A question of weight

[paul4147]

I'm part way through my first wagon build to P4 standards, a Red Panda BR lowfit. So far ive built the main wagon structure and added torsion bar suspension from MRD which was really easy to use, and now i need to weight it.

Is there an agreed amount of weight for an individual wagon and if so what is it? Or does everyone go their own way?

Thanks in advance for any help, and sorry if this has been asked before, I did try and check

Cheers

Paul

[John Donnelly]

I believe that 25g per axle is the normally stated value.

John

[Paul Townsend]

1.8 Grams +/- 20% per scale foot of headstock length is the recommendation from original P4 Society.

It has worked for me for 40 + years!

[JFS]

1.8 Grams +/- 20% per scale foot of headstock length is the recommendation from original P4 Society.

It has worked for me for 40 + years!

Don't you mean solebar length Paul? Headstocks are a pretty consistent length!

Best wishes,

[Guy Rixon]

1.8 Grams +/- 20% per scale foot of headstock length is the recommendation from original P4 Society.

It has worked for me for 40 + years!

That's interesting on three counts.

1. It gives about half the weight of "25g per axle" for a 15' wagon, so at least one of the two algorithms is not optimised.

2. I bet any cast wagon will be much heavier than 1.8g/foot.

3. 1.8g/foot might actually be achievable with plastic wagons! My current, scratch-built wagon is going to have lead solebars, "flitched" in plastic, to get it up near 50g.

[paul4147]

1.8 Grams +/- 20% per scale foot of headstock length is the recommendation from original P4 Society.

It has worked for me for 40 + years!

Thanks Paul, and to evertone else who has replied, maybe a daft question but if you have wagons of different length in the same train, they would be different weights, does this affect how the train as a whole works, will there not be issues with differently weighted wagons?

Cheers

Paul

[John Fitton]

Paul,

This has been discussed before but don't worry, you will always find active discussion on old topics as well as new! First, no, there is no universally accepted standard for vehicle weights, although the NMRA on the left hand side of the Atlantic has an extensive set of recommendations. For bogie stock I use their standard: 1 oz. plus 0.5 oz. per inch of car weight. Thus, a 10 inch car weighs in at 6 oz. Approximate conversion to metric: 168 grams, or about 42 grams per axle. As the US does not have a lot of non-bogie stock, I don't know if this recommendation applies to, say, 4-wheel wagons.

I think also it is important to take into account your particular railway. Planned gradients, locos, and especially, length of trains and anticipate running conditions. I have a large tail-chaser layout with long bogie trains and high speeds, and I weight all stock to the NMRA values. When I come to assemble my line up of Airfix minerals, I will post on this forum!!

Hope this helps!

John Fitton

[paul4147]

Hi John, thank you for your reply, the majority of stock i plan to build will be 4 wheeled wagons. I thought it must have been discussed before but i couldnt find it when i needed it as is always the way!

Its my intention to model my home town station of Castleford during the late 70's early 80's when i was a young lad fascinated with trains, and it never went away

Cheers

Paul

[billbedford]

Since, as far as I know, no one has produced any conclusive experimental evidence on this issue, you are free to choose whichever voodoo formulation you wish. However, experience tells me that 25g per axle is an impossible figure for small pre-grouping wagons made of plastic, unless the ballast is made of an exotic metal.

Members of CLAG have proposed that a figure of 4g per prototype ton should be used. This would give a weight of between about 28g (empty) and 96g (loaded) for a standard 16 ton BR mineral. From this, it seems to me, that a range of weights would work equally well and I suggest a looser criteria could be used, such as:

The wagon should be heavy enough that the springs/compensation works as intended, yet not so heavy that the bearing cause excessive drag.

[paul4147]

Thanks Bill, as someone new to P4 its others experience that is needed. So my BR lowfit will come in around 24g which seems a little easier to fit in such a small wagon.

Cheers

Paul

[John Fitton]

Re the comment about exotic material: I looked into the use of tungsten a few months ago, and whilst its price is eye-watering, the difficulty of obtaining small enough pieces was a real problem. It is used as fishing weights, but I can't imagine the effort required to hammer ovoid weights into rectangular blocks!

JF

[dal-t]

It is used as fishing weights, but I can't imagine the effort required to hammer ovoid weights into rectangular blocks!

No need. It also comes as paste, for moulding into shapes you desire - even available in a choice of colours, such as 'silty brown' or weedy green'. Ideal (if a tad on the expensive side) for keeping 'tailsitting' model aircraft on their wheels. Of course, it's more economical if you can find a local source and avoid internet postage charges - I review the 'fishing aisle' which all our local supermarkets sport whenever we do the weekly shop, but so far it has failed to reach this corner of rural France (think my last lot was ordered from Germany, and postage was about the same as purchase price, but just Google 'tungsten paste**').

Edit or 'Putty' - working across 3 languages can sometimes be a little taxing, particularly at 36°C!

[David B]

Is there an agreed amount of weight for an individual wagon and if so what is it? Or does everyone go their own way?

I asked a very similar question when I joined up in 2009.

For ballast, I have used lead shot but now find lead sheet (from a scrap place or builders' merchant) more amenable as I can cut it to shape and size and fit it in to all sorts of places. You can use double thickness where it will fit and if on a brass kit, tack solder it in place. On plastic I use a dab or two of No More Nails (tube from Staples). Whatever method you use, do not use PVA with lead in a confined space or you will have tears in a few years' time.

I have also used sand held in place with a diluted paper glue - remember Gloy?

[paul4147]

Is there an agreed amount of weight for an individual wagon and if so what is it? Or does everyone go their own way?

I asked a very similar question when I joined up in 2009.

For ballast, I have used lead shot but now find lead sheet (from a scrap place or builders' merchant) more amenable as I can cut it to shape and size and fit it in to all sorts of places. You can use double thickness where it will fit and if on a brass kit, tack solder it in place. On plastic I use a dab or two of No More Nails (tube from Staples). Whatever method you use, do not use PVA with lead in a confined space or you will have tears in a few years' time.

I have also used sand held in place with a diluted paper glue - remember Gloy?

I have heard of the reaction between lead and pva before so will def be avoiding that!

Sand is an interesting one, as my next wagon will be a BR sand wagon again from the red panda range, so that should be easy to weight with the load

Thanks again

Paul

[martin goodall]

For a long time, I worked on the assumption that wagons should weigh 3 - 4oz (85 - 115g). However, two GW cattle wagons I built in P4 over 30 years ago turned out on checking to weigh only 35g - 40g each. These performed satisfactorily (with compensation) for many years, even on the less than perfect track of my Crichel Down layout. I nevertheless proceeded on the assumption that P4 wagons without compensation (but some slop in the axleboxes) should be heavier, and all my fixed-axle P4 wagons were weighted to at least 3oz (85g).

I have now [June 2015] come round to the view that vehicles should be weighted as follows:

16-foot wagon (64mm) 50g [If running with W/M stock - 75g]
17’6” wagon (70mm) 50g [If running with W/M stock - 75g]
22-foot wagon (88mm) 65g [If running with W/M stock - 80g]
4w coach 80 – 100g
40-foot coach (160mm) 125g
50-foot coach (200mm) 150g
60-foot coach (240mm) 175g
70-foot coach (280mm) 215g

[Note the need to beef up the weight of any wagons that are likely to run with white-metal wagons, so that there is not too great a difference in their respective weights.]

The figures for coaches accord with Philip Hall’s views. He aims for at least 170 -180g (for a 60-foot bogie coach), which allows for sprung gangways and sprung buffers. So long as they are free running, once the train is moving a degree of momentum takes over. This weight and sprung buffers allows locomotives and tenders to be coupled to carriages properly i.e. buffer to buffer, not like a loose coupled goods train as so often seen. This last point is an interesting one, as it addresses an issue that I had not previously been able to resolve – the ‘loose-coupled’ nature of the connection between a loco and its train.

[billbedford]

I have now [June 2015] come round to the view that vehicles should be weighted as follows:

Perhaps you would like to expand on your criteria for choosing these specific weights, cos to me this seem like just plucking numbers out of the air, i.e. another voodoo formula.

[Philip Hall]

Martin is very kind to mention me, and it is true that for some years I have used quite heavy carriages to make sure the gangways and buffers work. But the inspiration for this, and for extending it to the engine/carriage coupling came from seeing Trevor Pott's "Churston", where I first witnessed the spectacle of a Castle slipping slightly to start a very heavy seven car train, as it had, like the prototype, to shift a coupled effective dead weight. Trevor achieves this with a selection of loose single link couplings dropped over both opposing coupling hooks, thus avoiding the trauma of fiddling about under gangways with a screw coupling. It's still fiddly, but you don't have to do it all day. The same single links are used between engine and carriages.

That seven car train had some very heavy vehicles built by Rodney Cooper, who I think doesn't usually weigh them, just like Trevor! I had a few of Rodney's carriages through my workshop for minor repair a while back, and one or two came in at 250g, which meant that train was probably over 2 kilos! From that time, I have always erred on the generous side with the weight. I should say that you probably don't need so much weight if your radii are generous; Trevor's are 3'6" or so. I use the same principle with Hornby carriages where I use a modified version of their close coupling system, which is slightly sprung.

The only trouble with the heavyweight approach is a possible difficulty with a full length train. Twelve on would be a whopping 4 kilos or more!

Philip

[Paul Townsend]

Don't you mean solebar length Paul? Headstocks are a pretty consistent length!

Best wishes,

Ooops yes of course!

The words should read " length over headstocks" so I have always taken that as meaning solebar length plus 2 x buffer beams thickness

[Paul Townsend]

1.8 Grams +/- 20% per scale foot of headstock length is the recommendation from original P4 Society.

It has worked for me for 40 + years!

That's interesting on three counts.

1. It gives about half the weight of "25g per axle" for a 15' wagon, so at least one of the two algorithms is not optimised.

2. I bet any cast wagon will be much heavier than 1.8g/foot.

3. 1.8g/foot might actually be achievable with plastic wagons! My current, scratch-built wagon is going to have lead solebars, "flitched" in plastic, to get it up near 50g.

I agree. The real-world effect of this is that plastic wagons are light...good when you have gradients as I do. All is well until you attempt to propel a train of mixed plastic and WM wagons.
Segregate them into separate trains if you propel over pointwork.

[dal-t]

All is well until you attempt to propel a train of mixed plastic and WM wagons.
Segregate them into separate trains if you propel over pointwork.

I guess we all recognise the sense of that empirically, but would anyone care to explain the theory behind the physics (preferably in terms non-natural scientists might manage to grasp)? It could help us get a handle on what should determine appropriate weighting, without resorting to Bill's voodoo formula(e).

[Guy Rixon]

All is well until you attempt to propel a train of mixed plastic and WM wagons.
Segregate them into separate trains if you propel over pointwork.

I guess we all recognise the sense of that empirically, but would anyone care to explain the theory behind the physics (preferably in terms non-natural scientists might manage to grasp)? It could help us get a handle on what should determine appropriate weighting, without resorting to Bill's voodoo formula(e).

When you propel a train, most of the force acts along the train and a small part pushes vehicles up or down or sideways. The up/down/sideways parts happen because the buffers are never perfectly aligned. Put your hands together fingertip to fingertip and push: you'll feel the effect.

If a vehicle is light, the upward force on it might be enough to lift it off the track. If there's a sideways component to the force - propelling on a curve - and a vehicle lifts more than the flange depth plus the suspension movement on that axle, then it will come off. The sideways component of the force will almost always be more than the vertical component, so the train will buckle sideways when the flanges clear. Do the thing with your hands and you can see this.

We can work out how much vertical force is needed to lift a vehicle - just its weight or some fraction of the weight near 50% if we're worried about lifting one end. We can work out very approximately the distribution of the loco's shunting force between propulsion and lifting by guessing the buffing geometry. What we then need to know is the force actually applied by the loco. This is approximately the rolling resistance of the train ahead (in the direction of shunting) of the suspect vehicle, plus a bit as the train accelerates.

You could say that the solid-lead wagons should always be marshalled ahead the light ones if you want to set back safely. Perhaps I won't build that whitemetal kit for the Met brake-van after all...

[Guy Rixon]

buffing-forces.001.png

Concerning the component of force acting vertically, it's just f sin(theta) as per the diagram above. I've drawn a fairly extreme case of buffer misalignment coupled with buffer-face curvature and got theta = 23 deg. For that geometry, the vertical force is 40% of the total force.

I don't know what figure to take for rolling resistance. Handwaving, suppose it's 3% of train weight. Now suppose we have a 40g wagon in propulsion between the loco and 250g of train. The total propelling force would be 7.5g and the vertical component would be about 3g for the geometry above. The wagon will not lift; it'll hardly rise on its suspension.

Now suppose we have 1kg of train ahead (say 10-15 metal wagons) and 3% rolling resistance. The vertical force rises to about 12g and the wagon will rise on its suspension, but it shouldn't be hoisted off the track. Postulate 5% rolling resistance, because the train is setting back over a tight curve, and the vertical force will be about 20g. Depending on how the buffer meet at each end of the light wagon, the lifting force may be all at one end (i.e., consider what happens if the light wagon's buffers are higher than the loco's and lower than the next wagon along the train). Now it's going to lift and come off.

The hand-waving above is for the easy case where there is no acceleration; i.e. the loco eases into the train so slowly you can't see it moving. This isn't realistic. If the loco slams into the train, the latter is going to accelerate away at, I speculate, a few hundredths of a gravity of acceleration. Each 0.01G of acceleration raises the propelling force by 1% of of the train weight, over and above the force needed to counter the rolling resistance. Add 0.05G to 3% rolling resistance and we may have bounced the first wagon off.

Note that the acceleration stress will be a lot worse with rigid buffers, or with sprung buffers that don't work properly. I speculate that the full-size railway avoids our problem because their buffers do work.

[grovenor-2685]

I suspect this is the reason a number of continental designs had one of each buffer pair flat so that the vertical component of the force was eliminated leaving only the horizontal forces to worry about.
PS On my steep grades I have seen vehicles supported only by the buffers with all wheels clear of the track, hills give all this a whole new dimension. Couplings in tension can also give a lift if the hooks are not at the same level, it's not just a pushing problem.

[Guy Rixon]

On my steep grades I have seen vehicles supported only by the buffers with all wheels clear of the track, hills give all this a whole new dimension.

Wierd! Why doesn't such a vehicle keep rising until the buffers lock? I don't see anything that will reduce the vertical force as the thing goes up. Or is it held in place by the friction between buffers as the track falls away beneath it?

[Noel]

I don't know what figure to take for rolling resistance. Handwaving, suppose it's 3% of train weight. Now suppose we have a 40g wagon in propulsion between the loco and 250g of train. The total propelling force would be 7.5g and the vertical component would be about 3g for the geometry above. The wagon will not lift; it'll hardly rise on its suspension.

Now suppose we have 1kg of train ahead (say 10-15 metal wagons) and 3% rolling resistance. The vertical force rises to about 12g and the wagon will rise on its suspension, but it shouldn't be hoisted off the track. Postulate 5% rolling resistance, because the train is setting back over a tight curve, and the vertical force will be about 20g. Depending on how the buffer meet at each end of the light wagon, the lifting force may be all at one end (i.e., consider what happens if the light wagon's buffers are higher than the loco's and lower than the next wagon along the train). Now it's going to lift and come off.

Guy, I assume that your calculations are for plain, level track, and that an irregularity in the track may be sufficient to derail a wagon between these two extremes and which would not derail if it had not met the irregularity? In other words, the standard of the track matters, and track faults can cause derailments with lighter wagons in mixed-weight trains?

Noel