# Track radius



## Terry Jackson (Jan 4, 2008)

What is the minimum radius I can get away with when running a USA SD70 engine? I am building a new layout and layout size is an issue.


----------



## chuck n (Jan 2, 2008)

I run mine on 10' diameter. There is a lot of overhang. It handles those curves, but doesn't look great.

Chuck


----------



## Treeman (Jan 6, 2008)

The bigger the better!


----------



## chuck n (Jan 2, 2008)

As Mike said bigger is better. What is the maximum radius you can use in your space? 

Chuck


----------



## NTCGRR (Jan 2, 2008)

So Terry, what did you decide? plus what switches are you using first? that will set the standard .


----------



## Peter Eaton (Mar 11, 2015)

Their ad says 8' diameter minimum. Peter


----------



## Treeman (Jan 6, 2008)

With the floppy leading and trailing axle they will run on an 8' diameter curve. They don't look very good doing it and some cars won't stay on the rail behind the loco.


----------



## Greg Elmassian (Jan 3, 2008)

What Mike said is important, while the loco will negotiate the 8' curve, it will normally derail the car it is pulling.

Get wider curves or a smaller loco, or you will be disappointed with the whole setup.

Greg


----------



## chuck n (Jan 2, 2008)

As I said earlier, I have 10' d curves. My SD70s will pull 40' freight cars, with truck mounted couplers, off the track when entering the curve from a straight track, no problems with body mounted couplers. They swing out more than the truck mounted couplers

Chuck

Here are some pictures of a USAT SD70 on an AristoCraft wide radius curve (5' r, 10' D). Note how far out the coupler is from the center of the track.










With body mounted coupler.










Entire engine on curve.


----------



## Jerry McColgan (Feb 8, 2008)

Radius or diameter of what? I don't think I have ever seen a really good description of what a manufacturer means when they refer to diameter or radius (I don't have a universal answer).

I would guess that when a radius or diameter is given they are referring to an imaginary line running in the center between the two rails. I don't think they could be referring to the radius or diameter of the inner rail or of the outer rail.

If I am fitting a track between brick walls (which I have had to do) The minimum diameter or radius would have to at least include up to the outer edges of the ties since they would have to physically fit. Even then, if the track will barely fit, most likely an 0-4-0 would not fit and certainly not the cowcatcher and back of the cab of a Mikado or Big Boy.
*

I think I can say without fear of contradiction that no brand of 4 foot diameter curves could make a circle that would fit on an 4' x 8' sheet of plywood - neither could one fit an 8' diameter circle on two sheets of plywood (8' X 8') - yet is that not exactly what someone might expect when they buy 4' or 8' diameter curves?*

*(It does not work with O gauge track either)*

In my opinion, with regard to this hobby, radius and diameter are relative terms to be used with caution because, as others have said, it all depends on whether the question applies to fitting within a space or to the ability of a locomotive and consist to negotiate the curve. The answer has to include the specifics of what is going to be run inside that space.

Throw in an adjacent siding and the question expands to what might be hit that is sitting on adjacent siding.

Throw in and elevated layout and the question expands to how much additional space your curve may need to give you a level of safety if something derails perhaps falling several feet to the ground.

Throw in a long coach on a sharp curve and the coach might be able to handle the curve but it might hit something inside the curve.

*Even the direction a locomotive is facing matters. An F-3AB will pull a boxcar around a sharper curve than an F–3ABA (or ABBA) because the rear coupler of that type of loco has a wider swing than the nose coupler.*

Diameter and radius?

Trick questions.

Marty asked an important question "what switches are you using first? that will set the standard." 

I once had to sell my Bachmann 10 Wheelers - not because they could not handle my LGB 4 foot diameter curves but because they could not handle my LGB 4 foot diameter switches (turnouts). 

Jerry


----------



## chuck n (Jan 2, 2008)

My understanding has always been that LGB's diameter/radius was measured using the centerline between the rails. I'm not sure about the others. If you have a confined space you really need to factor in the entire width of the track, not just the inside half.

Chuck


----------



## Totalwrecker (Feb 26, 2009)

All track is traditionally measured from the center line. Choosing between the inner and outer is moot with center as the choice.
Small scales use radius, I think Large scale chose Diameter because it sounded larger than radius does. 10'D is more impressive than 60"r, but they are the same.
John


----------



## Sjoc78 (Jan 25, 2014)

Typically the diameter or radius is measured to the center line of the track, thus an 8' diameter will overhang an 8' circle by about 3/4" Also those figures are not accurate as most track is actually designed and manufactured in metric so it's probably around 2400-2450mm diameter, I forget the exact number they use, I'm just doing the inch to metric conversion and rounding.

Also Chuck's photo's illustrate overhang really well; give yourself enough space for the loco to swing out wide of the curve. That will be mitigated for wider curves but often real railroads bent their rail and selected equipment that would run on a certain line based on the room they had. That's a big reason aside from cost that some railroads started as narrow gauge.


----------



## Terry Jackson (Jan 4, 2008)

On my old layout in Oklahoma my SD70 had problems on one curve derailing the first car behind the engine so that is the reason for wanting wider radius curves. 

Marty I still have the Aristo wide radius switches. Maybe I should sell some of them and buy some wider radius switches.


----------



## Greg Elmassian (Jan 3, 2008)

The Aristo wide radius switches are 10' diameter / 5' radius, they will be fine.

Of course "bigger" is better for operation.

Greg


----------



## Ted Doskaris (Oct 7, 2008)

On my two SD70s I body mounted Kadee 787 couplers in a swinging box arrangement in place of the factory couplers and was able to operate the loco pulling cars on 10 foot diameter track curves and negotiate my rail yard that includes an 8 foot diameter semicircle.

Shown below is the SD 70 loco coupled to a 40 foot car with Kadees on 8 foot diameter track circle.









If interested, see article, *USAT SD 70 MAC Experiences*, hosted for me on Greg E. Web site 

-Ted


----------



## chuck n (Jan 2, 2008)

Terry

What curves were you using? Were you using truck mounted or body mounted couplers? 

Chuck


----------



## NTCGRR (Jan 2, 2008)

When the #6 came out, I started a few cross overs at a time on main one. then on main 2 as I could afford it. Later the whole RR was #6 min and I sold the wide radius to others starting out. I used to keep any 3 axles on main line service only, until the upgrade was complete. Of course now I run 50ft radius min and don't care. LOL


----------



## Terry Jackson (Jan 4, 2008)

It was a very small radius. Not really sure what it was. I just formed the track to configure the corner. Body mounted couplers.


----------



## GaugeOneLines (Feb 23, 2008)

Terry,
You have to ask yourself if you want to be a scale model railroader or do you just want a train set in the garden. Advice given by others here is sound, the more space you have for a larger radius the better, trains on 5' radius curves look downright silly (unless it's 7/8" scale on 45mm) and toy-like. Me, my absolute minimum radius is 15' (30' dia) and 32" long coaches handle it nicely and look good.


----------



## chuck n (Jan 2, 2008)

Before you go much further, you need to know where you have been. Most makers of sectional track have their name and a part number molded into the underside of one or more ties. Turn your track over and see if there are any identifying marks. Armed with that information you can find out what curves you are presently using and help us In advising you on where to go.

Chuck


----------

