What back to back setting do you use?

Model and prototype rolling stock, locos, multiple units etc.
Julian Roberts
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Re: What back to back setting do you use?

Postby Julian Roberts » Tue Dec 26, 2017 7:59 am

Well I enjoyed swilling these ideas around in my head with the festivities.

I am certainly not worrying about 0.01mm Terry and I don't suppose anyone else is, in practice. But it is necessary to understand what is going on at the limits to get the best results.

An analogy might be the water gauge of a steam loco in service. Most train buffs probably know the water should be kept at half a glass. But actually it requires a lot of skill from the fireman to keep the level in just the right place, and on the way to achieving that skill one learns how far one can go in each direction and what it takes to avoid going there - learning the balance required

It took me a long time to understand the limits on the P4 BB setting. And to an extent I still have questions. There is some misleading stuff around. And a lot of concepts to get hold of once you start thinking about it.

It is absurd to suggest as was a while back that to ask the questions that have been raised here implies there is a megacrisis in P4 modelling.

But some people find P4 modelling doesn't work out of the box and I say good for you Tim for teasing out why that is the case for you. If these is any crisis it is when people are discouraged from rigorous thinking, which can involve numbers to the nearest 0.01mm, or worse become disillusioned with the whole thing. I suspect as the Roger Sanders article suggests that many people tolerate an element of unreliability of running as the price you have to pay for P4 visual realism, and moreover turn a blind eye to it. Personally I find that any derailment is intolerable.

"Are there any brave souls out there willing to admit the running they achieve is not good enough" Roger asks. "Has automatically rerailing stock become second nature"?

Great questions. I think zero derailments are achievable but if you never ask why they occur you can't cure them and if you're not allowed to talk about it here, like a family secret, the Forum is pointless.

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Le Corbusier
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Re: What back to back setting do you use?

Postby Le Corbusier » Tue Dec 26, 2017 8:41 am

Happy Christmas Julian. :thumb

Julian Roberts wrote:But some people find P4 modelling doesn't work out of the box and I say good for you Tim for teasing out why that is the case for you.


So far so good .... and still thoroughly enjoying the process ;)

I suspect that as skills and experience improve ... less and less 'mucking about' will be required ... but in the absence of either a local group I can currently spare the time to attend or an experienced friendly modeller on tap to run an eye over things - threads like this (and others) are just the thing :)
Tim Lee

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Jol Wilkinson
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Re: What back to back setting do you use?

Postby Jol Wilkinson » Tue Dec 26, 2017 10:31 am

Julian Roberts wrote:Well I enjoyed swilling these ideas around in my head with the festivities.

I am certainly not worrying about 0.01mm Terry and I don't suppose anyone else is, in practice. But it is necessary to understand what is going on at the limits to get the best results.

An analogy might be the water gauge of a steam loco in service. Most train buffs probably know the water should be kept at half a glass. But actually it requires a lot of skill from the fireman to keep the level in just the right place, and on the way to achieving that skill one learns how far one can go in each direction and what it takes to avoid going there - learning the balance required

It took me a long time to understand the limits on the P4 BB setting. And to an extent I still have questions. There is some misleading stuff around. And a lot of concepts to get hold of once you start thinking about it.

It is absurd to suggest as was a while back that to ask the questions that have been raised here implies there is a megacrisis in P4 modelling.

But some people find P4 modelling doesn't work out of the box and I say good for you Tim for teasing out why that is the case for you. If these is any crisis it is when people are discouraged from rigorous thinking, which can involve numbers to the nearest 0.01mm, or worse become disillusioned with the whole thing. I suspect as the Roger Sanders article suggests that many people tolerate an element of unreliability of running as the price you have to pay for P4 visual realism, and moreover turn a blind eye to it. Personally I find that any derailment is intolerable.

"Are there any brave souls out there willing to admit the running they achieve is not good enough" Roger asks. "Has automatically rerailing stock become second nature"?

Great questions. I think zero derailments are achievable but if you never ask why they occur you can't cure them and if you're not allowed to talk about it here, like a family secret, the Forum is pointless.


Julian,

I rather feel that you previous posts appear to contradict your claim that you aren't worrying about .01 mm. In reality, given the same set of wheels
and gauge, most people will get a different B2B within the levels of tolerance your have written about, depending on their "feel" for the fit between the rim and gauge.

In my experience, there are other factors, such as lateral wheel "wobble" that are more problematic than wondering what B2B tolerance to accept,- within the standards most of us have been used to working with for some years.

I'd also suggest that very few of us have ever made P4 work entirely satisfactorily "out of the box". We have all probably had to work up to an satisfactory level of expertise in various skills and abilities before we have achieved what we want. I also expect that we also don't achieve a level of running that we would wish for all the time, but that is affected by a whole range of factors, not just the wheel/track interface.

While challenging the accepted ways is admirable and often worthwhile, ultimately nothing beats actually getting on with it. I know a lot of excellent modellers who do just that and I believe you learn a lot more that way.

Jol

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Le Corbusier
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Re: What back to back setting do you use?

Postby Le Corbusier » Tue Dec 26, 2017 1:01 pm

Jol Wilkinson wrote:
Julian Roberts wrote:
In my experience, there are other factors, such as lateral wheel "wobble" that are more problematic than wondering what B2B tolerance to accept,- within the standards most of us have been used to working with for some years.
Jol


Happy Christmas Jol,

You could well be right .... as alongside changing my back to backs (which were outside the standards) my new 'mallet' B to B gauge allowed me to get rid of some wheel wobble into the bargain. :D

Another 15mins playing this morning and still no de-railments to date :thumb
Tim Lee

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David Thorpe
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Re: What back to back setting do you use?

Postby David Thorpe » Tue Dec 26, 2017 1:57 pm

Jol Wilkinson wrote:While challenging the accepted ways is admirable and often worthwhile, ultimately nothing beats actually getting on with it. I know a lot of excellent modellers who do just that and I believe you learn a lot more that way.


Well said, Jol. I set my B2B using a standard gauge and don't question (or measure) what it does. It has enabled me to build nine locos in P4, all of which work satisfactorily. Such derailments as I have are much more likely to be down to a track defect or misalignment than to B2Bs being out by a tiny part of a millimetre. I do sometimes despair about what newcomers considering changing to P4 must think when they read threads such as this.

DT

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jon price
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Re: What back to back setting do you use?

Postby jon price » Tue Dec 26, 2017 5:04 pm

First loco was 0-6-0. Mainly Trains rigid chassis. AG wheels. GW wheel press. Works fine. Beginners luck, or angels on a pinhead. You choose. Mind you I have managed to burn out the decoder, but that is a different story.
Connah's Quay Workshop threads: viewforum.php?f=125

Philip Hall
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Re: What back to back setting do you use?

Postby Philip Hall » Tue Dec 26, 2017 5:37 pm

David Thorpe wrote: I do sometimes despair about what newcomers considering changing to P4 must think when they read threads such as this.

DT


Well said. So many of the problems we have had over so many years with the public face of P4 can be laid squarely at the door that leads to over complication and overthinking. Here’s to Jol, just get on with it!

Philip

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Le Corbusier
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Re: What back to back setting do you use?

Postby Le Corbusier » Tue Dec 26, 2017 9:33 pm

Philip Hall wrote:
David Thorpe wrote: I do sometimes despair about what newcomers considering changing to P4 must think when they read threads such as this.

DT


Well said. So many of the problems we have had over so many years with the public face of P4 can be laid squarely at the door that leads to over complication and overthinking. Here’s to Jol, just get on with it!

Philip



Not wanting to stir the pot .... but :twisted:

I just 'had a go' without 'over complicating things' ..... It all looked good and I thought all was going fine .... then two of my stock running from one direction only derailed :cry: after much fiddling with the track to no effect .... this 'overcomplicating' thread along with some input from Keith sorted things nicely, simply and quickly .... confidence fully restored.

I disagree with Jol on his interpretation of Julian's musings .... the 0.001mm discussions appear to me to be part of his theories suggesting that such increments are nonsense ... indeed I thought he was suggesting that Back to Back so long as within tolerance is unlikely to be significant ;)

I like having a check list for when things don't go quite as smoothly as I hoped.
Tim Lee

Philip Hall
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Re: What back to back setting do you use?

Postby Philip Hall » Wed Dec 27, 2017 9:41 am

Tim,

I’m glad that the thread and advice has solved your problem quickly. I just feel, along with some others, that some of this stuff could just put people off, when it doesn’t have to. P4 is a set of standards and tolerances which generally do work.

It was interesting that you were, with your mallet gauge, able to tease out wheel wobble. I think I will have to get one! I suspect that many of the supposed back to back variations folk have are actually a result of wheels out of true. I use a GW adjustable gauge most of the time, set to 17.7mm, but I treat this as a sliding fit between the tyre edges, so I guess it may well actually be a little bit more. However, I check this at several different places around the circumference of a driving wheel, as well as trying to make sure there is no wobble. If it’s tight at one point it’s an indicator of where to tweak.

Philip

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Le Corbusier
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Re: What back to back setting do you use?

Postby Le Corbusier » Wed Dec 27, 2017 12:12 pm

Philip Hall wrote:Tim,

I’m glad that the thread and advice has solved your problem quickly. I just feel, along with some others, that some of this stuff could just put people off, when it doesn’t have to. P4 is a set of standards and tolerances which generally do work.

It was interesting that you were, with your mallet gauge, able to tease out wheel wobble. I think I will have to get one! I suspect that many of the supposed back to back variations folk have are actually a result of wheels out of true. I use a GW adjustable gauge most of the time, set to 17.7mm, but I treat this as a sliding fit between the tyre edges, so I guess it may well actually be a little bit more. However, I check this at several different places around the circumference of a driving wheel, as well as trying to make sure there is no wobble. If it’s tight at one point it’s an indicator of where to tweak.

Philip


Hi Phillip, :)

I found that the shape of the mallet worked well for me (I know Keith feels the 'L' shaped gauge does much the same - but this wasn't my experience). Because it is round with just the cut out for the axle, I found that on wagon and coach wheels (when resetting the B to B) they pressed down by hand nice and evenly and appeared to take out most of the wheel wobble (at least I find it hard to discern now). I haven't been able to try in the same way on my loco as I can't drop the wheels out .... will correct that with the next build.

Maybe we need another (technical) layer on the forum. I agree that some of these discussions leave me far behind .... But it is amazing how interesting they become when you have a problem to solve - and I suspect even if a thread just started with a plea for help, it would quite quickly become quite involved - as it seems to me that solutions to similar problems vary depending on who you talk to. This could be because the manifestation is the same but the root cause different? Anyway, I like the idea of building up a check list of things to do whilst building either track or stock - simply to ensure all bases are covered. Some of the theories may be complex, but the actuality of the various checks seem to me to be quite simple and straight forward.
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Re: What back to back setting do you use?

Postby Enigma » Wed Dec 27, 2017 3:13 pm

But how does the 'mallet' work on loco axles with gear boxes, bearings etc.? This is where the good old 'L' shape gauge must be called into action.

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Re: What back to back setting do you use?

Postby Le Corbusier » Wed Dec 27, 2017 3:21 pm

Enigma wrote:But how does the 'mallet' work on loco axles with gear boxes, bearings etc.? This is where the good old 'L' shape gauge must be called into action.


Good point ..... :thumb .. It would be that or two mallets, one either side (less convenient). I suppose it depends on whether I can find an 'L' with the same B to B to ensure consistency.
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zebedeesknees
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Re: What back to back setting do you use?

Postby zebedeesknees » Tue Jan 02, 2018 2:59 pm

We call ourselves scale modellers, right?

A bit of the dreaded mathematics:- The real b-b according to the drawings from North British in one of my O.S.Nock books is 4' 5 and 5/8". Or 53.625" for easier calculation. Multiply by 25.4 (Millimeters per inch) equals 1362.075mm. Divide by our scale, 76.2, equals 17.875mm. QED.

A little bloggery:- When I joined the Scalefour Society a long while ago, I received the starter bits, but fortunately as it turned out, mislaid the grossly overpriced bit of steel called the check rail gauge. It didn't matter, I innocently looked up the real dims, and scaled the check gap to 0.58mm. Figuring that 0.02mm wasn't going to amount to anything, I used some of the 0.6mm drill bits I had in stock to gauge the checks on the track I was building. But, again with innocent faith, I used the b-b gauge that the Society had sent to me.

Later, I joined the Central London Area Group, and took some of my stock to run on Tony Wilkins' 'Green Street'. When running through some of the crossings, an audible click could be heard, like the tick of a clock. "Ah, you know why that is don't you?" "Errr, no." "Well, you're using a P4 b-b gauge, and this track is to S4 standards!"

The difference, 0.008" or 0.2mm, doesn't sound like much, but my stock ran better there and on my own trackwork! A bit of luck, as Prem Holdaway turned me up a replacement b-b roller which is still on my bench, and it's a lot easier to re-gauge wheelsets than it is to alter all the checks I'd fitted. It is actually 0.703"...

Now for some contentious opinion:- The Scalefour Society is misnamed, 'cos scale it ain't; It should be the Peefour Society, and we need an Essfour Society to stop misleading the innocent ... This came about because of the ruckus at the beginning of it's existence, when certain members of the MRSG were hoping to make a commercial advantage out of new products, but when they found that the tolerances required to do the job properly were going to eat into profits they compromised. I know from personal conversation that Joe Brook Smith wasn't one of these.

Ted Scannell 2769

Julian Roberts
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Re: What back to back setting do you use?

Postby Julian Roberts » Wed Jan 03, 2018 8:55 am

Ted

:thumb ...........but Shh!! - some people get terribly upset about people talking numbers less than about 0.1! As for the dreaded maths - well it took me three goes to get the bottom grade of O level maths, but with a mobile phone calculator the scariest looking puzzles are easily calculated as you show, or you can even just ask Google for the answer.

I don't know, but would have thought people with the interest and perseverance to follow this would understand the difference between S4 and P4. What I find particularly interesting is your last paragraph, as I had believed what I understood was the standard interpretation of Scalefour Society history, that the extra clearance of 0.1mm in the flangeways and associated reduced BB was to enable more typical model railways of the time with minimum radius much lower than our commonly accepted(?) minimum advised 4 foot.

I'm not sure what the production tolerances are that would have had to be tightened? - assuming it is just as difficult or easy to make a gauge to 0.58 as to 0.68, for example. The differences of BB gauge people have mentioned here - are they not intentional, different interpretations of the official 17.67 - 75 range?

The flanges of the wheels? According to the Digest the tolerance range is 0.35 - 0.40. Again according to it, the real flange is 1 1/8", or 1.125", or 28.575mm. According to Alan Turner the real flange was allowed to wear 5mm, down to 23.575. Divided by 76.2 that comes to 0.375mm down to 0.309......(lots more numbers). So the real flange had more variation than ours (unless we run an intensive 24/7 service, and actually wear out the flanges, I suppose, as Allan Goodwillie did at his museum in Melrose on some of his stock) - at our scale that flange range would be 0.31 - 0.375. (hopefully I've put the correct numbers here.)

Seems to me that your BB gauge is pretty much just right at 17.856 for the slightly wider model flange at its 0.4 width. (Though to be really pedantic 18.25 minus 0.4 = 17.85, in the same way as 18.25 - 0.375 = 17.875). So BB + EF = CG, prototype and model. (Or to be super pedantic, I think the correct way of thinking is to say BBmax + EFmax is not greater than CG min?! :ugeek: :ugeek: ).

I wonder whether Tony Wilkins you might be persuaded to show us the article in Modern Railways you referred to earlier?

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Jol Wilkinson
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Re: What back to back setting do you use?

Postby Jol Wilkinson » Wed Jan 03, 2018 1:51 pm

It is one thing playing about with numbers on a bit of paper or a calculator, but building models and track to very small tolerances is another matter, especially when using gauges that rely on "feel" How tight or loose do you make the fit on a B2B gauge? How tightly do you press the rail against the flangeway gauge when assembling a point? How much run out does a wheel have, which makes it accurate at one position but not at 180 deg.?.

While Ted is realistic when talking about .1mm, I suggest that measurements of multiples of .001mm are getting to a measurement smaller than we can reasonably achieve, especially given the manufacturing tolerances of the products we can buy.

What Ted's typically provocative post does is simply confirm the need for consistently applied sets of standards.

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Le Corbusier
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Re: What back to back setting do you use?

Postby Le Corbusier » Wed Jan 03, 2018 3:20 pm

ding ding .... seconds out ..... round 7 :twisted:
Tim Lee

Tony Wilkins
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Re: What back to back setting do you use?

Postby Tony Wilkins » Wed Jan 03, 2018 4:18 pm

Be very careful reading Ted's post as he mixes metric and imperial measurements. When I and CLAG made our own B2B roller type gauges, we only possessed imperial micrometers, which measured to one thousandth of an inch 0.001". I calculated that the scale B2B dimension was near enough 0.703". One thou is the same as 0.0254mm, so 0.01mm is roughly two and a half times smaller. 1 thou is accurate enough for me. If you can work to better tolerances than this, you're a better man than I Gungadin.

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Julian Roberts
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Re: What back to back setting do you use?

Postby Julian Roberts » Thu Jan 04, 2018 9:06 am

I'm afraid people may not understand my point but time limits the amount I can re- explain myself.

I have tried to understand why my wildly inaccurate BB wheels DO stay on the rails. Ted's last post confirms what I suspected. The 17.67 P4 minimum is to enable compatibility with true scale S4 track. The wheels can just squeeze through pointwork. The clicks he heard are the flange backs against the wing and check rails.

In just the same way as he thus proves 17.67 is the absolute minimum S4 BB this shows 17.47 is the absolute minimum P4 BB and explains why one of my locos which has a wobbly front wheel that at its minimum is 17.47, operates trouble free.

At the other extreme this thread has been latterly all about the maximum BB improving running. I have consistently argued against working to the tiny tolerance where 17.75 is simultaneously the minimum and maximum, for the reasons you put in your last post Jol, and made my point that I find it's weight that is more the issue.

Why is 17.75 the max in P4? I had understood it to be because, plus a 0.40 flange, that was the CG dimension and if exceeded could result in a click or derailment on the nose of a crossing.

If in absolute scale the BB can be 17.87 then surely in P4 it can be 17.77, which surely would be good news to those (I am not one of them but I am very happy that they find it works for them) who believe the wider the BB the better the running.

All I am trying to do is understand the limits so that if by mischance I find my wheels are at those limits I can know what may or may not happen.

I don't know if you can relate to what I am trying to say as a modeller with probably a lot less experience than you. In my Crab thread I have verbalised the issue rather more on 31st Dec as I got some slightly narrow BB there.

I'm not a better man than Gungadin! Whoever he is...apologies if my meaning still isn't clear. I'm trying the get my wheels somewhere between 17.67 and 75 but find its not easy!
Last edited by Julian Roberts on Thu Jan 04, 2018 10:15 am, edited 1 time in total.

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Le Corbusier
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Re: What back to back setting do you use?

Postby Le Corbusier » Thu Jan 04, 2018 9:44 am

Julian Roberts wrote:I'm afraid people may not understand my point but time limits the amount I can re- explain myself.

I have tried to understand why my wildly inaccurate BB wheels DO stay on the rails. Ted's last post confirms what I suspected. The 17.67 P4 minimum is to enable compatibility with true scale S4 track. The wheels can just squeeze through pointwork. The clicks he heard are the flange backs against the wing and check rails.

In just the same way as he thus proves 17.67 is the absolute minimum S4 BB this shows 17.47 is the absolute minimum P4 BB and explains why one of my locos which has a wobbly front wheel that at its minimum is 17.47, operates trouble free.

At the other extreme this thread has been latterly all about the maximum BB improving running. I have consistently argued against working to the tiny tolerance where 17.75 is simultaneously the minimum and maximum, for the reasons you put in your last post Jol.

Why is 17.75 the max in P4? I had understood it to be because, plus a 0.40 flange, that was the CG dimension and if exceeded could result in a click or derailment on the nose of a crossing.

If in absolute scale the BB can be 17.87 then surely in P4 it can be 17.77, which surely would be good news to those (I am not one of them but I am very happy that they find it works for them) who believe the wider the BB the better the running.

All I am trying to do is understand the limits so that if by mischance I find my wheels are at those limits I can know what may or may not happen.

I don't know if you can relate to what I am trying to say as a modeller with probably a lot less experience than you. In my Crab thread I have verbalised the issue rather more on 31st Dec as I got some slightly narrow BB there.

I'm not a better man than Gungadin! Whoever he is...apologies if my meaning still isn't clear. I'm trying the get my wheels somewhere between 17.67 and 75 but find its not easy!


:thumb
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Noel
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Re: What back to back setting do you use?

Postby Noel » Thu Jan 04, 2018 11:43 am

Julian Roberts wrote:I'm not a better man than Gungadin! Whoever he is...


https://www.poetryfoundation.org/poems/46783/gunga-din
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Jol Wilkinson
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Re: What back to back setting do you use?

Postby Jol Wilkinson » Thu Jan 04, 2018 2:21 pm

Julian Roberts wrote:I'm afraid people may not understand my point but time limits the amount I can re- explain myself.

I have tried to understand why my wildly inaccurate BB wheels DO stay on the rails. Ted's last post confirms what I suspected. The 17.67 P4 minimum is to enable compatibility with true scale S4 track. The wheels can just squeeze through pointwork. The clicks he heard are the flange backs against the wing and check rails.

In just the same way as he thus proves 17.67 is the absolute minimum S4 BB this shows 17.47 is the absolute minimum P4 BB and explains why one of my locos which has a wobbly front wheel that at its minimum is 17.47, operates trouble free.

At the other extreme this thread has been latterly all about the maximum BB improving running. I have consistently argued against working to the tiny tolerance where 17.75 is simultaneously the minimum and maximum, for the reasons you put in your last post Jol, and made my point that I find it's weight that is more the issue.

Why is 17.75 the max in P4? I had understood it to be because, plus a 0.40 flange, that was the CG dimension and if exceeded could result in a click or derailment on the nose of a crossing.

If in absolute scale the BB can be 17.87 then surely in P4 it can be 17.77, which surely would be good news to those (I am not one of them but I am very happy that they find it works for them) who believe the wider the BB the better the running.

All I am trying to do is understand the limits so that if by mischance I find my wheels are at those limits I can know what may or may not happen.

I don't know if you can relate to what I am trying to say as a modeller with probably a lot less experience than you. In my Crab thread I have verbalised the issue rather more on 31st Dec as I got some slightly narrow BB there.

I'm not a better man than Gungadin! Whoever he is...apologies if my meaning still isn't clear. I'm trying the get my wheels somewhere between 17.67 and 75 but find its not easy!


Julian

I am confused by your reference to "wildly inaccurate BB wheels". I presume you mean Back to Back, rather than a Bill Bedford product.

In what way are they inaccurate. Is the B2B measurement you are using inaccurate or is the problem accurately setting them to whatever dimension you have chosen? If the former, I don't intend to get involved in a discussion. If the latter, then wherein lies the problem? Are the B2B gauges you have different to what you want? If so, a GW adjustable B2B gauge, set using a metric micrometer may be the answer.

My previous responses have been based upon the belief that there are a variety of factors, beyond the figures displayed on the calculator screen or in the Society's published standards, that can affect the results we get. One of the things I learned when undergoing my training as a Mechanical Engineer was that the results we can get with measuring tools is dependent on the operators feel. How tight is the fit of the elsewhere on the B2B gauge? How hard to you press the rail against the flange way gauge when assembling track. These and other variables may affect the results you get and render achieving tolerances of .001 2, 3, 4, or even .005 mm impractical.

My small selection of stock has been built over the years using, until recently, two identical "L" B2B gauges (provenance forgotten, although I think they are from the Society). More recently I bought the large Exactoscale 4CW801 gauge, as recommended for assembling their C&W wheels. I haven't measured any of them with either my digital vernier caliper or Starrett imperial micrometer. Nor have I measured the various track gauges I have used to build the small amount of track I have constructed. However I have, perhaps more by luck than judgement, got satisfactory running.

That are many variables that affect the results we can get. Rail profile, moulded chair accuracy, tyre profile, wheel eccentricity and concentricity, etc. These are usually outside our control and difficult to measure accurately. Hence my view - in which I may be alone - that worrying too much about tolerances that may be too small to measure/control consistently is not very productive.

Jol
Last edited by Jol Wilkinson on Fri Jan 05, 2018 9:52 am, edited 1 time in total.

Philip Hall
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Re: What back to back setting do you use?

Postby Philip Hall » Thu Jan 04, 2018 6:44 pm

I am very much with Jol here.

The ‘feel’ when you use any kind of gauge is something you only pick up with experience. When you use feeler gauges they are very aptly named. I have no engineering training but I have picked up over the years how to do it. When I check a back to back setting I slide the gauge in gently feeling the resistance or otherwise to the gauge.

It is also important to accept that a slightly wobbly wheel (and however much you might want to achieve a dead true, non wobbly wheel, you will for sure end up with one, however slight) will affect the back to back setting. The wheel will wander in and out of the proper value, however hard you try. This is precisely why the tolerances in the settings are so useful. I try to set wheels at 17.7mm and if they end up a bit on the high side, fine. The wheels WILL wobble, try as hard as you might. Even if you get them bang on to start with, they could well move over time. I for one have no intention of being lumbered with having to re-set b-b as the years pass.

To sum up, recently, I took some of my stock to a running session on the Epsom Club’s ‘Wadhurst’. During the afternoon stock from at least six different builders was charging around the layout. And I do mean charging, we had a couple of ‘Schools’ with eight Maunsells on the drawbar, none of them suspended in any way, with standard P4 wheelsets. The track has been down some twenty five years. There was only one derailment, and when the b-b was checked, one bogie wheelset on a LBSCR 4-4-2T was a tiny bit out. But it never derailed again, so it seemed that the wheel wobble thing might have been apparent.

For me, this says that the P4 standards work beyond question. But I still think it’s laudable that some amongst us are striving to understand what can go wrong, and how Things Might Be Improved.

Philip

Julian Roberts
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Joined: Sat Jan 09, 2010 11:33 pm

Re: What back to back setting do you use?

Postby Julian Roberts » Fri Jan 05, 2018 8:36 am

I will reply Jol but not today for time reasons. I will describe the problems I had simply getting a pair of wheels on to the correct BB (not Bill Bedford!) as below, the tender for the Crab
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Julian Roberts
Posts: 923
Joined: Sat Jan 09, 2010 11:33 pm

Re: What back to back setting do you use?

Postby Julian Roberts » Sun Jan 07, 2018 8:40 am

Thanks Noel for the link.

Jol - By BB I mean Back to Back.

When I started in P4 I had a P4 BB gauge from the EMGS. I had no means of measuring it and made three locos with it that ran perfectly as per my article in Snooze 199 and 200 with weight arranged to the guiding outer wheels. Then, big mistake perhaps, I obtained a bargain digital Vernier. (I already had a non-digital one). I found the wheels on all three locos were significantly under gauge, mostly the 17.55 to 65 area, and as I have said several times, one had a minimum BB of 17.47 at the narrowest point of its wobbly perimeter, but none of them ever derailed.

It is obvious when using it that the reading to less the nearest 0.1mm depends entirely how you use it. Pushing hard or less hard, getting it absolutely square...then the wheels actually slightly flex apart too at the rims with the slightest pressure. So a measurement accurate to 0.01 is impossible. But measuring say a piece of "0.45" wire, as claimed on the packet, I found the result is consistently 0.45, maybe 0.44 or 46 depending on push, but the tool itself gives reasonably consistent readings. As the battery fails the readings flash and then go weird. The measuring faces have to be clean as does what is being measured, and the zero-ing facility has to be used quite often. So I'm not getting worked up about whether it's dead accurate because, as you say, how I use it has a bigger bearing on the outcome.

So measuring just one place of a wheel circumference you can get pretty variable results and all you can say is that it is somewhere near the average of what you read. But it is possible to see exactly what you say, that any wheel is going to have quite a variability of BB as it is not possible to get it completely perpendicular to the axle and parallel all the way round. - Except if the variability happens to go in tandem and both wheels are off perpendicular the same way.

I thought I ought to get a Society BB gauge. The next loco I made had a problem of derailing on pointwork in the forward direction. I found the front wheel BB was overgauge, about 17.85 area, at least at one part of its wobbly perimeter. Still I didn't measure the new gauge and even when disassembling the wheel and regauging it with Araldite, it still turned out to settle around 17.80. Measuring the new gauge I found it varied from 17.67 to 75. So the problem was that I needed to have a different feel and push the wheels harder if I wanted to be sure they were not overgauge.

So why did the narrow to gauge locos never derail, even though they were at least 0.1 narrow, and more, while the wide to gauge loco did derail sometimes, even though it was only the same amount wide?

I don't know if you understand what I wrote in the last but one post.

So anyway now I know what's going on, and I have the simple task of putting a plain wheel, no quartering involved, into some plain bearings for a tender rear wheelset that is fixed, not rocking. What I want is no sideplay, as the Bachmann plastic body is only just wide enough for the wheels even after loads of scraping and filing, so they have to be exactly in the middle. So I spend at least an hour going over and over the maths to work out the washers I need over a chassis 14mm wide, and am pretty sure that the washers that purport to be 1mm and 0.5mm are exactly that. With the inside and outside boss, which I don't want to file away as the wheel needs to be robust, the axle should be exactly the right length. And I'm working on a 17.67BB assumption, so if I've miscalculated and it jams up I can go for 17.75.

I fix one wheel on with Loctite and get that settled and later do the second wheel having put it in place with washers. So, on it goes, then drop of Loctite on the outside before pushing it home against the gauge. Problem is the chassis is in the way so I can only get the gauge to bear against one side of the wheel. ARGGGHHH. It's against the gauge but now the digital vernier says it's 17.9 in one place, 17.6 in another. The Loctite may cure within about 10 minutes, I have heard it said. There is still a little sideplay. What I've learned is that I don't want it overgauge but underguage is OK. So I push the wheels a little more while twisting them. Now there' s no sideplay at all but they can still revolve freely - perfect. The maximum BB is down to around 17.75, the minimum is about 17.55. Yuk. Is that the best I can do? But with a cup of tea things seem to improve - is it me or is the wheel itself settling? Am I measuring it more accurately now or has it really changed? Who knows?

The point is, it stays on the track, trundling along like a caravan behind a car, and just as well in the reverse direction. Yet to test through any pointwork but I very much doubt there will be any problem. The actual problem is, how am I going to get the other two wheelsets to have any sideplay which they will need? I may have to resort to purposely making them narrow to gauge....

Martin Wynne's diagram says everything: (see his post Dec 15th) - I hope he won't mind my quoting it.
Martin Wynne diagram p4_b2b  .png
Martin Wynne diagram p4_b2b .png (9.26 KiB) Viewed 4233 times


I don't mean by the way any disagreement with your last post Philip! Of course the standards work! It's just that "to err is human", (I can't remember the rest of the quote - is it "To sin is divine?!) - and I am sometimes pretty poor at getting wheels within that window, and I think that it's knowing what sort of sin you can get away with that can help in extremis.

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Noel
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Re: What back to back setting do you use?

Postby Noel » Sun Jan 07, 2018 10:45 am

Julian Roberts wrote: It's just that "to err is human", (I can't remember the rest of the quote - is it "To sin is divine?!)


That's an interesting concept, at least for theologians. Actually it's "To err is human; to forgive, divine" - Alexander Pope.
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Noel


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