Peco OO/HO large radius point measurement query

[Grovenor]

Keith (Grovenor) I presently disagree. I used a formula to calculate the figure. It was online but I cannot find it at the moment. It's getting late! I also tested the result in practice with real points.

I haven't calculated the radius, no need to know, its just simple geometry, try making up a circle with 30 of the points and see if they are all the same radius! Actually you will probably see the effect clearly with 3 or 4. (Use the printed templates).

 

I'm also a little puzzled how two straight points with different substitution radii might join on a cross over and maintain the specified and desired track separation.

That has nothing to do with substitution radius, just with the 12 degree angle and offset.

 

Regards

[Grovenor]

Sorry Keith one other point, the length of the point you quote is surely the length of the straight leg, ie a section of the tangent, not the length of the arc? Correct me if I am wrong please?

Correct, as the arc length is not given, but the difference is negligible.

Regards

[RichardS]

Correct, as the arc length is not given, but the difference is negligible.

Regards

[RichardS]

Keith, I think talking at cross purposes. You're talking about the nominal radius whereas I have been discussing the substitution radius which is the same for all the points See Martin's post above.

[Grovenor]

Keith, I think talking at cross purposes. You're talking about the nominal radius whereas I have been discussing the substitution radius which is the same for all the points See Martin's post above.

Not at all, I am talking of the substitution radius, and Martin is wrong in this case! Unless his (and your) definition of substitution radius is different from mine.

My understanding of substitution radius is the radius of the circle that the turnout can be inserted into such that each end of the turnout is tangential to the circle. (It is a normal design feature of Settrack that turnouts are designed with substitution radii the same as the standard settrack curves).

With turnouts having a divergence angle of 12 degrees (one 30th of a circle) the formula to calculate the substitution radius is r = 30a/2pi (where a is the arc length).

For the three Peco turnouts: Long a = 259, hence r = 1237;  Medium a = 220, hence r = 1050;  Short a = 185, hence r = 883.  All in mm.

Regards

 

PS Actual numbers now filled in. Note, however as per Martin's further posts below not all of these are true substitutes without a kink at each end, and to make a full circle you may need 15LH and 15RH.

[Grovenor]

Martin Wynne said,

All Peco turnouts have an exit angle of 12 degrees at 1 inch (25.4mm) offset.

This means that all turnout sizes have the same substitution radius of 1162.24mm ( 45.76 inches ).

However the smaller sizes will need some additional approach track to reach the tangency point. On the other hand the large size turnout requires a bit chopped off to meet tangency.

IMHO its a very strange definition of substitution radius if you can't actually substitute without adding bits on or chopping them off, and it gives a very misleading impression of the radius, the figure given is neither substitution nor actual curve radius in the point, all of which, except the long point substitution radius, are significantly less.

See above.

Regards

[martin_wynne]

Not at all, I am talking of the substitution radius, and Martin is wrong in this case! Unless his (and your) definition of substitution radius is different from mine.

 

Hi Keith,

 

I think it must be. The only 2 variables needed to calculate the substitution radius is the angle turned through, and the offset dimension from the tangent at that angle.

 

Those two are the same for all Peco Streamline turnouts, 12 degrees and 25.4mm. So the substitution radius is the same for all of them. Called the External Geometrical Radius in Templot.

 

Where it gets confusing is that the toe of the turnout is not usually at the tangency point. In order to actually fit such a turnout into a perfect circle, some plain track must be added in some cases, or removed in other cases:

 

 

EGTP = External Geometrical Tangent Position

 

There is a video about substitution radius in Templot here:

 

 http://www.templot.c...os/subs_rad.exe

 

Set-Track turnouts are different, those do have the toe at the tangency point.

 

regards,

 

Martin.

[martin_wynne]

IMHO its a very strange definition of substitution radius if you can't actually substitute without adding bits on or chopping them off, and it gives a very misleading impression of the radius, the figure given is neither substitution nor actual curve radius in the point, all of which, except the long point substitution radius, are significantly less.

 

Hi Keith,

 

If you want to know the actual radius in the rails, you look at one of the other radius dimensions displayed in Templot.

 

For the Peco Large Radius turnout, the "turnout" radius is:

 

turnout-road centre-line radius (at turnout-curve) = 1137.88

 

This dimension makes no allowance for the deflection angle at the switch, which may or may not be relevant in determining which rolling-stock will run through.

 

regards,

 

Martin.

[rue_d_etropal]

a lot of big words, when it is just a simple case of radius and angles. I can imagine the original design team, coming up with something simple, ie a 4ft radius, a 3ft radius and a 2ft radius. What is odd, now I have redrawn my version of the point, is that there is an extra 3mm(1/8th in?) added to length of point. It does not really matter, and suspect there as some technical reason at the time. It does puzzle me why the info for the point suggests it is 5ft not 4ft radius.

[martin_wynne]

It does puzzle me why the info for the point suggests it is 5ft not 4ft radius.

 

As I said, it is called marketing-speak.

 

The internal geometrical radius (tangency to gauge-intersection) is 1176.10 mm  ( 46.3 inches ).

 

The substitution radius (tangency to exit) is 1162.24 mm  ( 45.8 inches ).

 

The actual radius in the rails would be: 1137.88 mm ( 44.8 inches ) using a 1:35 straight switch (which is an estimate on my part).

 

All of them are less than 4ft ( 48 inches ).

 

regards,

 

Martin.

[LNER4479]

To be frank the issues of PECO track have been discussed so frequently that if you are using RTR stock on 16.5mm you'd be better off enjoying your hobby rather than worrying about angles and geometries. PECO track works as evidenced by the 000's of layouts using it. Sometimes it's better to just accept things as they are.

At the risk of diving into the sort of discussion I usually only read from the sidelines, the piece of video at the bottom of this post may be of interest.

 

This is all Peco Code 100 and the principal curve in question is laid out on a constant 6 foot radius with transitions at either end. The first train seen (which was going a little faster than I thought it was when I set up the shot!) encounters in quick succession a trailing large radius Y, a facing large radius Y and a trailing single slip (adapted to fit the governing 6 foot radius). The second train, in the opposite direction, encounters a facing curved point, another modified, trailing single slip and a trailing curved point. I chose 6 foot radius for this curve as I considered that both the large radius Y and the outer curve of a curved point equated to approx. 6 foot radius.

 

Both locos are standard Hornby items but the stock is kit built.

 

I guess therefore that I am concurring with the above quoted comment

 

[Joseph_Pestell]

At the risk of diving into the sort of discussion I usually only read from the sidelines, the piece of video at the bottom of this post may be of interest.

 

This is all Peco Code 100 and the principal curve in question is laid out on a constant 6 foot radius with transitions at either end. The first train seen (which was going a little faster than I thought it was when I set up the shot!) encounters in quick succession a trailing large radius Y, a facing large radius Y and a trailing single slip (adapted to fit the governing 6 foot radius). The second train, in the opposite direction, encounters a facing curved point, another modified, trailing single slip and a trailing curved point. I chose 6 foot radius for this curve as I considered that both the large radius Y and the outer curve of a curved point equated to approx. 6 foot radius.

 

Both locos are standard Hornby items but the stock is kit built.

 

I guess therefore that I am concurring with the above quoted comment

 

 

A quick calc suggests that the nominally 6' radius Y is rightly advertised as such.

[martin_wynne]

Well you live and learn.

 

My previous statement that 30 Peco Small Radius turnouts would not join up if connected together was of course wrong:

 

 

The radius of a superimposed circle is 880.16mm ( 34.65 inches ).

 

I have no idea what this radius is called, nor have I ever considered adding it to the displayed info in Templot. But I will now think about adding it, so thanks to Simon for making me think about this.

 

 

It can't be called the "substitution" radius because the turnout could not be substituted into such a curve. That's because the rail joints are not actually located on the curve or tangential to it. They are not off by much, see at B above, but enough to make a rough joint and some misaligned track if you tried it.

 

The actual tangency point is at a position within the turnout, at A above. The white line is the superimposed circle.

 

I'm not sure where this leaves anyone designing a layout using these turnouts, but it's amusing to play with in Templot.

 

regards,

 

Martin.

[Pacific231G]

a lot of big words, when it is just a simple case of radius and angles. I can imagine the original design team, coming up with something simple, ie a 4ft radius, a 3ft radius and a 2ft radius. What is odd, now I have redrawn my version of the point, is that there is an extra 3mm(1/8th in?) added to length of point. It does not really matter, and suspect there as some technical reason at the time. It does puzzle me why the info for the point suggests it is 5ft not 4ft radius.

I'm pretty sure that for Peco the medium radius turnout was the basis from which the others were derived. In the 1950s points with a nominal three foot radius seem to have been almost standard for "average modellers" in 00 modellers who built their own. Laying a Peco medium radius over the template for an SMP three foot radius turnout gives a very similar geometry with a 12 degree crossing angle and the same one inch divergence (which was the BRMSB standard) the Peco turnout is though slightly longer from the nose to the point of the switch blades.

 

I've also been comparing Peco points with each other and was slightly surprised. The switch itself appears identical for the the medium and large radius points so the deflection angle would be the same. The switch blades are the same 72mm length for all three (and for the three way and Ys) but with a sharper radius for the small radius point as you'd expect. The crossing (frog) is identical for the small and medium radius points but is longer for the large radius with the track curved beyond the V to get to 12 degrees.

For mass production it obviously makes sense to use as many common components as possible but I've been wondering how  much this was/is also common practice in prototype PW circles and whether British practice was different from other countries.

 

I know that before high speed turnouts appeared SNCF settled on five standard crossing and three standard deflection angles* with  maximum speeds on the diverging route depended on the crossing angle. I happen to have the list of turnouts used in the old Bastille terminus in the 1950s where most of the turnouts had the tightest 1/7 crossing normally used only in yards and restricted to 40kph  (some were 1/9) but are listed as being long or short which makes it look like an order from a model catalogue. I have the impression though that in many countries pointwork had always been based on standard pre-fabricated units assembled on site rather than being hand laid in situ.   

 

 

* In the 1960s SNCF's five standard crossing angles ranged from 1/30 to 1/7 (with a sixth 1/6 for symetrical i.e. Y turnouts in yards)  and three standard deflection angles of 0deg18minutes (for 3000m radius), 0deg25minutes for 2000, 605 and 250m radius and 1degree for 190m radius turnouts. Speeds limits were controlled by the crossing angles and ranged from 125kph for a 1/30 turnout to 40kph for a 1/7 (and 30kph for the symmetrical 1/6)   It does strike me that most of us would consider a #7 turnout to be rather generous in 00 or H0.   

[rue_d_etropal]

Thing to remember is that the original Peco points were designed without the help of computers, so probably a bit of fudging involved. The small radius is pretty near to 2ft radius with a straight piece on end. The medium radius is 3ft with a shorter extension. The large radius does not need the extension, which is why I thought it might have been designed first, otherwise it is a bit of luck/coincidence that it fits. The point design has been modified over the years, but when I put my own drawings against actual points they match up.

When I am planning a layout I tend to start ith a simple sketch, and then try it out with templates and pieces of track. Given I am usually only building small layouts this tends to work best for me. When I have used layout design software in the past, it still needs the full size mock up, so quicker to miss out the software step, unless I happen to want a detailed drawing. The CAD program I use is capable of doing any layout design I require, and then can be saved off as an image, and 'painted, then printed off as image, not just a plan. I used this for the Scarborough dock mock up a few months ago.

[shortliner]

It has nothing to do with your original post, Simon, but I do know for a fact that PECO Setrack turnouts are currently a larger radius, and totally different length than the earlier ones - I have never set out to check the Streamline series to see if PECO changed them too. I have no knowledge regarding the thinking behind the change, unless it was to get all manufacturers ranges "Standardised", although it may have happened when the "radius zero, 10-and-a-bit-inch" curves were dropped from train-set ranges - although they were still available from ROCO until relatively recently.

[martin_wynne]

The first Peco Streamline turnout was shown at the Toy Retailers Fair in 1961, see:

 

From the Railway Modeller.

 

No dimensions given, but counting the timbers (24) makes it shorter than the current Small Radius turnout (25 timbers). Flexible switches from normal rail section, rather than the current loose-heel stampings. The moulded check rails are longer than current production.

 

Martin.

[Silver Sidelines]

To build my layout I drew up and cut out cardboard templates for the curves that I thought I might use. I used these to mark out the centreline of the track bed on the layout and then laid the track along the lines – by eye. This technique does not have the decimal accuracy of computer generated images but I would say is a good quick way of building a layout in a large area.
The following pictures I think are instructive and show one of my Peco Streamline Large Radius Points, which is then overlaid first by a five foot radius template and then by a four foot radius template. On the template the inner curves were drawn to 59 and 47 inches respectively whilst the outer curves were made 61 and 49 inches.

 

The Peco Large Radius point showing I think a smooth curve through the majority of the point.

 

 

The 59 inch radius of the five foot template looks to me to be a good match for the inside rail of the Large Radius Point.

 

 

The 47 inch inner radius of the four foot template looks to me to be far too tight for the inside rail of the Large Radius Point.

[Joseph_Pestell]

The first Peco Streamline turnout was shown at the Toy Retailers Fair in 1961, see:

 

From the Railway Modeller.

 

No dimensions given, but counting the timbers (24) makes it shorter than the current Small Radius turnout (25 timbers). Flexible switches from normal rail section, rather than the current loose-heel stampings. The moulded check rails are longer than current production.

 

Martin.

 

I am fairly sure that is an engineering prototype. I don't recall production Streamline points ever having flexible blades.

[martin_wynne]

I am fairly sure that is an engineering prototype. I don't recall production Streamline points ever having flexible blades.

 

After only about 2 or 3 years they changed to what they called the MarkII design, starting with the short Y-turnout. These had the holes for the clip-under point-motor and chamfered ends on the vee rails to make the dead frog area as short as possible. The rails were moulded in situ by insert moulding -- making it impossible to have flexible switches.

 

My recollection was that after the short MarkII Y-turnout appeared there was a long wait for the straight turnout to return to production in MarkII form, leading to various daft track plan designs being published comprised entirely of short Y-turnouts.

 

Martin.

[Colin_McLeod]

Peco medium radius point with 36" tracksetta template

[Grovenor]

Martin said,

 

 

I think it must be. The only 2 variables needed to calculate the substitution radius is the angle turned through, and the offset dimension from the tangent at that angle.

Those two are the same for all Peco Streamline turnouts, 12 degrees and 25.4mm. So the substitution radius is the same for all of them. Called the External Geometrical Radius in Templot.

Where it gets confusing is that the toe of the turnout is not usually at the tangency point. In order to actually fit such a turnout into a perfect circle, some plain track must be added in some cases, or removed in other cases:

Apologies Martin for saying you were wrong, but I still think the definition wrong! "Substitution" I don't believe appropriate if it involves modifying the substitute. Your term of "External Geometrical Radius" is better. In effect substitution can only be done exactly if the 'Intersection point' is exactly on the bisector of the angle, hence fixed for Peco points by the angle and offset. Mind you, if you use equal numbers of left hand and right hand points you would be able to generate the circle without the slight kinks at the joins.

Regards

Keith

PS, glad your circle had the same radius as my calculation.

[RichardS]

Post #38.

Martin said: "I'm not sure where this leaves anyone designing a layout using these turnouts, but it's amusing to play with in Templot. "

 

It may be, Martin, that it leaves us/them/whoever, (the vast majority probably)  as I suggested earlier, using PECO track as it's designers intended, not worrying about angles and things and just getting on and having fun with their trains.  ​

 

For those, who are fewer in number, like yourself, Grovenor, AndyID, Hayfield to mention but 4 contributors to these track threads, who have a great knowledge of the technical aspects of track design, wheels and geometries etc the detailed debates are probably stimulating. If nothing more they certainly illustrate to and educate others, and I use myself as the sole example here lest I cause offence to anybody else, about the subject. Though I don't fully understand it. Is life long enough?

 

Perhaps anybody who is curious about or questions PECO, or other, rtr track standards and geometries is already moving beyond using those products towards different track they might make themselves with, perhaps also, a gauge other than 16.5mm (I have deliberately not used the words: right, accurate, prototypical, or alluded to any ordained standards. :-) )​

​

For folk who want to plan a layout with any of the rtr track systems software such as Anyrail software may be helpful.

 

 

 

 

​

[RichardS]

Re Post #36 by LNER4479   ( sorry about this style but I can't get the reply quoting a post to work presently - it used to)

 

You layout looks great, It works for me, it works for you and your colleagues, people are smiling, having fun, What's not to like? And you've got a layout up and running. Hope your show went well. :-)

[martin_wynne]

It may be, Martin, that it leaves us/them/whoever, (the vast majority probably)  as I suggested earlier, using PECO track as it's designers intended, not worrying about angles and things and just getting on and having fun with their trains.

 

Hi Richard,

 

I was thinking there about the OP Simon who asked the question about Peco turnouts, presumably because he needs to know about angles and dimensions.

 

I agree that for most users the answers aren't relevant to their layout building. But it is odd that it is such a guessing game with Peco, when most manufacturers would simply post the drawings on their web site -- see Tillig for example: http://www.tillig.com/Produkte/produktinfo-85313.html

 

regards,

 

Martin.