Articulated beams?

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Wed Apr 12, 2017 7:08 pm

On the Flexichas Appreciation thread I think Andy proto87 stores was suggesting the same thing as a possible product for rtr conversions. A few pages back from latest.

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grovenor-2685
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Re: Articulated beams?

Postby grovenor-2685 » Wed Apr 12, 2017 7:18 pm

Russ Elliott wrote:
Armchair Modeller wrote:For a 2-axle chassis, this is an alternative configuration to the traditional fixed + rocking axle configuration.
a-3jaa0p3u.jpg
My impression from limited testing is that this alternative is more stable and requires less weight to stay on the track.

I agree. It's like an MJT torsion-bar CCU, for example. But it's a significantly different principle, namely where the rotation is in the pitch plane, i.e. 'longitudinal compensation' (and where one could argue the 'triangle' is irrelevant because of its symmetry over the wheelbase), as opposed to rotation in the roll plane, which is the traditional 'transverse compensation'.

I recollect this style being recommended by the S gauge fraternity for 4 wheel wagons many years ago and more recently by another group, perhaps the 2mm people.
Andy Reichert's proposal is different in as much as he is proposing pivots on both sides and both ends thus getting away from the asymnetry and also from the 3 point support principle.
Regards

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Russ Elliott
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Re: Articulated beams?

Postby Russ Elliott » Wed Apr 12, 2017 10:48 pm

Yes, I think we are in furious agreement, Keith. I wasn't suggesting Andy's proposal was the 3-point principle. (Pivots on both sides are functionally equivalent to a torsion bar admidships.) Andy's proposal, as I understand it, is an equalised one:

springy-equaliser3.png
springy-equaliser3.png (4.06 KiB) Viewed 1200 times

Not recommended for 4-wheel wagons though!!!

andrew jukes
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Re: Articulated beams?

Postby andrew jukes » Wed Apr 12, 2017 11:33 pm

Russ wrote
I agree. It's like an MJT torsion-bar CCU, for example. But it's a significantly different principle, namely where the rotation is in the pitch plane, i.e. 'longitudinal compensation' (and where one could argue the 'triangle' is irrelevant because of its symmetry over the wheelbase), as opposed to rotation in the roll plane, which is the traditional 'transverse compensation'.

This feels like nonsense because there is still a triangle and the CofG is quite near the triangle boundary. I suspect the real difference is that the forces that cause poor stability to become a derailment risk - which are probably buffing and coupling forces - are dealt with similarly at both ends. Both ends are at moderate risk instead of one end being stable and the other being high risk.

Russ may recall that 'longitudinal compensation' is how I've designed the ball bearing bogies for my Golden Age rakes. They are of course loaded exactly centrally and there are no buffing forces. They ride excellently.

Andrew

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Fri Apr 14, 2017 7:14 am

Russ Elliott wrote:Probably not. In qualitative terms, lateral stability is easy to explain (we know a CofG towards the left improves lateral stability but gives a silly axle weight distribution, whilst one towards the triangle apex improves that distribution slighty but makes the lateral stability critical), but I have always found it impossible to quantify such instability. I think there are aspects about the lateral CofG position, beams possibly wiggling laterally very slightly, and the effects of beams impinging on axles moving (albeit very slightly) laterally under sideplay that we don't fully appreciate. Essentially, our '3-legged stools' were always a lot more precarious than had been originally envisaged.

basic-compensation-triangle.png


Russ on another thread you and Martin Wynne were amusingly discussing the instability of 4 wheel wagon and how easy it is simply to blow it over. Scaling this up to full size one of you suggested it would take 33 million people to do the same.

A friend was describing to me a regular sight of
his childhood and one I never witnessed,, the view from a footbridge of an express steam train gradually approaching over two miles of straight track at 70mph. As the train neared part of the drama he could see was the side to side rolling of
the locomotive.

I can see very well that at full size the instability issue is critical, and that at model size it can also be where the vehicle is light, as a 50g wagon. But I have an 0-8-0 where the compensation is as for an 0-4-0 so the triangle apex is on the front axle. It weighs 400g though that weight comes from the WM castings without anything added so the CofG is not especially tailored to be low. I very much
doubt it could be blown over! But an aspect in the series of articles that I meant to come over with more emphasis than it did is that I gear all my locos to a 30mph-ish maximum to gain the best possible slow running. In these ways, and with the axle weighting that has been established here that I actually achieve, I suggest that my style of loco building might have sufficient for purpose stability,
and validity...?

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Russ Elliott
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Re: Articulated beams?

Postby Russ Elliott » Fri Apr 14, 2017 9:11 am

Julian Roberts wrote:I suggest that my style of loco building might have sufficient for purpose stability, and validity...?

Of course, Julian. There are hundreds maybe thousands of 3-point (or 4-point, still quite popular in some 7mm kits) compensated locos which perform perfectly adequately, and which are stable, and I refuse to criticise them. Suspension encompasses a broad church. All I have ever said is that if the CofG is not in a reasonable place, there could be instances where lateral stability is compromised. When Chris Gibbons did an initial etch of his 0-4-2T with a single beam (at the back), and asked me what I thought of it, I told him it might fall over on curves. Guess what - it fell over on curves. Chris then did a redesign with twin beams at the back. ("Russ, it works now!" "Yes, Chris. That's because the laws of physics.")

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Fri Apr 14, 2017 10:26 am

Will L wrote:Its a function of the much longer wheelbase. This means you need a lot more lateral flexibility on the middle axle, and it is more sensitive to vertical curvature of the rail head. I.e. it is likely to find itself suspended over any dip in the track, or standing on a high spot with the end wheels waving in the air. My last effort was really just a 4 wheel compensated vehicle much as you suggested, with the middle wheel W iron not supporting any vehicle weight but with a significant amount of lead attached keep it on the track, and laterally sprung to keep it somewhere
near the centre line of the vehicle. Worked OK in 00 but doesn't cope well with p4 sized flanges.
I've got a lot of GER 6 Wheelers in stock, if/when I get to the next one, it will be built sprung, which I'm pretty sure will work better, but there is still the question of how you arrange enough lateral
flexibility on the middle axle.


Will I see the point about vertical curve but I wonder if the lateral centreing springing was really needed? Once a centre wheel is on the track I can't see why it needs guidance given enough downward force. Unlike most wheelsets its axle will always be perpendicular to the rails and in line with the centre of any curve.

I have a six wheel guards van done in the way I described that has been faultless, and the layout does have some complex trackwork. Two tenders where the front four wheels similarly serve only a cosmetic function and work the same way have also been faultless. The wheelbase is less challenging but the principles must be the same surely? What is important is sufficient lubrication for the axle to slide easily sideways.

With a more contemporary springy approach I imagine the same idea of having a no more than sufficient force-weight on the middle axle might remain appropriate?

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Will L
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Re: Articulated beams?

Postby Will L » Fri Apr 14, 2017 5:13 pm

Julian Roberts wrote:Will I see the point about vertical curve but I wonder if the lateral centreing springing was really needed? Once a centre wheel is on the track I can't see why it needs guidance given enough downward force. Unlike most wheelsets its axle will always be perpendicular to the rails and in line with the centre of any curve.

I have a six wheel guards van done in the way I described that has been faultless, and the layout does have some complex trackwork. Two tenders where the front four wheels similarly serve only a cosmetic function and work the same way have also been faultless. The wheelbase is less challenging but the principles must be the same surely? What is important is sufficient lubrication for the axle to slide easily sideways.


Short wheel base 6 wheel vehicles are a very different proposition to a long wheelbase 6 wheel coach, and the lateral control springing I referred to was all to do solving 6 wheel coach problem (and not a very good solution either). It is certainly not necessary under 6 wheel guards vans and tenders. Just like your average small wheel 0-6-0 loco the wheels on this sort of vehicle are close enough together for them to go round most curves with little or no side play on the centre axle (and no GW either, in P4 at least). The vertical curvature thing shouldn't be a problem either.

With a more contemporary springy approach I imagine the same idea of having a no more than sufficient force-weight on the middle axle might remain appropriate?


You and I are never going to agree about this I'm afraid. I just don't agree that there is any general advantage in unloading the middle wheel in favour of the outer ones. After all, If the leading wheel is a bogie/pony truck it will generally be weighted less than the drivers (there are exceptions) but on any half decent chassis we expect to be able to get them to stay on as reliably as the drivers.

6 wheel coaches are however the exception. The vertical curvature issue may well require that suspension on the centre axle of a 6 wheeler should be that bit softer than the outer two so the vehicle doesn't get beached on the centre axle. Guy Rixon's observations earlier in this thread suggest this was true on the prototype.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Sat Apr 15, 2017 5:33 am

Will my remarks were on the 6 wheel coach problem. Of course these vehicles need much more sideplay and vertical flexibility on the centre wheelset than my six wheel guards van, and what I was suggesting was that, despite the much greater extent of possible movement, there is no need for any lateral centreing springing which might be counterproductive, and that given enough downward force the wheelset will stay on the rails without help. Of course this wheelset won't need any GW, given enough sideplay.

(I imagine it might be necessary for absolute proof against derailment to make an invisible and very unprototypical rebate or even hole above the centre wheels, and similar rebate on the inside of
any footstep etc, all depending on the vertical and horizontal radii to be encountered.)

Yes Guy's observations give a good excuse for you to lighten the force on the centre wheelsetwhich seems to me essential on the model coach, so long as there is sufficient to keep it down firmly.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Tue May 02, 2017 5:23 pm

As a semi tongue in cheek footnote to Andrew's critique of my Snooze 199 article: I don't dispute his demolition job on my reasoning regarding sideways forces scaling differently, not that it would cause me to change my approach which works for me. But I found this while looking up "Wheel Profiles".

Fri Jun 20, 2014 9:18 am
viewtopic.php?f=20&t=3749&hilit=wheel+profiles&start=75

Alan Turner wrote:
andrew jukes wrote:
Most of these 'scale effects' work in our favour. How can we run trains at a scale 100mph on 4ft radius uncanted curves when the full-size railway needs a one mile radius curve with 6 inches of cant?

Regards

Andrew


You of course have to define what you mean by a scale 100mph. If you mean the model covers a linear scale 1 mile in 36 seconds then that is not scale speed because you have not scaled time.

Try running the train at 8 times the "scale" 100mph if you want to represent the dynamics of the system. I don't think somehow that your model will still be on your 4ft rad uncanted curve for very long. Just like the real thing.

regards

Alan


Will L wrote:
Alan Turner wrote:You of course have to define what you mean by a scale 100mph. If you mean the model covers a linear scale 1 mile in 36 seconds then that is not scale speed because you have not scaled time.


Are, another of those familiar little chestnuts rolled out for re-examination. There is no such thing as scale time.


JFS wrote:
Will L wrote:
Alan Turner wrote:You of course have to define what you mean by a scale 100mph. If you mean the model covers a linear scale 1 mile in 36 seconds then that is not scale speed because you have not scaled time.


There is no such thing as scale time.


True, but that does not negate what Alan said which was that in order to represent the dynamics you would have to "scale" time. And what he said does not negate what Andrew said which was that the scale effects work in our favour.

So every one is right! There have been many decent PhDs on this topic - none of them written by me!

Best wishes,

Howard


I am not supposing Alan or Howard would defend my bodger's approach to roadholding either!


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