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DT
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Discussion Starter · #1 ·
The cant of a railway (also referred to as superelevation) is the difference in elevation between the two rails. A cant which is not equal to zero results in a banked turn, allowing vehicles to traverse the turn at higher speeds than would otherwise be possible.

On railways cant helps a train steer around a curve, keeping the wheel flanges from touching the rails, minimizing friction and wear.

The amount of cant must be chosen for a given speed, and if trains traverse the turn at different speeds, the cant ceases to serve its purpose, and can lead to damage. As a result, a compromise value of cant must be chosen during design.

The maximum value of cant (the height of the outer rail above the inner rail) for a standard gauge railway is about 6 inches (150 mm).

Ideally, the track should have railroad ties (sleepers) at a closer spacing, and a greater depth of ballast to accommodate the increased forces exerted in the curve.

At the ends of a curve, the amount of cant cannot change from zero to its maximum immediately. The cant must change (ramp) gradually in a track transition curve. The length of the transition depends on the maximum allowable speed - the higher the speed, the greater length is required.

The main functions of cant are

* better distribute load across both rails
* reduce rails and wheel wear
* neutralize effect of lateral forces
* improve passenger comfort



So this is my attempt at superelevation. I'm using the Tillig (part 86613) banking strips that I bought from Lokshop. I find that if I lead the rail off the flat foam underlay on the flat straight onto the thick edge of the banking strip then the levels are maintained quite well.

Note that the inner rail on the left is a passing siding. Not designed for high speed and therefore is laid on flat foam underlay. You can see that the two outer rails on the right are laid on the banking strips.

These (below) are the Tillig banking strips seen edge-on.



[Definition source: wikipedia]
 

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In depth idiot
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It does look good, but is it just me, or is it the outer track on the right that is laid flat?. There is inevitably a twist or'wind' on the track in the transition from straight and level to fully canted curve. I try and accomodate this smoothly over the 400 - 500 mm of transition curvature from straight to the constant radius of the curve, as this comfortably exceeds any vehicle length so the wind never risks lifting a flange clear. (Technically, the inside rail should drop 1mm, the outside rail rising 1mm, so that centre of gravity of the vehicles stays at near constant height, but this is something of a counsel of perfection in model form.) Once the track is ballasted and has a train on it the appearance will be very striking, particularly if there are vehicles standing on an adjacent uncanted track.
 

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Looks excellent Doug.

Thanks for the info.

Anyone know if such technique has always been used or is a modern addition to the prototype

TimP
 

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Just another modeller
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*** It does look good doesn't it - and model trains look really, really nice on a superelevated curve too, so I'm looking forward to seeing a video sometime soon! Well done Doug.

Tim - its always been there, although possibly somewhat less scientifically calculated and laid in place than it is now - I have many old photo's (1920~1939) of my chosen layout area where the track is visibly superrelevated or a train is clearly "leaning into a curve"

Kind regards

Richard
 

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DT
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Discussion Starter · #7 ·
Going around the bend at full speed is impressive. The tilt is noticeable, but I asked my kid if he noticed anything and he said "no", but he wasn't looking for tilt. I think he expected lights and fireworks or something. I'll post a video soon.

I'm very happy as I have completed exactly half my mainline. Loco yard up to the mainline, return-loop and half the exposed mainline. I tidied the track completely, removing tools, wires debris and all the rest of the junk that accumulates when you're working on the benchwork. It really does look good.

My wife deserves a medal for allowing me to use this room in the garage for a layout. I have a massive area to play in, and when the mainline is complete then the trains will certainly be tested to their limits.
 

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Does anyone know if anything similar is available for N Gauge ??

I've been giving this matter some thought for a while and was contemplating an extra layer of 1mm thick insulation (the sort that goes under wood flooring) under the outside rail. This would, however, still leave the matter of transitions but I haven't come up with a satisfactory solution for that so far, other than just packing up the sleepers with ballast.
 

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QUOTE (poliss @ 22 Nov 2008, 04:54) <{POST_SNAPBACK}>For N Gauge there is the Kato superelevated track.

Unfortunately my whole design is based around Peco Code 55 and I have already bought the track.

I suppose, however, that the angle of 'cant' is constant for all scales so it would be possible to just use narrower strips of the Tillig material.
 

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QUOTE (Expat @ 22 Nov 2008, 06:42) <{POST_SNAPBACK}>Unfortunately my whole design is based around Peco Code 55 and I have already bought the track.

Hi Expat,

Nothing unfortunate using code 55 it looks a a lot better and prototypical

The Tillig looks fine in narrower strips ......... or use some very thin balsa sheet to pack evenly the edge of the track , cut over size on the outer edge the glue applied nearer to the track and not under the oversized part a blade held at an angle afterwards to cut away the excess forms a camber for the ballast the watered down pva holding the ballast in and soaking into the balsa underneath.
 

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QUOTE (Expat @ 22 Nov 2008, 08:06) <{POST_SNAPBACK}>Does anyone know if anything similar is available for N Gauge ??

I've been giving this matter some thought for a while and was contemplating an extra layer of 1mm thick insulation (the sort that goes under wood flooring) under the outside rail. This would, however, still leave the matter of transitions but I haven't come up with a satisfactory solution for that so far, other than just packing up the sleepers with ballast.

*** Hi Trevor

Those Tillig strips are an excellent answer for flat top baseboards but if you are using a cookie cutter approach as I do it is simple with nothing to spend at all....

I actually add superelevation to the strip of wood that the track is laid on - done this way, the track bed has a completely natural transition as it twists from flat to superelevated as the wood twists very naturally from its last flat & secured point.

On double track you should theoretically split the trackbed and do each separately, but its not essential - we are building a gentle cant, not a banked race track.

I make my track securing risers from a bit of 20x40mm glued to a 12mm ply strip to form an L. (easy way to make a lot of them is to Screw & Glue a long length of each together then cut into 100mm risers with a drop saw). I then do a slight "shaving cut" on the face where the two meet/the track bed will go to make a perfectly flat surface.

I have two ways of making the angle happen using these.

(1) Secure it fully to the trackbed, gently twist it to the correct angle as it is screwed into place....
(2) install it square to the baseboard member, and then add a packer of the right thickness under the trackbed one side then screw it down tight to force the twist to happen.

Both work 100% of the time - I usually use 2 for the first riser and 1 for the rest if access is good.

Because I always use risers everywhere (even my flat areas are raised above baseboard frames) then my cost is effectively zero money and only slight added effort for adding superrelevation!

regards

Richard
 

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I use the Peco 55 track on Gaugemaster/Noch ballasted underlay and insert a strip of styrene of the appropriate thickness under the outer rail on curves. In my view this gives a good representation of a modern superelevated track with a good ballast shoulder, though it would be less appropriate for steam-age track. However I think the styrene strip would still work under normal ballast.

I agree you have to watch the transitions to and from superelevated track - best to check that your longest fixed wheelbase vehicle runs through at more than maximum speed without derailing, before you fix anything down.
 

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DT
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Good advice Edwin. I see nothing wrong with raising the outside rail with card or something similar. I glue my track down with PVA and use thumb-pins to hold it in place. You can test the track before glueing as long as the pins aren't too close to the inside edge of the rail. Raise the outer rail by a couple of mm on the curve and by say 1mm on the transition. If you attach your track to the baseboard with pins though, this may cause problems and bend the sleepers.

What these Tillig track banking strips are effectively doing though is raising the outer rail a little and dipping the inner rail a little. The transition is not so severe and no danger of bending the sleepers.
 

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Found a good picture of gentle elevation.

 

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Unfortunately the information supplied to Wikipedia is incorrect. The statement "On railways cant helps a train steer around a curve, keeping the wheel flanges from touching the rails, minimizing friction and wear" is factually wrong.

Cant does not help to steer a train around a curve. The converse is true. Cant does not keep the wheel flanges from touching the rails, minimizing friction and wear. The converse is true. Cant causes flange friction and wear.

It is cant deficiency which provides steerage to help a train around a curve, keeping the wheel flanges from touching the rails, minimizing friction and wear.

The author can be forgiven for getting it wrong because what might appear obvious is not necessarily true.

The wheels of a rail vehicle are rigid on the axle and do not behave like wheels of an automobile which can rotate independently. Essentially, railway wheels want to travel in a straight line. When a bogie or 4-wheeled wagon traverses a curve the leading wheels run along the outside (high) rail while the trailing pair of wheels run along the inside (low) rail. The leading wheel flange is at an angle to the high rail and literally chews into it. Because the friction between the leading outside wheel and the rail is high, the inside leading wheel is forced to rotate faster than the vehicle is travelling causing mushrooming of the rail head. Strangely if the cant is increased the effect becomes worse. Increasing the cant only tips the trailing wheels harder against the low rail which in turn increases the angle at which the leading outside flange attacks the high rail. Sidewear on the high rail increases as does mushrooming of the low rail. Wheel/rail squeal also increases.

If the cant is reduced sufficiently, causing cant deficiency to increase, the trailing pair of wheels will lift away from the low rail. This reduces the angle at which the bogie or wagon traverses the curve. In turn this reduces the angle at which the leading outside flange attacks the high rail.

It might seem difficult to comprehend but reducing the cant actually reduces rail wear and rail/wheel noise. The life of rails on numerous curves has been extended significantly by taking this approach. Curves at Northam (Southampton) and Wandsworth Common are two that come to mind.

Normal design practice in the UK is for only two thirds of the equilibrium cant to be applied to the track. The remaining third is taken up as cant deficiency. This practice will not be found in any design rules but might appear in some guidance notes. The reason there is no standard is because it is not possible to set a mandatory minimum speed at which trains will traverse a curve. Where trains run at a variety of speeds or signals impact on operation the application of cant can be a difficult decision. In BR days it was mandatory that all changes of cant were approved by the regional Chief Civil Engineer.

Paul Plowman

About me: http://www.mrol.com.au/AboutUs.aspx
 

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I think what Paul is saying applies only up to (or possibly only beyond) a certain point.

Disregarding the flange for a moment, the wheel tread is actually tapered like a slice of a cone. At low speeds on gentle curves the wheelset can position itself laterally so that the wheel radius at the point of contact is greater on the outer rail than on the inner one, proportionate to the difference in radii of the two rails. Thus neither wheel will be slipping, despite the outer wheel having to travel further. If the conicity of the wheel and rail are correct then they will settle down in an equilibrium position.

If you increase the speed (without increasing the curvature or adding cant) then centrifugal effects push the wheelset outwards from the position described above, so that a wheel will be slipping by a tiny amount and therefore cause wear on the wheel and the rail. Adding some cant will fully correct for this effect at a particular speed, over-correcting at lower speeds and under-correcting at higher speeds. The cant must be chosen to suit the mix of traffic types and speeds.

Below a certain radius, the conicity of the wheel is not enough to compensate and flange contact comes into play. This is when the effect Paul describes starts to happen. The radius below which flange contact happens depends on various things including cant and conicity, but it's generally in the hundreds of metres.
 
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