What back to back setting do you use?

[Noel]

Yes I think the load would have been greater on the rear wheels than the front with the CofG so far back (it must have been more or less at the rear wheel prior to extra weight being added at the front)

It can't have been behind the rear axle, or the loco would simply have rotated about the axle and lifted the other wheels off the track. However, the train being pulled exerts a force on the loco; since couplings are commonly above the axle height, this force will tend to produce rotation about the rear axle which lifts the front axle [and vice-versa if the loco is running in reverse]. This was much more of a problem before pinpoint bearings; nevertheless it may have been a factor with the C of G that far back.

[Julian Roberts]

Tony
Very interesting re Deltics. Surprising design...? -to have same bogie and motors as 37.

Only nowadays with Pendolinos and Voyagers do very fast trains seem to be able to accelerate as rapidly from standing as suburban ones.

[Julian Roberts]

Noel the loco in question was running light. Also must emphasise it has run very well on the club layout and "Kettlewell" without any derailments. This bit of track is for freight only and that is why this mostly passenger loco had not gone there in normal use, so the issue had not previously arisen.

[Alan Turner]

Not sure what my DMU stuff says about previous discussion. Think each car was powered, maybe 2 out of 3. Not 1 out of 3. That was SR dmus Thumpers etc.

Just wanted to quickly say about CofG

Difficult to get it in the middle as model steam locos tend to be light towards the front (I have found).

More importantly, 4-4-0 or 0-4-4 I dont want the CofG in the middle of the coupled wheelbase. But in middle of loco. Lots of weight at the front of 4-4-0 can be balanced by a weighted tender resting on loco giving win win situation of loads of weight on driving wheels.

As regards my spreadsheet you can put the CoG anywhere, within the wheel base, you like (within reason). You must ensure however that the CoA and the CoG coincide to obtain the correct wheel-load output. That's simply a requirement of the method of analysis.

regards

Alan

[Will L]

...Only nowadays with Pendolinos and Voyagers do very fast trains seem to be able to accelerate as rapidly from standing as suburban ones.

For the same reasons, lots of powered axles distributed through the train means lost of adhesion available to apply lost of power to the track to accelerate the train away.

[Enigma]

Did all these steam and diesel locos all have the same back to back measurement for their varying wheel sizes?

[Tony Wilkins]

Tony
Very interesting re Deltics. Surprising design...? -to have same bogie and motors as 37.

Hi Julian.
Yes, it does seem a bit strange, but the bogies of the 37s and Deltics were interchangable. The only difference that I am aware of is that the traction motor blowers were uprated on the Deltic to cope with the greater power and these would have been housed within the body of the loco and so external to the bogies. The Deltic would have typically had 10 to 12 Mk1 coaches behind it so twice the weight of the 6 suburbans behind the 31. The 31s were much more sure footed and could apply full power at a lower road speed.
Re the DMU. If they were Cravens twins, which were the staple units for many years, they had one power car and one trailer car per set and often ran as 4 or 6 car sets, so every other car powered. Incidentally only the inner two axles were driven on the power cars. I have seen them described as 2-2-2-2s. (1A-A1) would be a better description.

Incidentally you can tell that technology and I don't always see eye to eye.
Regards
Tony.

[Tony Wilkins]

Did all these steam and diesel locos all have the same back to back measurement for their varying wheel sizes?

I would be very surprised if they weren't.
However, there was article published in Modern Railways December 1965 about experiments BR were conducting with Heumann tyre profile wheels. It is a most enlightening piece. Whereas the then standard type profiles had a 2 point contact with the rail when the flange contacted the rail, the Heumann profile only had one and ran much more smoothly as a result. There were some wear diagrams included which show that most of the tread wear took place at the root of the tread flange interface, so the BB dimension remained largely unchanged. When tyre turning became necessary the Heumann profile tyres required much less material to be removed to return them to their original shape. Perhaps the most interesting thing was that the BB dimension for the Heumann profile wheels is given as 4' 5 7/16" instead of the conventional 4' 5 5/8" for standard profile tyres.

Regards
Tony.

[Martin Wynne]

Perhaps the most interesting thing was that the BB dimension for the Heumann profile wheels is given as 4' 5 7/16" instead of the conventional 4' 5 5/8" for standard profile tyres.

Illustrating my point that the BB dimension is determined by the wheel flange profile. All this endless talk about BBs for P4 (and for EM and 00) is meaningless without a quoted dimension for the effective flange thickness.

For example there is talk of increasing the "official" BB for EM from 16.5mm to 16.6mm. Whereas for those in EM who nowadays like to use widened RTR wheels, the optimum BB is 16.4mm, a reduction from 16.5mm, not an increase. Otherwise with RTR flanges 0.8mm* thick the 17.2mm check gauge is exceeded.

*NMRA RP-25/110 wheel profile

Martin.

[Armchair Modeller]

Like most of these kinds of discussion, it seems to me it is unlikely to get anywhere, other than round in circles. Those that shout loudest or have the strongest opinions are not necessarily those best qualified to make a positive contribution.

It would be a big help if the Committee could work out a way of establishing what the best standards should be, in the light of current knowledge and experience. This may simply be a confirmaton of the existing standard or some modifications as appropriate. The Society could then certify products as meeting the standards or not. Manufacturers could be encouraged (nicely, of course) to comply.

We could then all be confident that we are buying compatible products that work well in P4 - or not.

[Tony Wilkins]

Like most of these kinds of discussion, it seems to me it is unlikely to get anywhere, other than round in circles. Those that shout loudest or have the strongest opinions are not necessarily those best qualified to make a positive contribution.

It would be a big help if the Committee could work out a way of establishing what the best standards should be, in the light of current knowledge and experience. This may simply be a confirmation of the existing standard or some modifications as appropriate. The Society could then certify products as meeting the standards or not. Manufacturers could be encouraged (nicely, of course) to comply.

We could then all be confident that we are buying compatible products that work well in P4 - or not.

I am inclined to agree with you although I think the standards were largely agreed many years ago as quoted in the Scalefour digest and elsewhere. I see no reason to believe that what worked then should not work equally well now. If anything, with improved manufacturing techniques, the standards should be easier to comply with now.
Regarding manufactures, they can either be given the societies endorsement or not depending on whether they meet the required standards.
Tony.

[Enigma]

Did all these steam and diesel locos all have the same back to back measurement for their varying wheel sizes?

I would be very surprised if they weren't.
However, there was article published in Modern Railways December 1965 about experiments BR were conducting with Heumann tyre profile wheels. It is a most enlightening piece. Whereas the then standard type profiles had a 2 point contact with the rail when the flange contacted the rail, the Heumann profile only had one and ran much more smoothly as a result. There were some wear diagrams included which show that most of the tread wear took place at the root of the tread flange interface, so the BB dimension remained largely unchanged. When tyre turning became necessary the Heumann profile tyres required much less material to be removed to return them to their original shape. Perhaps the most interesting thing was that the BB dimension for the Heumann profile wheels is given as 4' 5 7/16" instead of the conventional 4' 5 5/8" for standard profile tyres.

Regards
Tony.

It wasn't a serious question - I was trying to get back on topic! Which it sort of appears to have done.

Perhaps the S4 (and EM?) Soc. should behave more like the NMRA and issue conformity certificates for products that fit the bill.

[Armchair Modeller]

I am inclined to agree with you although I think the standards were largely agreed many years ago as quoted in the Scalefour digest and elsewhere. I see no reason to believe that what worked then should not work equally well now. If anything, with improved manufacturing techniques, the standards should be easier to comply with now.
Tony.

That was my feeling too, Tony. Yet a number of people have recently jumped on the bandwagon of suggesting that there is some kind of megacrisis in P4 modelling. That stock habitually falls off. That something urgent needs to be done about it. That this or that tweak is all that is necessary. Why didn't anyone think of this before, etc. etc.

Someone needs to restore confidence for those of us who thought everything was fine.

[Le Corbusier]

I am inclined to agree with you although I think the standards were largely agreed many years ago as quoted in the Scalefour digest and elsewhere. I see no reason to believe that what worked then should not work equally well now. If anything, with improved manufacturing techniques, the standards should be easier to comply with now.
Tony.

That was my feeling too, Tony. Yet a number of people have recently jumped on the bandwagon of suggesting that there is some kind of megacrisis in P4 modelling. That stock habitually falls off. That something urgent needs to be done about it. That this or that tweak is all that is necessary. Why didn't anyone think of this before, etc. etc.

Someone needs to restore confidence for those of us who thought everything was fine.

Is there a crisis? I got the impression that because different people have different strengths and weaknesses (so different issues arise) there were just a plethora of options to try in order to determine what works for you?

[Armchair Modeller]

In reference to the standards, that shouldn't be necessary. One standard ought to suit all. That is why I joined the Scalefour Society and started modelling in P4. People shouldn't have to mess around and try their own solutions. It ought to work 'straight out of the box', so to speak.

[Martin Wynne]

In reference to the standards, that shouldn't be necessary. One standard ought to suit all.

There certainly should be and is only one standard called P4.

But no-one is obliged to follow it if they don't want to. They are entirely free to invent their own version if they prefer, and it may turn out to be better than P4.

But it will NOT be P4. If the details are published here or elsewhere they should be given a distinctive name other than P4. Anything less is unfair to beginners trying P4 for the first time and trying to make sense of half a dozen different dimensions all quoted as being for P4.

Martin.

[martin goodall]

Railway Modeller, December 2017, page 1074 (at the bottom of the page).

[Armchair Modeller]

In reference to the standards, that shouldn't be necessary. One standard ought to suit all.

There certainly should be and is only one standard called P4.

But no-one is obliged to follow it if they don't want to. They are entirely free to invent their own version if they prefer, and it may turn out to be better than P4.

Martin.

Fully agree Martin - but some people seem to be suggesting that the existing standards don't work and putting forward tweaks that they imply should replace the existing standards.

There have also been references to wheels, rail, BTB gauges etc being sold as P4 when in fact they are not strictly adhering to the standards. I appreciate that we rely on the goodwill of a limited number of suppliers, but it would be nice to be able to know in advance if a product meets the standards rather than have to buy them and measure/test them for ourselves.

[Le Corbusier]

In reference to the standards, that shouldn't be necessary. One standard ought to suit all. That is why I joined the Scalefour Society and started modelling in P4. People shouldn't have to mess around and try their own solutions. It ought to work 'straight out of the box', so to speak.

I wasn't trying to suggest that we should need/want to stray from the standards as set out in the digest - though there does appear to be a degree of variability within the gauges on sale (mine was way out), and there has been some suggestion that the rail profiles now available ( and even some wheel profiles) might mean playing around a bit within the standards might be required for good running.

However, standards are only as good as an individuals skill to implement them. With varying abilities and differing strengths, fault finding and correction is important to discuss and will inevitably vary. I have found interesting the suggestion to set back to back dimensions at the higher end of the standard and that this is reported to have been effective on Sidmouth (It also appears to have solved my own issues - might this be something to do with the reported degradation of the rail profile). Equally, I have found interesting Julian's findings relating to the beefing up of loading and focussing the loads towards the outer sets of wheels for improved track holding - obviously this has worked for him. Same with the advice that if using C&L/Exacto chairs one should use the Exacto roller gauges and not the triangular gauge to avoid gauge narrowing.

As a beginner I have tried to apply the standards as accurately as I could using the correct gauges .. however without the above input I could well be somewhat disillusioned with it all by now.

[Tony Wilkins]

I got the impression that because different people have different strengths and weaknesses (so different issues arise) there were just a plethora of options to try in order to determine what works for you?

Here I think, we have the nub of the issue. Peoples abilities vary considerably. Give half a dozen people a pair of wheels and a BB gauge and I suspect that the settings they achieve will vary slightly, we are not all engineers. The P4 standards are the P4 standards, as Martin states, anything else cannot be, by definition.
The P4 standards have been proved to work by 100s of working layouts built successfully over the years, but a degree of Quality Control and self discipline is required to achieve perfect running, which may be beyond some to achieve. This is not to denigrate these people, but accept the reality. Many years ago, I tried to make a loco with a sprung chassis. It derailed with monotonous regularity. I took out the springs and put equalising beams in and it ran perfectly. That does not mean that springing does not work, just that I couldn't get it to. Some of the smoothest running coaching stock I have seen run was sprung.
Perhaps some of the derailments witnessed have been wrongly ascribed to incorrect BB setting. I have witnessed occasions where a train will complete a circuit several times and then something derails. Replaced on the track everything runs OK until the same item derails again. A thorough investigation reveals no obvious cause therefore adjust the BB. This may not be the underlying cause, but that is deemed the answer. I have used the phrase before but we are dealing with an envelope of probabilities and just occasionally a limit is exceeded. Understanding this may not be easy and rectifying it even harder, but it should be possible.
Why did I choose to work to S4 standards? It was not because I thought that P4 wouldn't work, it was because I preferred the exact scale dimensions given in the original Protofour table but not adopted for commercial use. I can understand exactly why the MRSG did this given the leap of faith that was required at the time. Perhaps if they had had the courage of their convictions and gone the whole hog, we would not be having this discussion now.
Regards
Tony.

[Will L]

I agree with Tony and think that there is a danger becoming obsessed with one a particular factor like the BtB. When you get a hard problem which derails everything its usually not to difficult to nail it down to a particular problem. When you get a transitory problem there are usually a number of factors at work which all combine occasionally to produce a problem. By tweaking one of them at random you may apparently fix the problem for now, but the other underlying factors remain. So the hapless modeller may believe he had an issue with what ever he tweaked, when in reality there may a other factors which were just as much to blame as the one he fixed. P4 requires skill yes, but persistence and consistency are probably the key part of that, and generally come with experience so don't expect to get it all right all that quickly.

[Julian Roberts]

Completely agree with last two posts. C Pendlenton said to our group it's the combination of single faults that on their own seem harmless which causes derailments or accidents. Seems to me ditching the 00 gauge mentality is part of it. It's not just a matter of sticking on compliant P4 wheels on a 00 gauge model on compliant P4 track, although that complies with the "standards" and can work. With tiny flanges weight becomes more critical because of the formula I gave in my original article which I will add here later. (Flange climb will occur if the sideways forces acting on the wheel are greater than the downward) - that will hold true in 4mm scale even if the forces do scale down 3 times as much as linear dimensions.

Were the exact scale dimensions given in the original Protofour table the same as those now given in the Digest Tony? I suggested earlier that BB minimum could be as low as 17.75 yet give working results. I wonder if you would concur? - Given the official P4 flange envelope of 0.35-40 the max would be 17.85 not 17.89 as stated there surely to stay within the formula that BB + EF =/《 CG

[Julian Roberts]

From the RAIB report Angerstein Junction

[Tony Wilkins]

Were the exact scale dimensions given in the original Protofour table the same as those now given in the Digest Tony? I suggested earlier that BB minimum could be as low as 17.75 yet give working results. I wonder if you would concur? - Given the official P4 flange envelope of 0.35-40 the max would be 17.85 not 17.89 as stated there surely to stay within the formula that BB + EF =/《 CG

Hi Julian.
Yes, they were the same.
I can honestly say that I have never considered the tolerances involved with the exact scale equivalent dimensions. I use a BB gauge of 17.87mm and it works. This does of course go hand in hand with flangeways that are nominally 0.58mm rather than 0.68mm for P4 (ignoring gauge widening).
Since the scale dimension over checks is 17.67mm it should be theoretically possible for a BB just greater than this to pass through, but I would not recommend it. So yes a BB of 17.75 mm will work. However, if one has a set of standards, they should be precisely that and not deviated from or problems may arise that have not been foreseen. Additionally the wheels used must be of an acceptable standard and I have a draw full of reject wheels that I need to go through sometime. The biggest problem has not been with the tyre profile (although some are) but eccentricity and wobble.

The other part often overlooked is the rail profile. The wheel / rail interface is very important and everything else follows from it. New Bullhead rail had a head radius of 12" and corner radii of about 1/2". New Flatbottom rail has a head radius of 9" with similar corner radii. The change was made because this is what it was found that they wore to in service as wheel and rail tended to wear to a common profile regardless of their starting profiles. The original Kings Cross rail had a more rounded head than the rail we have today, which is virtually flat but does have a small radius to the corners. How near to scale that corner radius is I cannot tell. The other thing is that two samples of Bullhead rail that I have just measured are 0.90 and 0.91mm wide, which is as near as dam it scale. I had expected it to be over width due to the problems some have experienced with functional chairs. Additionally real rail is canted in at an angle of 1 in 20. Plastic chairs do this, but soldered track is usually vertical, so the contact point will be on the inner edge rather than centre of the head. A squarer rail edge will if anything push the contact point further in toward the centre of the track equivalent to making the effective flange wider. All these factors play a part in how the wheel and rail interact.

Sometimes though, I think we get carried away by the minutia of all this. It is after all supposed to be a hobby.
Regards
Tony.

[Terry Bendall]

Sometimes though, I think we get carried away by the minutia of all this.

Yes we do indeed Tony. That of course may appeal to some and that is fine but for most of us building models that work and look good is what it is all about and would I suggest give more pleasure than worrying about the last 0.01mm.

Terry Bendall