Track spacing calculator

[MartynJPearson]

Given a radius of a curve and the dimensions of my largest piece of rolling stock, I am trying to determine the minimum radius of a parallel curve, such that passing rolling stock on the parallel tracks don't collide.

 

I'd had a stab at the maths myself, then found an NMRA paper (https://www.nmra.org/sites/default/files/standards/sandrp/pdf/tn-7_track_center_and_obstacle_clearance_methodology_jul_2017.pdf) describing the calculations. However, is anyone aware of any online tool that does these calculations? I believe Templot does it, but as I'm not building track, just laying it I thought that might be a more complicated route to an answer.

 

Thanks in advance!

[Dungrange]

1 hour ago, MartynJPearson said:

However, is anyone aware of any online tool that does these calculations?

 

No.

 

1 hour ago, MartynJPearson said:

I believe Templot does it, but as I'm not building track, just laying it I thought that might be a more complicated route to an answer.

 

Templot sort of answers your question.  You draw a curve of your given radius and then add a test vehicle to it for which you define the length, width and distance between the bogie pivots plus a clearance envelope and the software draws parallel lines.  You can then make a second track at whatever offset you want and add your test vehicle to that track and you'll see whether or not you achieve your desired clearance or whether the two envelopes overlap and you can then redraw your outer radius as appropriate.

 

The alternative is to do it using a piece of track and the stock you actually want to use along with a pencil to mark things out!

 

[giz]

2 hours ago, MartynJPearson said:

 

I'd had a stab at the maths myself, then found an NMRA paper (https://www.nmra.org/sites/default/files/standards/sandrp/pdf/tn-7_track_center_and_obstacle_clearance_methodology_jul_2017.pdf) describing the calculations. However, is anyone aware of any online tool that does these calculations?

What about the spreadsheet that the paper describes?

https://www.nmra.org/sites/default/files/standards/sandrp/pdf/curved_track_center_and_obstacle_clearance_assistant_jul_2017.html

 

It's under RP-7 in the recommended practices.

 

https://www.nmra.org/index-nmra-standards-and-recommended-practices

 

 

[martin_wynne]

Here's a video clip showing the running clearance being adjusted in Templot:

 

https://flashbackconnect.com/Default.aspx?id=NEZusxJ3GUOLWCQ8XTWt7g2

 

Ask on the Templot Club forum for more info.

 

Martin.

[St Enodoc]

The NRMA method seems unnecessarily complicated. There are simpler mathematical ways to calculate the increased clearance required on curves, of which the most common uses the versine formula to calculate the "end throw" of a vehicle (on the outside of a curve) and the "centre throw" of a vehicle (on the inside of a curve).

 

To save you the trouble, some values were published in the Scalefour Digest many years ago. As I don't know what spacing you are using on straight track, I've presented these in the form of the necessary increase from the straight track spacing to maintain the same clearance between vehicles in 4mm scale:

 

Inner track radius (mm) => increase in spacing (mm)

 

Infinity (straight track) to 8800 => 0

8800 to 5867 => 1

5867 to 2933 => 3

2933 to 1760 => 5

1760 to 1257 => 7

1257 to 978 => 9

978 to 800 => 11

800 to 677 => 13

 

Hope that helps.

[DCB]

Your biggest problem will be in finding your worst case items of rolling stock. More modern items are better than the Triang-Hornby era but some of them crabbed along the track about 3mm out of line with 2mm play between wheel backs and chassis.  You can design so these monstrosities never clash or so they just clear in ordinary service.  It makes quite a difference.  The problem is by the time you have derived all the data you need to do the calculations by experimentation you already have the answer.  Personally I'm quite happy to accept I can't use coaches more than 57ft long or overscale width stock (OO 9F / Britannia)  if it lets me run 44mm spacing on straights and through pointwork.  For me over scale gaps between tracks ruins many other wise fine UK layouts.  Then again my current project mixes set track with 44mm spacing to save length so I am not exactly mainstream.

[St Enodoc]

12 minutes ago, DavidCBroad said:

Your biggest problem will be in finding your worst case items of rolling stock. More modern items are better than the Triang-Hornby era but some of them crabbed along the track about 3mm out of line with 2mm play between wheel backs and chassis.  You can design so these monstrosities never clash or so they just clear in ordinary service.  It makes quite a difference.  The problem is by the time you have derived all the data you need to do the calculations by experimentation you already have the answer.  Personally I'm quite happy to accept I can't use coaches more than 57ft long or overscale width stock (OO 9F / Britannia)  if it lets me run 44mm spacing on straights and through pointwork.  For me over scale gaps between tracks ruins many other wise fine UK layouts.  Then again my current project mixes set track with 44mm spacing to save length so I am not exactly mainstream.

My straight track is spaced at 45mm centre to centre (full size was/is 11ft 2in) and I increase that dimension on curves by the amounts I listed above.

[Joseph_Pestell]

3 hours ago, St Enodoc said:

My straight track is spaced at 45mm centre to centre (full size was/is 11ft 2in) and I increase that dimension on curves by the amounts I listed above.

45mm centres makes a big improvement to the look of the layout in most cases.

 

Appropriate for you as the broad gauge Cornwall Railway was single track. Elsewhere on the GW, track centres are often wider reflecting a former double track in broad gauge.

[MartynJPearson]

Many thanks for the information.

 

8 hours ago, giz said:

What about the spreadsheet that the paper describes?

https://www.nmra.org/sites/default/files/standards/sandrp/pdf/curved_track_center_and_obstacle_clearance_assistant_jul_2017.html

 

 

Doh, I'd seen the reference in the paper but for some reason, failed miserably to find that page!

 

5 hours ago, St Enodoc said:

To save you the trouble, some values were published in the Scalefour Digest many years ago. As I don't know what spacing you are using on straight track, I've presented these in the form of the necessary increase from the straight track spacing to maintain the same clearance between vehicles in 4mm scale:

 

My application is a 90 degree bend in a fiddle yard. By working out how far apart I have to space the tracks, I can work out how many storage loops I can accommodate in the space, so that is really useful - in fact I think it has meant that I could well have space for an extra loop. That will go down well when Mrs P says "Haven't you already got enough trains?" and I can point to the empty loop...!!

 

 

[St Enodoc]

13 minutes ago, MartynJPearson said:

My application is a 90 degree bend in a fiddle yard. By working out how far apart I have to space the tracks, I can work out how many storage loops I can accommodate in the space, so that is really useful - in fact I think it has meant that I could well have space for an extra loop. That will go down well when Mrs P says "Haven't you already got enough trains?" and I can point to the empty loop...!!

Good idea. Keep the wider track spacing on the straight bits too (so all the tracks are parallel all the time) and you'll have more room to get your fingers in when putting trains on or taking them off the track.

[Il Grifone]

24 minutes ago, MartynJPearson said:

Many thanks for the information.

 

 

Doh, I'd seen the reference in the paper but for some reason, failed miserably to find that page!

 

 

My application is a 90 degree bend in a fiddle yard. By working out how far apart I have to space the tracks, I can work out how many storage loops I can accommodate in the space, so that is really useful - in fact I think it has meant that I could well have space for an extra loop. That will go down well when Mrs P says "Haven't you already got enough trains?" and I can point to the empty loop...!!

 

 

 

How can you possibly have enough trains?