OO-SF tolerances and RTR wheel profiles

[Captain Kernow]

This thread is intended for anyone who has a layout with at least one set of points, built to OO-SF standards (ie. where the gauge is reduced from 16.5mm to 16.2mm through the common crossing, in order to produce a more realistic set of checkrail gaps) and who operate RTR locos, without any modification to the wheels.

 

On my OO 'cameo' layout 'Bethesda Sidings', I have one crossover built to OO-SF standards. The rest of the pointwork on the layout had already been built (for other, abortive projects) prior to the layout being conceived. The OO-SF crossover was built especially for the layout, as I wanted to try OO-SF out.

 

The crossover was built using the correct OO-SF gauges and very careful attention was paid to ensure that all clearances and the gauge were correct. The crossover was then bench-tested using a variety of locos, some with etched brass chassis and Markits wheels and some RTR locos, prior to being laid on the layout, painted, weathered and ballasted etc.

 

All locos ran through the crossover with no problem during the bench testing. This would have been sometime during 2018.

 

The layout was subsequently finished. Progress was documented on it's own thread in the Layouts section on RMWeb.

 

Since the completion of the layout, I have expanded the pool of locos intended for use on it. In some cases, new RTR locos (ie. recent releases) have been bought.

 

Whilst some of these new RTR locos run through the OO-SF crossover with no problem, some products from three separate manufacturers have not. All have come to a stand (in one case derailed each time) when the wheelsets encounter the 16.2mm gauge. This actually happens just prior to the location of the check rails, where the gauge has been narrowed from 16.5mm to 16.2mm.

 

I have re-checked the gauge of the points and I am satisfied that it is still 16.2mm, so nothing has changed since the points were built.

 

I have also tested some of the affected locos on another set of OO-SF points, which I bought from another (late) modeller, who built many OO-SF points while he was alive and the same binding and stalling happens there as well. The gauge on these other points is also bang-on for OO-SF.

 

I have been assured by two of the three manufacturers that their wheels conform to RP25-110 standards.

 

However, I have been faced with locos from three OO RTR manufacturers that do not seem to be compatible with OO-SF.

 

The solution I have adopted is as follows:

 

- Manufacturer 1 - loco was returned to retailer, as it was a very bad runner anyway (even after running in) and the credit used for something else

 

- Manufacturer 2 - loco will have the RTR wheelsets removed and Markits driving wheels substituted (with 3mm axles). Hopefully this will result in a viable loco

 

- Manufacturer 3 - wheels were removed, final drive gears removed (these were diesels) and the gears then mounted on Black Beetle coach wheels from Branchlines - this results in locos that run very happily through the OO-SF pointwork

 

I would stress that the majority of the OO RTR locos that I have tried, do actually run through with no problem.

 

In all cases (from the three affected manufacturers and the unaffected manufacturers), the back-to-backs are 14.5mm.

 

I did have a problem with a loco where the back-to-backs were 15mm (instead of 14.5mm), which also had problems (but not from manufacturers 1 - 3). The retailer changed the loco for me and the replacement, with 14.5mm back-to-backs, ran OK.

 

You might wonder why I don't modify the OO-SF crossover, to allow the locos from manufacturers 1 - 3 to work OK. Well, I didn't discover the problem until the layout had been completely finished, all track laid, ballasted and scenery in place all around. It's simply not worth the effort and aggravation to disrupt the layout to such an extend for a small number of locos, when I already have sufficient 'good' locos to operate the layout.

 

In conclusion, I would be very interested to hear whether anyone else has encountered problems of this nature.

 

Thank you.

[PMP]

One thing to bear in mind, is the only agreed industry standard is the BRMSB standards of the 1950’s. Obviously in real terms the manufacturers of that era aren’t extant, and there isn’t a similar contemporary trade organisation operating today for Uk 4mm OO RTR equipment. Or indeed for other Uk mainstream scales. 

The ‘North American’ hobby has the NMRA which liases with manufacturing, and has established a practical set of standards concerning technical aspects of the hobby, which many of the manufacturers adopt. Quite a few also make referrals to those standards in their marketing of products emphasising compatibility with those standards. 
 

With the lack of a similar body and agreement within the manufacturers to universally adopt a set of UK standards, variations are commonplace and will continue to be so. The core selling requirement for the greater majority of Uk 4mm RTR products is that they will run on set track, and negotiate 2nd radius curves and pointwork.

Edited December 12, 2023 by PMP
Addition

[Dungrange]

1 hour ago, Captain Kernow said:

I did have a problem with a loco where the back-to-backs were 15mm (instead of 14.5mm), which also had problems (but not from manufacturers 1 - 3). The retailer changed the loco for me and the replacement, with 14.5mm back-to-backs, ran OK.

 

I'm just about to start building my first 00-SF turnouts, so I can't comment on the problems you have identified.  However, it was my understanding that if the wheelsets are to RP25-110 standards then the correct back-to-back dimension is 14.4 mm (ie wheels with flanges 0.8 mm thick).  14.5 mm back to back would be correct for Romford /Markits wheels with a flange thickness of 0.7 mm.  The problem you are describing sounds as though the back-to-back (or rather front-to-front) dimension is too big.

[Buhar]

Given the B2B is ok, I'm wondering if it's the width of the flange on the rogue models that's causing the binding or derailing.  I'll be interested to see how this develops.

 

In the interest of fairness, 00-SF was designed for the finer wheels usually found under kit-built locos.  The fact that most modern RTR runs through well ended up being a useful by-product. (Sort of like Marmite, a by-product of beer that divides opinion.)

 

Alan 

[Izzy]

I’ve been bemused in recent years to encounter RTR OO locos with different wheel standards, flange width/depth & overall width, under the same loco I.e. bogie/driving/tender. Indeed no two locos wheels from any maker have been exactly the same dimension wise either. 

 

Bob

[spamcan61]

1 minute ago, Izzy said:

I’ve been bemused in recent years to encounter RTR OO locos with different wheel standards, flange width/depth & overall width, under the same loco I.e. bogie/driving/tender. Indeed no two locos wheels from any maker have been exactly the same dimension wise either. 

 

Bob

Might well be made by different manufacturers (as in the wheels, not the complete item) with different 'standards'.

[PMP]

26 minutes ago, Buhar said:

Given the B2B is ok, I'm wondering if it's the width of the flange on the rogue models that's causing the binding or derailing.  I'll be interested to see how this develops.

 

You may well be right regarding the flange width, but it has to be understood that the models aren’t rogues, they just have a different wheel profile/dimensions to those that do work through OOSF track.

Edited December 12, 2023 by PMP
Addition

[Captain Kernow]

1 hour ago, Dungrange said:

I'm just about to start building my first 00-SF turnouts, so I can't comment on the problems you have identified.  However, it was my understanding that if the wheelsets are to RP25-110 standards then the correct back-to-back dimension is 14.4 mm (ie wheels with flanges 0.8 mm thick).  14.5 mm back to back would be correct for Romford /Markits wheels with a flange thickness of 0.7 mm.  The problem you are describing sounds as though the back-to-back (or rather front-to-front) dimension is too big.

What I will say is that one of the manufacturers advised me that their wheels were to 'RP25-110, with a b2b of 14.5mm'.

 

I think that the 0.1mm potential difference on the flange is not the issue here, rather the 'front-to-front' measurement, as you expressed it. I think this is the case, because the binding problems start on a short piece of 16.2mm track, before the first wheelset gets to the checkrailed area.

 

Of course, 14.4 + .8 = .8 = 16mm, whereas 14.5 + .7 = .7 = 15.9mm. If a b2b measurement of 14.5mm is combined with 0.8mm flanges, then we get 16.1mm. Even though such differences are very small, what's clear is that a difference of 0.3mm (ie. 16.2mm as opposed to 16.5mm) is making all the difference here.

 

[Captain Kernow]

1 hour ago, Buhar said:

Given the B2B is ok, I'm wondering if it's the width of the flange on the rogue models that's causing the binding or derailing.  I'll be interested to see how this develops.

I think the width of the flanges is contributing to the problem, as described above, even though the difference is very small, in that it's making the 'front to front' measurement of the flanges too big.

 

1 hour ago, Buhar said:

In the interest of fairness, 00-SF was designed for the finer wheels usually found under kit-built locos.  The fact that most modern RTR runs through well ended up being a useful by-product. (Sort of like Marmite, a by-product of beer that divides opinion.)

I didn't realise that it was designed for the finer wheels, as you describe them, although it was admittedly such wheels that initially were used on my workbench tests of the crossovers.

 

[Captain Kernow]

1 hour ago, Izzy said:

I’ve been bemused in recent years to encounter RTR OO locos with different wheel standards, flange width/depth & overall width, under the same loco I.e. bogie/driving/tender. Indeed no two locos wheels from any maker have been exactly the same dimension wise either. 

I've said on this forum before that I found this problem with some Bachmann steam locos about 20 years ago. I even asked Alan Gibson (when he still owned the company that bears his name) for a drop-in wheelset for an OO loco, because the driving wheel profile was difference from other Bachmann products at the time. The locos were the Standard 5 4-6-0 and an ex-LMS 'Crab' 2-6-0, the driving wheels of both 'bounced' on the chairs of my C&L flexi-track.

 

I also have a Bachmann WD 2-8-0, where I had to change the bogie and tender wheels, due to the same problem (substituted Markits wheels), although fortunately the driving wheel profile was compatible with the chaired track.

 

[Captain Kernow]

1 hour ago, spamcan61 said:

Might well be made by different manufacturers (as in the wheels, not the complete item) with different 'standards'.

One other thing that one of the manufacturers told me was that their loco was made in the same factory as that of one of the other manufacturers I mentioned.

 

This left me wondering whether individual factories have more leeway that may possible be desirable, in setting wheel profile standards?

 

[Captain Kernow]

1 hour ago, PMP said:

You may well be right regarding the flange width, but it has to be understood that the models aren’t rogues, they just have a different wheel profile/dimensions to those that do work through OOSF track.

Quite right, Paul, which is why I am not naming the manufacturers concerned.

 

What I would advise, however, is that anyone contemplating building OO-SF pointwork make sure that all the stock that may conceivably run over it, is actually compatible with the 16.2mm gauge.

 

Of course, this cannot cover future releases, so I would advise people just to be aware that this could be a potential problem.

 

I don't wish to denigrate OO-SF, it's an intriguing idea, but I will not be building any more points to that standard.

 

As some may know, I model in both OO and P4. Although I am in no way trying to make out that P4 is in any way a superior choice, one thing that I do find comforting is that there is a laid-down standard with the latter for wheels and track....  I assume it is the same for the current EM (18.2mm) standards.

 

[Dungrange]

8 minutes ago, Captain Kernow said:

As some may know, I model in both OO and P4. Although I am in no way trying to make out that P4 is in any way a superior choice, one thing that I do find comforting is that there is a laid-down standard with the latter for wheels and track....  I assume it is the same for the current EM (18.2mm) standards.

 

My understanding is that as far as track is concerned 00-SF is simply the EM track standards minus 2mm.  The problem is that wheelsets to RP25/110 do not comply with the EM wheel standards: they comply with the NMRA standards for HO.  It's just a happy coincidence that what is technically an oversize wheel can still get through the EM standard checkrail gap provided the back-to-back dimensions are set accurately.  Ideally, all 00 wheels should be replaced by finer ones that have a profile that aligns with the EM standards, but I understand that for most models that isn't strictly necessary (which I think is what you are saying you have found).  00-SF could therefore be described as hybrid between 00 and EM.

[34theletterbetweenB&D]

1 hour ago, spamcan61 said:

Might well be made by different manufacturers (as in the wheels, not the complete item) with different 'standards'.

 

36 minutes ago, Captain Kernow said:

One other thing that one of the manufacturers told me was that their loco was made in the same factory as that of one of the other manufacturers I mentioned.

 

This left me wondering whether individual factories have more leeway that may possible be desirable, in setting wheel profile standards?

 

These are to the point.  So it says 'Well Established Brand' on the box.

 

That could mean, wholly designed, all parts manufactured and assembled in a single facility in our ownership. 

 

Ranging all the way to an assembly of parts to our design specification made in all sorts of locations, assembled somewhere else and packed in boxes carrying our branding.

 

[polybear]

Sounds like you need to knock up a BEF Gauge; this should help:

 

https://www.rmweb.co.uk/forums/topic/98904-oo-sf-back-to-back-dimension/#entry1878333

 

Out of interest, what Locos were giving you problems?

[Captain Kernow]

11 hours ago, polybear said:

Sounds like you need to knock up a BEF Gauge; this should help:

 

https://www.rmweb.co.uk/forums/topic/98904-oo-sf-back-to-back-dimension/#entry1878333

 

Out of interest, what Locos were giving you problems?

I hadn't seen that before, but it is interesting.

 

The problems I have discovered are not connected with clearance gaps between check/wing rails and running rails, it's more to do with the distance between the outside of the flange (ie. where the wheel tread curves into the more vertical face of the flange itself). When certain RTR wheels encounter plain line laid to 16.2mm gauge.

 

Although both the Markits wheels I used to test the points (which were fine) and the problematic RTR wheels both had b2bs of 14.5mm, the RTR wheels had thicker flanges, which meant that they were not happy on 16.2mm track, whereas the thinner flanges of the Markits wheels were fine.

 

I wasn't going to mention the manufacturers concerned, as in other respects they produce very fine models and I didn't want to adversely affect people's opinion of them, but at the same time, anyone modelling in OO-SF who expects to run any of the affected locos is going to be disappointed.

 

I'd be happy to share the 'guilty' locos by PM, please let me know if you would be interested.

 

Thanks.

 

[Captain Kernow]

Just now, Captain Kernow said:

The problems I have discovered are not connected with clearance gaps between check/wing rails and running rails, it's more to do with the distance between the outside of the flange (ie. where the wheel tread curves into the more vertical face of the flange itself). When certain RTR wheels encounter plain line laid to 16.2mm gauge.

 

Although both the Markits wheels I used to test the points (which were fine) and the problematic RTR wheels both had b2bs of 14.5mm, the RTR wheels had thicker flanges, which meant that they were not happy on 16.2mm track, whereas the thinner flanges of the Markits wheels were fine.

As such, it's got me thinking that this problem is not something that could necessarily have been forseen, when OO-SF was first mooted.

 

[polybear]

51 minutes ago, Captain Kernow said:

I hadn't seen that before, but it is interesting.

 

The problems I have discovered are not connected with clearance gaps between check/wing rails and running rails, it's more to do with the distance between the outside of the flange (ie. where the wheel tread curves into the more vertical face of the flange itself). When certain RTR wheels encounter plain line laid to 16.2mm gauge.

 

Although both the Markits wheels I used to test the points (which were fine) and the problematic RTR wheels both had b2bs of 14.5mm, the RTR wheels had thicker flanges, which meant that they were not happy on 16.2mm track, whereas the thinner flanges of the Markits wheels were fine.

 

I wasn't going to mention the manufacturers concerned, as in other respects they produce very fine models and I didn't want to adversely affect people's opinion of them, but at the same time, anyone modelling in OO-SF who expects to run any of the affected locos is going to be disappointed.

 

I'd be happy to share the 'guilty' locos by PM, please let me know if you would be interested.

 

Thanks.

 

 

@Martin Wynne actually suggests a B2B of 14.4mm for RTR wheels (though wheel profile, thickness etc. also plays a part here - hence the suggestion of checking with a BEF Gauge) - see here:

 

https://85a.uk/00-sf/setting_00_wheels.php

 

50 minutes ago, Captain Kernow said:

As such, it's got me thinking that this problem is not something that could necessarily have been forseen, when OO-SF was first mooted.

 

 

The idea of 00-sf is around 50 years old now, so probably not.....

 

https://85a.uk/00-sf/history.php

[Not Jeremy]

I know why it was done etc, but the idea of further reducing an already hideously narrow gauge is complete anathema to my way of thinking. Goodness knows who or what seduced CK into dabbling with it. What with him being such a good constructor of track and all over so many years!

[Captain Kernow]

10 hours ago, Not Jeremy said:

I know why it was done etc, but the idea of further reducing an already hideously narrow gauge is complete anathema to my way of thinking. Goodness knows who or what seduced CK into dabbling with it. What with him being such a good constructor of track and all over so many years!

Kind words, old chap. Why indeed was I seduced into building the points to OO-SF? I really can't remember what came into me at the time, but it's definitely a case of 'once bitten, twice shy' from now on.

 

[polybear]

Hi CK,

 

I've just been skipping thru' your layout thread "Bethesda Sidings" in an attempt to try to understand what may be going on.

Firstly, if I'm correct you mentioned the layout was built to "00 Finescale" track standards; this confuses me a bit because the DOGA 00 Finescale Track Standard has a Crossing Flangeway dimension of 1.0mm (the same as 00-sf) - however this requires a minimum B2B dimension of 14.8mm, meaning RTR Locos etc. would require adjustment.  However, the DOGA Intermediate Standard states a CF of 1.2mm, with B2B of 14.4mm.  Do you recall what dimensions you used for the 00 part of the layout?

 

I suspect that constructing a BEF Checking Gauge would be most enlightening (all it takes is three short lengths of rail soldered to a scrap piece of copperclad board, using the appropriate 00-sf track gauges); it may well be that the offending locos just require a very slight adjustment of their B2B settings in order to play nicely.

 

It should be remembered that one of the main attractions of 00-sf is compatibility with both Kit built wheels (Markits, Gibson etc.) as well as with most modern RTR locos and stock without the need to adjust RTR B2B dimensions from the normal 00 dimensions.  Naturally if some manufacturers start shifting the goalposts with regards to wheel profiles then this may well have an effect.

 

Finally, you mention that the Turnout causing problems is an A5; I generated one in Templot and the radius comes out at 597mm. 

00-sf is basically EM minus 2mm, meaning the "rules" for EM apply to 00-sf as well; the suggested minimum radius for 00-sf is 750mm on Templot (though this isn't by any means set in stone).  I did note that all the Locos mentioned on your layout thread that had issues were short wheelbase ones though, perhaps with the exception of the Manor - it's not as though you were having issues with Pacifics, 9F's etc.

 

I have a test board (basically a Y-shape) using a B7 (might be B8) turnout (I'm unsure of the radius, but it won't be less than 750mm anywhere) - and as yet I've yet to encounter any Loco that would fail to go thru' it smoothly (and that includes a kit built 9F where the seller said he was only parting with it because it couldn't cope with the curves on his 00 Layout).

 

HTH

Brian

 

p.s. Very nice Layout, by the way ......must have a read thru' the fifty pages......

Edited December 16, 2023 by polybear

[Izzy]

Mention has been made of wheels to RP25/110 profile. If you look at the NMRA standards I think this may well be where the problems lay when 16.2mm is used. 

 

This profile is that recommended for OO/HO where the flangeway gap is set at 1.22mm using the NMRA track standards. The actual wheel flange is 0.73mm so in theory it should pass through the OO-FS reduced flangeways of 1.0mm. 

 

But this doesn't take any account of the root radius which may cause the wheel to rise off the rail head if the wheelset is too wide to gauge and then they can end up running on the root rather than the tread. Obviously the wheel b-t-b or back-to-flange front measurement is also key here. 

 

Unified wheel and track standards are obviously best. This is where track standards such as P4 and 2FS score. One gauge, one wheel profile etc. Alter them at your discretion and expect them not to work if you don't know and understand what you are doing or parts are outside the tolerances built into the original standards you are altering. The same is really true of all these gauge narrowing dodges to overcome either running or looks issues. 

 

I've built a small layout in O-MF/31.5mm. This seemed easier than what I did previously of increasing the b-t-b to obtain reduce flangeways. It works quite okay. But O-SF/31.25mm? Hm, I'm not so sure it would work for me, pushing things a bit too far. Reducing slackness is one thing, removing it altogether quite another. 

 

Bob

 

 

[Not Jeremy]

Short radius points, and sharply curved track actually requires gauge widening, which would appear to make some sort of mockery of any slavish adherence to a specified gauge in all instances.

 

I do understand the point of the gauge tightening in OO SF, before I am given any lectures, but sharp curves, including those through point work will require a more nuanced and flexible approach. That’s what the prototype does and the suggestion that EM or even P4 “won’t work” at sharper radii is plain wrong.

[PMP]

5 hours ago, polybear said:

 

.  Naturally if some manufacturers start shifting the goalposts with regards to wheel profiles then this may well have an effect.

 

There’s no manufacturer shifting goalposts.

There are no agreed industry standards set for Uk 4mm OO gauge wheel profiles. This is why there are significant variations between manufacturers, and even within manufacturers ranges.

[polybear]

7 hours ago, Not Jeremy said:

Short radius points, and sharply curved track actually requires gauge widening, which would appear to make some sort of mockery of any slavish adherence to a specified gauge in all instances.

 

I do understand the point of the gauge tightening in OO SF, before I am given any lectures, but sharp curves, including those through point work will require a more nuanced and flexible approach. That’s what the prototype does and the suggestion that EM or even P4 “won’t work” at sharper radii is plain wrong.

 

Sharp radii will work - but will have a bearing on what Locos can be used; that's why the likes of Hornby etc. give guidance on what curves are suitable for their Loco range.  Just to add to the fun their are other factors that come into play as well, such as side play on wheelsets - which would explain why one kit built 9F might go round 24" curves and another won't.