What back to back setting do you use?

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

Postby Le Corbusier » Thu Dec 21, 2017 6:08 am

Julian Roberts wrote:Edited previous post

Tim yes it must have been even further back!

Individual guitar type wire springs for each wheel


Julian,

I am interested to discuss this further.

Did you measure the weight at each wheel in your set up? I would be very interested to know what is going on at each axel. I assume from what you say the C of G may be somewhat incidental as according to your snooze article hypothesis it is the increased weighting over the front ( and presumably rear) axels along with the increase in weight over all that is the critical factor. Why were the 'individual springy beams' to the front wheels beefed up? Was this to do with the increased weight added at the front? If so was there any issue with the loco sitting level? Were there any changes made to any of the other beams? I assume that the weight at the rear over one or other of the axels would be even greater than at the front given the C of G? Presumably prior to adding the weight the front of the loco was very lightly loaded with the C of G even further back and perhaps explains the derailment almost on its own?

Trying to get my head around what you think might be happening. Would I be right in thinking that if the weight is different at each axel, then the spring at each axel needs theoretically to be a different thickness to counter the loading and ensure the loco sits level ( or at least front and back as there might be a degree of bridging at the middle)

Do you think the change to the B to B had any effect?

CSBs ..... I might be being a little thick here and perhaps Will or Ross or anyone really could advise?

If I understand CSBs correctly then the C of G needs to be placed at the point to coincide with the spread sheet ... which is often near as damn it central.

However, if to achieve the correct overall balance the weight is (of necessity because of the limited positions to apply weight) placed eccentrically within the loco such that the front end is loaded and the rear is loaded even more (but nearer to the fulcrum) such that the whole is in balance as far a C of G is concerned but eccentric as far as loading at each wheel is concerned (prior to springing) with the centre wheels having less weight .... would the CSB springing (being continuous) equalise this out such that the weight is evenly distributed across the wheels? Or do loadings still increase at the different wheels according to the distribution after the springing? If this is the case would the weight have to be re-adjusted to ensure an even distribution so depression of the beam is not eccentric and the loco sits level?

Sorry this is straying a little from Back to Back dims.
Tim Lee

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

Postby Julian Roberts » Thu Dec 21, 2017 7:47 am

Will reply later Tim. Quickly, the loco isn't mine so don't know all the answers on those questions.

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

Postby Le Corbusier » Thu Dec 21, 2017 8:07 am

Le Corbusier wrote:
Hi Will,

I understand the point about the weight.

But I thought that there were two other factors going on. Firstly as you mention, friction. I understood that increasing the number of either coupled or powered wheels meant that you had more friction and so could use more of the power? Secondly, I thought that the issue with larger wheels was the tendency to slip as the rotation at the circumference meant that it was far too easy to put too much power on. The introduction of gearing meant that you could use small wheels and still achieve high running speeds. So a diesel could actually put much more power down on to the track because it had more points of contact and the wheel circumference was small and so the gradual application of power was more easily controlled (again aided by the gearing). Presumably the fantastic torque characteristics of an electric motor at low revs further helps with all of this (true to a lesser extent with diesel)


Still vaguely wondering if my understanding is correct here?
Tim Lee

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

Postby Jol Wilkinson » Thu Dec 21, 2017 12:07 pm

Tim,

although it is many years since I did any formal engineering, I recall that a steam engine cylinder develops effort dependent on applied steam pressure. So maximum pressure produces maximum force and hence maximum torque can be produced at zero rpm.

The torque at the wheel/rail "interface" is dependent on the ratio between the crank radius and wheel radius (just like a pair of gears).

The greater the number of wheels and the greater the adhesive weight, then the greater the tractive effort (for a specific engine power), as the energy is transmitted through more points of contact (just think two or four wheel drive cars). However the effort that can be transmitted is dependent on the Coefficient of Friction. If that is low, then no matter how much adhesive weight you have, slipping will occur when you try to transmit too much energy.

Jol

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

Postby Le Corbusier » Thu Dec 21, 2017 1:52 pm

Jol Wilkinson wrote:Tim,

although it is many years since I did any formal engineering, I recall that a steam engine cylinder develops effort dependent on applied steam pressure. So maximum pressure produces maximum force and hence maximum torque can be produced at zero rpm.

The torque at the wheel/rail "interface" is dependent on the ratio between the crank radius and wheel radius (just like a pair of gears).

The greater the number of wheels and the greater the adhesive weight, then the greater the tractive effort (for a specific engine power), as the energy is transmitted through more points of contact (just think two or four wheel drive cars). However the effort that can be transmitted is dependent on the Coefficient of Friction. If that is low, then no matter how much adhesive weight you have, slipping will occur when you try to transmit too much energy.

Jol


Thanks Jol,

So in lay mans terms ....

Controlling power output with steam is not particularly subtle and so with large drivers it is easy to spin them as the speed of the rim is exponentially increased in relation to its size? and the power required to turn the larger wheel might place it right on the edge of effective traction?.

Smaller drivers such as in the goods loco's have less tendency to slip because the circumference moves more slowly for the same speed at the crank meaning the power is more controllable ... plus less power is required to start the wheel rolling?

Multiple drivers are better, because each pair of wheels increases the rail contact and therefore positions to apply the power? (Why Spinners were abandoned as train weights increased)

Weight is good as it increases friction as does sanding?


So modern electric traction will be better than steam because gearing means that smaller wheels can be used whilst not impacting on maximum speed. Using smaller wheels means you can have a lot more of them meaning more points of contact to put the power down. Smaller wheels means that less power is required to turn the wheel and so getting the train moving is easier and less prone to slippage. Finally electric is better because it is much easier to control the application of power as an electric motor is a more subtle engine and power can be gradually applied?
Tim Lee

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

Postby Noel » Thu Dec 21, 2017 3:16 pm

Le Corbusier wrote:Controlling power output with steam is not particularly subtle and so with large drivers it is easy to spin them as the speed of the rim is exponentially increased in relation to its size? and the power required to turn the larger wheel might place it right on the edge of effective traction?.Smaller drivers such as in the goods loco's have less tendency to slip because the circumference moves more slowly for the same speed at the crank meaning the power is more controllable ... plus less power is required to start the wheel rolling?


Also, steam engine bearings were quite primitive by modern standards, so the faster the wheel rotated, the greater the risk of failure. Bigger wheels meant lower rotational speeds and reduced the risk for high speed locos. Roller bearings were experimented with on UK steam engines from the late 1940s, but these experiments were never really followed through as it was becoming obvious that steam had no future. Standard 9Fs had plain bearings, which led to authority's concern about the effect of the rotational speed of 5ft wheels at 80+ mph on these bearings. Mainline UK diesel and electric locos have roller bearings, which are much more reliable, and much less prone to problems arising from rotational speeds, so smaller wheels are acceptable.
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Le Corbusier
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Re: What back to back setting do you use?

Postby Le Corbusier » Thu Dec 21, 2017 4:42 pm

Noel wrote:Also, steam engine bearings were quite primitive by modern standards, so the faster the wheel rotated, the greater the risk of failure. Bigger wheels meant lower rotational speeds and reduced the risk for high speed locos. Roller bearings were experimented with on UK steam engines from the late 1940s, but these experiments were never really followed through as it was becoming obvious that steam had no future. Standard 9Fs had plain bearings, which led to authority's concern about the effect of the rotational speed of 5ft wheels at 80+ mph on these bearings. Mainline UK diesel and electric locos have roller bearings, which are much more reliable, and much less prone to problems arising from rotational speeds, so smaller wheels are acceptable.


Thanks Noel ... appreciated.

Tim
Tim Lee

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Guy Rixon
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Re: What back to back setting do you use?

Postby Guy Rixon » Thu Dec 21, 2017 7:15 pm

More wheels does not give better adhesion per se. Adding wheels allows a greater, total adhesive-weight when the weight on each axle is limited by the track.

The main reason for large, coupled wheels is not the bearings, it's the piston speeds; consider that the carrying wheels are much smaller and their bearings still work. Trying to run at very high crankshaft revolutions makes too much linear friction in the cylinders. Best case, the pistons and valves wear too quickly. Worst case, the engine seizes at speed (as did Leader when Mr. Bulleid failed to sort out the lubrication to the sleeve valves).

Piston speed is still an issue in IC engines; it's why American engines with long piston stroke have great torque but won't rev and short-stroke European engines rev like crazy. In a conventional steam engine, with no gearing, the problem gets even worse as axle speed is crankshaft speed. Large wheels are the simple way out.

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Will L
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Re: What back to back setting do you use?

Postby Will L » Thu Dec 21, 2017 7:45 pm

Le Corbusier wrote:.... I understood that increasing the number of either coupled or powered wheels meant that you had more friction and so could use more of the power? Secondly, I thought that the issue with larger wheels was the tendency to slip as the rotation at the circumference meant that it was far too easy to put too much power on. The introduction of gearing meant that you could use small wheels and still achieve high running speeds. So a diesel could actually put much more power down on to the track because it had more points of contact and the wheel circumference was small and so the gradual application of power was more easily controlled (again aided by the gearing). Presumably the fantastic torque characteristics of an electric motor at low revs further helps with all of this (true to a lesser extent with diesel)


Tim

You were due a reply, it’s just that life has been a bit full at the moment.

I didn’t want to show up the holes in my physics, so I’m not going to try and justify this in detail but in outline the coefficient of friction is the ratio between the weight of an object resting on a surface and the size of the push needed to get it to move, and, perhaps counter-intuitively, isn’t dependant on the area of contact*. It is more or less constant for any given pair of materials, given they are both clean smooth and dry. The basic approximation for the maximum pulling power of a single loco wheel is the weight carried times the co-efficient of friction. So long as the load to be pulled doesn’t exceed this maximum, the wheel will pull the load, if not it slips. The pulling power for the loco is the sum of all the driven wheels (and if you would like to understand more implications of this see this post). Therefore if you want to increase then load that can be pulled you can either add more wheels, or you add more weight to the wheels you've got. Doubling either will (approximately) double the load that can be pulled. Given the real railway was limited to the weight on a wheel they had to go for more wheels. We are not so constrained.

* OK this does fly in the face of what we think we know about cars tyres, but this basic approach to the coefficient of friction only applies to smooth objects sliding on smooth surfaces, like steel wheels on steel rail. Car tyres and roads are anything but smooth. Car tyres can also be quite sticky giving a coefficient greater than 1 which is rather odd when you think about it.

As to wheel sizes, I have never before heard a suggestion that the adhesion available to a wheel being affect by its diameter, and I can’t see why it should be. The forces involved in adhesion all happen at the interfaces between the wheel and the rail and act along the surface of the rail. The diameter of the wheel seems irrelevant to me.

Steam locos have big heavy reciprocating bits and the bigger the driving wheels the slower these bits have to travel, which is a good thing saving on all sorts of stresses strains and wear. The advent of diesel and electric propulsion removed that problem and allowed the use of a wheel size both big enough not to have to revolve too quickly and small enough to conveniently got into a bogie. Creep control on modern heavy haul freight locos is down to the micromanagement of the power delivered to each axle to ensure it never passed the adhesive limit.

As to the characteristics of the various drive systems, electricity and steams both share the useful characteristic that they can apply maximum torques while stationary*. The odd one out is diesel which has to have some revs on before you can draw any power, hence the needs for a clutch in a purely mechanical drive, and the preference for the diesel engine driving a generator which powers an electrical motor on the axle.

* So electric cars are better than diesel/petrol at accelerating from a standing start.
Last edited by Will L on Thu Dec 21, 2017 9:20 pm, edited 3 times in total.

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Re: What back to back setting do you use?

Postby Will L » Thu Dec 21, 2017 8:02 pm

Posted in hast, I forgot the link which will take a while to find and I'll edit out any spelling mistakes later.

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

Postby Le Corbusier » Thu Dec 21, 2017 8:47 pm

Will L wrote:As to wheel sizes, I have never before heard a suggestion that the adhesion available to a wheel being affect by its diameter, and I can’t see why it should be. The forces involved in adhesion all happen at the interfaces between the wheel and the rail and act along the surface of the rail. The diameter of the wheel seems irrelevant to me.

Thanks Will.

I suspect I am confusing the gearing implication of the wheel with the power applied at the track. I was thinking that because full power could be applied through the rods when stationary it would generate far more speed from a standing start on a larger wheel than with a smaller wheel, so long as the crank throw was similar and therefore it would be much more likely to spin the wheels. In the archive films I have watched I regularly see pacifics spinning there wheels, but can't recall ever having seen a goods loco doing the same - it may be I have just not watched the right films.

I also assumed that goods vehicles had smaller wheels as top speed was unimportant and smaller wheels would require less power to turn ...similar to a lower gear in a car and so the power could be applied with more control. Thus larger weights could be pulled.
Last edited by Le Corbusier on Thu Dec 21, 2017 9:55 pm, edited 1 time in total.
Tim Lee

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Will L
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Re: What back to back setting do you use?

Postby Will L » Thu Dec 21, 2017 9:21 pm


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Will L
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Re: What back to back setting do you use?

Postby Will L » Thu Dec 21, 2017 10:06 pm

Jol Wilkinson wrote:The torque at the wheel/rail "interface" is dependent on the ratio between the crank radius and wheel radius (just like a pair of gears).


True, but the adhesion available and hence the amount of torque that can actually be used to pull trains is independent of all this.

Le Corbusier wrote:.. In the archive films I have watched I regularly see pacifics spinning there wheels, but can't recall ever having seen a goods loco doing the same - it may be I have just not watched the right films.


Pacifics were more photogenic, and they had this problem, when you apply power the forces transmitted through the loco caused it to "sit down" on the springs of the rearmost driving wheels. A pacific (4-6-2) has carrying wheels behind the rear driver so you get transfer of weight onto the pony truck and off the front drivers. The Bulied Pacifics were notoriously inclined to slip on starting because of this and also it is said that as a result a Royal Scott (4-6-0) could start a larger load up Camden bank than could any of the LMS pacifics.

Anyway, accelerating hard with a heavy train isn't really freight loco territory. While most 0-6-0s had quite enough power to slip their wheels, getting a heavy loose coupled freight train moving was a very different trick than setting out with style on a fancy express. Your goods train driver tries to ensure all the couplings are slack, then he tries to start one wagon at a time. So the technique is to just get the train rolling and keep it at that speed until all the wagons had joined in. Doing it any other way had severe repercussions for the guard. On those occasions when they had to restart a train on a raising grade, with the couplings tort, they slipped all right.

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Re: What back to back setting do you use?

Postby Le Corbusier » Thu Dec 21, 2017 10:10 pm

Will L wrote:Missing link now added under the phrase and if you would like to understand more implications of this see this post

Thanks will .... brilliant, clear and concise.

So in summary ... if the weight adds up to the same total and is evenly distributed .... whether there are 2, 4 , 6 or 8 drivers is irrelevant, the tractive power will be the same? However, spreading the load over a number of axels allows heavier weighted locos to travel on the track as the individual axle weight is much reduced? is this why the Midland Singles were eventually phased out .... because the axle loading on the track would become unsustainable?
Tim Lee

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

Postby Julian Roberts » Fri Dec 22, 2017 7:16 am

After that most interesting discussion what would interest me to know in relation to model locomotives is: on my 0-8-0T Q1 how much greater load could it pull if all the wheels were equally loaded as against the way I made it, where the outside guiding wheels i.e. front and rear are loaded as for a compensated 0-4-0, and the inner two wheelsets are sprung down simply through the action of top acting wipers. Theory might be able to give the answer. It weighs 400g. As far as I am concerned it holds the track perfectly and pulls quite enough for my requirements.

One memory I have is of being on a dmu to Hitchin in 1976. At Stevenage a Deltic hauled passenger train was given the right away exactly the same moment as mine. We pulled away much more quickly through the four gears and it was about a mile out and well into 4th gear say about 45mph that the express finally and with that thrilling Deltic scream started to gradually overtake us and pull away. How much greater its acceleration continued to be in its higher speed range was evidenced by the way the signals behind it as we passed them were, not so long after, showing green while we gradually trundled up to our maximum 70mph. In my memory of being on Deltic hauled trains the acceleration was always quite tentative at low speed as though the electric traction motors might be burned out if all the power were to be laid down....?

More recent equivalent observation is that to me a Class 90 (top speed 110) has better acceleration than a Class 91 which has a top speed of 140mph IIRC. On both classes you can hear the squeal of the creep control as wheels under controlled slip extract every drop out of the coefficient of friction available on a contact patch the size of an old 5p piece for each wheel (according to what I've understood from of Roger Ford in Modern Railways)

Another interesting thing. I was at the NRM recently. By the Evening Star 9F is a notice with facts etc. It says that the Class through its successful use on 90mph passenger trains disproved the then normal acceptance that bigger wheels were needed for high speeds on steam locos. I have no great knowledge about this so don't say this to disagree with anything said above, just report what I saw.

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

Postby Julian Roberts » Fri Dec 22, 2017 7:50 am

Tim yesterday I only briefly answered you. Until I do properly may I refer you to the thread Articulated Beams. You will find a discussion on my Snooze article. Basically it is an attempted demolition job on what I wrote by Andrew Jukes and others. It attempts to prove that because I quite evidently had no particular detailed understanding of centre of gravity considerations the locos I had built couldn't work, were likely to fall over, derail, have wheels going up and down in all sorts of strange aberrant behaviours, and generally I was talking rubbish. I did learn a thing or two and it was an interesting exercise in public scrutiny, but the proof of the pudding can only be in the eating, and without demonstrable action all theorising can only be that. It was my loco that did stay on the track in the two videos posted a while back, the link is
20171209 132448
built according to my principle of unequal compensation weighting, which means it hugs the track whatever the imperfections of the latter. And incidentally I have found on my current project that with a coil spring you can't have the same precise control of axle weighting because the spring's force varies so much with the up and down movement, which is what I really dont need. So for me compensation is a no brainer absolutely perfect way to suspend a model loco on the track, but I have yet to experiment with CSBs.

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Re: What back to back setting do you use?

Postby dal-t » Fri Dec 22, 2017 8:18 am

As a dedicated non-engineer I have steadfastly refused to bring my 'O' level physics into this debate - or the 'facts' that I grew up with, in the steam loco era - but I will contribute this from personal experience. Having stood lineside while Brits, Coros and Spaceships all roared past at 90mph+, regardless of impact on whitemetal bearings I would always go for the one with the biggest wheels. The 8Ps always seemed effortlessly in control; the Class 7s a bit breathless and a lot less surefooted; and the 9Fs totally out of control, rods flailing, steam spraying and frames bending all over the place (oh, sorry, that last bit should have been aimed at another thread) as they were pushed along by their load. As I often remark in other contexts, the fact that something can do something doesn't mean that it should.
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Jol Wilkinson
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Re: What back to back setting do you use?

Postby Jol Wilkinson » Fri Dec 22, 2017 9:12 am

I wonder if there is too much navel gazing and theorising at times.

Getting back to basics I would suggest that roughly equal loading (weight) on the driving wheels, biased slightly to the outer axles, will provide optimum traction and "road holding". Overall weight should be slightly less than that which will stop the driving wheels from slipping when the loco is against a stop, to avoid stalling and burning out the motor.

Don't ask for a treatise, formulas, etc. on these views, they are just based on what I have found works best.

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

Postby Le Corbusier » Fri Dec 22, 2017 9:20 am

Julian Roberts wrote:Tim yesterday I only briefly answered you. Until I do properly may I refer you to the thread Articulated Beams. You will find a discussion on my Snooze article. Basically it is an attempted demolition job on what I wrote by Andrew Jukes and others. It attempts to prove that because I quite evidently had no particular detailed understanding of centre of gravity considerations the locos I had built couldn't work, were likely to fall over, derail, have wheels going up and down in all sorts of strange aberrant behaviours, and generally I was talking rubbish. I did learn a thing or two and it was an interesting exercise in public scrutiny, but the proof of the pudding can only be in the eating, and without demonstrable action all theorising can only be that. It was my loco that did stay on the track in the two videos posted a while back, the link is
20171209 132448
built according to my principle of unequal compensation weighting, which means it hugs the track whatever the imperfections of the latter. And incidentally I have found on my current project that with a coil spring you can't have the same precise control of axle weighting because the spring's force varies so much with the up and down movement, which is what I really dont need. So for me compensation is a no brainer absolutely perfect way to suspend a model loco on the track, but I have yet to experiment with CSBs.


Julian,

I followed the debate you mention and have also read your article a couple of times. I found it all very interesting. I have to say that I assumed your theories were arrived at by trial, experimentation and observation. I therefore assumed that they worked and improved the track holding as advised .... so what I would be really interested in would be to know why they appeared to work rather than why theoretically they didn't - ie, to look at your locos before the adjustments you made, and then after and to understand what it was that had been corrected to ensure the good running you observed and what was maybe incidental. It may be that something is going on which has not actually been discussed, or it may be that some of the feared repercussions at the scale of a 4mm model become incidental ( like the eccentric forces on a loaded flying buttress).

Out of interest - another structural example I recall similar to the flying buttress analogy is the small stone voussoir bridges that required engineering for tank traffic in France during WWII. The solution was to load the arch equally along its full length with hundreds of sand bags. This meant that the static downwards force (self weight of the bridge) was increased to such an extent that the eccentric live loading of the moving tanks became incidental.

I assume that on your eccentrically loaded locos, the debate above about pulling power is not really relevant because the actual loads mean that evenly the lightest loaded of the axles has a sufficient load to ensure we are fine on a 4mm scale model ... and I assume if scaled up it would likely be well over the route limits of the full size network..
Last edited by Le Corbusier on Fri Dec 22, 2017 9:35 am, edited 1 time in total.
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Re: What back to back setting do you use?

Postby Julian Roberts » Fri Dec 22, 2017 9:24 am

Le Corbusier wrote:
Julian Roberts wrote:Edited previous post

Tim yes it must have been even further back!

Individual guitar type wire springs for each wheel


Julian,

I am interested to discuss this further.

Did you measure the weight at each wheel in your set up? I would be very interested to know what is going on at each axel. I assume from what you say the C of G may be somewhat incidental as according to your snooze article hypothesis it is the increased weighting over the front ( and presumably rear) axels along with the increase in weight over all that is the critical factor. Why were the 'individual springy beams' to the front wheels beefed up? Was this to do with the increased weight added at the front? If so was there any issue with the loco sitting level? Were there any changes made to any of the other beams? I assume that the weight at the rear over one or other of the axels would be even greater than at the front given the C of G? Presumably prior to adding the weight the front of the loco was very lightly loaded with the C of G even further back and perhaps explains the derailment almost on its own?

Do you think the change to the B to B had any effect?

CSBs ..... I might be being a little thick here and perhaps Will or Ross or anyone really could advise?

If I understand CSBs correctly then the C of G needs to be placed at the point to coincide with the spread sheet ... which is often near as damn it central.

However, if to achieve the correct overall balance the weight is (of necessity because of the limited positions to apply weight) placed eccentrically within the loco such that the front end is loaded and the rear is loaded even more (but nearer to the fulcrum) such that the whole is in balance as far a C of G is concerned but eccentric as far as loading at each wheel is concerned (prior to springing) with the centre wheels having less weight .... would the CSB springing (being continuous) equalise this out such that the weight is evenly distributed across the wheels? Or do loadings still increase at the different wheels according to the distribution after the springing? If this is the case would the weight have to be re-adjusted to ensure an even distribution so depression of the beam is not eccentric and the loco sits level?

Sorry this is straying a little from Back to Back dims.


Hi Tim
The builder of the loco who I will call Mr D is a more typical member of the Society reticent about putting pen to paper in a digital way. Also I'm trying not to say the chassis kit maker's name as I don't want to in any way blacken his reputation. In fact to me the fact that every wheel has its own spring that can be height adjusted as well as have different thickness of guitar string to give different spring character is the best possible advert for his style of suspension.

I can't fully answer for Mr D's every thought but we had an email conversation following the meeting where I took the video. Basically I suggested to him the problem might be that the weight wasn't concentrated enough on the front wheels. At first I suggested heavier springs on the front but then realized lighter centre ones on the centre axle would be a better option. However by then he had changed the front springs. I suggested to him this might not work except in combo with more weight at the front - which it was possible for him to increase. He also, being aware of this thread, increased the BB. (I had not appreciated the CofG was so far back till the second test which was more than a week later.)

So whether any one of those actions would have achieved the desired result is open to question.

So as I said previously the C of G is obviously important/vital, but so blindingly obvious it scarcely needs to be mentioned (except that's disproved by the fact I'd not thought about it with Mr D's loco at first!). But a full understanding does help, I have realized from the Articulated Beams thread. My lack of full understanding of it doesn't invalidate my article, because I was writing there what I had been doing for ten years achieving what I wanted to achieve. Of course scepticism is perfectly legitimate as anyone can claim any darn thing they want and people may believe them. I wanted people to know what I had found because I thought it might help!!!!

I don't think the sitting level of the loco has been changed. No other beams were changed. 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) and the imbalance of the loco could have explained or gone a long way to explaining the derailment prone nature of the loco as was.

Trying to get my head around what you think might be happening. Would I be right in thinking that if the weight is different at each axel, then the spring at each axel needs theoretically to be a different thickness to counter the loading and ensure the loco sits level (or at least front and back as there might be a degree of bridging at the middle)


Yikes this is getting a bit headbanging for me! Getting the loco sitting the right height is difficult with compensation and another ball game with springing. Maybe have a look at my ramblings on my Crab build thread but that's work in progress still (being somewhat delayed by this indulgence in the immediate gratification of seeing visual results so quickly on a digital screen.) I'd basically suggest the CofG needs to be at the centre of the fixed wheelbase on a model steam loco, so the springs would be equally strong on my "guiding" wheels (and in my terms lighter on the centre ones. Whether that is a practical proposition with CSBs is of interest to me and where Will disagrees with me...which is where the other thread talking about weight on wheels is interesting to me (Engineering Injuneering and Knitting)). Normal CSB theory is I think equal weighting on all driving wheels to maximize haulage power - which as this thread has revealed is another commonly misunderstood business in our little world. As Will has said, it maximizes haulage power for a given weight, but it's the amount of weight that matters most for actual amount of pull haha. As my video there shows https://m.youtube.com/watch?v=Q3JINX9Gnv0 each wheel on the real thing weighs enough to depress the length of rail between three sleepers by an inch or two if there is a void under one of them. That could not possibly happen in our scale. So we have far less weight on each wheel that IMHO we really need for properly reliable track holding that will not rely on perfect track (in my opinion a uselessly idealistic objective), so IMHO we need to maximize it on the guiding wheels, though intermediate ones still need enough to keep them on the rails too as they are needing to move sideways on curves.

I think that has answered your last paragraph. Hope so. Oh sorry - well I don't know if the BB alteration on its own would have done the trick. Mr D did all the alterations at once so I couldn't tease all this out. Others may have a go. But the thing about BB is that every time you fiddle about with it the wheels get looser so it's going to become less and less reliable, as they may alter themselves - for all the clever dick work getting them to a precise 17.75 (at one point of their circumference - bet it won't be all the way round!) With a steam loco the quartering will be likely to be spoiled with this fiddling about, and if wheels are at all loose likely to be out soon, if not already made so by the fiddling about. Only on the new (as of a few years ago) design Exactoscale wheels can the BB be adjusted without fear of this problem. I lock the wheels on a steam loco with Araldite and steel pin as soon as they're on the axle and I just don't believe all this stuff about BB for the reasons I have already said, and my proven experience which arose from originally having a below spec BB gauge. - So long as the BB is not more than 17.75. (The 6ft 6ins rear wheel on my 4-4-0 is larger than that at one point of its wobbly state of being, so one day it will come off when going tender first when it happens to coincide with a crossing V, but that hasn't happened so far...)

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Re: What back to back setting do you use?

Postby grovenor-2685 » Fri Dec 22, 2017 9:55 am

Normal CSB theory is I think equal weighting on all driving wheels to maximize haulage power -
That is entirely up to the individual builder, the CSB chassis mounts can be spaced to distribute the weight as preferred and I would suggest designing for the centre axle to carry a bit less that the end axles.
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Re: What back to back setting do you use?

Postby Le Corbusier » Fri Dec 22, 2017 10:04 am

grovenor-2685 wrote:
Normal CSB theory is I think equal weighting on all driving wheels to maximize haulage power -
That is entirely up to the individual builder, the CSB chassis mounts can be spaced to distribute the weight as preferred and I would suggest designing for the centre axle to carry a bit less that the end axles.
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At the risk of sounding totally thick ......

I understand that the distribution of weight can be adjusted via the positioning of the mounts ... but how does the weighting on the actual loco affect all of this. Does the CSB springing automatically distribute this weight according to the design irrespective of its position ... or do you first need to get it evenly distributed. As per my note above, I tend to find that the positions available for loading up the body dictate that while I can get it to balance ie C of G at the centre of the body, the actual weight is eccentrically positioned.
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Re: What back to back setting do you use?

Postby Tony Wilkins » Fri Dec 22, 2017 12:09 pm

[quote="Julian Roberts"

One memory I have is of being on a dmu to Hitchin in 1976. At Stevenage a Deltic hauled passenger train was given the right away exactly the same moment as mine. We pulled away much more quickly through the four gears and it was about a mile out and well into 4th gear say about 45mph that the express finally and with that thrilling Deltic scream started to gradually overtake us and pull away. How much greater its acceleration continued to be in its higher speed range was evidenced by the way the signals behind it as we passed them were, not so long after, showing green while we gradually trundled up to our maximum 70mph. In my memory of being on Deltic hauled trains the acceleration was always quite tentative at low speed as though the electric traction motors might be burned out if all the power were to be laid down.....[/quote]

I have a similar memory but with a Brush type 2 class 31 on a six coach suburban train. We left Kings Cross at the same time as the Deltic hauled express and by the time we exited Gasworks tunnel were well ahead of the Deltic. We came to a stand at Finsbury park about half a minute before the Deltic roared through on full power. The Deltics were splendid machines for their time, but at 99 tons for 3,300 HP were light on their feet. There were incidences of them getting stuck on the climb up through Gasworks tunnel in their early days and their regulators were modified with more control notches at the slow speed end to give better traction control. They were also capable of making the wheel slip light flicker at speeds into the 80s.
As they had the same bogies and traction motors as the class 37s (with half the Horse Power), I suspect that the amount of power that could be applied was carefully limited to prevent overloading the traction motors at the bottom end.

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Re: What back to back setting do you use?

Postby Le Corbusier » Fri Dec 22, 2017 12:59 pm

Tony Wilkins wrote:[quote="Julian Roberts"

One memory I have is of being on a dmu to Hitchin in 1976. At Stevenage a Deltic hauled passenger train was given the right away exactly the same moment as mine. We pulled away much more quickly through the four gears and it was about a mile out and well into 4th gear say about 45mph that the express finally and with that thrilling Deltic scream started to gradually overtake us and pull away. How much greater its acceleration continued to be in its higher speed range was evidenced by the way the signals behind it as we passed them were, not so long after, showing green while we gradually trundled up to our maximum 70mph. In my memory of being on Deltic hauled trains the acceleration was always quite tentative at low speed as though the electric traction motors might be burned out if all the power were to be laid down.....


I have a similar memory but with a Brush type 2 class 31 on a six coach suburban train. We left Kings Cross at the same time as the Deltic hauled express and by the time we exited Gasworks tunnel were well ahead of the Deltic. We came to a stand at Finsbury park about half a minute before the Deltic roared through on full power. The Deltics were splendid machines for their time, but at 99 tons for 3,300 HP were light on their feet. There were incidences of them getting stuck on the climb up through Gasworks tunnel in their early days and their regulators were modified with more control notches at the slow speed end to give better traction control. They were also capable of making the wheel slip light flicker at speeds into the 80s.
As they had the same bogies and traction motors as the class 37s (with half the Horse Power), I suspect that the amount of power that could be applied was carefully limited to prevent overloading the traction motors at the bottom end.

Tony.[/quote]
Thats interesting Tony,

Presumably as far as the DMU was concerned it would always have it over a traditional loco scenario because by definition there are multiple engines and so the number of coaches actually being pulled are relatively small (2 at most?). By contrast the Deltic had to get eight or so coaches on the move? So presumably all the above discussions come into play.
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Re: What back to back setting do you use?

Postby Julian Roberts » Fri Dec 22, 2017 2:47 pm

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.


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