# Track Layout Questions on Radius



## BalsaBob (Oct 21, 2013)

Greetings All, I hope to start my layout soon and have a few questions about the turn/curve radius of the track. I think I need to know the answers to these questions before I can begin to start designing the track plan.

1. What should the minimum radius be for the curves .... 6 ft diameter ? 8 feet diameter ? I plan on running primarily diesel locos (none of the long old time steam engines).

2. What should the minimum diameter be to allow the track to cross over or under itself (like in a climbing circle) ? 

Thanks. Bob


----------



## chuck n (Jan 2, 2008)

I'll offer an answer on 1, use the largest diameter, radius, that will fit in the space. You will never regret it. A grade of 2 or 3% is a reasonable limit. Too steep a grade will add to performance problems and lead to excessive gear wear. You will need a rise of about 10". This will permit most engines and cars to pass under the bridge. You will need to do the calculation. Chuck


----------



## BigRedOne (Dec 13, 2012)

Welcome! 

Just like the prototype, larger radius and slighter grade is best. 

Large, modern diesels probably warrant 10 foot or more - depends on what engine and cars you wish to run, and whether you care about appearance in addition to function. Curves add drag, so they also limit your train length. 

Minimum diameter of a climbing loop is really limited by the resulting gradient. The shallower the gradient, the more train you can pull and the more realistic the appearance.


----------



## BalsaBob (Oct 21, 2013)

Thanks Chuck. I can actually make the diameter(s) as big as 12 feet. But, I would rather make them smaller if I can ... since the larger I make the diameters, the longer the walls have to be, and the more fill I will need (I plan to elevate the layout about 2 1/2 feet high). 

Bob


----------



## chuck n (Jan 2, 2008)

My layout has 10' diameter curves. Some of my longer engines and cars look a little funny on the curves, but they run. Given the choice I would have gone larger, but in 1993 larger sectional curves weren't available. I would have had to bend my own. Chuck


----------



## SD90WLMT (Feb 16, 2010)

Well Bob...make a shopping list of your locos of choice FIRST.... 
Then decide how long trains will be.... 
Be practical here... let these make the choices to dictate curves.. 

If it is larger it may take a couple extra days to fill. 
If your design causes issues we warned you about, you'll wind up being stuck, and not as happy as you "hoped to be".... 

Here I'll probably get laughed at..... Ha! There ...I beat you guys to it.. 
Yesterday..... I laid out a new curve.. Yes.. 
It has a 75' Radius/ 150' Diameter..... 

.... Bob .... good luck.. keep asking ?'s here!! 

Dirk


----------



## SD90WLMT (Feb 16, 2010)

Even using a 12 ft diameter. The grade will fall between 2.5 to 3% { quick math } if you build a basic figure 8. 

Dirk


----------



## chuck n (Jan 2, 2008)

As a guide, I run 1:20.3, 1:22.5/24, and 1:29. All but the 1:29 look reasonable on the 10' diameter curves (reasonable, but not great). The overhang with the 1:29 is much more noticeable. Freight trains with F3s and 40' cars aren't a visual shock, but passenger trains with USAt streamliners and USAt GG1 or SD70 Macs have a lot of over hang. The added two feet from 10 to 12 would make a big difference, in my opinion. 


Chuck

PS, modern diesels tend to look more out of place on 10' diameter curves than the older longer steamers. I do not notice it nearly as much with my Aristo Mallet than I to with the GG1 and the SD 70 Macs.


Here is the overhang of a SD70 Mac (USAt) on a 10' diameter curve.


----------



## vsmith (Jan 2, 2008)

It depends on what dismals your planning. A GP9 will work better on a smaller diameter than a SD70. My personal preferences are to use smaller diameter track so I can add spurs and sidings etc. But I would agree with the suggestions to decide NOW what your biggest planned (or wished) acquisition (loco and cars) will be and then build around that.


----------



## BigRedOne (Dec 13, 2012)

Posted By vsmith on 23 Oct 2013 07:13 PM 
dismals 

Yep, that they are.

Go with steam!


----------



## BalsaBob (Oct 21, 2013)

Actually, making a logging and/or mining layout was another of my desires. My overall layout space is approx. 12' X 30' and I will be limited by 10' diameter curves. The smaller steam engines of this era can tolerate smaller track radius and steeper grades .... correct ? 

Bob


----------



## chuck n (Jan 2, 2008)

It is not just the smaller stream engines that run on 10' diameter curves. I run my Aristo Mallet on 10' diameter curves without any problems. I'm not sure how it would do on 8' diameter curves, Any engines suitable for a mining/logging rr would work. Four and five foot diameter curves can be a potential problem for all but the smallest engines, 0-4-0. Chuck


----------



## bicyclexc (Mar 31, 2010)

This year I built my layout using mostly 10' diameter curves, and 8' diameter on the reversing loops because that's all the space I had. I'm now already daydreaming about the next incarnation of this layout because I want to up the curves to something even bigger, 15-20' diameter. I'm finding its difficult running body mounted couplers around 8' diameter curves, especially on longer diesels like mentioned above by other folks. They say 10' diameter to scale represents just about the tightest curve possibly found on a real railroad, (loading platforms, wye leads yard tracks etc) so it can BARELY be called a prototypical curve, definitely not a main line curve. 

Go with as much as you can get away with . Like I said, in some places I could only get 8'.


----------



## BalsaBob (Oct 21, 2013)

Another question .... What is the minimum difference (in diameter) in curves, to run one inside the other ... 2 feet ? 1.5 feet ? If I plan have a 10 foot diameter curve for the inside track, can the outside track have an 11.5 foot diameter ... or is a 12 foot diameter needed ? Thanks again. Bob


----------



## BigRedOne (Dec 13, 2012)

You're probably safe with 1.5 feet difference in diameter, which gives you 9 inches center-to-center. 

Again, this is dictated by your equipment - though you may also wish to give an allowance for equipment you don't yet know you want, or the possibility of hosting others at your railway. 

I'd err on the side of more clearance, for things like inconsistency of bending, track not flat or shifting, rocking motion of some rolling stock, etc.


----------



## Gary Armitstead (Jan 2, 2008)

Posted By BalsaBob on 27 Oct 2013 07:24 AM 
Another question .... What is the minimum difference (in diameter) in curves, to run one inside the other ... 2 feet ? 1.5 feet ? If I plan have a 10 foot diameter curve for the inside track, can the outside track have an 11.5 foot diameter ... or is a 12 foot diameter needed ? Thanks again. Bob Bob,

Check out this site......very useful information regarding your questions. http://www.urbaneagle.com/data/RRstddims.html

Remember, clearance for cars and locomotives on curves, depends a great deal on the LENGTH of said rolling stock and locos. AND rather than being hampered by the manufacturers SET diameters and radii, think about getting a "rail bender", bend your rail and track and make your curves custom fit to your layout.


----------

