Trains.com

Subscriber & Member Login

Login, or register today to interact in our online community, comment on articles, receive our newsletter, manage your account online and more!

Minimum radius curves and the reason to avoid them...

4097 views
20 replies
1 rating 2 rating 3 rating 4 rating 5 rating
  • Member since
    April 2009
  • From: Sweden
  • 1,468 posts
Minimum radius curves and the reason to avoid them...
Posted by Graffen on Thursday, July 1, 2010 7:13 PM

Hi all, I just had a few thoughts that I would like to share.

The previous layouts I´ve had, have been in HO scale and mostly on the smallish side and I had tricked myself every time that I needed as many spurs as possible and that on the  smallest area possible.

It is an equation that doesn´t really work.especially when you start to barter with yourself regarding the radii you will have, I settled mostly for 18", and that wasn´t always so smart.

The problems all came one after one other. Locos derailing, cars derailing, unable to operate anything longer than a 50´car... and worst of all, a nightmare to operate on as it was so cramped.

I felt the need to start on new layouts as I grew tired on the recent ones....The plans for a "better" layout is always in the works, right?

In the end I got an epiphany when I built a model railroad for the local museum, there it was spacious and I was to make it with lots of landscape and a minimum radius of 30". What a difference! 

I scrapped my layout at home and started on a shelf layout instead, no more sharp curves. no turnouts smaller than a # 5 and a trackplan that is possible to expand in the future if I would wish. The space exists as a shelf layout is much more space saving than a roundy.round layout.

Now I can buy all the equipment I like as the layout doesn´t set the boundaries, only the era I´m modeling Big Smile.

It really makes the hobby (and the layout) much more enjoyable to me, as I don´t have to worry about what I can´t do but instead of what I can do.

So if you are in the position of planning a new layout. Avoid the small radius curves and tight turnouts. Plan for the future and I will promise you that it will provide you with benefits.

Hope it helps someone Wink.

Swedish Custom painter and model maker. My Website:

My Railroad

My Youtube:

Graff´s channel

  • Member since
    January 2004
  • From: Frisco, TX
  • 483 posts
Posted by cordon on Thursday, July 1, 2010 7:55 PM

Smile

Yea, verily.

I had some 18-inchers on my previous HO layout.  Ultimately, I found out that it is very difficult to lay a perfect 18-inch curve with flex track.  I investigated by using a truck with an indicator made of a 12-inch piece of thin balsa wood (about as thick as two pieces of wire in parallel) glued cross-wise to the truck.  As I moved the truck along the curve, the indicator revealed several places where the radius varied considerably.

The indicator should point to the spot on the layout that is the center of the curve.  Where it points ahead of that spot, it indicates a local radius greater than intended.  Where it points behind, it reveals a local radius less than intended.  These tiny variations in radius were very difficult to see until after my tool pointed them out.  Every problem I had with derailing on the supposedly 18-inch radius curves was at one of the locations that the tool revealed.

I toyed with the idea of using sectional track, but then we moved to a different house where I have much more room.  My answer is the same as yours - change to a radius significantly larger than the "minimum" for your rolling stock.

Smile   Smile

 

  • Member since
    February 2005
  • From: Los Angeles
  • 1,619 posts
Posted by West Coast S on Thursday, July 1, 2010 8:11 PM

That's why my next layout will use 60 - inch as the minium and # 10 turnouts wherever possible, might have to compromise and use # 8's at industrial locations.

Dave

SP the way it was in S scale
  • Member since
    April 2009
  • From: Sweden
  • 1,468 posts
Posted by Graffen on Thursday, July 1, 2010 8:22 PM

With that minimum radius there is no danger of derailments due to that at least, I even think that high speed reversing a long train through the # 10 turnouts won´t be a problem.

Swedish Custom painter and model maker. My Website:

My Railroad

My Youtube:

Graff´s channel

  • Member since
    November 2008
  • 1,205 posts
Posted by grizlump9 on Thursday, July 1, 2010 9:04 PM

 whatever radius you settle on, don't forget the easements.  even a short easement is better than none at all and you can compromise on the curve radius a bit to fit them in and still be ahead in the game.

 in the real world we were always gun shy about shoving a long, heavy train backwards, especially through a crossover.  too many times things folded up like Lawrence Welk's accordian. thanka you boysa.

grizlump

  • Member since
    February 2005
  • From: Southwest US
  • 12,914 posts
Posted by tomikawaTT on Friday, July 2, 2010 12:22 AM

grizlump9

 whatever radius you settle on, don't forget the easements.  even a short easement is better than none at all and you can compromise on the curve radius a bit to fit them in and still be ahead in the game.

Words that should be carved in jade and filled with gold!  An easement can be the difference between silky operation and a burlap sackful of problems.

 in the real world we were always gun shy about shoving a long, heavy train backwards, especially through a crossover.  too many times things folded up like Lawrence Welk's accordian. thanka you boysa.

grizlump

My test for newly laid track, whether it be a perfect tangent or a puzzle palace of double slip switches, is to back my designated derailment test train through it at 2x track speed.  If it makes it, I'm good to go.  If it doesn't, the problem is isolated and fixed NOW.  Good enough - isnt (Unless you set good enough as the closest possible approach to perfection.)

My solution to the minimum radius question was to erect a test spiral and run everything into it until the 'not past here' point was located.  Then I eased the radius to something I was sure everything could live with.  Of course, not every car can handle certain routes.  Everything I own is happy on 610mm radius curves, but only those items that were good down to 300mm will ever be allowed onto the railed goat trail that leads uphill to my collieries.  Actual minimum radius is 350mm, too tight for most of my mainline power, my longer freight cars and all of my DMU.  But then, there's no reason for any of those to be on that stretch of rail anyway.

On the visible part of the main curves will be wider - for looks, not for operating capacity.

As for overloading the bowl of spaghetti - more than one of my earlier layouts had more, and more complex, trackwork in less space than I've designed into my double-garage filler.  I'd rather have it look right than see if I could cram in every possible noodle.  (In going from a single stall to the whole garage, I added a total of four turnouts to the visible track...)

Chuck (Modeling Central Japan in September, 1964)

  • Member since
    January 2004
  • From: Frisco, TX
  • 483 posts
Posted by cordon on Friday, July 2, 2010 12:56 AM

Smile 

grizlump9
 whatever radius you settle on, don't forget the easements.  even a short easement is better than none at all and you can compromise on the curve radius a bit to fit them in and still be ahead in the game.

 

I use easements everywhere, but I have to be careful to allow for the extra distance between the tangent tracks that they require.  For example, if I use an 18-inch radius, 180-degree half loop with easements, my two tangent tracks will be separated by 18 x 2 inches plus the easements (typically 1/2 inch each), for a total of 37 inches.

Looking at it the other way, if I plan the tangent tracks first, the curve connecting them will be a bit sharper with easements than without them.  If I am not working with radii near the minimum, the difference is not important.  This should be the case for the radii mentioned above.

Smile   Smile

 

  • Member since
    March 2007
  • 2,751 posts
Posted by Allegheny2-6-6-6 on Friday, July 2, 2010 1:02 AM

 You couldn't be more right on the money, yes a lot of locomotive manufactures advertise their engines can run on 18" curves sure they can and they look hideous while doing it and you will have to set sign post signals and utility pols so far back when your articulated go around the 18" bends so they don't get wiped out. A lot of guys fail to understand the word "minimum" in my interpretation it's just enough to get by. So unless your only running maybe a small consolidation locomotive and such 18" radius curves are a no no.

Just my 2 cents worth, I spent the rest on trains. If you choked a Smurf what color would he turn?
  • Member since
    September 2004
  • From: Dearborn Station
  • 24,184 posts
Posted by richhotrain on Friday, July 2, 2010 4:55 AM

Couldn't agree more with the OP.

Sadly, a lot of beginners getting into this hobby, including me seven years ago, are unaware of the real world requirements for smooth and flawless operation.  An 18" radius may be possible with a switcher engine and 40' freight cars but even then derailments are a distinct possibility if anything less than #6 turnouts are used.  This is not to mention track work and the problems caused by humps, kinks and other associated track laying problems.

Now on my 3rd layout and planning a 4th, my basic rules are flex track only, 30" minimum radius, easements for sure, and disciplined testing of all track from the outset.

Yeah, I know, a lot of modelers have limited space to plan a layout.  Well, sorry to say, if HO scale is going to be used, you are going to be disappointed trying to run much of anything on 18" radius, or 22" or 24" for that matter, #4 turnouts, and sectional track.

Just my opinion based on a lot of negative experience before I got smart.

Rich

Alton Junction

  • Member since
    December 2004
  • From: Bedford, MA, USA
  • 21,418 posts
Posted by MisterBeasley on Friday, July 2, 2010 6:33 AM

At the same time, you need to be realistic and manage your expectations.  I started with a 5x12 table layout, and I've incorporated that into the larger plan now under construction.  I've gone to using longer turnouts, where possible, but since I was already using 18-inch curves, I saw no reason to avoid them.

Yes, it is difficult to get a good 18-inch curve with flex track.  But, with a bit of perserverence, it can be done.  I use a gauge, and I also pre-measure the curve and lay the roadbed using sectional track, so that the flex will pretty much do what I want when I lay it down.  But, when you think about it, it's just as difficult to lay a perfect 30-inch curve.  It's just that if you dip down to 28 inches somewhere, you won't derail.  If that 18-inch curve gets down to 16, you're going to have trouble.

I grew up in the Transition Era, and that's what I model.  So, I have mostly 40-foot cars and 4-axle diesels.  I've selected short passenger coaches that can just barely deal with the curves, but by taking care with the trackwork and turnouts, I can run them reliably all day long.

It takes an iron man to play with a toy iron horse. 

  • Member since
    November 2002
  • From: Colorado
  • 4,075 posts
Posted by fwright on Friday, July 2, 2010 8:44 AM

MisterBeasley

At the same time, you need to be realistic and manage your expectations.....

Yes, it is difficult to get a good 18-inch curve with flex track.  But, with a bit of perserverence, it can be done.  I use a gauge, and I also pre-measure the curve and lay the roadbed using sectional track, so that the flex will pretty much do what I want when I lay it down.  But, when you think about it, it's just as difficult to lay a perfect 30-inch curve.  It's just that if you dip down to 28 inches somewhere, you won't derail.  If that 18-inch curve gets down to 16, you're going to have trouble.

I grew up in the Transition Era, and that's what I model.  So, I have mostly 40-foot cars and 4-axle diesels.  I've selected short passenger coaches that can just barely deal with the curves, but by taking care with the trackwork and turnouts, I can run them reliably all day long.

 

Make mine another vote for the road less traveled.

18" radius became the defacto HO standard back in the 1950s.  Both because it fit well on a 4x8, and because the prototypes for most models were a lot smaller.  40ft and 36ft freight cars were the norm, as were 4 axle diesels (E's and PA's excepted).  For those who wanted passenger car operation, several manufacturers made shorties to take the 18" curves.  While big steam was available, it was accepted and known that big steam models didn't run on 18" radius curves.  The Mantua Mikado was about the biggest anybody expected on a 4x8 with 18" radius.  IIRC, the Bowser Challenger was the 1st to break the understood rules.

Go back further to the 19th Century, and things are even better.  Freight cars were commonly in the 28ft - 34ft range.  The 2-8-0 was the standard big power on the grades in the mountainous West.  In flatter areas, the 4-4-0s and 4-6-0s could pull passengers at higher speeds.  A 65ft turntable (9" in HO) suffices.  Train lengths were in the range where we can accurately model them.  Running 50ft open platform passenger cars on 18" radius is the same as running 80ft passenger cars on 29" radius - and I don't have diaphragms to deal with.

I'm not advocating over-crowding with track.  One of my biggest challenges in designing my small layouts is forcing reductions in the number and amount of track in a plan - it always starts out as too much.  Getting the crowding in only the right places to accurately model the 19th Century takes some studying of photos. 

Rail-served industries could be much smaller because there was no truck competition to switch to for smaller loads.  There were still plenty of behemoth industrial concerns, but there were many small towns that were nearly totally rail-served.  The team track was mandatory in every town, as was LCL and "express" service.

As for laying 18" radius curves in HO, I've found it quite practical as long as I don't use Atlas "Super Flex" track.  Either use sectional track, flex track that will hold a pre-bend, or hand lay (again, pre-bending the rail).  That's how I avoid the dip down to 16" radius that commonly happens with Atlas flex.  Handlaid track with code 70 and 55, and even code 40 rail is of course a much more realistic model of 19th Century track.

As Mr B said, small radius track can be just as reliable as large radius, especially if you keep your rolling stock commensurate with the radius.

Fred W

....modeling foggy coastal Oregon, where it's always 1900....

  • Member since
    September 2004
  • From: Germany
  • 1,951 posts
Posted by wedudler on Friday, July 2, 2010 11:38 AM

 Minimum radius depends on the engines. Brass engines with rigid frame need a bigger radius. I think, with more than 36'' you're on the safe side.

I've tested the radii for my narrow gauge start with a test track and my friend's engines.

 Wolfgang

Pueblo & Salt Lake RR

Come to us http://www.westportterminal.de          my videos        my blog

  • Member since
    September 2002
  • From: North Carolina
  • 1,905 posts
Posted by csxns on Friday, July 2, 2010 7:34 PM

cordon
easements

Can you tell me what a easement is and if you can show me one.Dont understand how one works.Thanks.

Russell

  • Member since
    February 2002
  • From: Reading, PA
  • 30,002 posts
Posted by rrinker on Friday, July 2, 2010 8:24 PM

 I guess the title really shoudl be "Too small a minimum radius and why to avoid it". If you set you minimum radius to somethign your equipment can easily handle, there's no real reason to avoid having curves of that radius. I set mine to 24" because all the stuff I run comfortably handles that radius, and I have no problem makign curves that small. Just no smaller - that's the whole point of it being a 'minimum' radius. Nothing smaller, but larger is ok where it fits.

                                      --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

  • Member since
    January 2004
  • From: Frisco, TX
  • 483 posts
Posted by cordon on Saturday, July 3, 2010 8:30 AM

Smile 

csxns

cordon
easements

Can you tell me what a easement is and if you can show me one.Dont understand how one works.Thanks.

 

An easement is a transition section between a curve with a fixed radius and straight (tangent) track.  The actual radius in the transition section is not fixed, but varies smoothly from the radius of the curve to a very large or infinite radius where it joins the tangent track.  The point of having an easement is to avoid running a train directly from straight track onto a curve at a single point on the track.  The train can "ease" into and out of the curve.

Sectional track does not have easements.  One plugs a straight section into a curved section, and there is no smooth transition.

If you do a search in these forums on "easement" you will see a wealth of information from people much smarter than I am about it, as well as many proposed methods of laying them on model RRs.  I prefer the simplest method.

My comment referred to the fact that an easement requires a lateral displacement of the track that a connection without an easement does not need.  Imagine a semicircle of sectional track with a 20-inch radius joining two straight portions to make a "U."  For purposes of this example imagine that the curve consists of six sections of 20-inch radius and that each straight portion consists of three straight sections.

The centerlines of the straight portions will be 2 x 20 = 40 inches apart.  In order to make an easement one has either to reduce the radius of the semicircle by a small amount or to move the straight portions of the track farther to the outside by a small amount.

So, imagine at one of the transition points lifting up and removing one curved section (of sectional track) and one straight section without disturbing the placements of the remaining sections.  Then move the entire straight portion a small amount towards the outside, crosswise to the direction of the track.  What's a "small amount?"  You will find many recommendations, but for this example I'll use half a track gauge.  Then imagine connecting a piece of flex track between the remaining curved and straight portions without disturbing their locations and making the connections and the transition curve as smooth as possible by eyeball, letting the flex track follow its own natural shape to meet the joints smoothly.

This would be a "simple" easement.  It's not what the real RRs do, but it works for me.  You can do the same thing if you are using flex track by first drawing your fixed radius curve and your displaced tangent centerlines and then putting down a long pre-assembled section of flex track so it "looks" good.  On my model RR it runs well if it looks good.

My point in the earlier post was that if you are planning an oval in HO with easements into 20-inch radius curves, then the tangent portions of your oval will be about 41 inches between centerlines instead of 40 inches.  You need to account for this in your layout plan.

Smile   Smile

  • Member since
    August 2008
  • 357 posts
Posted by EM-1 on Saturday, July 3, 2010 1:54 PM

Just a bit of an opinion.  Maybe worthwhile, maybe not.

Each of us has a certain amount of space to work with.  We have to fit the best compromise between what we'd like to have and what we have available.  And if we give up trying to build something we'd like to do because someone else says that's the wrong way to go, we may never achieve a reasonable degree of satisfaction.

I have a 9' X 13' room that I'm looking at right now.  That means a table about 6 X 13 with two sides against walls maximum.  I've read that I'd be better off with an around the walls layout.  Certain features of the room prevent that.  The rolling stock I've collected, much of it scratch or kit bashed, often requires at least a 24" radius (HO), so I'm planning on 26" main.  Transitions? probably not.  Shelf switching layout?  Possible option, but I've built a few, and while there's a certain satisfaction, there are still times I want to watch one of my articulateds just running free.  Can't do that with a point to point shelf.  same with the multi-truck flat cars and articulated passenger cars I have.

N scale to get the decent radii curves?  I'm a 66 year old stroke survivor.  I am about to sell my collection of N.  Too small any more for me to work with.

Probably the most derailment free pike I've ever built in HO was a 4' X 6' L, using 15" curves and snap switches.  That was in the days of oversize flanges, Mantua loop-hook couplers, mostly 40' freight cars, and Globe (original Athearn) passenger cars.  Even my first Mantua 2-8-2 kit loco could squeeze around on slow speeds.

You can look at the pictures of these super size works of art in the mags, you can drool over the fantastic work some people have done, you can wish for the room for a 30' X 50' layout, but in the end, you have your own situation, and you have to do the best you can with what you have to work with, beit  a collosal Z guage layout in a barn with several scale miles of uninterrupted mainline run, a 10' X 11' HO scale L shaped switching layout,, or even a small G switching layout in a window box.  Or a small oval in a briefcase.  Or file cabinet drawer.

It's your layout, your world, you are the only one who really needs to be satisfied.  There's a lot of good advice out there, pick and chosse that which seems to fit your situation the best Adapt it to what you have to work with.

It's a hobby, a relaxation.  Do it your way.

  • Member since
    July 2006
  • From: west coast
  • 7,631 posts
Posted by rrebell on Saturday, July 3, 2010 3:38 PM

I try for 22" but 18" works fine (everything I run is 40' or less) and #4's have not been a problem but I try to go #6. The key is how good is your trackwork and you need to tune all your switches.

  • Member since
    June 2003
  • From: Culpeper, Va
  • 8,202 posts
Posted by IRONROOSTER on Saturday, July 3, 2010 8:50 PM

EM-1
...

Probably the most derailment free pike I've ever built in HO was a 4' X 6' L, using 15" curves and snap switches.  That was in the days of oversize flanges, Mantua loop-hook couplers, mostly 40' freight cars, and Globe (original Athearn) passenger cars.  Even my first Mantua 2-8-2 kit loco could squeeze around on slow speeds.

....

 

The flanges do make a difference.  I find my 40' cars in S have no problem with 19" radius curve (equivalent to about 14" in HO) with the hi rail flanges, but derail frequently with scale flanges. On a 24" radius curve they both work fine.  I'm going with scale because I have the room for a minimum 33" radius curve, but if I were tight on space I'd use the hi rail.

Enjoy

Paul

If you're having fun, you're doing it the right way.
  • Member since
    July 2006
  • From: Sorumsand, Norway
  • 3,417 posts
Posted by steinjr on Saturday, July 3, 2010 9:32 PM

EM-1
I have a 9' X 13' room that I'm looking at right now.  That means a table about 6 X 13 with two sides against walls maximum

 Just curious. I'm trying to picture how to fit in a 6 x 13 foot table in a 9x13 foor room in such a way that it has two sides against walls, and I am not quite able to picture it in my head.

I assume your 6x13 foot table is not orientented in a 9x13 feet room in such a way that you will have to crawl on the floor under 6 feet of layout to get from one 18" wide aisle to another 18" wide aisle?

I also suspect that the answer may have something to do with how you use the word "room".

Smile,
Stein

 

 

  • Member since
    August 2008
  • 357 posts
Posted by EM-1 on Saturday, July 3, 2010 10:35 PM

That's a 9' X 13' floor space in a 9' X 24' semi finished basement area.  A layout would be built against the outer concrete block walls, leaving me a 3' aisle to a divider wall to the laundry/furnace/pantry area.  The other end would open onto a small bar and general entertainment area. 18" aisles would be a little tight, especially for the access to the furnace combustion area and a small closet.

 And, I remember the hoopla about the RP-25 flange contour promising to reduce derailments.

 I guess I could have stated it better.

I think my original point is still valid: do the best you can in the space you have available.  If you can't do it "right", do it the way that works best for you.  Long as you have fun doing it.

  • Member since
    July 2006
  • From: Sorumsand, Norway
  • 3,417 posts
Posted by steinjr on Saturday, July 3, 2010 11:01 PM

EM-1
That's a 9' X 13' floor space in a 9' X 24' semi finished basement area.  A layout would be built against the outer concrete block walls, leaving me a 3' aisle to a divider wall to the laundry/furnace/pantry area.  The other end would open onto a small bar and general entertainment area. 18" aisles would be a little tight, especially for the access to the furnace combustion area and a small closet.

 18" aisles would have been very tight. Which is why I was curious. 

 Thank you for your answer.

 Stein

 

 

 

Subscriber & Member Login

Login, or register today to interact in our online community, comment on articles, receive our newsletter, manage your account online and more!

Users Online

There are no community member online

Search the Community

ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT
Model Railroader Newsletter See all
Sign up for our FREE e-newsletter and get model railroad news in your inbox!