Geometry of curved point.

[edcayton]

Can someone please enlighten me as to how these work please? Are both legs true arcs? What radius are they and are they "tangential" for want of a better word? I'm sure the answer is in Templot, but I'm not clever enough for that, so need words of one syllable (or pictures). Is the Peco setrack one the same?

 

Thanks in anticipation

 

Ed

[Joseph_Pestell]

In terms of geometry, I really don't know. Theoretically it is impossible so  they must have cheated somehow.

 

I have always found them very unreliable as a facing point and advise against them.

[28XX]

I asked this question to Peco technical 20 years ago before the interwebby and got a fudged answer. In short they are a mixture of curves of varying radii and straights. Normal mortals should work with them empirically, as you say, for further edification see Templot.

[Hilux5972]

Basically from what I can work out, the inner track is 2nd radius while the turnout section is 2nd and a half radius. Since the turnout connects a 2nd to a 3rd radius curve that's the best I can come up really lol. Personally I avoid them and just stick to the standard straight points or the express straight points.

[66C]

First of all - which curved point?

 

For OO gauge:

 

The setrack version is a combination of a short straight and second radius curves.

 

The streamline version is a fairly good combination of the two different radius curves.

 

Bear in mind that drawings are available from the Peco website if you need to see if they fit your track layout.

 

Regards.

[Joseph_Pestell]

First of all - which curved point?

 

For OO gauge:

 

The setrack version is a combination of a short straight and second radius curves.

 

The streamline version is a fairly good combination of the two different radius curves.

 

Bear in mind that drawings are available from the Peco website if you need to see if they fit your track layout.

 

Regards.

 

As this is in the Hornby subforum I think that we can safely say that we are talking the nominally R2 Setrack type.

[Hilux5972]

One would assume he is referring to the R8074 and R8075 points.

[66C]

One would assume he is referring to the R8074 and R8075 points.

 

In that case the Hornby web site has drawings of all their track available here:

 

http://www.Hornby.com/media/pdf/Track-Geometry-PDF.pdf

 

Regards.

[edcayton]

In that case the Hornby web site has drawings of all their track available here:

 

http://www.Hornby.com/media/pdf/Track-Geometry-PDF.pdf

 

Regards.

 

Thanks for that Bill. So it would seem that the inside (longer) leg is radius 2 and the outer (shorter) is radius 3? That is helpful, but are they both true curves from the toe of the point? and also I repeat my question does the Peco point conform to the same geometry?

 

Thanks to all who have shown interest in this.

 

Ed

[Hilux5972]

According to AnticsOnline the Peco ST244 is equivalent to Hornby R8074 so yes they match.

[34theletterbetweenB&D]

... but are they both true curves from the toe of the point? and also I repeat my question does the Peco point conform to the same geometry?...

You can answer the first question by just sighting along the rails. It is easy to see on the standard R2 set track point that the diverging road through the crossing is a sum of straightish and curved sections, averaging R2 between the two connectors so that the piece substitutes for a constant radius R2 curved piece. Not owning a curved set track point I cannot do this for myself, but I'm betcha 'twill be much the same.

 

The Peco version will very similar because it has to adhere to the substition radius to work; but they may have chosen a different 'mix' to get there.

[66C]

Thanks for that Bill. So it would seem that the inside (longer) leg is radius 2 and the outer (shorter) is radius 3? That is helpful, but are they both true curves from the toe of the point? and also I repeat my question does the Peco point conform to the same geometry?

 

Thanks to all who have shown interest in this.

 

Ed

 

Hi Ed

 

Once upon a time Triang-Hornby as they were then published the details of the curved points as part of the information in their track plans book.

 

The inside curve is equivalent to one and a half second radius curves - that is 33.75 degrees.

 

The outer track is a short straight section approximately equivalent to the standard track spacing - that is 67mm, followed by a standard second radius curve - that is 22.5 degrees.

 

If you compare sizes from the Hornby diagram you should be able to see this.  It does not help, however, that the half second radius curve used in the curved crossover is not labelled - this does not help a prospective purchaser.

 

As far as I know Peco have made their set track to the same dimensions so as to be compatible with Hornby - I suspect, if I remember correctly, that this was to sell nickel-silver rail into the set track market when the Hornby rail was still steel.

 

Regards.

[sub39h]

I can confirm that Peco set track curved points sold today and Hornby curved points are exactly the same. I swapped a Hornby curved point for a Peco one on a train set I made for my cousins' kids a year or two ago.

 

Beware however that older Peco points (not from this century I don't think) have a different geometry. Basically if you're buying new don't worry about it, they are completely interchangeable.

[RAF96]

Just looking at the Hornby geometry download and it is plain to see that R8075 has its inner leg rad = R2 and R8074 has its outer leg rad = R3.

 

The legs in between when used as a curved R2-R3 cross over mean there is a gap that by eyeballing it suggests could be bridged by R643 (R2) but from feedback seen some say it fits and others say it doesn't and they had to use a bit of flexy. 

 

Logically one would assume the outer leg of R8075 is great than R2 but less than R3 and the inner leg of R8074 is the same, (i.e. R2-1/2) which is probably why R643 isn't a good match.

Edited April 4, 2016 by RAFHAAA96

[MrTea]

The bit that the Hornby PDF doesn't show is how to use them other than as a crossover between R2 and R3 tracks. There's actually a useful diagram on the back of the packaging. I'll try and take a photo later and post it on here.

[andyman7]

I can confirm that Peco set track curved points sold today and Hornby curved points are exactly the same. I swapped a Hornby curved point for a Peco one on a train set I made for my cousins' kids a year or two ago.

 

Beware however that older Peco points (not from this century I don't think) have a different geometry. Basically if you're buying new don't worry about it, they are completely interchangeable.

The older Peco Setrack curved points were indeed of a different geometry - they were based on 1st radius curves.

[MrTea]

Ed,

 

Hopefully this diagram (via Model Railways Direct) will help?

 

I got a couple of these the other week and they make quite a nice curved loop.  The only thing you have to watch for is that the frogs are quite long and therefore wheels do drop into the gap as they pass over.

 

Ian

[edcayton]

Thanks for all the replies so far. The comment about old Peco points being 1st radius is useful, I think that may be part of my confusion. My son keeps telling me I am SO last century, that may be part of the explanation.

 

Ed

[cromptonnut]

Interesting, when I'd used the setrack curves in a fiddle yard setting before I'd just put 8074 and 8075 straight together without a 643 in the middle ... I wonder if that's why I had no end of derailments with certain locos.

 

Funny what you learn 20 years later... RTFM springs to mind

[57xx]

The Peco curved point on my test track has a short straight section in it around the tip of the switch rails, it is not a uniform curve. Not sure which part number it is, but it is Code 100.

[cessna152towser]

When I built my layout in 1989 I needed to use a curved point in the station throat to fit the track plan onto the available space.   Used a Hornby one which I later replaced with Peco Setrack which was an exact fit.   I guess the curvature on both forks of the point is at best second radius as it is at the limit of tightness for some of my Hornby and Bachmann steam outline locos.    I have found that some locos are either left or right handed in that some will only negotiate the point with their smokebox pointing towards the fiddle yard while others can only go through it with the smokebox pointing away from the fiddle yard.

Edited April 6, 2016 by cessna152towser

[Pete the Elaner]

I have found that some locos are either left or right handed in that some will only negotiate the point with their smokebox pointing towards the fiddle yard while others can only go through it with the smokebox pointing away from the fiddle yard.

Back to backs are well worth checking. They are often badly out on brand new models & curved points will highlight the issues more than straight ones.

[lazythread]

Athough billed as second radius, the inner path seems to be a composite, and not a true arc. In places it's more first radius than second, particularly around the frog. Bachmann and Hornby 9Fs baulk at them, as do many other medium / largish steam locomotives, they're ok on the outer path.

[Akerbill]

I found this useful - from Hattons Widnes.

[GeraldH]

This very informative webpage illustrates the potential uses of Settrack curved points, as well as regular Settrack points.

 

https://jonscaife.com/model-rail/track-geometries-for-00-gauge-Hornby-setrack/