Sharp radius points in OO

[Captain Kernow]

This is a copy of the post that I made in the 'Help & Advice' section.

 

I am interested in finding out what people's experiences might be of running modern RTR and/or RP25 wheels through kit-built or scratchbuilt OO point work, specifically A4 and A5 radius points.

 

I appreciate that something small like an industrial 0-4-0T should be OK, but what about slightly larger locos, like 0-6-0Ts from the main line companies (eg. 57XX pannier, Jinty etc.)?

 

Also, if the points were to be built to OO-SF standards, does this make any practical difference, please?

 

Many thanks.

[Joseph_Pestell]

A4 and A5 are not radii but crossing angles. Doubtless Martin W will be along soon to explain better than I can.

 

How well a larger loco will run through such tightly curved pointwork will be as much a function of how much slop there is in the chassis as of the profile of the wheels. Finer wheels should help a bit though as would adoption of a narrower gauge on the plain track so that the loco does not weave about.

[Gordon A]

A4 in 00 comes out at 14" radius.

 

Not sure if this is of some use:

 

You could draw a 14" radius curve, draw a chord = to the wheelbase of the outer axles of the 0-6-0.

Measure from the position of the middle axle to the curve. That is the sideplay you will need in the middle axle to negotiate that curve.

 

Even if you have the side play, it does not mean that the loco wont climb the inside rail and come off the track.

 

Gordon A

[Izzy]

If you do as Martin often suggests and use the non-prototype model switch available in Templot, apart from being much shorter, with a A4 the ruling radius comes out around the 17" mark. I would think that 4-SF (00-SF) pointwork, if set to 16.2mm throughout and not just in the crossing area, would give less leeway with increasing tightness of radius with larger/longer wheelbase locos compared to 16.5mm. Obviously the larger the wheels are the more difficult it also becomes because of the increased angle the flanges assume relative to the rail head. Quite a few variables to consider taking into account any wheel profile differences, but it is sometimes quite surprising just how tight a radius you can 'get away with' if needed and still obtain decent running.

 

Izzy

[Captain Kernow]

Thanks everyone so far.

 

I've had another look in my 'filing system' and have now found the OO and OO-SF templates for the points I need. I will probably settle for A5 turnouts.

[Dungrange]

00-SF has a minimum track gauge of 16.2 mm.  However, for curves below a radius of, I think, around 750 mm (ie 2' 6"), gauge widening should be employed using a three point gauge during construction, which means that in practice there should be little difference between a 16.5 mm gauge turnout (00-BF) and a gauge widened 00-SF turnout.

[nswgr1855]

00-SF has a minimum track gauge of 16.2 mm.  However, for curves below a radius of, I think, around 750 mm (ie 2' 6"), gauge widening should be employed using a three point gauge during construction, which means that in practice there should be little difference between a 16.5 mm gauge turnout (00-BF) and a gauge widened 00-SF turnout.

A typical 3 point gauge makes minimal difference to the track gauge. They are not long enough. My experience is you do not need to gauge widen because most small scale models have over scale axle end play.

 

To determine the practical minimum radius have a look at the AMRA minimum radius standard http://amra.asn.au/standards/

 

Cheers,

 

Terry Flynn

[Poor Old Bruce]

Even if you have the side play, it does not mean that the loco wont climb the inside rail and come off the track.

 

I would suggest there is something fundamentally wrong if your wheels come off the inside rail. Most curving forces are created where the leading wheels (or leading coupled wheels) pushing against the outside rail of a curve so that is where derailment will occur.

[Izzy]

A typical 3 point gauge makes minimal difference to the track gauge. They are not long enough. My experience is you do not need to gauge widen because most small scale models have over scale axle end play.

Cheers,

 

Terry Flynn

 

I will disagree. On straight track maybe, not otherwise.

 

Using a 'typical' 3 point gauge - where the distance between the two legs is roughly twice the track gauge - significant under-gauging will result with the single leg on the outside of a curve, and the opposite with it on the inside ( the normal way of providing gauge widening). The amounts obtained in either situation will depend on/be proportional to the severity of the curvature. My experiences with using finer track and wheel standards in a variety of scales/gauges is that gauge widening can prove a distinct advantage.

 

Izzy

[Izzy]

I would suggest there is something fundamentally wrong if your wheels come off the inside rail. Most curving forces are created where the leading wheels (or leading coupled wheels) pushing against the outside rail of a curve so that is where derailment will occur.

 

Not neccessarily. It is often the case that with tight curvature, rather than the leading outside wheel riding up the rail head, the inside middle wheel will do so. I have often found this with some models. The wheel flange/tread profile seems to affect this as well as the weight of the loco/coach involved ( 6-wheel coaches seem prone to this when fixed axle chassis are used).

 

Izzy

[nswgr1855]

I will disagree. On straight track maybe, not otherwise.

 

Using a 'typical' 3 point gauge - where the distance between the two legs is roughly twice the track gauge - significant under-gauging will result with the single leg on the outside of a curve, and the opposite with it on the inside ( the normal way of providing gauge widening). The amounts obtained in either situation will depend on/be proportional to the severity of the curvature. My experiences with using finer track and wheel standards in a variety of scales/gauges is that gauge widening can prove a distinct advantage.

 

Izzy

 

Clearly you have not done the maths. Basically to gain suitable gauge widening for a 0-6-0 locomotive with no end play your 3 point gauge needs to be as long as your locomotive wheelbase. I do not know of any 00-SF 3 point gauges that are 70mm long to suit a GWR pannier tank loco. If you need to gauge widen to get a model around a curve, you are using curves to sharp for the length of model.

 

Cheers,

 

Terry Flynn.

[hayfield]

Terry

 

To be fair, don't most models have a bit of side play, especially if sharp radii curves are to be used?

[Izzy]

Clearly you have not done the maths. Basically to gain suitable gauge widening for a 0-6-0 locomotive with no end play your 3 point gauge needs to be as long as your locomotive wheelbase. I do not know of any 00-SF 3 point gauges that are 70mm long to suit a GWR pannier tank loco. If you need to gauge widen to get a model around a curve, you are using curves to sharp for the length of model.

 

Cheers,

 

Terry Flynn.

 

Ah, that's the problem, my mistake, I tend to deal in generalities based on practical experience rather than specifics based on theory. OO is only one specific gauge width irrespective of the other standards used with it, whereas I was referring to 3-point gauges in general terms covering any track radius and track standard, as OO-SF/4-SF has been mentioned.

 

Izzy

[Poor Old Bruce]

I would suggest there is something fundamentally wrong if your wheels come off the inside rail. Most curving forces are created where the leading wheels (or leading coupled wheels) pushing against the outside rail of a curve so that is where derailment will occur.

 

Not neccessarily. It is often the case that with tight curvature, rather than the leading outside wheel riding up the rail head, the inside middle wheel will do so. I have often found this with some models. The wheel flange/tread profile seems to affect this as well as the weight of the loco/coach involved ( 6-wheel coaches seem prone to this when fixed axle chassis are used).

 

Izzy

 

Derailments on plain line generally come down to the relationship between lateral and vertical forces acting at the wheel-to-rail interface. So either there is not enough weight on the wheel which derails or there is too much lateral force which could well be down to limited later clearances or too sharp curves or both.

[nswgr1855]

Ah, that's the problem, my mistake, I tend to deal in generalities based on practical experience rather than specifics based on theory. OO is only one specific gauge width irrespective of the other standards used with it, whereas I was referring to 3-point gauges in general terms covering any track radius and track standard, as OO-SF/4-SF has been mentioned.

 

Izzy

 

The theory works for all scales and any 3 point gauge less than the rigid wheel base of any six or more coupled locomotive will not compensate enough. In practice most models built in small scales have over scale end play, thus negating the need for gauge widening in small scales. Also most standards used have a track gauge already with gauge widening built in. The exceptions are the close as possible standards like scale 7.

 

Cheers,

Terry Flynn.

[Izzy]

The theory works for all scales and any 3 point gauge less than the rigid wheel base of any six or more coupled locomotive will not compensate enough. In practice most models built in small scales have over scale end play, thus negating the need for gauge widening in small scales. Also most standards used have a track gauge already with gauge widening built in. The exceptions are the close as possible standards like scale 7.

 

Cheers,

Terry Flynn.

 

That's fair enough, I understand. The 2FS track I have recently built with gauge widening using a 3-point gauge to allow a couple of ex-Midland 0-6-0's to get aound the tighter curves - which they couldn't do otherwise - is all purely my imagination. Nothing more to be said really.

 

Izzy

[nswgr1855]

That's fair enough, I understand. The 2FS track I have recently built with gauge widening using a 3-point gauge to allow a couple of ex-Midland 0-6-0's to get aound the tighter curves - which they couldn't do otherwise - is all purely my imagination. Nothing more to be said really.

 

Izzy

It's expected if you are using curves with a radius sharper than 5 times the length of your six coupled locomotives. The extra sharp curves are really only appropriate for a 0-4-0 if you want your models to look realistic. Sounds like you are in toy train set territory.

[avonside1563]

How tight do you want it to be? Sometimes the prototype can be even better than the model... Take a look at 1:24 in this video. (Oh and the loco is my profile name!)

 

[Izzy]

It's expected if you are using curves with a radius sharper than 5 times the length of your six coupled locomotives. The extra sharp curves are really only appropriate for a 0-4-0 if you want your models to look realistic. Sounds like you are in toy train set territory.

 

Aren't we all?

 

After all the prototype started applying 1/4" gauge widening at 10 chains (660' = 8.6'@4mm), 1/2" at 7 chains (462' = 6'@4mm), and 3/4" at 5.5 chains (363' = 4.7'@4mm). But our toy trains generally use coarser track and wheel standards to try and offset the inevitable use of tighter than prototype radius but the time still comes sometimes when it becomes neccesary.

 

Hm, 5 times the wheelbase.  As the radius is about 10 times wb this doesn't quite seem to work out. Does this take wheel dia into account?

 

 

Izzy

[nswgr1855]

How tight do you want it to be? Sometimes the prototype can be even better than the model... Take a look at 1:24 in this video. (Oh and the loco is my profile name!)

 

 

Nice video. The 4 way turnout is a beauty.

 

The AMRA minimum radius standard suggests you can go as tight as 3 times the locomotive length being an 0-4-0. For example, using my  1/76 scale Hornby Pug  as an equivalent locomotive, it is 85mm long. This means it can get around 85mm x 3 = 255mm (10") radius curves.

 

Cheers,

 

Terry Flynn.

[nswgr1855]

Aren't we all?

 

After all the prototype started applying 1/4" gauge widening at 10 chains (660' = 8.6'@4mm), 1/2" at 7 chains (462' = 6'@4mm), and 3/4" at 5.5 chains (363' = 4.7'@4mm). But our toy trains generally use coarser track and wheel standards to try and offset the inevitable use of tighter than prototype radius but the time still comes sometimes when it becomes neccesary.

 

Hm, 5 times the wheelbase.  As the radius is about 10 times wb this doesn't quite seem to work out. Does this take wheel dia into account?

 

 

Izzy

 

 

No, the AMRA standard says 5 times the vehicle length. So what is your radius and six coupled locomotive length?  I have no trouble getting my 1/76 scale GWR 0-6-0 (124mm 0ver buffers) through 00-SF turnouts, no gauge widening with a minimum radius of 610mm (24"). This is just under the AMRA standards recommended minimum. The AMRA calculation is 124mm x 5 = 620mm. I have removed most of the side play on the outer axles, but left the end play on the centre axle as delivered from Bachman. This radius is still much sharper than your prototype examples, so I am not being unreasonable. Wheel diameter is not a factor if the front flange angle is not to steep. Also the bigger the driving wheel the longer the locomotive will generally be, naturally compensating by producing a larger minimum radius using the formula from the standard.

 

Cheers,

 

Terry Flynn.

[Izzy]

 I have no trouble getting my 1/76 scale GWR 0-6-0 (124mm 0ver buffers) through 00-SF turnouts, no gauge widening with a minimum radius of 610mm (24").  This is just under the AMRA standards recommended minimum.

Cheers,

 

Terry Flynn.

 

 

Yes, but the main and original thrust of this thread concerned A4/A5 00-SF/4-SF point work with radius of around the 14-17" mark. Quite a bit under these AMRA standards you quote - whatever they are.

 

My concern remains your assertion that 3-point track gauges do not produce noticeable/usable gauge widening, which remains at odds with my and many others experiences given the recommendations to use them for this purpose that were given.

 

Izzy

[jim.snowdon]

No, the AMRA standard says 5 times the vehicle length. So what is your radius and six coupled locomotive length?  I have no trouble getting my 1/76 scale GWR 0-6-0 (124mm 0ver buffers) through 00-SF turnouts, no gauge widening with a minimum radius of 610mm (24"). This is just under the AMRA standards recommended minimum. The AMRA calculation is 124mm x 5 = 620mm. I have removed most of the side play on the outer axles, but left the end play on the centre axle as delivered from Bachman. This radius is still much sharper than your prototype examples, so I am not being unreasonable. Wheel diameter is not a factor if the front flange angle is not to steep. Also the bigger the driving wheel the longer the locomotive will generally be, naturally compensating by producing a larger minimum radius using the formula from the standard.

 

Cheers,

 

Terry Flynn.

It may do, but it is the wheelbase that has to do with curvature and gauge widening, and if you refer to full size practice, the published reckoning for a wheelbase of about 10', which covers most wagons and 4-coupled industrial locomotives, the desirable minmum was 120' radius, or 480mm for 4mm scale, with widening based on the formula W = 44/R, where W is the widening in inches, and R the radius in feet. For the record, that came from Summerson's Platelayer's Guide, as published in 1895.

 

It is also why the LMS Fowler 0-6-0 Dock Tanks, with a wheelbase of 9' 6" and an overall length of 27' 6" will go round distinctly tighter curves (intentionally) than an MR/LMS 3F 0-6-0 tank with a wheelbase of 16' 6" and an overall length only 3' 6" greater than the Dock Tank.

 

Jim

[Dungrange]

Clearly you have not done the maths. Basically to gain suitable gauge widening for a 0-6-0 locomotive with no end play your 3 point gauge needs to be as long as your locomotive wheelbase. I do not know of any 00-SF 3 point gauges that are 70 mm long to suit a GWR pannier tank loco. 

 

The theory works for all scales and any 3 point gauge less than the rigid wheel base of any six or more coupled locomotive will not compensate enough. In practice most models built in small scales have over scale end play, thus negating the need for gauge widening in small scales. Also most standards used have a track gauge already with gauge widening built in. The exceptions are the close as possible standards like scale 7.

 

Whilst I agree with your logic that a 3 point gauge would have to be as long as the wheelbase of a locomotive to totally negate the need for any end play in the locomotive chassis, this is to some extent irrelevant.  I do not agree that by extension there is therefore no need to gauge widen when talking about radii that are below the Australian standards that you quote - ie circa 400 mm in 1:76 scale (particularly in 00-SF and the closer to scale gauges).  That is what is being discussed in this thread.

 

If there was absolutely no end play in the locomotive chassis, then gauge widening would be required at much larger radii.  However, as we drop below prototypical curve radii, there is no immediate need for gauge widening on what are considered 'generous' curves by model railway standards.  This is because, as you highlight, most standards have a certain amount of 'slop' in the track / wheel standards and some end play in the locomotive chassis.  This combination is sufficient down to 'train set' curves in 00.  However, when changing from 00-BF to 00-SF you are removing 0.3 mm of that side play in the track gauge, which tightens things up.  This therefore means that you can't realistically expect a locomotive that was designed to go around a 400 mm curve with a track gauge of 16.5 mm to go round the same curve with a track gauge of only 16.2 mm without widening the gauge (or modifying the amount of end play in the locomotive) .  This is the purpose of the 3 point gauge and whether or not it is as long as the wheelbase of a locomotive is largely irrelevant.  All the 3-point gauge needs to do is increase the track gauge as the radius reduces - which it does.

 

Personally, I've never used a 3-point gauge, but then I have an absolute minimum radius of around 920 mm in the layout that I am building, with most radii being closer to 1.5 m or more.  With these type of curves, I don't need a 3-point gauge, but that's not what is being discussed in this thread.

[Captain Kernow]

Thanks for all your comments so far.

 

The A5 I built a couple of years ago works fine with all the tank locos that I will be running over the layout. I've now started building an A5 crossover in OO-SF and will probably install some gauge widening on the first set of points and check how that goes.