Here's a funny story.

RichardS

Member
Sadly it's not funny ha-ha, but funny peculiar... and could very well have it's basis in the ignorance of this scribe. (I'll take no bets!) However, I'm hopeful that somebody here can explain that which I have discovered.

The recipe is as follows:
Take one software program (in this case AnyRail).
To which add the Peco Code 100 Streamline Track System in HO/OO.
Blend with the need to fit a quart into a pint pot of a track plan.
and garnish with one puzzled user (in this case me)

I appreciate that this is probably pretty 'beans on toast' to the regulars on here who work with higher class goodies such as Templot and C&L etc to create their cordon bleu creations.

As I was using AnyRail, and I've used it several times before, I noticed that my parallel tracks were not actually always parallel. I was using a track centre spacing of 50mm which is what I thought the standard spacing was supposed to be. Investigation revealed that the actual spacing is 52mm (about). There does not seem to be any public affirmation of what it really is.

The other issue I confronted was that of bringing a curve back off the turnout to create a parallel track for sidings say.

Reading the data on the PECO website I learned the following:

Large Radius Point: Nominal Radius: 1524 mm 60 ins Angle: 12 degrees
Medium : 914 mm 36 ins 12
Small : 610 24 ins 12

I wasn't interested in the small radius units but have included them for completeness.

MY kind of logic tells me from this that I need a piece of track angle 12 deg and radius 1524mm for a large point. Apply this to the program add a parallel track and ..... it doesn't line up. Nearly but not quite. I suspect it's something in the program and has to be lived with.

But being a curious fellow I explored further.

If I see the words 'nominal radius' I assume that this is an approximate figure, in fact I learn that the curve through the point is actually a compound curve and the quoted figure is an amalgam of various radii. Nevertheless, one would expect 30 of the said items (12 degrees x 30 = 360) joined together to produce a circle with a nominal radius of 1524, 914 or 610mm.

So using AnyRail I designed the necessary arrangements to test the hypothesis and measured the results with the ruler tool in AnyRail at a scale of 1:4. What I found has puzzled me greatly.

The circle of large radius points has a radius of approx 1220mm about 4ft. No where near 5ft. The mediums produced 1040mm about 3ft 5ins, which is bigger than the quoted 'yard', while the smalls came up with 2ft 10 ins or thereabouts considerably bigger than their stated nominal size.

Of course the variation could be due to errors in AnyRail, or user errors, but I have done things twice. It seems very strange to me.

Btw - the set track points do exactly what it says on the tin.

I'd welcome any observations. No doubt it's all to do with geometry.

(and why does it take so long for what I'm typing to be rendered on this message board?)
(strangely I've edited the message after posting and the edit window works normally)

Thanks
 

martin_wynne

Western Thunderer
Hi Richard,

The Peco track-centre spacing is nominally 2 inches (50.8mm) with metal rail joiners. This dates from the original design of about 1960. If you use insulating rail joiners with a bit of plastic separating the rail ends, the spacing increases a bit.

The dimension you need for a full circle of turnouts is called the "substitution radius", also called the "external geometrical radius" in Templot.

Naturally Peco don't publish it, but it is actually 1162.34mm (45.76") radius*. To make them fit into such a curve you need to chop 13.77mm off the toe end of the turnout (the point-blades end).

Here's a diagram from Templot showing that. A 12-degree length of 45.76" radius, superimposed on a Peco Large Radius turnout:

peco_subs.png

*this is simple geometry and nothing to do with Peco. A 12-degree angle at offset 25.4mm calculates to

R = 25.4 / ( 1 - COS(12) ) = 1162.34 mm

regards,

Martin.
 
Last edited:

RichardS

Member
Thank you Martin (again). I have to be honest although I was allegedly reasonable at maths I never, ever understood Cos/Sin/Tan and so forth. I suppose it's because nobody ever explained it. Which is a long winded way of saying that while I recognise the factors I haven't a clue what it means, so, I trust you. (I do actually):)

So, although the arrangement of 30 LR points in AnyRail appears to be circular it is in fact a 60 sided polygon, of which 30 of the sides are straight and 13.77mm long. (That's assuming the software is clever enough to reflect this) The other 30 sides being curved.

Now I have the terms 'substitution radius' and 'external geometrical radius' I have had a look on line and see that once again this is a question which you have fielded before in other places. Apologies for causing you more repetition. I just didn't know what to look for.

The 'external geometrical radius' seems to me to be a pretty key number for many people who are trying to slot OTS points into their layout and puzzling why 'it don't always work as they expect'. I had always assumed that the 'nominal radius' quoted by the manufacture was, essentially, the radius. It is and it isn't. I think the manufacturer should make this clearer in their data. Although I suspect that for most people it is of little interest as they 'plug and play.'

Thanks for the Templot references. I'm still getting to grips with your s/w. For the friend I'm 'helping' (an imprecise term) this time, I'll continue with AR, it'll be close enough, though I will experiment with radii and so forth to see what happens.

Nice thing about railway modelling is that there's always something new to learn. Recent research has shown that people who undertake activities that require new learning as they get older are less likely to develop memory loss. (BBC website)

Now to get my 'O' level maths books out, trouble is I can't remember where they are.....

Regards

Richard
 

oldravendale

Western Thunderer
Nice thing about railway modelling is that there's always something new to learn. Recent research has shown that people who undertake activities that require new learning as they get older are less likely to develop memory loss. (BBC website)

Richard

I guess that's probably correct, although I find it helps if I write new stuff down.

The problem is that I can't remember where I put the notes when I need to refer to them.........

B
 

RichardS

Member
Coming back to my original problem, the reverse turn in AnyRail for a siding off a point, I reason that two pieces of track are needed a 12 degree arc with a radius of 1162.34 and a short straight of 13.77mm. I shall test it.
 

RichardS

Member
It works, of course the short straight is theoretical and not actually needed.
I shall now calculate the substitution curves for other units. (I'm sure it's been done but I'll only learn if I do it myself)
 

RichardS

Member
Doh! The substitution radius is the same for all three radii of point, as the angle and is the same. I'll get there one day!
The attached picture might be of interest to somebody.Peco Streamline Substition Radii.jpg
 
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