# Minimum radius



## Gramps (Feb 28, 2016)

I'm looking to expand my shelf layout by continuing it along a second wall. This involves installing a 90 degree turn. My longest unit is an 85' Budd RDC. I know that the rule of thumb is to go with the largest radius but space is tight. I can live with the overhang on the curve but what is the smallest radius I can get away with without derailments?:dunno:


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## Chet (Aug 15, 2014)

It could depend on how well the RDC tracks. My tightest raduis is 32" with the majority in the high 40's, and have no problems pulling a passenger train of Walther 85' cars around the layout. That also look good. I imagine that you could get away with 24". If you have any flex track, you can test how tight a curve the RDC requires. 

I do not use sectional track at all. I started my layout hand laying code 70 track and turnouts and then started using Shinohara turnouts and flex track. Back in the mid 70's when I was stuck in an apartment I buld a small N scale layout that could be slid under a bed and the guys at the local hobby shop turned me onto flex track and haven't touched sectional track since.


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## Old_Hobo (Feb 20, 2014)

I would say that a 22" radius (or larger, if possible) should give you trouble-free operation.....


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## LateStarter (Mar 12, 2016)

85-footers?
If couplers are set back to allow diaphragm contact, 22" radius will likely be a problem.
But if not, it might work, but might still be dicey. My MTH sleepers will accommodate 22", but only barely, with the couplers set outward... and they're 80-footers.


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## Gramps (Feb 28, 2016)

LateStarter said:


> 85-footers?
> If couplers are set back to allow diaphragm contact, 22" radius will likely be a problem.
> But if not, it might work, but might still be dicey. My MTH sleepers will accommodate 22", but only barely, with the couplers set outward... and they're 80-footers.


It's a single RDC with nothing coupled to it.


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## CTValleyRR (Jul 26, 2014)

Charlie Comstock, former editor of Model Railroad Hobbyist, maintained that your minimum radius should be 2.5x the length of the longest piece of equipment that will use it. That rule would give you a 30" min.

However, I've found that rule very conservative. No guarantees, of course, but I would say 22" is fairly low risk. Especially if nothing is going to be coupled together.


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## DonR (Oct 18, 2012)

Without seeing it, my guess is that your RDC uses
4 wheel trucks. Since it will not be pulling any
cars, you could well get by with 18" radius but it
won't look good...as you note the ends will bulge
out and the car will 'hang' over the side of the
track on the inside of the curve. Keep scenery
at a distance.

The best you can do is go for the widest radius
your space will afford.

Don


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## Old_Hobo (Feb 20, 2014)

DonR said:


> The best you can do is go for the widest radius
> your space will afford


This!

Always the best advice! :thumbsup:


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## Gramps (Feb 28, 2016)

Old_Hobo said:


> This!
> 
> Always the best advice! :thumbsup:


The key words being "...your space will afford."hwell:


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## mesenteria (Oct 29, 2015)

CTValleyRR said:


> Charlie Comstock, former editor of Model Railroad Hobbyist, maintained that your minimum radius should be 2.5x the length of the longest piece of equipment that will use it. That rule would give you a 30" min.
> 
> However, I've found that rule very conservative. No guarantees, of course, but I would say 22" is fairly low risk. Especially if nothing is going to be coupled together.


I add my own experience to this worthy opinion above, with the reason that stated minimums are sometimes incorrect. Walthers, unfortunately, is the perp in both instances I can immediately recall.

About 12 years ago, they advertised a #7.5 curved turnout in their Walthers/Shinohara series. Don't get me wrong...they're qood quality turnouts, but their advertised radii were incorrect. And not just by 3/4", either. They were to be 32" and 28" for the outer and inner routes respectively. The reality was that they were much closer to 30 and 24. That made them useless, after I purchased, and began to install, four of them. These were not cheap. I had to hack them up and widen both routes to get them to accommodate my Walthers heavyweights, which run over 80 scale feet.

About those same heavyweights: Walthers say they need 24" radius minimum when coupled. Not so. They need closer to 27" because those darned diaphragms get in the way of each other in tight curves. It gets worse when you're shoving the cars in reverse moves through those tighter curves because the slack bunches, bringing the diaphragms that much closer together.

When not coupled, long cars or an RDC should do okay, provided you allow good gauge loading near the tracks (use your NMRA clearance gauge, and mebbe add another 0.25" on curves tighter than 26" [I would]). If you are going to tow autoracks and other longer modern cars, then just tack up some curves of flex on a bit of plywood, couple some cars together, and trail and shove at least two of them along the curves of various radii. You'll soon have some hard data to use to fashion curves.

I cut a template with a minimum radius for the entire layout using some scrap plywood. Draw our an arc of about 40 degrees and then make an 'almond sliver' template with about 16 inches of arc. Always check your iffy curves with the template's curved edge on the centerline for the track.


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## CTValleyRR (Jul 26, 2014)

If you don't want to make your own templates as mesenteria described above, you can get commercially made radius templates. They are made of wood, plastic, or metal and slide between the rails to ensure your radii are correct.


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## CTValleyRR (Jul 26, 2014)

I went back and read the Charlie Comstock article last night. His rule of thumb takes into account not only the physical cornering of the trucks and wheels, but also overhang and clearance for scenery and structures. The ends of a long car will overhang the outside of a curve while the center will overhang the inside of a tight turve.

He further recomends 3× car length for acceptable inside overhang, 4x for acceptable outside overhang, and 5x for trouble free coupling / uncoupling on curves. "Acceptable" to him means "doesn't look so unrealistic as to spoil the illusion."


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## Gramps (Feb 28, 2016)

Thanks for the responses especially the tip on the template. I'm going to go with 24" radius. :thumbsup:


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## santafealltheway (Nov 27, 2012)

Doesnt need to take up too much space. Inside is a 28" radius, outside is probably a little bigger.

shelf is about 10" deep on the walls. the outside of the corner is 2 feet from the wall corner.


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## 89Suburban (Jan 4, 2017)

I am running 4 axle diesels and 40' & 50' cars on 15" Bachman EZ Tracks.


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## tiger (Dec 16, 2015)

By the way, where is the radius measured? Centerline of the track?


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## Old_Hobo (Feb 20, 2014)

tiger said:


> By the way, where is the radius measured? Centerline of the track?


Yes......


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## SBRacing (Mar 11, 2015)

89Suburban said:


> I am running 4 axle diesels and 40' & 50' cars on 15" Bachman EZ Tracks.


15"R is a very tight radius. And I am to assume you cannot run a lot of cars with them struggling in the radius.

Sent from my SM-G892A using Tapatalk


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## dinwitty (Oct 29, 2015)

As a kid we had an O-27 layout, we had RDC's, we had these steeple cab engines, but the couldnt pull, no magnatraction, the RDC's did, so they were our freight locos. We had some long trains with them but I bring this up because of car type/weight. Heavier cars like boxcars and lighter cars like flat cars, you could not put the flatcars at the front because they would get pulled off the track by their lack of weight on the curves, so we always had the flat cars at the end of the train. O-27 converted to HO makes this about over 6 inch radius. 15 inch radius is not so dasterdly for 40-50 ft cars. Or engines with 4 wheel trucks. 85 foot cars need wide radius couplers or coupler system, that may work or not on 18 inch, you will need a serious wide swing coupler. Equipment like this will be happier on 22 inch or higher. I made my own decision to go for 24 inch radius standard since I don't have the room to deal with 36 and make the layout I want to make. But in certain industrial areas I will run 15" but restrict what I run on it by design. Its all how you design and plan and operate.


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## SBRacing (Mar 11, 2015)

dinwitty said:


> Its all how you design and plan and operate.


That's 100% correct. My reply may have come across the wrong way. dinwitty said it best. If you are happy operating with a 15" R that's grate but if in the future u want a larger loco, like SD48 or even a big boy. You may need a larger radius. I don't know your space your working with so you may want to try using a track program like anyrail. There is a free version you can get. The layout I'm planning for a nursing home is an oddball radius at 17.5" R. I'll be running a few box cars maybe an old tanker and a few open top hoppers. With either a 0-6-0 or an AB unit and nothing else so that radius will work great. However I'm more of a modern person so my personal layout is going to have a 24" minimum at (At some points 40) and #8 turnouts. Because I love intermodals and auto racks.

Sent from my SM-G892A using Tapatalk


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## 89Suburban (Jan 4, 2017)

SBRacing said:


> 15"R is a very tight radius. And I am to assume you cannot run a lot of cars with them struggling in the radius.
> 
> Sent from my SM-G892A using Tapatalk


I am in beta testing right now only have 11 cars so far. Will be adding more when I have the funds.


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