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Reverse curve radius help/suggestions needed


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I’m considering doing a pair of reverse curves for my Sleaford East Junction layout which 90% of the time, my stock happily sits in the hidden sidings.  The only issue I have is I want to run my version of the Boston steel train with a Hornby Class 60 and 12 (hopefully more) Bachmann BYA wagons which will be fitted with Bachmann E-Z Mate/Kadee couplers, so hence the reverse curves.

 

As I don’t have a great deal of space, could some give me some sensible suggestions of a minimum radius which could used without causing coupling or derailing problems.

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I think the absolute radius is less of an issue than being sure to ease it as you change from one curve to another. Set track users were always advised to try and use a straight piece between opposite curves but if using flexi track you want to try and transition out from the sharpest point of the first curve and then flow gently into the opposite radius curve.

Edited by andyman7
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I think the question is actually about return loops. I don't know the answer though, probably quite large to allow a long train to not be derailed inwards by the loco pulling it.

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16 hours ago, andyman7 said:

I think the absolute radius is less of an issue than being sure to ease it as you change from one curve to another. Set track users were always advised to try and use a straight piece between opposite curves but if using flexi track you want to try and transition out from the sharpest point of the first curve and then flow gently into the opposite radius curve.

More important is to put a length of straight track in the middle, it doesn't need to be very long.

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6 minutes ago, Guy Rixon said:

But if the radius of the transition curves goes to infinity at the point where they meet, does that not do the same as a straight length? How much straight would one need?

If you can transition the radius then you don't need a straight, but that option isn't available to set track users. 
If the question is about a reverse loop I can't help with the specifics of the loco/rolling stock but it is worth pointing out that the 'minimum 2nd radius' can be adapted if transition curves are employed from the straight - the biggest risk of buffer locking comes when the leading vehicle enters a sharp curve and swings out before the vehicle behind aligns. A transition curve (as used on the real railway) eases all vehicles in line into the curve. Even on a train set type board, easing the curves in improves the look of how trains go round no end!

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I've occasionally wondered if you could fudge a transition curve using set track by going 4th - 3rd - 2nd radius in series. Might even be able to use 1st in there with such an arrangement as the change from 2nd isn't that big.

 

Or it might not work at all.

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38 minutes ago, Guy Rixon said:

But if the radius of the transition curves goes to infinity at the point where they meet, does that not do the same as a straight length? How much straight would one need?

About 3 or 4 inches of straight is enough to make  a big difference in running but anything is better than nothing and an abrupt change of direction.

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1 hour ago, Guy Rixon said:

But if the radius of the transition curves goes to infinity at the point where they meet, does that not do the same as a straight length? How much straight would one need?

 

50 minutes ago, Michael Edge said:

About 3 or 4 inches of straight is enough to make  a big difference in running but anything is better than nothing and an abrupt change of direction.

 

I'd say the desired length of straight is a function of the length of stock that is being operated.  In the case of four wheel wagons, the desired length is the wheelbase of the longest wagon or the distance between the axle on one wagon and the nearest axle on the next.  In the case of bogie stock, the desired length is the distance between bogie pivots or the distance between the bogie pivot on one wagon and the nearest bogie pivot on the next.

 

That therefore means if most stock operated on the layout is older four wheel wagons, then three or four inches is probably adequate, but if operating longer bogie wagons, as @jools1959 is proposing, then somewhere in the region of six or seven inches may be more appropriate.  In the set-track geometry, that would correspond to a single standard straight.

 

However, if using transition curves, then I'd probably double that length and not bother with the straight - ie as long as there was a minimum circa 12" transition between two opposite curves, it should be okay for most stock.

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1 hour ago, Guy Rixon said:

But if the radius of the transition curves goes to infinity at the point where they meet, does that not do the same as a straight length? How much straight would one need?

Thatr's mathematically true but it also means that the actual straight is infinitely short.

I found this an unclassified MoD document "Permanent Way Design and Maintenance Policy and Standards on railway construction" published in 2009. 

 

7.2.8 Transitions between Reverse Curves
The term "reverse curve" is used to denote the situation where a curve of one hand is
followed by another of the opposite hand, with either no intervening length of straight,
or a length of straight which is less than the distance between the axles or bogies of
a given vehicle.
To prevent buffer locking a length of straight or transition curve must be provided
between reverse curves equal to at least the length of the longest wagon used, eg
freight wagon 11.25m, container flat 13.5m, MOD Warwell 14.3m.
The transitions between reverse curves must be designed with the same rates of
change on both sides of the reverse, to give a constant rate of change of curvature
and, within the limits of rounding, cant and deficiency through the transition.

 

Having a wheelbase length between reverses makes sense because otherwise one end of the vehicle is turning one way and the other the other way. The golden rule being that nothing should happen to a vehicle suddenly and changes of motion should be as gradual as possible.


 

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24 minutes ago, Pacific231G said:

Thatr's mathematically true but it also means that the actual straight is infinitely short.

I found this an unclassified MoD document "Permanent Way Design and Maintenance Policy and Standards on railway construction" published in 2009. 

 

7.2.8 Transitions between Reverse Curves
The term "reverse curve" is used to denote the situation where a curve of one hand is
followed by another of the opposite hand, with either no intervening length of straight,
or a length of straight which is less than the distance between the axles or bogies of
a given vehicle.
To prevent buffer locking a length of straight or transition curve must be provided
between reverse curves equal to at least the length of the longest wagon used, eg
freight wagon 11.25m, container flat 13.5m, MOD Warwell 14.3m.
The transitions between reverse curves must be designed with the same rates of
change on both sides of the reverse, to give a constant rate of change of curvature
and, within the limits of rounding, cant and deficiency through the transition.

 

Having a wheelbase length between reverses makes sense because otherwise one end of the vehicle is turning one way and the other the other way. The golden rule being that nothing should happen to a vehicle suddenly and changes of motion should be as gradual as possible.


 

That sounds about right, my suggestions are just based on many years of experience.

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2 hours ago, Zomboid said:

I've occasionally wondered if you could fudge a transition curve using set track by going 4th - 3rd - 2nd radius in series. Might even be able to use 1st in there with such an arrangement as the change from 2nd isn't that big.

 

Or it might not work at all.

Not ideal but clearly better than going straight into the sharpest curve. I wonder whether  a transition setrack curve would be possible? Fortunately, it's perfectly possible to mix Setrack with code 100 flextrack. 

I find it interesting that Peco appear to incorporate transitions through the closure rails in their Streamline points which actually makes the minimum radius rather smaller than other points of the same length using circular curves. Given that we generally use back of the gasworks sidings points on main running lines this would makes sense and I assume they did quite a lot of work going right back on what gives better running.

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4 hours ago, Guy Rixon said:

But if the radius of the transition curves goes to infinity at the point where they meet, does that not do the same as a straight length? How much straight would one need?

I have  a minimum radius of 30" I used the 'create transistion curve' feature in the excellent ANYRAIL programme ( which has a free trial version). I printed a full size version of the transition curve and cut a hardboard templet that fitted between the rails and used this create transition curves at the start and end of these curves.    

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16 minutes ago, Pacific231G said:

I wonder whether  a transition setrack curve would be possible

Geometrically it should. Straight to R2 in 22.5* would still be pretty sharp though. And it works need a companion straight to R3 to go with it. Probably wouldn't sell enough since the kind of people who worry about transition curves are generally not the same people who buy much in the way of set track.

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23 minutes ago, Vistisen said:

I have  a minimum radius of 30" I used the 'create transistion curve' feature in the excellent ANYRAIL programme ( which has a free trial version). I printed a full size version of the transition curve and cut a hardboard templet that fitted between the rails and used this create transition curves at the start and end of these curves.    

An excellent idea and far easier than the mathematical formulae that MRN and MRC used to fill umpteen pages with.

I understand that a 30" curve with proper transitions will give far better running than a 36" curve without. Interestingly, 30"  for curves and 36" for points were the BRMSB recommended minima for 00 and H0 way back in the mid 1940s. They tended - especially nominally three foot radius points- to become the standard rather than the minima used by many modellers who built their own track thereafter. 

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2 hours ago, Pacific231G said:

Having a wheelbase length between reverses makes sense because otherwise one end of the vehicle is turning one way and the other the other way.


Exactly the approach used on industrial NG railways laid out using full-sized set-track, including military field railways.
 

Such railways usually use only four basic components: straight panels; curved panels of a single radius; LH turnouts; and, RH turnouts. By inserting a straight panel between reverses, they operate happily at decent speeds over ‘toy train set’ layouts. For 4W locos and wagons, that straight panel can be very short; if bogie wagons are used it needs to be longer, unless speed is seriously restricted and very particular coupling arrangements are used (The Guinness railway in Dublin was a case of this).

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4 minutes ago, Nearholmer said:


Exactly the approach used on industrial NG railways laid out using full-sized set-track, including military field railways.
 

Such railways usually use only four basic components: straight panels; curved panels of a single radius; LH turnouts; and, RH turnouts. By inserting a straight panel between reverses, they operate happily at decent speeds over ‘toy train set’ layouts. For 4W locos and wagons, that straight panel can be very short; if bogie wagons are used it needs to be longer, unless speed is seriously restricted and very particular coupling arrangements are used (The Guinness railway in Dublin was a case of this).

Indeed. I've often though that Frank Hornby's O gauge tinplate track and indeed some of the locos and  wagons were based on Decauville's portable railway (or more probably the licensed version made by Hudsons) the dished pressed sleepers in particular look remarkably similiar and, apart from the rail section, the whole thing was almost a model of an industrial 2ft/60cm portable railway- which for use on the nursery carpet was really hat it was- rather than a toy version of a main line railway.

I have a couple of reprints of Decauville catalalogues and they do look rather like a larger scale Hornby 0 gauge catalogue.

 

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Hornby tinplate track was a ‘value engineered’ copy of German tinplate track (fewer sleepers), which did indeed look like Decauville/Fowler/Hudson/ Koppel etc.

 

I think my affinity for industrial NG really embedded when I saw an entire spare turnout leant up behind a shed at our local brickworks, it was, as you say, precisely like the ones at home!

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On 16/05/2021 at 10:59, Zomboid said:

I've occasionally wondered if you could fudge a transition curve using set track by going 4th - 3rd - 2nd radius in series. Might even be able to use 1st in there with such an arrangement as the change from 2nd isn't that big.

 

Or it might not work at all.

 

Works excellently even if each bit is only a half a coach long 

 

Don

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  • 3 weeks later...

Confused with this reverse curve problem. This is part of a layout I am designing which uses Kato HO scale 490mm radius turnouts in the fiddle yard. Am I right  that there will be problems with wagons/coaches traversing the section above the red 100mm ruler. The wagons will be mostly 4 wheel and the coaches Mk1's and Staniers

 

1294968419_KATOHO490rPOINTS.jpg.4d1f941d72d823c70a7b0a8a92656b7f.jpg

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19 minutes ago, wasdavetheroad said:

Am I right  that there will be problems with wagons/coaches traversing the section above the red 100mm ruler.

 

1294968419_KATOHO490rPOINTS.jpg.4d1f941d72d823c70a7b0a8a92656b7f.jpg

 

I'd change the 'will' to 'may'.  Yes, there is a risk that you may have problems with the reverse curve shown (particularly if you are snaking through the tracks), but it's very much stock dependent and you have reverse curves in all crossovers (as we all do).  You refer to the curved road of the turnout being 490 mm radius, which is greater than the second radius (438 mm) employed in the set track geometry and this works in many train sets.  I think 490 mm is about third radius.  The stock that you're referring to is also not likely to be as problematic as larger stock, since the wheelbase is shorter, the overhang less and end throw is also likely to be less than some other items of stock.

 

My only advice is to test your particular track and stock combination before you commit too heavily to the plan.  You're not going to get a definitive answer any other way, as it's the combination of track, stock dimensions, as well as buffer size and position and couplings (type and spacing) that will define whether or not you actually have a problem.  My gut feeling is that you'll probably be okay, but if you could incorporate a short straight between the toes of the turnouts, then I would probably look at doing so.

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On 16/05/2021 at 10:59, Zomboid said:

I've occasionally wondered if you could fudge a transition curve using set track by going 4th - 3rd - 2nd radius in series.

 

Peco's 00 gauge Setrack range includes the 11.25° ST238 "special curve" which has a radius of 859.6mm (a tad under 3ft), which could be used as the first element of a Setrack "transition curve".  (AFAICS it's basically there to complement the Setrack Y point, allowing the cliched "single track splitting symmetrically around an island platform" - which reeks of 'train set' - to be constructed.)

 

Hornby's equivalent is the 852mm radius R628.

Edited by ejstubbs
Unfortunate typo
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1 hour ago, Dungrange said:

 

I'd change the 'will' to 'may'.  Yes, there is a risk that you may have problems with the reverse curve shown (particularly if you are snaking through the tracks), but it's very much stock dependent and you have reverse curves in all crossovers (as we all do).  You refer to the curved road of the turnout being 490 mm radius, which is greater than the second radius (438 mm) employed in the set track geometry and this works in many train sets.  I think 490 mm is about third radius.  The stock that you're referring to is also not likely to be as problematic as larger stock, since the wheelbase is shorter, the overhang less and end throw is also likely to be less than some other items of stock.

 

My only advice is to test your particular track and stock combination before you commit too heavily to the plan.  You're not going to get a definitive answer any other way, as it's the combination of track, stock dimensions, as well as buffer size and position and couplings (type and spacing) that will define whether or not you actually have a problem.  My gut feeling is that you'll probably be okay, but if you could incorporate a short straight between the toes of the turnouts, then I would probably look at doing so.

Thanks for the advice, I read through the posts again and checked the turnout finding there is a 20mm straight section between the end of the point blades and the end. that gives 40mm of straight built in and I think I can add an additional 60mm section giving 100mm. All the stock will have medium Hornby tension locks fitted. I actually like the 'train set' appearance :wacko:

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5 hours ago, wasdavetheroad said:

Confused with this reverse curve problem. This is part of a layout I am designing which uses Kato HO scale 490mm radius turnouts in the fiddle yard. Am I right  that there will be problems with wagons/coaches traversing the section above the red 100mm ruler. The wagons will be mostly 4 wheel and the coaches Mk1's and Staniers

 

1294968419_KATOHO490rPOINTS.jpg.4d1f941d72d823c70a7b0a8a92656b7f.jpg

There will, add a short length of straight track between the points - it doesn't have to be very long 2 or 3 inches will help.

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When I built my layout (16.5 mm gauge) I put a 4 inch straight between opposing curves or roughly 2nd radius. This scheme was intended for small shunters and four-wheeled stock but has rather surprisingly worked out fine with most of my RTR locos and wagons up to about 8 inches long.

 

I have since discovered, if you intend to run a Rapido APT-E (tilting) the straight section must be as long as a coach. Otherwise the tilting mechanisms are trying to go in opposite directions at the same time. And bogie coaches having a close-to-scale ride height really do need a straight as long as themselves, even though they will negotiate the curves at each end without trouble.

 

- Richard.

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