Flexi Chassis an Appreciation

[grovenor-2685]

In the specific 4mm scale case, the c of g may stably reside within the symmetrical diamond shape area between the four body support points.

So, is that the constraint? ie 4 points must be in the form of a symmetrical diamond.
Hence, as you say, nbg for bogie vehicles, or even long 4 or 6 wheelers. And how would you apply it to a loco with more than 4 wheels?
Regards

[proto87stores]

In the specific 4mm scale case, the c of g may stably reside within the symmetrical diamond shape area between the four body support points.

So, is that the constraint? ie 4 points must be in the form of a symmetrical diamond.
Hence, as you say, nbg for bogie vehicles, or even long 4 or 6 wheelers. And how would you apply it to a loco with more than 4 wheels?
Regards

My idea of long bogie vehicles is the ~85 ft US variety. I don't anticipate any particular problem for long UK 4 wheel vehicles or past 6 wheel ones. On the bogie Amfleets shown earlier, I use 4 point body springing instead, which saves mounting body springs separately.

4 point equalization body support only means that you support the body on the centres of the transverse equalizing beams at the ends and the uppermost longitudinal equalizing beams on the sides. That creates the diamond area for absolute stability, which is far more helpful than the asymmetric 3 point triangular area. That's also what I described in my post to Will regarding the 0-6-2 N7. And that I intend to use the prototypical model sideframes as the uppermost beams.

Hope that clarifies.

Andy

[Jol Wilkinson]

No, confused as ever. How about a drawing?

Jol

[Will L]

I will admit it took me a while to satisfy myself that Andy's 4 point equalisation as demonstrated in this post would do what he claimed, but after a while getting my head around what was happening, I can see that it does.

I don't claim to entirely understand the theory of all this. or how exactly it will work when extend to more complex vehicles (with the exception of vehicles on 2 4 wheel bogies), but at this stage I would not bet against Andy achieving it. However, while it remains largely theoretical for anything more than 4 wheels at a time the jury must remain out and I think we have to await a practical demonstration to find out how easy it is to do and how well it works. Only when there are practical vehicles running round do we find out if, as a concept, there are unforeseen consequence we need to be aware of, much as three point compensation has proved to have. Remember that bogies built Andy's way already do exist and appear to run well.

[Terry Bendall]

Only when there are practical vehicles running round do we find out if, as a concept, there are unforeseen consequence we need to be aware of

And provided that a suitable test track is available either in P4 or P87 we can make space available at Scaleforum for the concept to be proved.
Build it, show it, prove it!

Terry Bendall

[Jol Wilkinson]

I will admit it took me a while to satisfy myself that Andy's 4 point equalisation as demonstrated in this post would do what he claimed, but after a while getting my head around what was happening, I can see that it does.

I don't claim to entirely understand the theory of all this. or how exactly it will work when extend to more complex vehicles (with the exception of vehicles on 2 4 wheel bogies), but at this stage I would not bet against Andy achieving it. However, while it remains largely theoretical for anything more than 4 wheels at a time the jury must remain out and I think we have to await a practical demonstration to find out how easy it is to do and how well it works. Only when there are practical vehicles running round do we find out if, as a concept, there are unforeseen consequence we need to be aware of, much as three point compensation has proved to have. Remember that bogies built Andy's way already do exist and appear to run well.

Will, thanks for that reminder.

So, not unlike the Prickly Pear wagon underframe that Paul Tasker developed. A concept since used - if I get Andy R's idea right - by John Redrup on an etched London Road Models underframe for use with the Ratio LNWR wagon kits (the top u/f in the picture).

In the PP/LRM design the pivoted side beams are replaced by horizontal springs/beams, which have an element of flex. The downside of that is that a heavy white metal vehicle body could lean to one side, not a problem with the Ratio plastic wagon body or Andy's rigid beams.

I have seen recommended compensation systems on 7mm six coupled locos for a rigid axle and a pair of axles on two side beams. How that is supposed to work I wouldn't know.

Jol

[Paul Townsend]

I have seen recommended compensation systems on 7mm six coupled locos for a rigid axle and a pair of axles on two side beams. How that is supposed to work I wouldn't know.

Jol

But it does!
I have such a loco in P4 using that arrangement, built 40 years ago by a chum.

Its suspension is not as tolerant of poor track as CSB, but it mostly stays where I want it.

[Philip Hall]

I built a Crownline Bulleid Pacific a very long time ago which was designed to have this very arrangement. I think I modified it to a conventional arrangement of fixed axle/single beam on the other pair, simply because I couldn’t see how it was going to work. As Paul has an engine that does work like this, I could have saved myself the bother. The engine still exists, having passed from its original owner to new management, and now runs on St Merryn.

Philip

[grovenor-2685]

I have seen recommended compensation systems on 7mm six coupled locos for a rigid axle and a pair of axles on two side beams. How that is supposed to work I wouldn't know.

It converts a rigid 6 wheeler into a rigid 4 wheeler, it gets over the ptimary 6 wheel problem of end wheels lifting off if the centre set is on a hump, but does not cover for twist in the track. Just like rigid 4 wheelers a lot depends on how much slop there is in it, and how good the track is.
Regards

[Will L]

So, not unlike the Prickly Pear wagon underframe that Paul Tasker developed.

Er only superficially, as Andy's system does not include anything springy. The Prickly Pear item suffers from too hard/too soft a sprig issues just like Andy tends to go on about, in this case with reason as you have noted. I suspect Andy would say that as the ends of the axles are connected by rigid beams which pivot/rock about a fulcrum point at their centre, it is very different. See below for Andy's picture of his etch from a few pages back. The big holes are to do with his use of ball bearing bearings rather than pinpoints (lets not go there again)

[Proto87stores]

Andy

A couple of weeks ago, there were a few points you made that I wanted to chew over further. However life has got in the way a bit, hopefully today I will finally get to respond, but perhaps not to all of them at once.


From this post viewtopic.php?f=37&t=5273&p=57883#p57847

I have looked over the impressive galley at http://www.clag.org.uk/csb-gallery.html . And great and prolific work it all is. However, that's not a valid argument to claim that any one them would have better static or dynamic performance than an equivalent fully 4 point equalized version. In fact, it's had to imagine how a system that doesn't keep constant equal weight on all drivers, can ever reach the tractive and track holding performance of an equivalent appropriately equalized vehicle.

I'm not sure I or anybody else has ever claimed that, given that the operative word was better. All systems which are designed to reliably deliver an equal share of the vehicles weight on each wheel are going to deliver pretty equal performance in this respect, and any that delivers what it claims is unlikely to be all that different, particularly when compared with vehicle with no suspension at all which is where most model railway vehicles are.

Comparing CSB and full equalization, and thinking of the static situation to start with, I will even agree that only fully equalized vehicle such as you propose will always deliver equal weight if the track isn't entirely flat. Where as a sprung system can only claim that on flat track. But then, railway track is, in general, pretty flat so I would suggest that the occasions when this difference would prove significant would be few. Anyway the road holding and tractive performance are hardly an issue when static.

The dynamic situation is more interesting. As your equalisation must pass some part (probably half) of any upward or downward motion of any wheel directly to the vehicle body, I'm afraid the idea that the weight on each wheel remains equal fails as surely as it does on a sprung vehicle. At the point a wheel meats a high spot it has to raise the body, and to do that it must accelerate it upwards. This acceleration force can only act on one place, the wheel rail interface and will be in addition to the weight normally carried. Then once the body has got as far upwards as it needs to, it will still be travelling upwards and the deceleration under gravity which happens next will equally cause the effective weight on the wheel to drop below normal. Of course, the same is true of a sprung system, but I would claim that the whole point about a sprung system is to reduce/minimise this effect.

Now of course you are going to go on about hard and soft sprig rates, and whether the spring rate of a CSB can be correctly matched to the need. So I will come to that next

I'm pretty much in agreement with all the above. I think the most upward movement to the simplest 4 wheel chassis body by a single wheel would only cause 1/4 of that motion to the c of g , but otherwise, yes.

Note that if you follow Bill's argument that practical bumpiness is minor, then 1/4 of minor displacement isn't much to see. But it will be there regardless. Hence my happiness with body springing if needed.

Andy

Andy

[proto87stores]

In this posting viewtopic.php?f=37&t=5273&p=57883#p57532

Unless you know the vehicle weight and then choose the needed spring rate first, you have no idea whether you are designing in very hard springing, medium springing or very soft springing for that vehicle weight And only once you have the spring rate, can you then decide to how to implement that spring, bearing in mind it can only be achieved as some a combination of spring length and appropriate wire diameter.

And BTW, the overall effective vehicle spring rate (ratio of weight against spring rate) is what dramatically affects and determines the track holding reliability of a sprung vehicle.


For any given spring rate, thick wire needs to be longer, thin wire needs to be shorter. Obviously you are limited in that you have to choose a wire length that will fit the space available. FINALLY, only then can you set the wire support point heights so that the axle bearing, with the wire you are using, is around the suspension movement mid point.

What you can't say/don't know is whether the springing provided by a CSB is to hard, to soft or perhaps just right, and I think that shows a failure to appreciate the implications of a CSB design.

---------------------------

I think it much more likely shows that the typical CSB designs work regardless of spring rate. And the very significant implication of that is, that when running, their apparently satisfactory track holding is therefore due to some other effect of Physics/Mechanics than linear springing. Which I why I am so curious to know exactly how they actually work. If you understand that having a "hard" spring rate is virtually indistinguishable from having a "rigid" chassis, then it's clear that something about CSB operation is not yet fully understood.

That means I'm going to skip the following three paragraphs, as they really pertain to supporting the existing CSB thinking and introduce some other ideas for consideration later

--------------------------

Correctly applied, whatever the final weight of the vehicle, all its springs are depressed by the same amount while carrying an equal proportion of the weight. This you can do at the design stage before the tue weight is know. What can't be varied is the location of the CofG. Then all you need to do when the true weigh is known is chose a wire size that gives you the static spring depression you're after (normaly 0.5mm).

The implications of all that being that the spring rate (i.e. force required to deflect spring/applied weight = a constant value) is effectively the same for all vehicles designed this way.

As they will all have the same characteristics, the question is not whether the suspension on any particular vehicle can be considered hard medium or soft (relative terms anyway which give no guidance on what we actually need), as they will all be the same. The question is, how suitable is it for use on a rail vehicle use, and, if it is right for one it will be right for all.

Now we are in the field of experience and we find that locos design this way perform very nicely thank you with nothing to suggest that the resulting spring rate is in any way unsuitable for loco on a railway, no matter how the characteristics of the design were decided upon. So perhaps we did fail to consider the physics of the interaction of racing car suspension (yes I've watched the video) but that hasn't stopped us produce highly functional results that work, from a method that by design gives result with consistent and repeatable characteristics. Which, by the by, is something no other springing method can claim.

Obviously CSB's have many quite happy users and the obvious comment is why fix what isn't broken.

My first answer is that CSB's still require horn blocks and height adjustments, so don't lend themselves to a "just put it all together and run it kit", or a simpler means of upgrading a RTR vehicle, or even a potentially economic manufacturing method for factory suspension fitted RTR vehicles.

My second is that the apparent performance of a CSB seems to match the "holy grail" performance of a model equivalent of the prototype's highly non-linear, leaf springs. But there is no obvious visible mechanism to make that so. And therefore I have to question whether a CSB is really actually a springing system?

Discussing that last point will take more than a few sentences, so I'll move on before tackling that here.

Andy

[Proto87stores]

Don't agree, perhaps we were lucky, or perhaps going for a system that ensures the static deflection is half the variable dynamic movement is exactly the right basis to judge if the correct spring rate is in use. The proven results and the fact they both theory and practice show they are consistent and reproducible suggest... not so risky after all and definitely not unsubstantiated.

Not too risky . But, as per my previous post, I have a feeling the prevailing current theory doesn't agree with the practice.

Andy

[Proto87stores]

... the 4 point suspension of full equalization that is the natural progression of Flexichas.

I think that depends on what you think is the most important characteristic of the Flexichas method. If you focus entirely on the use of levers to ensure that all the wheels are fully in contact with then track, then yes full equalisation is the natural progression of Flexichas,

However for me this was only part of what Flexichas was all about. What appeal to me, and what got me started on this thread an awful lot of pages ago, was that, most of all, it was a whole series to techniques which enabled the average kitchen table modeller like me with no machine tools, to produce chassis which run reliably and well. And it is the progression on this same path to even better running that makes me think that CSB is the natural extension of Fkexichas.

There you are John, nicely back on topic I think.

Here is a picture of a commonly misunderstood "sprung bogie". To an observer seeing this for the first time, it appears that the bogie bolster is sprung and that the springs handle the routine imperfections in the track.

However, close inspection and handling reveals the following:

1. The springs are actually pre-loaded by the ends of the bolster being forced into the gap between the tops of the springs and the upper edge of the side frame. I.e. they are partially compressed, even when the car is unweighted. This means in practice, that no further spring compression movement can occur until either, the unlikely overall weight of the car exceeds 8 oz., and no “bump smoothing” occurs except for those instances when the upwards bump force exceeds the local pre-load value. - which is 2 oz at each end of the bogie bolster. (e.g. non-linear "Hard" springing only if large bumps ).

2. The top surface of the end of the bolster has a slight ridge which is pressed up against the bottom of the upper edge of the side frame by the double springs. This allows the side frame to slightly rotate left or right around the end of the bolster. This rotation requires almost no force to happen, because as the side frame rotates, the double springs alternately compress and release, the force on one being almost completely counter balanced by the extension of the other, and the net force is close to zero. Therefore the side frames actually function as "equalizing beams" for all normal running conditions and the springs perform no other function except to act as the pivot for the equalizing beams.

3. This is therefore an "equalized bogie" with extra provision to only smooth extremely high shocks. But it's model performance is very similar to having prototypical type non-linear leaf springing. And of course CSB’s apparently have similar characteristics.

--------

So Equalization is like having a combination of very soft springing to allow easy movement for small bumps and low resultant height change, but with hard springing counterbalancing the vehicle static weight and keeping it stable at the correct static height without needing any adjustment or shimmying.. Adding pre-loaded additional hard “real” springing only to control excessive movement prevents large shocks being passed upwards to the body.

Both Will and I published you tube clips showing a vehicle being pressed down by hand to demonstrate that its body was sprung. It’s interesting that both videos appeared to show a similar (strong?) downward force being applied before the vehicles dipped. And no “shimmies” in either case. Yet the Amfleet car body was supported by the same 8 springs shown in the freight bogie. I.e. At one end, it needed a greater than 4 oz push to cause the downward movement. And that would be quite hard springing for a model of that weight.

Admittedly hazarding a guess that Will’s model needed a similar heavy push, then it could be construed that the springiness of the CSB’s is not the method that small bumps are handled., and that instead some form of self equalization is occurring partially or instead.

We already know that CSB’s only work if the wire slides freely between the support pins. The wire is much too strong to stretch between fixed points. So the entire springing capability is limited to the extension or shrinking of the loops of sliding wire above each wheel. For the static support of the vehicle it’s reasonable to accept that the loops expand (as springs) until the overall vehicle weight is balanced.

The usually suggested way that equalization occurs in a CSB is by the principle of springy beams. However, any beam that is pivoted in its centre, only “springs” if a changing load is applied to both ends of the beam at once. Then it stabilizes in an equilibrium bent shape and becomes effectively rigid at that loading. If the load at one only end changes, then the springy beam will try to rotate as a rigid beam until a new equilibrium position is reached.

This rotating beam action is I believe identical to the rotation of the side frame in the “equalized bogie” example above. The only case that the wire behaves as a spring or ”springy” beam is when both adjacent wheels try lift simultaneously. I.e. the static case, not any dynamic one. Which now suggests that bumps at individual wheels are handled by the rotations of the statically loaded wire sections, acting as linked rigid equalizing beams. (like soft springs) And if greater shocks are received, then the appropriate loops expand by wore sliding and then acting as “strong” springs.

So my conclusion is that CSB’s are seriously misnamed and their dynamic operation not yet fully understood. In the dynamic case, they are fundamentally a single wire acting as a series of flexibly joined dynamically rigid equalizing beams. And as such they pass the movement of small bumps upward to the vehicle body in almost exactly the same manner and amount as would a series of separate rigid beams. They only time and manner they act truly as springs is in the static balancing of the body.

Before I leave this up for others’ comment, I will say that attempting to explain a different account of the how “CSB’s” work in no way detracts from the current modellers’ experiences that say these are a very satisfactory performance solution so far for 4mm scale vehicle suspension. And I am in agreement with that. But I now believe that that is due to their being a fundamentally a form of equalization more than a form of springing.

A couple of interesting experiments would be (1) to adapt an existing CSB to use completely flexible (non springy) wire and a suitable return spring (like I understand the Varney system was), and see how, if at all, the performance changes. And (2) use similar springy wire but cut it above each horn block so that the system only contains individual springy beams and test that also.

Andy

[Will L]

Hi Andy, fighting to find time enough for a thoughtful reply as usual

Here is a picture of a commonly misunderstood "sprung bogie"…

Not entirely sure how the bogie arrived in response to my post, but while we’re here…

I tend to agree that as a practical solution, equalisation suits bogies very well, and that randomly applied coil springs rarely if ever really spring anything, except in extremis. I have to admit that the next 6 plus bogie vehicles on my to do list will all have nothing more than basic bogie to body compensation (just like your Fast and Easy Car Stabilisers) and fixed axles, as this all works pretty well even in P4 and the bogies have been built for some years.

There are in fact several variety of bogies available in the UK which from your point of view are effectively equalised more than sprung, starting from the MTJ variety. Unfortunately, they all share the same aesthetic fault, the lack of the solid frame member across the ends so prominent on the prototype. Not a problem on the average freight wagon bogie even in the UK, but enough to make me seek something different under my UK pre-grouping coaches should fixed axles prove troublesome.

A response re your thoughts on the functioning of CSBs follows.

[proto87stores]

The bogie "Arrived" because I was using it as an example of the visual appearance of model linear springing, and actually fitted with apparently linear springs --- but which behaves as though it has the excellent prototype's non-linear leaf springing track holding. And that performance is due to equalization for most bumps and strong (still linear but pre -loaded) springs only for reducing the shock of really big ones.

And it seems to me, that with closer examination, the mechanics of CSB's are actually very similar. And so therefore is the satisfactory performance experienced.

My example of using a (handy) clearly outside framed model bogie was misleading in that I'm only suggesting using the same inside frames with interior beams structure across all situations: 4 wheel bogies, 6 wheel bogies, 4 and 6 wheel wagons and all steam loco wheel arrangements. I think that's different from what I perceive as the current Scalefour situation of using inconsistently different suspension types in different vehicle types, almost willy-nilly. Leaving each modeller to solving performance issues on a case by case basis.

And for the common UK outside rigid frame bogies with hornblocks, as you mentioned, I'd expect those to be cosmetic on the outside of the equalized inside framed mech.





Here is the way I made the equalized inside mechs up for my Bachmann BR Mk1's.

[Will L]

Hi Andy as promised further thoughts.

For anybody else inclined to want to read on, the following contains detailed consideration of how a CSB fitted loco may or may not work. Such discussions amuse me, but your not obliged to read it and I promise you any failure to read or understand what follows will in no way effect how well your P4 train set works.

Both Will and I published you tube clips showing a vehicle being pressed down by hand to demonstrate that its body was sprung. It’s interesting that both videos appeared to show a similar (strong?) downward force being applied before the vehicles dipped. And no “shimmies” in either case. Yet the Amfleet car body was supported by the same 8 springs shown in the freight bogie. I.e. At one end, it needed a greater than 4 oz push to cause the downward movement. And that would be quite hard springing for a model of that weight.
Admittedly hazarding a guess that Will’s model needed a similar heavy push, then it could be construed that the springiness of the CSB’s is not the method that small bumps are handled., and that instead some form of self equalization is occurring partially or instead.

As to the downward force needed to further compress a CSB, no need to guess. As you have pointed out in the past, a CSB spring is not progressive. Therefore, as the full weight of the loco is required to compress the springs by 0.5mm, then to get a further 0.1mm compression on all wheels will require a push equal to 1/5 of the loco weight, etc. etc. The difference between my loco and your coaches was that the loco was already standing on springs matched to the weight of the loco and designed to be compressed by a given amount, and it is movement in the springs that produces the desired road holding performance; while your coach springs are not so matched to the vehicle weight, are not required to provide the road holding, and may well have been barely compressed at all until you pressed on the end.

...We already know that CSB’s only work if the wire slides freely between the support pins. The wire is much too strong to stretch between fixed points. So the entire springing capability is limited to the extension or shrinking of the loops of sliding wire above each wheel. For the static support of the vehicle it’s reasonable to accept that the loops expand (as springs) until the overall vehicle weight is balanced.

The usually suggested way that equalization occurs in a CSB is by the principle of springy beams…

I think that perhaps describing to you what CSBs do as equalisation was never a good idea. From where you are coming from, equalization has a very specific meaning about the equal distribution of weight which I don’t think is entirely what CSBs do. They certainly transfer weight from one wheel to the next, but the distribution will only be equal if the wheels are on level track.

…However, any beam that is pivoted in its centre, only “springs” if a changing load is applied to both ends of the beam at once. Then it stabilizes in an equilibrium bent shape and becomes effectively rigid at that loading. If the load at one only end changes, then the springy beam will try to rotate as a rigid beam until a new equilibrium position is reached.

This is where we are going to part company. The CSB is continuous through a series of fulcrums so it clearly isn’t going to pivot around anything, and wheel loads are applied to the beam at a point partway between two fulcrums, so I’m unclear where your idea of applying force “at one end” comes from. So I don’t accept your conclusion…

... that CSB’s are seriously misnamed and their dynamic operation not yet fully understood. In the dynamic case, they are fundamentally a single wire acting as a series of flexibly joined dynamically rigid equalizing beams. And as such they pass the movement of small bumps upward to the vehicle body in almost exactly the same manner and amount as would a series of separate rigid beams. They only time and manner they act truly as springs is in the static balancing of the body.

The only way any wheel moves relative to the chassis is by changing the curvature of the bit of the beam it is in contact with. Curving the beam is exactly where the springiness comes from, so changing the curvature means the load being applied to the wheel has changed. In my mind there is no doubt that irregularities in the track will result in slightly different deflections in each spring segment, which further implies slightly different loadings on each wheel. Certainly, true in the static situation.

I think what may be on your mind is that in the dynamic case, the shifting weight balance resulting from uneven track would mean a that the CSB need to move significantly through the fixed fulcrums to take up the different profiles needed, and that you doubt that can happen fast enough to accurately follow the track profile. Hence you think that some other mechanism explains the fact that CSBs do work.

I agree there is a potential friction issue involved in the fact that the length of CSB required must change as the loads on the individual wheels change and thus must slide across (some of) the fulcrum points. For this reason, Russ Elliot has always been keen to advise that fulcrum points are lubricated, but personal experience says this is unnecessary.

In the static situation it is quite clear that my CSB fitted locos do sit down on their springs as the CSB theory would lead you to believe, and any friction between wire and fulcrum point is insufficient to show any noticeable reluctance to settle to the full static deflection. This is of course when most of this movement must occur and movements in the dynamic case will inevitably be small in comparison. Then as taking more weight on one wheel inevitably means taking less on another, the necessary movement is likely only to involve a subset of fulcrums.

Any failure of this sliding movement to occur, or to not occur fast enough, would inevitably compromise the locos ability to deal with track irregularities. Now remember this is P4 we are talking about where the scale flange size means we become only too painfully aware of locos that have problems in this area. Finally remember my very first CSB loco, which did do much to convince me CSB were the way to go, was an 8 coupled loco with a long rigid wheelbase, the very thing most likely to show up any difficulty in keeping wheels firmly on the track, and with additional sets of fulcrums to exacerbate this problem if it was a real concern.

In passing, I did used to wonder if these movements of the CSB wire through the fulcrums would cause the wire to work one way or the other during an exhibition day. For a while I used to check for evidence of such movement, but I never found any.

[Flymo748]

Thanks to Will, and to Andy, fo keeping a conversation up across some nineteen pages of postings and pictures.

With Will's final reply, we seem to have come to the point of having said as much as is possible to say on the subject. It's an appropriate time to down pens and go back to doing some modelling.

Cheers
Paul Willis
Deputy Chairman