OO track geometry.
 

As some might know I am not an expert modeler, although I have just got out again a double-oval track I made a few years back. I have been amazed over the last three weekends that my kids (11 & 6) have switched off the telly and watched some authentically very dubious trains going round-and round- and round for hours. (e.g. GWR pannier tanks pulling LMS express passenger coaches with half a dozen Southern Railway vans on the back:D...........could it be turning into the S&D perhaps?

Anyway, I am looking at some minor track modifications and want some geometry. I know I could just wait for the bits when they arrive in the post, but I want to do some technical drawings, to get tangents and transition spirals. ( I'll leave out the superelevation calcs for now !) I've tried Hornby website - but no good. I dont want to buy a manual or a computer programme just for this.

Right then. Here's the maths homework...........There is a Hornby standard curved point which both legs appear to be 2nd radius. i.e arcs of two circles of radius 438mm. The centre of each of these circles will be at a different position. How far apart are the centres? 67mm perhaps?????

These things can be a bind! You can introduce a small degree of flexibility by removing some of the webbing from between the sleepers....just remember to make all joins on curves a smooth transition.....

I will have the answer on Saturday - if the excellent Hatton's mail order performs as usual. (I will also have a Queen Mary brakevan in Southern brown too :D )

I used to do highway design, but all the formulae were orginally derived from railway work. Pre-computers, we even drew our highway drawings with Railway Curves ! (I still remember the awesome day, when as a young trainee engineer I was finally trusted to borrow the company's mahogany box of brass railway curves from the Chief Draughtsman.... It sounds totally Victorian, but was only 1977)

Hmmm, do you think Parabolic transitions, or cubic-spline? And can Peco flexi track cope with the difference? ............Just give me a big enough hammer, and enough track pins :eek: !

I have studied the curved turnouts from the Hornby website. Apart from having strange arcs (22.5°ext. and 33.75°int.) the turnouts should be no different from other manufacturers' principles, i.e. the external leg is a straight equivalent to the distance between track centers plus a standard
1/4 r.angle arc, and the internal leg a constant radius arc. We know the internal leg is 438 mm radius because there is a compensation track of 11.25° in such radius. There is no point making it in 505 mm radius.
One can so develop the junction from single to parallel tracks in a 90° arc.
Cheers from Dario

Thanks dario and zwicky.

The aim was to see if I could squeeze in a loop line inside the second radius with enough space for an island platform, and without decresing the curve below 2nd radius. I have pasted a diagram below.

http://i8.photobucket.com/albums/a27...unk/Layout.jpg

Having now got all the bits and spent all day with bits of cotton and drawing a spider's web of arcs on the board, the answer to my question is 61mm (+/- 2mm) Incidentally one factor is that Hornby have used straight point blades rather than curved, which gives a slight flattening from a true circle.

I have used the "springy bit of piano wire technique" to let the new track loop find its own geometry (aka cubic-spline) rather than trying to set out elipses with two nails and a loop of string (almost y = x(squared) curves)

This isn't some fancy exhibition layout by the way. Just some old plywood and track for the kids to play with.

All this talk of "cubic-splines" and "the external leg is a straight equivalent to the distance between track centers plus a standard 1/4 r.angle arc" has got me totally baffled.:confused:
Now I know why I use Peco Flexitrack.......you just bend it to fit in the available space:)
P.S. Did your Queen Mary brakevan arrive from Liverpool with all its wheels intact or was it on bricks? :D
John

If you are going to stick with standad Hornby track you would be better to use 3rd radius track on the outside of that side of the layout. you need about 5" of additional width but you can get an island platform between the two. You could set up a complete loop which would add even more interest.

You need 1 single and 1 double 3 radius curve to set up the bay and double that to set up a loop (and another curved point)

I have spent hours experimenting with various combinations and it does help you to understand the geometry better. The Hornby Track Plans book is very helpful as well.

I am now in the process of designing a new layout which will use Peco Steamline track.

The other useful tool is the Tacksetta range of metal templates. I am now drawing the proposed plan out full size on wall paper backing paper and it amazinzing how it helps!

Hope this is useful.

John.

Well it is finished and the curves look great (to me) It only took me about 6 hours pin bashing :( I may have a new record in the number of times I had to remove and realign every pin.

The bad news is that, as dozens of model railway builders have found before me, curved points are a pain. My Bachmann Royal Scot sweeps solidly and majestically round the flexi track curves at full speed before launching into the air at the points. The pannier tank just falls over on its side at slow speed.

My attempts to reshape the plastic frog to stop the wheels hitting it were a failure, and my attempt to curve the point blades has put the whole thing out of gauge. These goes ten quid. :( I will try a Peco reaplcement and see if it is any better.

The reason why I was limited to second radius was that my board is only 1.0m wide, although if I ever rebuild it it will have be a 3rd radius width,

I agree, the "Tracksetta" templates are invaluable when using flexitrack.
John.

Being an N-scale modeller I didn't realise that Hornby's OO curved points were so "touchy" Is it a compatability problem? Will Hornby's own locos run over them smoothly?
John.

The usual problem with Hornby curved points is back to back wheel measurements. Both Hornby and Bachmann are not as careful as they might be with their settings. My Hornby 9F was so bad I snet it back to Horby! The good news is that I got it back within the week and it has been fine since then. It also runs fine on my club's layout which uses code 75 rail.

The back to back measurement should be around 14.5 mm........... I think!

On my new layout I have only used curved points where I have to!

It's a pity that OO curved points are so problematic.:( In N Gauge I don't know what I would do without curved points? There are at least 8 Peco curved points on my present layout and locos and stock of many different makes can traverse them smoothly at "well above" scale speeds.
John.

I have Bachman:- Pannier Tank, Royal Scot, Ivatt class 2 tank (new), and Hornby Black 5.

The intial problem was with the long rigid wheelbase of the pannier tank and the Scot hitting the frog. As in hitting a solid wall.... clunk.

I then shaved a bit (well a lot) off the frog and then found that the wheels started to drop into the much larger frog. So much so, that the pannier tank laid down on its side for a rest!

Pushing the Scot tender through the points gave noticably increased rolling resistance from a "pinching" effect. There is some side play in the Bachman wheels, but nothing like the Hornby Black 5

I then tried to ease out the gauge - but by this time I was using a cold chisel and lump hammer ;).

We spent the rest of the afternoon running all of the locos through the points OK in the trailing direction, once I had wedged the blades up with cardboard strips, but didnt try facing direction again.


Anyway, mindfull of probably some amazing scenic layouts in this forum , i darent show any photos. But imagine:- green painted MDF, with a grey paint ribbon as ballast for my layout that has to be carried out to the garage and stood on its end when not in use. My layout is as attached:-

http://i8.photobucket.com/albums/a27...nk/Layout2.jpg

ShedCat, don't be so critical of your layout :) As I said in another thread there are many people who miss out on all the fun that railway modelling can bring simply because their sights are set too high. You don't need a monster, all-singing, all-dancing layout to enjoy watching your favourite locos running. As it stands your track diagram gives plenty of operating potential. I hope you can sort out the passing loop problem.
John.

I would echo what Swisstrains is saying. Get in there and keep the train running. I have spent too much time fiddling around although the time spent on the full size plan will hopeully pay off. The Peco templates are great and they also do them for the set track which is the same geometry as Hornby/ Bachmann.

[IMG]F:\DCIM\100OLYMP\P9290113[/IMG]

This image shows work in progress on final full size design for my new layout using Peco and Tracksetta templates.

I cannot get the hang of inserting images into posts so will upload the photo in the Railway modelling galary

Here's the direct link to John H-T's layout planning image.
http://www.railwayforum.net/gallery/...php?photo=1604

Thanks John.

John H-T.

Corrr ........£10 + £10 + £10 (etc).... Some model railway shop owner is going be holidaying in the Bahamas this Xmas.. ;) (And there is going to be a sudden shortage of turnouts for the rest of us.)

As far as my design is going:- I think the Peco double curve turnout has been slightly better than Hornby, although there is not a lot in it really in terms of derailments. And I have proved by literally hours of juggling with the full sized pieces that it is impossible to do what I wanted with standard radius componants. It sort of works, but all my track joints have that irritating 1mm gap between the rails on one side.

I think Swisstrains had the best advice early on in this thread......forget the maths and get some flexitrack and just bend it till it looks right. Good tip. I'll remember next time.

Corrr ........£10 + £10 + £10 (etc).... Some model railway shop owner is going be holidaying in the Bahamas this Xmas.. (And there is going to be a sudden shortage of turnouts for the rest of us.)

Where do you buy your points, I certain will not be paying £10 for Code 75 24" radius points and thats before discount as long as I buy a few at a time!

Anyway base board first. Watch this space!

Still you can't deny John, there is some pretty complex pointwork going on there! Looks good. :)

Thanks Trev.

By the way the title for the layout is Wilford Junction for Wilford Wells.

Drat, I was forgetting the Wholesale discount, but I'll quickly claim that I was including the point motors and the interlocking. :D :D

But as a modeling beginner myself, I am seriously impressed.

I came across this article today. It appears to explain why many modellers have trouble with their locos and rolling-stock derailing on Hornby pointwork.
Being an N-scale modeller myself I haven't needed to try any of the solutions offered but it makes interesting reading.

Thanks for that John. it looks to be a useful site for other topics as well.

Best wishes,

John H-T