I did run a search for this, but nothing came up with what I am curious about.  With locos that have a curve limit to them, is that an absolute?  If for example, you have one that says 72 inch radius, if you use trackage with the cubic spiral design of a real railroad  with decreasing radius, say starting out with 072 and going to 054 in the middle, will the loco handle it but just look bad doing it?  This is for a loco that is not a rigid frame, and also has blind drivers.

Original Post

Yes, in many cases it is an absolute failure and the wheels will derail on a tighter curve.

In some cases it's couplers and tenders or other things that hit

Some can do a tighter curve than advertised, but there is also just as much that when they say 0-72 minimum, that's the true minimum.

In fact, I can tell you from personal accounts, my own home layout is 0-72 on the outside loop running Atlas track. I have seen one of my curves where I might have ever so slightly compressed the curve and that was enough to cause issues with a few 0-72 required engines and tenders. the tender would ride up and you could see it trying to derail at slow speed or even just pushed by hand. I had to take all my screws out of the track, run that tender around to find the desired actual smooth radius, and then lock down the track. When they say 0-72, sometimes they mean it.

Last edited by Vernon Barry
@CALNNC posted:

I did run a search for this, but nothing came up with what I am curious about.  With locos that have a curve limit to them, is that an absolute?

Pretty much yes.

If for example, you have one that says 72 inch radius,

Well, in O-3 Rail, that "72 inch radius" would actually be 0144 curves.

if you use trackage with the cubic spiral design of a real railroad  with decreasing radius, say starting out with 072 and going to 054 in the middle, will the loco handle it but just look bad doing it?

Most likely NO!

This is for a loco that is not a rigid frame, and also has blind drivers.

I can not imagine a steam locomotive model without a "rigid frame" that requires 072 minimum curves.

@CALNNC posted:

I did run a search for this, but nothing came up with what I am curious about.  With locos that have a curve limit to them, is that an absolute?  If for example, you have one that says 72 inch radius, if you use trackage with the cubic spiral design of a real railroad  with decreasing radius, say starting out with 072 and going to 054 in the middle, will the loco handle it but just look bad doing it?  This is for a loco that is not a rigid frame, and also has blind drivers.

If you make easements and compound curves it can work.  For example going from 072 to 048 to two 036 and back out the same way on the other end instead of a curve made up of all the same.  Varying lengths, of course.  Or using flex track.  Going into switches putting a straight before the switch does wonders to "straighten out" the loco as it enters so it is less likely to derail.

Ken Hoganson's book, 21 Great Track Plans for Compact Layouts does a good job explaining this, but I am sure there are others as well.

John

Last edited by Craftech

I'm surprised that you didn't find anything in your search, as this question comes up almost every month.  This conversation is often confused by diameter vs radius.  Toy train track is designated by diameter,  Hence O72 is 72 inches in diameter.

The short answer is no, but certain models will work on smaller diameter than designated.  This may be influenced by the type of track, the quality of the track work and even the weight and nature of the consist.  Switches are an issue.  To me not worth investing in engines that require more diameter than you have on your layout.

You guys trying to cram locomotives that are not designed for tighter curves crack me up!……there’s reasons they put a minimum curve rating on the box. Vernon touched upon the most obvious. Add tender draw bar swing, tether swing, locomotives kinking against the tender, the list goes on & on, ……and then the same guys are on here wondering why this is broke, or that’s broke. Chances are the manufacturer tested the product and determined the best curves to run the thing on reliably. If you want to push the envelope, then be my guest, …..I have yet to see on a box: O54 MINIMUM CURVE, BUT MAYBE YOU COULD CRAM IT THROUGH O36…..

Pat

@harmonyards posted:

You guys trying to cram locomotives that are not designed for tighter curves crack me up!……there’s reasons they put a minimum curve rating on the box. Vernon touched upon the most obvious. Add tender draw bar swing, tether swing, locomotives kinking against the tender, the list goes on & on, ……and then the same guys are on here wondering why this is broke, or that’s broke. Chances are the manufacturer tested the product and determined the best curves to run the thing on reliably. If you want to push the envelope, then be my guest, …..I have yet to see on a box: O54 MINIMUM CURVE, BUT MAYBE YOU COULD CRAM IT THROUGH O36…..

Pat

Yes. The minimum "radius" (actually diameter in 3RO measurements) is just that - geometrey, not a suggestion or opinion. Some locos can be cheated through smaller-than-spec curves, but that is because the companies want to be conservative and not disappoint. Getting one of these designs (the "cram-it-through" model) is purely the luck of the draw. I wouldn't put money on it.

If you’re building a layout or thinking of purchasing a large engine I would go with the minimum curve recommended by the manufacturer. Believe it or not they do test these engines. Don’t get disappointed.

@harmonyards posted:

I have yet to see on a box: O54 MINIMUM CURVE, BUT MAYBE YOU COULD CRAM IT THROUGH O36…..

I checked all my boxes, it's not on mine either!

We sometimes forget that real railroad have minimum curve radius problems also.  A lot of real locomotives could only run on certain divisions of their railroads.  The UP had to enlarge curves and clearances to run the Big Boy.

From a modeling perspective, trying to run a large locomotive or scale passenger cars through small radius curves looks un-prototypical.  The overhang is large and the model may hit nearby scenery.  Even O72 is sharp when running scale length equipment.  NH Joe

The minimum listed is what is needed without modification to the unit.

My 773 will run O31, as advertised, but it is tight.  You have to be careful how much wear you are causing also.   The key on a helix and the like is will it do it 100% reliably.

My shay minimum curve O42 listed is absolute.  Binding the gears on the drive train is not good.   On my S1 it is the couplers, they are too short for the smaller curve.  Putting a longer coupler on worked, but looks funny.

Real world example.  Went to Virginia City many years ago.  Their tourist train was running with a diesel.  They had a tunnel in a small hill collapse.  They decided to run the track around the hill instead of fixing the tunnel.   It was a sharper curve as a result.   The steam loco ran through the curve, but soon caused axle bearing damage.  We had a nice engineering conversation as we rode the train.

This is going to come across as heresy.

My memory goes back to when the common 3 rail choice was between O-27, O-31, O-42 and O-72.  In those days if something was rated O-42 it meant it did not work on O-31.  It did not necessarily mean it would not reliably operate on Super-O's 36" curves.  The manufacturer just did not test at that point.  I suspect in those days items rated O-72 would operate reliably on curves somewhat tighter than O-72.  The manufacturer just didn't test at points between 42 and 72.

I suspect that same principal applies today, but manufacturers are testing at different points.   We see items rated O-54 often enough that I am confident most manufacturers test at that point.  If I see something made today rated at O-72 I would not assume it could reliably operate on O-54.  I don't recall seeing anything rated O-63, so I don't know if they specifically test at that point.  Could something rated O-72 reliably work on O-63?  Maybe...or maybe not.  It comes down to whether you want to spend \$2-3k on a loco to find out.

I would like to point out to also check the catalog or manufacturer website for minimum curve as sometimes its not correct on the box.

Example, the Lionel PRR 2-10-0 I1. I have the short tender variation. It's advertised in the catalog as "Short Tender O54" "Long Haul Tender O72".  On the box it says O72 Minimum Curve, which is incorrect for the version I bought.

The Lionel PRR 2-8-2 L1 is advertised in the catalog as having a minimum curve of O42. The box says O54 minimum. O42 is the correct minimum curve.

This only seems to be a Lionel problem. They put a generic sticker on the box for whatever version is the longest, not necessarily the version that's inside the box.

Last edited by Stinky1

CALNNC, I know you're asking about the locomotive, but, while concentrating on the locomotive successfully making it around the curve, it's easy to overlook your purpose for owning the locomotive.  Others in earlier posts in this thread have pointed out that overhang at the front and rear of diesels (and additionally between the engine and the tender of steam engines) are neither implied nor stated on the box by the manufacturer.  And then there are the really long engines, which also have "middle overhang".

Unless you want to run nothing but light engines here's what you really need to know:  Will the locomotive, coupled to cars, successfully negotiate an O-whatever curve?  If the locomotive has rear overhang, it can exert enough lateral force to derail a long car coupled to it (sometimes any car coupled to it).  Also, the manufacturer's stated minimum radius does not apply to reverse curves of that radius having no intervening straight track (such as in a turnout to a parallel track).  Those can be tricky, usually proportionate to the length of the engines and cars.

As one who has, more than once, stepped over a \$10 bill to pick up a dime, I'm hoping to help you avoid doing the same, by offering an expanded view for thoughtful consideration.  Best of luck.

Last edited by Number 90

Sometimes the Lionel Web Site has it wrong. I picked up a used "Uboat" (U36) to run on my O31 curves because the Lionel web site listed it as "Minimum Cure: O31". It was actually larger and I had an engine that did not work at all on my layout.

@Larry.PRR, if your engine is indeed the U36B from the MPC era to this date (the 14-inch length model), it will not only run on 031 curves but 027 as well. Lionel has made some other U boat variations like the U30C and the U28C that are scale length, although Lionel advertises these engines as being able to make a 031 curve.

Again if your loco is one of the what is now called "traditional" 36B's, could be the wires going through the sheet metal frame to the trucks are too tight - not having enough play in them to allow the trucks to swing to and fro easily. Loosening the slack in those wires can make the difference.

There's one more issue: Tubular track has a bit more "flexibility" than does FasTrack. Sometimes when folks are building layouts, they'll pull in a little bit on curved diameters to make everything fit and join together. One would think logically, that shouldn't make a difference. But with some locos and rolling stock it can.

Regardless of what type of product from Lionel: Starter variety or high-end, locomotives always look better and likely run better on larger curves. Lionel advertises minimum curves because not everyone has a stadium sized layout. The tighter a radius a locomotive can negotiate can drastically increase its' sales potential.

Many of the starter set locos in Lionel's recent catalogs are listed as 031 minimum curve. In that Lionel no longer makes 027 tubular track, one could say that would be the reason. Except that some locos and a lot of the traditional rolling stock are STILL listed in catalogs as 027 minimum curve. So the argument that Lionel no longer makes 027 track doesn't hold much weight given that.

Many of the locos that currently come in Lionel starter sets have been around for years and were included in starter sets that came with 027 tubular track before Lionel developed FasTrack. Even the somewhat more recently tooled FT diesel came in starter sets with 027 track. And some of the engines that Lionel got tooling from MTH, like the Dash-8, that is now LionChief/LionChief+ will also run on 027 curves.

Didn't mean to start a fire storm, thought that some real world experience may have lead to observations that a  loco rated for a certain curvature might not always be true.  A couple did note that to be the case.  Many times things today are carrying specs to assure 100% no issue function so somebody won't sue the manufacturer if they fail to perform on things like tighter curves.  A curve with an easement going into a place that is not visible, making things look good when you can see it, but when behind scenery or under something, where you find out you need to tighten  a curve a bit to make it fit or you will have to redesign the whole thing, sometimes works.  When I was laying out a double track arch bridge thinking I only had to worry about the pilot on a Big Boy, proved the pilot made it just fine, but the back of the cab did not when exiting the bridge that had a 72 in radius curve involved.  The right of way crew had to come out to do some blasting to straighten out the track.

Last edited by CALNNC
@CALNNC posted:

Didn't mean to start a fire storm, thought that some real world experience may have lead to observations that a  loco rated for a certain curvature might not always be true.  A couple did note that to be the case.  Many times things today are carrying specs to assure 100% no issue function so somebody won't sue the manufacturer if they fail to perform on things like tighter curves.  A curve with an easement going into a place that is not visible, making things look good when you can see it, but when behind scenery or under something, where you find out you need to tighten  a curve a bit to make it fit or you will have to redesign the whole thing, sometimes works.  When I was laying out a double track arch bridge thinking I only had to worry about the pilot on a Big Boy, proved the pilot made it just fine, but the back of the cab did not when exiting the bridge that had a 72 in radius curve involved.

Again, a "72 in radius curve" is actually 0-144 (diameter) in 3-Rail terms.

The right of way crew had to come out to do some blasting to straighten out the track.

72" radius, 144" diameter, all the same and '72' saves a keystroke.  Haven't seen a loco box that said 144" diameter minimum yet.

@CALNNC posted:

72" radius, 144" diameter, all the same and '72' saves a keystroke.

Except, you didn't type 72,you typed 72", which means something quite different in the model railroad world.

Haven't seen a loco box that said 144" diameter minimum yet.

No, but how about "072 minimum"? Please remember, in the 3-Rail "toy train" world, track curve demensions are always given in DIAMETER measurements. On the other hand, 2-Rail O Scale, HO Scale, and N Scale all use radius dimensions for curves.