have to duck under at the end to get inside the room. If I was to use the 36" rad. or 6' circles staggered up the wall I could avoid the ducking under.I have tried a removable section and don't like that either. I have a few C truck 2 rail engines,1 Atlas SD40 and 3 MTH 8-40 CW. But I also have a few Atlas 8-40Bs I could hide the 36in circle partially in a mt and multi tunnels openings like opening along a large hill. I think that would take the eye off the tight curve as one facinates over the train ducking in and out of tunnels.  It all depends on how well these engines can navigate that 36" rad. I know Atlas say 3 rails with the sliding frame can but worry how well a fixed truck will as some may be too rigid and jump off.

Phill