P4 scale limitations.

Model and prototype rolling stock, locos, multiple units etc.
doggeface

P4 scale limitations.

Postby doggeface » Tue Jul 31, 2012 5:15 pm

Having devoted a little while to reading most of the comment and opinion regarding P4 v EM flange sizes and the antiquity of the P4 wheel profile I find myself wondering why the scale weight needs to be amplified so much to achieve scale traction!

My example being a 1/76 model of a 70 tonne prototype loco which would weigh in at about 160 gms. As most models come in at about twice that weight and still seem too light for steady adhesion (especially on rigid frames) it would appear that other factors have a greater effect than mere weight.

At present it takes me some weeks of study and measurement to commission a point such that most locos will transit without tripping over their boot laces. This mostly due to overnight changes in geometry in all planes coupled with Brook - Smith solder failures and plastic chair adhesive failures.

The vertical ripples which appear from time to time are the hardest to detect or explain. It is almost as if my hobby is working on the total destruction of my ego!

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Tim V
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Re: P4 scale limitations.

Postby Tim V » Tue Jul 31, 2012 5:53 pm

You are confusing scale, gauge, mass and gravity.

P4 is a gauge, the scale is 4mm.

As we can't scale gravity - gravity remains constant in our universe, the right weight for any vehicle is - just enough :!:
Tim V
(Not all railways in Somerset went to Dorset)

doggeface

Re: P4 scale limitations.

Postby doggeface » Tue Jul 31, 2012 6:26 pm

Too true Tim, just like time- very unforgiving! In the world of engineering modelling we do have to modify materials , gases and whatever in order to better reproduce conditions for the item under study. Gravity does not change but the weight is modified by the use of lighter materials and air substituted by lighter gases to suit.(water replaced by gases etc)

As we seem to accept the total scaling irrespective of the forces involved which my be 2nd or 3rd order and higher (as a function of unit length) it perhaps calls up the need for something to be modified which will compensate -- I believe that Martin Goodall has made this point in a truly empirical manner. I shall soldier on but be forced to follow the route of making the track geometry many times less variable than the prototype could ever hope for especially as it is going to be many times more rigid than the 4ft could ever be and remain safe.

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Russ Elliott
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Re: P4 scale limitations.

Postby Russ Elliott » Tue Jul 31, 2012 6:50 pm

doggeface wrote:Having devoted a little while to reading most of the comment and opinion regarding P4 v EM flange sizes and the antiquity of the P4 wheel profile I find myself wondering why the scale weight needs to be amplified so much to achieve scale traction!

Traction is not related to flange profile. Try approx 4g per prototype ton.

The vertical ripples which appear from time to time are the hardest to detect or explain.

That's inherent with any track, model or prototype. That's why we have wheel suspension. A balanced suspension will also optimise traction.

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Guy Rixon
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Re: P4 scale limitations.

Postby Guy Rixon » Sat Jan 05, 2013 2:09 pm

doggeface wrote:Having devoted a little while to reading most of the comment and opinion regarding P4 v EM flange sizes and the antiquity of the P4 wheel profile I find myself wondering why the scale weight needs to be amplified so much to achieve scale traction! ...


I looked up friction on Wikipedia. The page there suggests that for both static friction (in play when a train starts) and kinetic friction (when the train is moving), the frictional force is directly proportional to the force pressing the surfaces together. The scaling factor - coefficient of friction - doesn't depend on the size of the bearing surface, so the scale of the model doesn't come into the equation. The weight you need to pull a train depends on the relative coefficients of friction in the axle journals and at the wheel-rail interface.

I suspect that a full-size railway has a relatively low coefficient for the journals and a higher one at the rail surface, given that the quality of finish in the bearings will be much finer. Further, I suspect that scaled down trains have relatively smoother rails and poorer bearings. I know that pin-point bearings are supposed to counter this, but I don't think they work as well in P4 when we fit clever suspensions (certainly not when I build the suspensions). Slop in the bearings seems to increase the friction.

There's also the friction on curves, which will be vastly higher in our tightly-coiled layouts.

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Will L
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Re: P4 scale limitations.

Postby Will L » Sat Jan 05, 2013 4:54 pm

guyrixon wrote:I looked up friction on Wikipedia. The page there suggests that for both static friction (in play when a train starts) and kinetic friction (when the train is moving), the frictional force is directly proportional to the force pressing the surfaces together. The scaling factor - coefficient of friction - doesn't depend on the size of the bearing surface, so the scale of the model doesn't come into the equation.


Sorry but no, as the other factor in the equation, weight, varies with the cube of the liner dimension which we scale. So scale has a very big impact.

Will

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Guy Rixon
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Re: P4 scale limitations.

Postby Guy Rixon » Sat Jan 05, 2013 5:59 pm

Will L wrote:
guyrixon wrote:I looked up friction on Wikipedia. The page there suggests that for both static friction (in play when a train starts) and kinetic friction (when the train is moving), the frictional force is directly proportional to the force pressing the surfaces together. The scaling factor - coefficient of friction - doesn't depend on the size of the bearing surface, so the scale of the model doesn't come into the equation.


Sorry but no, as the other factor in the equation, weight, varies with the cube of the liner dimension which we scale. So scale has a very big impact.

Will


But we're talking here of the ratio of engine weight to train weight. For given surface quality, one gram of adhesive weight gets you x gram-force of traction whether it's a model or a full-size locomotive. Similarly, one gram of weight in the train gets you y gram-force of resistance. The ratio x/y, which determines if the engine can start the train, doesn't change with the scale. But we actually, empirically need heavier engines relative to the train weights, hence my hypothesis about the coefficients being different in the models.

John Fitton

Re: P4 scale limitations.

Postby John Fitton » Sat Jan 05, 2013 7:51 pm

Personally I go for the NMRA standards for bogie rolling stock, and with Tim V's approach for locomotives: as much lead as will fit. (paraphrasing of course.)

John Fitton

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Will L
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Re: P4 scale limitations.

Postby Will L » Sun Jan 06, 2013 1:09 am

guyrixon wrote:But we're talking here of the ratio of engine weight to train weight. For given surface quality, one gram of adhesive weight gets you x gram-force of traction whether it's a model or a full-size locomotive. Similarly, one gram of weight in the train gets you y gram-force of resistance. The ratio x/y, which determines if the engine can start the train, doesn't change with the scale. But we actually, empirically need heavier engines relative to the train weights, hence my hypothesis about the coefficients being different in the models.


You are assuming that the ratio x/y between loco weight and the train weight remains the same what ever the scale, and I don't think you can make that assumption, not the least because they are not made from the same material as the original. Then while it may be reasonable to suggest that the maximum possible tractive effort available from both scale model and prototype locos can be derived directly from weight and the coefficient of friction between wheel and rail, the same can't be said for either the starting or rolling resistance of the train which, on flat track, is all to do with the friction between axle and bearings which is highly dependant on the nature of the bearing.

Not sure I agree your empirical experience, as a good model loco is generally able to pull a longer train than the prototype. However poor chassis design can mean that a loco doesn't get anywhere near the theoretical maximum. Look back over the old Deputy Chairman's cup loco performance completion results in say snooze issue 110 to see how variable this can be, even for locos which, presumably, the owners though ran pretty well.

Will

Terry Bendall
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Re: P4 scale limitations.

Postby Terry Bendall » Sun Jan 06, 2013 9:21 am

doggeface wrote:At present it takes me some weeks of study and measurement to commission a point such that most locos will transit without tripping over their boot laces. This mostly due to overnight changes in geometry in all planes coupled with Brook - Smith solder failures and plastic chair adhesive failures.


I started actively modelling to P4 standards in 1974 and since then have built quite a lot of pointwork, using the Brook Smith method, including single and double slips and tandem turnouts. Whist some tweaking has often been necessary is is certainly never taken as long as this to get the pointwork to an acceptable running standard - by acceptable I mean nothing falls off.

When testing pointwork my usual test is to try to push a wagon through by hand so the weight of the wagon does not make that much difference. Without wishing to sound unkind, if you are having problems to this extent, then I wonder if you need to check your construction techniques.

It sounds like you are using a mixture of rivets and functional chairs. I think it was Iain Rice that devised this method and I have used it successfully myself. However there are some people who would question the idea, and there is at least one on this forum. If rivets are used in every sleeper the rail is held very firmly and is unlikely to move much, either in the guage or in the length. If you are using functional chairs for most the the sleepers there is a lot more scope for movement, since there will be a reletively long length of rail that could move, but which will be held firmly (or al least should be) by the rivets at each end of the length. Therefore it will be more prone to movement, going out of gauge and buckling. Track built using the Exactoscale method is only held by the functional chairs and any movement in the length of the rail is unlikely to cause problems since it can move just as on the prototype.

When building track it is important to check that the top suface of the rail is flat and I do this by laying a 300mm steel rule on edge along the top of the rail after it has been fixed in place. There is usually sufficient flexibility in the rule to bend it to deal with curves and the rule will spring back straight. When soldering rail to rivets you need to press down on the track gauge to make sure that the boittom of the rail is tight to the top of the rivet. If using functional chairs glued to sleepers than you need to put a weight on top of the rail to ensure that the rail is bedded into the chair and that the chair is pressed firmly onto the sleeper. It would also be worth while making sure that the rail is level when supported on rivets and in chairs.

Soldered joints can and do fail but if you follow the basic rules, they should be OK. The rules are clean metal, close fitting joint and correct temperature. Clean the rivet heads with a file after they have been put in the sleepers until you have a bright surface. Clean the underside of the rail using a glass fibre brush. All metal will have an oxide coating even if it looks clean. If you run a finger over the surface after cleaning you will deposit grease from the skin so clean it again. Apply a thin layer of solder to the rivet to "tin" the surface. Put the rail in place and solder in position. I always use a flux even though I use cored solder that contains flux and it may be necessary to add some more solder. Some of the track that I built 38 years ago is still in use and is still working so somehting must be right.

If there are problems with the track, the answer is to solve the track problems rather than looking at weight or suspension methods and I would say that large (i.e. long) steam locos will almost always need some sort of compensation/springing to work successfully.

Hope this helps.

Terry Bendall

clive mead

Re: P4 scale limitations.

Postby clive mead » Tue Jan 08, 2013 1:29 pm

Let me start off by saying I know nothing about adhesion, but knowing nothing has never stopped me in the past from putting in my two pennyworths.

The weight stationary prototype locomotive would surely indent the rail and the wheel rims, giving greater surface area of contact between rail and wheels and thus increase the adhesion available. This would help in getting everything moving, but once movement has been achieved this advantage is lost.

We can scale down a prototype in size and weight but the rail and wheel rims remain about the same strength and hardness as the prototype, so no indentation on our layout.
If this is true (comments please) this means you can never replicate prototype adhesion, at least from a standing start.

Having said all that, and not wishing to shoot myself down, it could also be argued that the prototype load of coaches/wagons would also indent the rail and their wheels and so would need an extra pull, to get them going.

Clive Mead

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Guy Rixon
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Re: P4 scale limitations.

Postby Guy Rixon » Tue Jan 08, 2013 1:56 pm

Will L wrote:
guyrixon wrote:But we're talking here of the ratio of engine weight to train weight. For given surface quality, one gram of adhesive weight gets you x gram-force of traction whether it's a model or a full-size locomotive. Similarly, one gram of weight in the train gets you y gram-force of resistance. The ratio x/y, which determines if the engine can start the train, doesn't change with the scale. But we actually, empirically need heavier engines relative to the train weights, hence my hypothesis about the coefficients being different in the models.


You are assuming that the ratio x/y between loco weight and the train weight remains the same what ever the scale, and I don't think you can make that assumption, not the least because they are not made from the same material as the original. Then while it may be reasonable to suggest that the maximum possible tractive effort available from both scale model and prototype locos can be derived directly from weight and the coefficient of friction between wheel and rail, the same can't be said for either the starting or rolling resistance of the train which, on flat track, is all to do with the friction between axle and bearings which is highly dependant on the nature of the bearing.

Not sure I agree your empirical experience, as a good model loco is generally able to pull a longer train than the prototype. However poor chassis design can mean that a loco doesn't get anywhere near the theoretical maximum. Look back over the old Deputy Chairman's cup loco performance completion results in say snooze issue 110 to see how variable this can be, even for locos which, presumably, the owners though ran pretty well.

Will


No, I'm not assuming that. This is all about why it might not be appropriate to scale the train weight in the same ratio as the locomotive weight. It's not my evidence - it's Clive's observation that the model engines don't pull as well as the prototype. And I'm not claiming that the coefficient of friction in the model rolling-stock is the as in the full-size train; if you actually read my first post in this thread you'd see that I actually speculate about why they might differ. I don't claim to be any kind of an expert in this; I'm just floating some ideas for debate. That debate would be easier if you actually responded to what was posted rather than attributing statements that I never made.

However, I take your point that some model engines do pull better than their prototypes and I am prepared to accept that poor weight distribution in the locomotive matters more than coefficients of friction. It would still be interesting, to me at least, to know if there theoretical limits on what we can expect from an optimal chassis.

FWIW, over the weekend I thought of another factor that the simple model of friction ignores. Under high enough pressure, surfaces of the same material will tend to bond together; this is why shafts and journals are not supposed to be of the same metal. Is it possible that that a full-size locomotive increases its adhesive friction by this effect while a model does not? If the model's weight is scaled down as the cube of the scale ratio, and if the wheels have the same profile as the prototype, then the pressure on the rails in the model will be less by a factor equal to the scale ratio.

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Russ Elliott
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Re: P4 scale limitations.

Postby Russ Elliott » Tue Jan 08, 2013 6:33 pm

guyrixon wrote:It would still be interesting, to me at least, to know if there theoretical limits on what we can expect from an optimal chassis.

If using perfectly clean steel tyres on steel rail, you might be looking at a theoretical maximum tractive effort of 50% of the tractive weight. In practice, materials are never perfectly clean, and you would be lucky to achieve 25 to 30%, and probably down toward the 20% or less area if one or both is nickel. Prototype haulage could be in the sub-10% range on very greasy track or in heavy rain. Don't forget the prototype had sanding gear, often needed in starting a train.

billbedford

Re: P4 scale limitations.

Postby billbedford » Wed Jan 09, 2013 10:02 am

The erstwhile Deputy Chairman's Cup competitions showed that the very best designed loco frames were able to give a drawbar pull to adhesion weight of around 25%. This can only be improved by non-finescale techniques such as traction tyres.

Personally I find this whole debate fairly sterile. There are so many places where the prototype and model fundamentally differ that are never considered, e.g. pin-point vs grease bearings, model vs prototype track radii, loaded vs unloaded weights, etc, that make the whole exercise academic.


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