Sensible minimum radii for reliable continuous running

[flexible_coupling]

Hello all,
In the next two months, I will be in a position where I can seriously consider "the big railway".... the time-frame having been moved forward significantly is an upswing of a move across the country, where the new location has an unusually large garage space (essentially single-car width, double-car length) which seems quite conducive to a good-sized project.

Basic arithmetic on my part suggests that I may be able to succeed in my desire to have continuous running. Locomotive stock is highly likely to subsist of small Bo-Bo diesels (no larger than a Hymek) and six-coupled tank locos. Already having an understanding of the various considerations with S7 - slight gauge-widening on sharp curves etc. - I put the question out into the ether for some experienced response - will a 4'6" radius continuous curve (to turn around to a fiddle-yard in the middle of a loop), if laid carefully and partnered with appropriately well built locomotives, be generally regarded to be "safe" for relatively small locomotives at not-outrageous speeds? Is it reasonable to take it down to 4' radius? I'd love to hear some responses from people that have tried/seen such measures in action. I'm at the really preliminary planning stages here.

 

[Dikitriki]

I can't answer from a S7 point of view, but for FS, one of the Heyside group has a continuous 4'6'' radius roundy roundy.

All the locos have to be built from the outset with this sort of curve in mind, but I have run my West Country there, and the Dean Goods and 128 in my last Heyside running session post come from that layout.

Richard

 

[Phill Dyson]

Will your Peak & Class 40 go round that curve Richard?

 

[Tim V]

It was just such a restriction that put me off 7mm. I have built a layout in 4mm (p4) on 3' radius. The amount of work to get it to run, plus severe restrictions on what does and doesn't run means the answer for you is a guarded yes. But have you thought of another way? I have a garage 27' x 17', I consider that too small for a 7mm layout.

 

[Dikitriki]

Will your Peak & Class 40 go round that curve Richard?

Hi Phill,

Since neither are as yet chipped, therefore not even tested round Heyside, I don't know. However, I would suggest that it is extremely unlikely, the problem being the swing of the front idler wheels.

Richard

 

[Phill Dyson]

Hi Phill,

Since neither are as yet chipped, therefore not even tested round Heyside, I don't know. However, I would suggest that it is extremely unlikely, the problem being the swing of the front idler wheels.


Richard

Thanks Richard,

The reason I ask is I am building a Peak at the moment at the moment & I was wondering about mine getting round my tightest curve of a shade under 6'?

So will your peak be unable to run on Heyside?

Phill

 

[Heather Kay]

I think the problem, whatever the chosen gauge, is tight radii. Dock and industrial networks had tight radii, but the consequence of that is the locos tended to be short wheelbase in order to cope. The LMS "standard" 0-6-0 dock tank attributed to Fowler and built in the early 1930s had all kinds of mechanical contrivances in order to allow it to negotiate extremely tight curves, including a form of Cartazzi truck for the trailing driven axle! If you look at the loco, the 4ft-something wheels are as close together as possible, with large overhangs and low-slung outside cylinders. It's one of my favourite locos because it looks so ungainly!

The accepted minimum radius for S7 is 6ft, 5ft 6in at the absolute extreme. Even that is pushing things. Don't expect a pacific or a large diesel to be happy negotiating anything less than 6ft except at really low speeds, and even then it'll look very uncomfortable doing it. If there is a train involved, expect coupling and buffer locking problems, too.

If you have your heart set on big locos, by all means follow it. I am quite taken by the new Heljan 40, but conveniently my bank manager isn't. Sadly, the space available to you means such locos will be display items, taken out for an occasional run on a larger railway, until you have the good fortune to move somewhere with more real estate. Smaller locos, with smaller wheels and short wheelbases will be all you can manage on a minimum space layout with tight radii, so embrace the challenge!

 

[JimG]

Phil,

I would go along with TimV. I model in S scale - roughly 2/3 the size of 7mm - which uses exact scale track and wheel standards, and four foot radius is our minimum recommended radius for most running but larger radii would be better. Going below four foot radius is possible, but does require a lot of gauge widening in the track and the locomotive chassis being optimised for the smaller radii - typically making sure that there is sufficient side play in axles of six coupled locos. Eight and ten coupled chassis would be out of the question.

I would have thought that four foot radius in S7 is pushing it for all but small industrial 0-4-0 locomotives and short wheelbase rolling stock, and four foot six inch radius is probably still a bit of a gamble. If I remember correctly in my S7 modelling days some years ago, six foot radius was the recommended minimum which probably means that five foot to five foot six inches was pushing it. It might be worth building a test track with a section of your tightest radius in it to see how your locos and stock will perform before starting building the layout. Your four wheeled bogie diesels will probably be OK provided there is enough clearance to let the bogies swing, but you might find problems with any six coupled loco chassis, or any 4-4-0 or 0-4-4T chassis with the overhang over the bogies.

Jim.

 

[Dikitriki]

Given that the OP referred only to small Bo-Bo diesels and 6-coupled tanks, it's worth laying a 4'6'' reverse curve plank to empirically test.

I referred to larger locos in my original post on the basis if larger locos can be squeezed round in FS, smaller locos should be possible in the same radius in S7. I do actually have an S7 loco, though I keep quiet about it - a Jinty - and it would be interesting to see what that can go round comfortably.

Richard

 

[Osgood]

A possible solution:

Utilise the bulk of the space for a stunning Mountain Ash industrial layout incorporating whatever radius curves will fit, keeping the main line as an end-to-end backdrop

 

[mickoo]

looking at it another way, do you really need a 4'6" curve?

Why does the fiddle yard have to be in the middle of the layout, can the fiddle yard be underneath one of the sides, sufficiently low enough for access (say 12" headroom), then all you need is a ramp of sufficient shallow gradient and follows the main curves above and rises up to join the rest of the layout.

 

[OzzyO]

IIRC when Bob Essery was building Dewsbury Midland in S7 he had to use gauge widening on 6' curves. This was with locos using full width frames (in 7mm) as the layout was set about 1890-1900 the largest M.R. loco that he would run was the M.R. 4F and that only had a 16' 6" W. B..
So I would say use the largest curves that you can.

OzzyO.

PS. I also think that if you go much under 6' rad. it starts to look a bit toy like in 7mm.

 

[flexible_coupling]

A possible solution:

Utilise the bulk of the space for a stunning Mountain Ash industrial layout incorporating whatever radius curves will fit, keeping the main line as an end-to-end backdrop

This is precisely what I'm plotting, as I suspect you've followed my prior progress! The 'continuous run' would, in fact, only be for circling around the coal wagons, with a 57xx pannier being the largest loco on that side of the fence. A section of "BR" would be represented end-to-end... I had initially mused over having the 'main line' circle around too, but that would severely inhibit my motive power options! I would like to have continuous running.... but it isn't the be-all and end-all. I don't think I will have the width in the garage to get much greater than 5' at a pinch on the outer arcs.

As I only own one loco at present, a Fowler diesel which will turn on a dime, the rest of the small locos can be built with more side-play behind drivers than most S7 modellers would go for, pre-empting tight operating conditions. For me, wheel and track cosmetics are the lure of S7... I'm not overly bothered with exacting standards on frame width (hopefully such a viewpoint doesn't get me expelled from the class!)...

 

[mickoo]

I'm not overly bothered with exacting standards on frame width (hopefully such a viewpoint doesn't get me expelled from the class!)...

Tut tut, that' s not the S7 way

Most, somewhere, to some extent, accept compromises in our modelling and rule 1 applies here, my train set so I'll do what ever I like and the anorak police know where the door is

If your layout is primarily industrial then I think 4'6" is rather generous

 

[Osgood]

…. For me, wheel and track cosmetics are the lure of S7... I'm not overly bothered with exacting standards on frame width (hopefully such a viewpoint doesn't get me expelled from the class!)...

Me too. Maybe if I had waited a little longer I might have gone the route of superfine32 or whatever it is called, as in Heyside (?). But too late to turn back now - S7 it is, feeler gauges and all!

 

[Dikitriki]

Me too. Maybe if I had waited a little longer I might have gone the route of superfine32 or whatever it is called, as in Heyside (?).

O-MF.
MF = medium fine

 

[Eastsidepilot]

I have built a small section of gauge widened and check railed S7 track set at 6'-6" radius for testing. An 0-6-0 goes through it quite happily and the S7 LNWR A class 0-8-0 I built also went through.
But bear in mind that the 0-8-0 had flangeless tyres on one axle.
I would recommend a check rail as this stops the wheels on the outside rail from trying to climb up the rail and de-railing.

Col.

 

[iploffy]

Does the layout have to solely remain in the shed/ garage could you not have a return loop through one of the walls to ease the problem of tight curves.

 

[Mike Sheardown]

Sometimes, tight curves can be the only way to achieve the kind of layout you want.

However, I don't see this as a problem, since, as long as you can get your stock to run round these tight radii, the offending curves can always be hidden.

I plan to use 'hidden' 5' curves frequently to make my own layout work (7mm fine scale), though transition curves into the 5' radii can soften the impact on stock and reliable running. I also model pre-grouping, so shorter wheel-base stock is generally the order of the day, which also helps.

On my previous test-track layout, I even went down to 4'6" radii without problem in one section, though this was only for a short distance.

Hope this is helpful

Mike

 

[Scale7JB]

Another thing to consider is a continuous check rail, this will allow for tighter radii, and looks the part on tight curves too..

JB.