Re: Flexi Chassis an Appreciation
Posted: Tue Jan 23, 2018 8:59 pm
No, confused as ever. How about a drawing?
Jol
Jol
Will L wrote: 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
Will L wrote: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.
Jol Wilkinson wrote:Will L wrote:
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
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 Wilkinson wrote:So, not unlike the Prickly Pear wagon underframe that Paul Tasker developed.
Will L wrote: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#p57847proto87stores wrote:
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
Will L wrote:In this posting viewtopic.php?f=37&t=5273&p=57883#p57532Proto87stores wrote: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.
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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
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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.
Will L wrote: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.
Will L wrote:Proto87stores wrote:... 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.
Proto87stores wrote:Here is a picture of a commonly misunderstood "sprung bogie"…
Proto87stores wrote: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.
... 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.