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Do I need a transition curve?


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  • RMweb Gold

Do I need a transition curve in a hidden storage area?  I am not concerned about what it looks like, I just want to avoid derailments when propelling.

 

Key information:

4mm scale

00 gauge

3-link couplings

fixed buffer heads (worst case assumption for the sake of this question)

915mm (3') minimum radius

Slow speeds.

 

I have done a quick test using a coach (I don't have a long, fixed wheelbase vehicle to hand) and a wagon and did not get buffer lock but it was close, so I thought it best to check what people thought based on practical experience.

 

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  • RMweb Gold

Thanks David.

 

I wouldn't consider not having a transition curve elsewhere on the layout, it's just that it would make things easier in this one spot if I could get away without one.  If enough people think it is a bad idea then I'll play safe and stick with a transition curve because derailments would bug me.

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Always better to have a transition curve if you are going to do a lot of propelling. But this won't stop buffer locking on the tightest part, if that is a problem?

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  • RMweb Gold
22 minutes ago, Mike Storey said:

Always better to have a transition curve if you are going to do a lot of propelling. But this won't stop buffer locking on the tightest part, if that is a problem?

Are you saying I am likely to get buffer locking on a 915mm radius curve anyway Mike, even if I do have a transition curve?

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  • RMweb Gold
24 minutes ago, Miss Prism said:

You'll need extra long 3-link couplings on that kind of radius with non-sprung buffers.

 

Can you quantify 'extra long' in this context please?

 

I am intending to use homemade 3-links rather than, say, Smiths, and they will be longer anyway but it would be handy to know what to aim at.

 

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3 minutes ago, teaky said:

Can you quantify 'extra long' in this context please?

 

You'll have to find that out by trial and error on your particular coach lengths.

 

They'll look silly on straight track of course.

 

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Buffer locking between vehicles of similar lengths can only occur on transitions between:

 

straight and a radius,

two different radii

reverse curves

 

Once you have a safe transition curve into and out of a long arc of a tight radius curve, you shouldn't get buffer locking on the curve no matter how tight the radius.

 

However, if you have a steam or similar modelling situation with a short loco coupled to long coaches, then you can get buffer locking too tight radii, even if you do have transitions in and out. 

 

Andy

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It's a curve that leads into another tighter curve so the overall effect is that the entire curve gets progressively tighter, rather than being of a single consistent radius. 

Edited by Anglian
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  • RMweb Gold
1 minute ago, rka said:

What is a transition curve, if I might ask?

Sometimes referred to as an easement curve.  Where a straight joins a curve it takes the form of a length of track that starts almost straight and gradually tightens until it reaches the radius of the curve.

 

In the real world, a train wants to continue in a straight line and gradually easing a train into a curve prevents the mass flinging it off the rails.  We can get away without transition curves with train sets flinging the models from straight to curve because there is little mass involved.  However, if not using tension lock couplings or similar, which prevent buffers touching, then there is a risk of buffer lock where the head of one buffer goes past the other and then gets hooked behind it and leads to a derailment.  Plus it looks daft because the curves are unrealistically tight.

 

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2 hours ago, Andy Reichert said:

Buffer locking between vehicles of similar lengths can only occur on transitions between:

 

straight and a radius,

two different radii

reverse curves

 

Once you have a safe transition curve into and out of a long arc of a tight radius curve, you shouldn't get buffer locking on the curve no matter how tight the radius.

 

However, if you have a steam or similar modelling situation with a short loco coupled to long coaches, then you can get buffer locking too tight radii, even if you do have transitions in and out. 

 

Andy

Thanks Andy.

 

Some clarification and some further questions, if I may.

 

"between vehicles of similar lengths"

  - I wasn't concerned about this for coaches since I intend to construct some couplings similar to those used by Tony Wright and others which will prevent coaches getting too close.  Perhaps not totally realistic when propelling but I'm not expecting to propel coaches very often other than in the storage areas.

  - For wagons it does not seem to be a problem.  I have done a small test using two unconverted RTR wagons with couplings removed but still with the original, fixed buffers.  The buffers do not appear to overlap by less than half their face at any point.

  - I was under the impression that the greatest risk occurred when propelling wagons of differing lengths and/or with longer overhangs i.e. greater throw at the ends.  Am I wrong about this?

 

"straight and a radius"

  - Exactly the scenario I am concerned about.

 

"two different radii"

  - Not applicable for me.  I have adopted a 915mm minimum radius and where a curve opens out or tightens it will do so gradually.

 

"reverse curves"

  - I only have two examples of this.  As part of the main running lines there is a reverse curve but I intend to include a short straight section in the middle, long enough for two wagons or two coach bogies, plus I do not expect to propell around this section.  The other example is a crossover but the radii of the points involved are much greater than my 915mm minimum so I am not expecting any problems especially at slower speeds.

 

"Once you have a safe transition curve ..."

  - I do not have any straight to curved sections that do not include a transition curve.  There is only one possible exception which is why I asked the question at the start of this thread.

 

"... steam ... a short loco coupled to long coaches ..."

  - I do have this risk.  Do you think my 915mm minimum radius is a problem?

 

 

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1 hour ago, teaky said:

Thanks Andy.

 

Some clarification and some further questions, if I may.

 

 

"... steam ... a short loco coupled to long coaches ..."

  - I do have this risk.  Do you think my 915mm minimum radius is a problem?

 

 

 

It's probably easier for you to check that. UK models Draw accurate card templates of the particular vehicle outlines from above. Place on top of  drawing of track situation lining up centres or bogie pivots on track centre lines. See where the buffers end up.  CAD is even better and quicker if you have it.

 

Andy

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On 13/01/2020 at 17:45, teaky said:

Are you saying I am likely to get buffer locking on a 915mm radius curve anyway Mike, even if I do have a transition curve?

 

I would doubt that would be a problem on 3ft radii. But, despite the theory related above, it can occur with different weights and different lengths (especially bogie to single axle). I know I have had problems on my test track, with 3ft curves but no transitions, but only with certain formations when propelling. Unless you have perfectly uniform stock with exactly equal buffer lengths, there is an element of the Law of Sodde. I guess you have to find out by trial and error, assuming you have tested the theory first!

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On 13/01/2020 at 12:23, rka said:

What is a transition curve, if I might ask?

 

See the effect of a 24" long transition curve from straight to 48" radius. Note the ends of the MK1 coaches do not swing across each other.

 

 

 

Andy

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The tolerance between wheel and rail is rather slack in 00 and allows the buffer heads to move sideways rather more than on the prototype. This was one of the reasons I decided to switch to EM in the distant past as I wanted to use scale sized 3 link couplings rather than the buffer bar and single ended tension lock type couplings I had been using with only limited success. I planned on using 4' 6" minimum curves and made the links slightly oversize (a scale 12" inside rather than 10"). as otherwise all the buffers would have required springing (I contented myself by just springing the coupling hooks*). A move to Italy meant that no more than one board got built. (The space was no longer available....). Maybe one day....

 

*I only had about 30 wagons at the time so it wasn't a great problem. My 1920s stock remains EM, but the 1950s has relapsed to 00 with Peco/HD couplings and I ignore the overlarge inter-vehicle gap. Mk I coaches help here as the buffers are retracted out of the way.

 

Laying out transition curves requires some complicated mathematics. I have to admit to doing it by eye. 'If it looks right it is right!'

https://en.wikipedia.org/wiki/Track_transition_curve

Edited by Il Grifone
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5 hours ago, Andy Reichert said:

 

See the effect of a 24" long transition curve from straight to 48" radius. Note the ends of the MK1 coaches do not swing across each other.

 

 

 

Andy

Thank you

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Andy's videos show admirably how the coaches sort of glide through the transition from straight to curve and curve to straight with very little end throw going out of line with the next or previous coaches.
 

i was watching a European model video (not one of my videos) on YouTube earlier, and it showed very well (by accident) the effects of not having transition curves. Pay particular attention to the double deck set being propelled into a loop at 4:44.

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That's more the result the result of a violent reverse curve than lack of transition curves. On the real thing, the pointwork would have started further back towards or even before the road crossing.

 

The VW van must be one of these https://shop.carson-modelsport.com/carson_en/search/?s=samba

 

As to the rest of it:  I must try harder, I must try harder, I must try ....

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I'd say the answer to Teaky's original question is dependent on what sort of layout he/she is building.  I would recommend using transition curves on a layout with flexible track, and in fact flexible track will naturally lie in a transition curve state anyway unless you lay it very carefully to specifically avoid this, which sounds daft as you are specifically avoiding a means of making your layout more realistic.  But if the layout is using set track, I'd recommend not using transition curves, and thus staying within the confines of the track system geometry.  

 

With flexible track, one is able to lay curves of much larger radius than with set track, and these inevitably look more realistic, and more realistic still with the transitions.  But attempting to lay tight curves, say anything less than about 30", runs the risk of the natural tendency of the flexible track becoming much sharper in the middle of the curve than it is designed to cope with, possibly pulling the rails out of gauge and providing running problems. If you need curves sharper than flexi can cope with, set track is much more reliable, easier to lay accurately, and will not cause running problems so long as you do not exceed the recommended minimum radius for the stock (does it make sense to 'exceed a minimum?).

 

On my current layout, the most successful I've ever managed, I did not make my usual meticulous scale plan beforehand to see what I could fit in the space.  I have never been able to build a layout aboslutely accurately to one of these plans in over half a century of modelling, and I'm not that bad at track laying.  On this layout, I drew a general arrangement rough track plan and laid the flexible track allowing it to 'sit' where it wanted to.  The only measurement used was for the loco release headshunt at the terminus, 3 inches longer than my largest loco, a 42xx.  The result is very good running and gentle curve throughout the scenic part of the layout which emulates many real locations; I am happy with the overall look of it.  Once you get into the fiddle yard, I use set track to save as much space as possible, but it has no place out where it can be seen...

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  • RMweb Gold

Thanks Johnster.  I'm not intending to use any set track and will not be laying curves less than 915mm (36") radius.  I will use a Tracksetta to ensure curves are consistent throughout their length plus a combination of homemade template and human eyeball to get the transition curves right.

 

My original question related only to one specific location within a hidden storage area where it would be easier to fit things in if I did not need the extra length required for a transition curve.  I was concerned though that I might get buffer lock when propelling.  Do you use 3-link couplings and, if so, do you have any problems with buffer lock in your fiddle yard?

 

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21 hours ago, Il Grifone said:

That's more the result the result of a violent reverse curve than lack of transition curves. On the real thing, the pointwork would have started further back towards or even before the road crossing.

 

The VW van must be one of these https://shop.carson-modelsport.com/carson_en/search/?s=samba

 

As to the rest of it:  I must try harder, I must try harder, I must try ....


Not to mention it jumping the level crossing barriers at 4.32.........;)

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My short answer is YES

Going from a curve to a yard the layout of the points can make a big difference

Of the 2 examples below I would use the upper one, which has a transition curve before the first point

In the lower layout there 2 reverse curves A-C & B-D

A straight at position B would reduce/eliminate the reverse curve A-C, but you loose siding length

The RH plan shows that extra width is needed when a translon curve is used

 

 

72404020_hidsid2.jpg.41008f6a5ff0c901c60c97bd579c0400.jpg 

The drawing may be a bit rough but they are only to show concepts 

John 

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