Double Track Centre in Curves for new TT layout

[justintime66]

I am planning to build a TT:120 layout in the attic and having settled on a general plan I am pondering what double track centres to use in the curves.  Minimum radius in current plan is 550mm with transition curves in most places. I will be using Peco track. Medium radius points give a "natural" double track centre of 35.5mm (at least as far as SCARM library goes). Critical Rolling stock will be A4, pullman / BR Mk 1 coaches and HST with MK 3 coaches. Intention is that HST is only modern train, so at least I don't need to be driven by clash between two sets of Mk.3 coaches.

I am currently using geometry to calculate theoretical separations between rolling stock at my planned radii. I'll confirm some of my maths with test tracks when A4, pullmans & peco track arrive this week. But unfortunately the crucial piece in the jigsaw (Mk.3 coach) is not due for release until summer 2024 (strange given Class 43 locos available this autumn according to Hornby).The Peco track geometry would give a gap between parallel running stock of 11.5mm on straights. So practical question what should minimum gap be for planning purposes (giving a descent margin for build & running tolerances and bearing in mind I won't be able to test with Mk.3 when laying track / building some of bridges).

I have considered setting double track spacing in curves so that the 11.5mm gap between critical case stock in curves is maintained. However, that would increase track separation more than necessary and the gap of 11.5mm would only be visible when both HST & pullman trains were running.

I note that Hornby's new trackwork has 43mm centres (same as Tillig I think). So 43mm may be the safe answer. But tracks do look a bit too far apart when I put my current limited rolling stock Vent van, brake van & petrol tanker) on 43mm separated tracks. 

[andyman7]

I'd be inclined to flex the track spacing so that it is wider on curves - plenty of prototypical situations allow the track spacing to diverge a bit. Given that you are already planning transition curves the adjustment in spacing can be achieved with a natural look

[Flying Pig]

My guess is that you would be fine using the Peco track spacing at that radius.

 

I can get a better intuitive feel for things by scaling up to 00 (a factor of 120/76.2).  A track spacing of 35.5mm becomes 56mm in 00, which is rather more than the 50mm given by a Peco Streamline crossover.  Or, going the other way, the 00 spacing of 50mm is equivalent to just under 32mm in TT:120.  So the Peco track spacing in TT:120 is a little more generous than in 00, pro rata.

 

BTW standard gauge lines in Britain were traditionally laid to minimum centres of 11' 2" - 28.4mm in 1:120 scale.  This was the minimum though and often tracks were further apart (and had to be in some circumstances).

 

As to the curve radius, a curve of 550mm in TT:120 scales up to 866mm or 34" in 00.  I doubt that would cause problems on double track at 50mm spacing, even with Mk3s.

 

The caveat is the possibility that some parts of TT:120 rolling stock need to be wider than scale (due to scale gauge, but overscale wheel treads).  However, these are likely to be the bits that wrap around the wheelsets (e.g. bogie frames and cylinders) and not the bits that swing and overhang on curves.

[Chimer]

I'm guessing, but assume (!) the Hornby track separation is driven by (a) their points turning out 22.5 degrees as per usual for sectional track and (b) their choice of radii, which will be tight.  The Peco points presumably (!)  only turn out something like the 12 degrees they use for 00 Streamline, and you will presumably (!) be using gentler curves, except maybe for 90 degree turns in hidden corners where it's easy to open out the track spacing by extending the straights on the outer circuit and using the same radius curves on both (as drawn many times in other threads by @DCB).  So I'd stick with the separation generated by the Peco crossovers as my baseline.

[PeterStiles]

Hornby TT:120 point are 15 degrees - the geometry is different to OO.

[Flying Pig]

1 hour ago, Chimer said:

I'm guessing, but assume (!) the Hornby track separation is driven by (a) their points turning out 22.5 degrees as per usual for sectional track

 

No, Hornby TT:120 points are unique in the context of UK sectional track systems in that they don't fit in with the radii and angles used for plain curves (which are themselves also unique as I understand things).  I think they have broadly matched the geometry of Tillig points, which seems unnecessary to me and means existing sectional track plans and patterns can't be transferred to TT.  A very odd decision imo. 

[justintime66]

On 06/06/2023 at 16:41, Chimer said:

I'm guessing, but assume (!) the Hornby track separation is driven by (a) their points turning out 22.5 degrees as per usual for sectional track and (b) their choice of radii, which will be tight.  The Peco points presumably (!)  only turn out something like the 12 degrees they use for 00 Streamline, and you will presumably (!) be using gentler curves, except maybe for 90 degree turns in hidden corners where it's easy to open out the track spacing by extending the straights on the outer circuit and using the same radius curves on both (as drawn many times in other threads by @DCB).  So I'd stick with the separation generated by the Peco crossovers as my baseline.

Yes Peco TT points turn 11.25 degrees.

Edited June 12, 2023 by justintime66

[Chimer]

Message to self:  Never assume!!