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.