Flexi Chassis an Appreciation

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proto87stores

Re: Flexi Chassis an Appreciation

Postby proto87stores » Mon Dec 18, 2017 5:57 pm

billbedford wrote:
Will L wrote:3. The CSB spread sheets came about because of the need to work out where the fixed fulcrum points must go.


We need a spreadsheet because the deflections on a beam with more that two supports is mathematically indeterminate. ie there aren't enough equations to be able to solve the problem.


And there I was thinking that suspension bridges were designed rather than injuneered by trial and error. :o

I'm still more interested in your response on spring rates vs. weight though.

Andy

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Re: Flexi Chassis an Appreciation

Postby billbedford » Mon Dec 18, 2017 8:05 pm

Proto87Stores wrote:However, what doesn't seem to be mentioned much on the forum is that merely fitting springs to wheel bearings does not itself provide a springing motion unless the spring rate is low compared to the weight of the vehicle supported by that wheel.. For those using horizontal wires, what seems to be the only consideration, even for CSB's, is balancing the wire thickness to achieve the required 50% static displacement position. But the thickness of the wire and its length between fixed supports affects its spring rate. You can therefore accidentally, or deliberately end up with different spring rates while still achieving the 50% balance position.


Sorry. 50% of what exactly?
Bill Bedford
Mousa Models
http://www.mousa.biz

proto87stores

Re: Flexi Chassis an Appreciation

Postby proto87stores » Mon Dec 18, 2017 9:36 pm

billbedford wrote:
Proto87Stores wrote:However, what doesn't seem to be mentioned much on the forum is that merely fitting springs to wheel bearings does not itself provide a springing motion unless the spring rate is low compared to the weight of the vehicle supported by that wheel.. For those using horizontal wires, what seems to be the only consideration, even for CSB's, is balancing the wire thickness to achieve the required 50% static displacement position. But the thickness of the wire and its length between fixed supports affects its spring rate. You can therefore accidentally, or deliberately end up with different spring rates while still achieving the 50% balance position.


Sorry. 50% of what exactly?


grovenor-2685 wrote:Somewhere among Bertiedog's postings in the RMweb archive there was a better description of the Varney system complete with an illustration from a Varney catalogue. If applied as intended it should not hold everything on the bottom bump stop, the adjustment allows the spring force to be set so it rides at the mid point like a CSB.
Regards


From Scalefour Suspension digest

5 Requirements for model locos

The main requirement for good running consists in maintaining continuous and consistent contact between all wheels and the track:

Continuous contact is achieved by allowing wheelsets to have vertical movement with respect to each other; this vertical movement is achieved by hornblocks or axleboxes (or tubular bearings) that can move up and down in the loco frames or chassis (see figure 1).


Bill,

This is the absolutely basic stuff of working suspension.

As per Keith's above, and others earlier posts on this topic, CSB's and and other spring systems, even such as your wagon springs, typically want and assume that the static on perfectly flat track spring position for each wheel is at least approximately at the 50% point of it's up-down movement - So that there can be Up-down movement.

I thought we'd moved on enough so that I could show that lightweight vehicles, such as you just previous suggestions for empty open wagons, need very much softer spring rates than normally weighted vehicles.

Andy

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Re: Flexi Chassis an Appreciation

Postby grovenor-2685 » Mon Dec 18, 2017 11:07 pm

So what's new? We have mentioned innumerable times that the diameter of the spring wire is adjusted to get the required static deflection with the actual vehicle weight, so the lighter vehicle gets a softer spring.
Regards

proto87stores

Re: Flexi Chassis an Appreciation

Postby proto87stores » Tue Dec 19, 2017 3:02 pm

grovenor-2685 wrote:So what's new? We have mentioned innumerable times that the diameter of the spring wire is adjusted to get the required static deflection with the actual vehicle weight, so the lighter vehicle gets a softer spring.
Regards


billbedford wrote:

proto87 wrote:
That's the fundamental difference between equalization and springing hitting a bump. A sprung wheel sees proportionally increasing resistance as the wheel tries to rise, while an equalized wheel sees no difference in resistance as the wheel rises. Basically it is free to follow the rail height profile without incurring extra loading. Hence the track holding excellence.

billbedford mistakenly replied:

You have that backwards. An equalized wheel has to lift the whole of the weight it supports as soon as it hits a bump, while as sprung wheel only has to deflect the spring and the energy stored in the spring deflection will lift the supported weight at a later time.



Actually I was trying to get Bill to understand the basics of there being an independently controllable dynamic spring deflecting rate by picking the spring length and dia. in co-ordination, as opposed to just achieving static balance with the spring length that happens to fit easily in the chassis.

As I understand the general spring fitting and CSB instructions, after you have set the balance by choosing the wire dia., you have no way of then choosing the dynamic spring rate and you are stuck with the one that happens to come with the spread sheet fixed fulcrums and the wire dia. that balances the weight.

It's highly unlikely that there is some "Goldilocks" like situation for Spring Physics in 4mm scale, that the dynamic spring rate you end up with happens to be just the right value for the dynamic springing of each wheel on that vehicle.

Andy

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Will L
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Re: Flexi Chassis an Appreciation

Postby Will L » Wed Dec 20, 2017 12:33 am

Proto87Stores wrote:While the subject of vehicle weight is currently being discussed, I would like to try and re-stress the importance of understanding the effect of spring rate in relations to vehicle weight.

So to recap the basics:

In order to safely ride over even small bumps and twists in track, each wheel must be able to independently and freely move up and down relative to the chassis/body of the vehicle.

This movement can be achieved by a) equalization/flexichas moving beam/levers, as already well covered, or by b) relatively free springing.

However, what doesn't seem to be mentioned much on the forum is that merely fitting springs to wheel bearings does not itself provide a springing motion unless the spring rate is low compared to the weight of the vehicle supported by that wheel.. For those using horizontal wires, what seems to be the only consideration, even for CSB's, is balancing the wire thickness to achieve the required 50% static displacement position. But the thickness of the wire and its length between fixed supports affects its spring rate. You can therefore accidentally, or deliberately end up with different spring rates while still achieving the 50% balance position.....


Andy

While in the past I would have tended to agree with your basic thesis (details omitted for clarity), that it was hard to match the characteristics of a sprung suspension system to the needs of a 4mm steam outline loco, and most of the attempt in the past did suffer from the sort of issues you described. None the less in the 3000 or so years of human wheel transport history, there are very few occasions where a wheeled vehicle hasn’t been improved by fitting some sort of sprung suspension, the issue being how best to achieve it. I think that the time has come even for 4mm scale steam outline models and that, in your certainty that CSBs really can't work as well as I would like to think they do, you seem to be missing a couple of salient fasts.

1. When you use the spread sheet to set up a CSB, the whole point is that you have designed a springing system that ensures, for the weight of a given loco, the springs over each axle will be depressed by the same amount (half the designed full movement) with the same load applied at each axle. I would have said that, however you push your balloons, this implied the same effective spring rate for each spring. As the result is that the required amount of deflection is produced by the same load for each spring segment, for the life of me I can't see why you think that any one span will suddenly become more or less resilient that any other over the rest of the permitted movement range.

2. I'm not the only person who builds these things, a number of competent modellers have adopted them and find that they work well as advertised*. This is no longer a theoretical exercise, there is practical experience to back it up. Which, with respect, is more than can be said for your concept of a fully equalise multi axle 4mm steam outline chassis. I'm not saying you can't make it work Andy, just that you haven’t yet, and, till you do, you can't demonstrate that it is any way simpler to produce or more effective in action than the CSB based locos we already have running round in ever increasing numbers. When you have achieved it, I expect the performance will be good enough to make it comparable with a CSB based loco, and the deciding factor as to whether it gets commonly adopted will end up being how easy it is to implement.

*OK I'm sure you can find people on RMWeb who have never built a CSB who tell you why it doesn’t work, and probably somebody who has built one but made a pigs ear of it, but then there is always one isn’t there.

Changing topic, I would also like to have a think about the uses for ball bearing bearings, but so as not to loose the discussion at the end of a very long thread I've started a new one called Low Friction Bearings in 4mm

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Re: Flexi Chassis an Appreciation

Postby billbedford » Wed Dec 20, 2017 12:27 pm

proto87stores wrote:Actually I was trying to get Bill to understand the basics of there being an independently controllable dynamic spring deflecting rate by picking the spring length and dia. in co-ordination, as opposed to just achieving static balance with the spring length that happens to fit easily in the chassis.


What is there to understand?

If the dynamic deflection under load is less than the designed static deflection any combination of spring rate and weight is going to accommodate the dynamic deflection. And because CSBs are partially equalised the dynamic deflection can even exceed the static deflection on some axles without detriment.
Bill Bedford
Mousa Models
http://www.mousa.biz

Proto87stores

Re: Flexi Chassis an Appreciation

Postby Proto87stores » Sun Dec 24, 2017 4:26 pm

billbedford wrote:
proto87stores wrote:Actually I was trying to get Bill to understand the basics of there being an independently controllable dynamic spring deflecting rate by picking the spring length and dia. in co-ordination, as opposed to just achieving static balance with the spring length that happens to fit easily in the chassis.


What is there to understand?


Actually all the following common-sense, and long established very basic engineering :(

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.

All the above is the very fundamental basis of why and how springing works. Yet none of that spring rate importance is explained in the suspension digest, nor the 4 point suspension of full equalization that is the natural progression of Flexichas. And nowhere is there any numeric table of worked out spring rates for the various wire lengths and sizes recommended for 4mm model use, so that individuals can quickly find the right wire size for their particular vehicle weight and application.

So your particular justification for recommending switching from Flexichas to springing, without first knowing and understanding this stuff, seems particularly risky and unsubstantiated.

If the dynamic deflection under load is less than the designed static deflection any combination of spring rate and weight is going to accommodate the dynamic deflection. And because CSBs are partially equalised the dynamic deflection can even exceed the static deflection on some axles without detriment


If you don't know or care how much dynamic deflection you get when you hit a bump, then sure any off the wall wire size suggestion will do. But that's how you get unexplained derailments for vehicles that are too light weight, or just wrongly sprung. If you don't the know the numbers your proposed solutions will give, they can't be depended upon to be the right answers.

And separately: Of course, CSB's do not correctly partially equalise an 0-8-0 axle configuration, or greater, even though they are frequently recommended here for such. Anyone who bends a piece of wire running through 5 fixed supports will see that.

Andy.
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proto87stores

Re: Flexi Chassis an Appreciation

Postby proto87stores » Wed Jan 10, 2018 5:53 pm

Will L wrote:
Andy

While in the past I would have tended to agree with your basic thesis (details omitted for clarity), that it was hard to match the characteristics of a sprung suspension system to the needs of a 4mm steam outline loco, and most of the attempt in the past did suffer from the sort of issues you described. None the less in the 3000 or so years of human wheel transport history, there are very few occasions where a wheeled vehicle hasn’t been improved by fitting some sort of sprung suspension, the issue being how best to achieve it. I think that the time has come even for 4mm scale steam outline models and that, in your certainty that CSBs really can't work as well as I would like to think they do, you seem to be missing a couple of salient fasts.

1. When you use the spread sheet to set up a CSB, the whole point is that you have designed a springing system that ensures, for the weight of a given loco, the springs over each axle will be depressed by the same amount (half the designed full movement) with the same load applied at each axle. I would have said that, however you push your balloons, this implied the same effective spring rate for each spring. As the result is that the required amount of deflection is produced by the same load for each spring segment, for the life of me I can't see why you think that any one span will suddenly become more or less resilient that any other over the rest of the permitted movement range.


See my immediately previous answer to Bill. Whether for an individual wheel, or the whole loco, the spring rate you end up with is the one that statically balances the weight for the support spacing, not necessarily the one that could/should have been chosen to give the best dynamic smoothing of bumps. Physics is an uncompromising Mistress. "You only get what you weigh for", not "everything comes to those who weight" ;)


Will L wrote:
2. I'm not the only person who builds these things, a number of competent modellers have adopted them and find that they work well as advertised*. This is no longer a theoretical exercise, there is practical experience to back it up. Which, with respect, is more than can be said for your concept of a fully equalise multi axle 4mm steam outline chassis. I'm not saying you can't make it work Andy, just that you haven’t yet, and, till you do, you can't demonstrate that it is any way simpler to produce or more effective in action than the CSB based locos we already have running round in ever increasing numbers. When you have achieved it, I expect the performance will be good enough to make it comparable with a CSB based loco, and the deciding factor as to whether it gets commonly adopted will end up being how easy it is to implement.


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.

Will L wrote:
*OK I'm sure you can find people on RMWeb who have never built a CSB who tell you why it doesn’t work, and probably somebody who has built one but made a pigs ear of it, but then there is always one isn’t there.


I've learned not to be a reader of advertising funded forums. :(
Will L wrote:
Changing topic, I would also like to have a think about the uses for ball bearing bearings, but so as not to loose the discussion at the end of a very long thread I've started a new one called Low Friction Bearings in 4mm


Slow be it ;)

Cheers

Andy
Last edited by John McAleely on Wed Jan 10, 2018 7:06 pm, edited 1 time in total.
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Re: Flexi Chassis an Appreciation

Postby billbedford » Thu Jan 11, 2018 9:38 am

proto87stores wrote:See my immediately previous answer to Bill. Whether for an individual wheel, or the whole loco, the spring rate you end up with is the one that statically balances the weight for the support spacing, not necessarily the one that could/should have been chosen to give the best dynamic smoothing of bumps. Physics is an uncompromising Mistress. "You only get what you weigh for", not "everything comes to those who weight" ;)




Some worked examples would be useful here, just to prove your point.
Bill Bedford
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proto87stores

Re: Flexi Chassis an Appreciation

Postby proto87stores » Thu Jan 11, 2018 3:47 pm

billbedford wrote:
proto87stores wrote:See my immediately previous answer to Bill. Whether for an individual wheel, or the whole loco, the spring rate you end up with is the one that statically balances the weight for the support spacing, not necessarily the one that could/should have been chosen to give the best dynamic smoothing of bumps. Physics is an uncompromising Mistress. "You only get what you weigh for", not "everything comes to those who weight" ;)




Some worked examples would be useful here, just to prove your point.




This explains why spring rate matters most for dynamically handling bumps. Your favorite subject. ;)

Andy

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Re: Flexi Chassis an Appreciation

Postby billbedford » Fri Jan 12, 2018 9:21 pm

Oh dear, more road vehicle stuff. No wonder you are confused.

The problem with this stuff is that railways, in general, keep any bumps in the track to a minimum. So we can ignore ignore most of what he said, apart from one thing that he implied, but didn't say explicitly. This was that if the wheels are to stay on the track/road the dynamic deflection has to be less than the static. Something I may have mention here before.
Bill Bedford
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proto87stores

Re: Flexi Chassis an Appreciation

Postby proto87stores » Sun Jan 21, 2018 1:39 am

billbedford wrote:Oh dear, more road vehicle stuff. No wonder you are confused.

The problem with this stuff is that railways, in general, keep any bumps in the track to a minimum. So we can ignore ignore most of what he said, apart from one thing that he implied, but didn't say explicitly. This was that if the wheels are to stay on the track/road the dynamic deflection has to be less than the static. Something I may have mention here before.


The worked example fits your previous question precisely. The wheel shown is assumed solid, and simply sprung with it's own Hooke's Law spring, so exactly matches the effect on an identical simply sprung model railway wheel as per your own wagon construction.

I'm certain there is no difference in the Laws of Physics for a railway vehicle as compared to an automobile. Explaining basic Physics is better done as a separate topic to save space and time. I'm assuming that basic knowledge when posting on this one.

As to the relative bumpiness of railway track, I agree for the prototype. But typical hand laid model track is far bumpier relatively. Calcutta Sidings video posted on that recent topic, being a example.

Andy

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Re: Flexi Chassis an Appreciation

Postby Will L » Sun Jan 21, 2018 9:21 pm

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

proto87stores 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

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Re: Flexi Chassis an Appreciation

Postby John McAleely » Sun Jan 21, 2018 10:25 pm

As this thread wakens up again, we'd like to remind people posting to be thoughtful in their responses. There is lots to read on this thread, and many excellent contributions. Some recently seem to over-focus on the individual posters, and not the topic at hand. Please be mindful to address the topic.

(And apologies for the out-of-order posts sometimes. Our non-member contributors have to wait for a member of the moderating team to read and approve the posts)

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Will L
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Re: Flexi Chassis an Appreciation

Postby Will L » Sun Jan 21, 2018 10:30 pm

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

Proto87stores 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.

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.

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Re: Flexi Chassis an Appreciation

Postby Will L » Sun Jan 21, 2018 10:50 pm

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

Proto87stores wrote:...Yet none of that spring rate importance is explained in the suspension digest, nor the 4 point suspension of full equalization that is the natural progression of Flexichas.

If you mean the The principles of model locomotive suspension on the CALG site, this was written some while ago now and all but the most basic CSB thinking post-dates it. No surprise it doesn’t cover your recent thinking on equalisation either. Is equalisation the natural progression of Flexichas? I’ll come back to that. While I'm inclined to agree that a discussion of spring rate does have a place when considering individual springs, as shown above, the need is avoided in CSB thinking by designing in consistent reproducible spring performance.
...And nowhere is there any numeric table of worked out spring rates for the various wire lengths and sizes recommended for 4mm model use, so that individuals can quickly find the right wire size for their particular vehicle weight and application.


While that may be literally true, providing the computer age equivalent is exactly what the spread sheets are doing. There may not be published tables (they would be horrendously complicated if they were, and not, I fear, quick to use), but the maths to produce such a thing exist in the spread sheets. However, just like log tables, these days we use computers to do the sums from first principles every time.

So your particular justification for recommending switching from Flexichas to springing, without first knowing and understanding this stuff, seems particularly risky and unsubstantiated.


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.

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Re: Flexi Chassis an Appreciation

Postby Will L » Sun Jan 21, 2018 11:09 pm

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.

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Re: Flexi Chassis an Appreciation

Postby grovenor-2685 » Sun Jan 21, 2018 11:24 pm

Proto87stores wrote:... the 4 point suspension of full equalization that is the natural progression of Flexichas.


Full equalisation has, for many years, been based on 3 point suspension, with the simple analogy of the three legged stool sitting firmly on any floor while the 4 legged chair will not.
How have things changed such that 4 point suspension now does what 3 points used to do?
Regards

proto87stores

Re: Flexi Chassis an Appreciation

Postby proto87stores » Sun Jan 21, 2018 11:55 pm

grovenor-2685 wrote:
Proto87stores wrote:... the 4 point suspension of full equalization that is the natural progression of Flexichas.


Full equalisation has, for many years, been based on 3 point suspension, with the simple analogy of the three legged stool sitting firmly on any floor while the 4 legged chair will not.
How have things changed such that 4 point suspension now does what 3 points used to do?
Regards


Without the necessary pausing to answer Will's inputs first and properly, I suggest this can be answered quickly and simply. Model railway vehicles are rectangular in nature and are expected to travel stably and safely in both possible directions. Say L and R. 3 point suspension often acts differently in the two directions and, as reported frequently in the past, that made many situations apparently dangerously unstable in one direction only, particularly for relatively lightweight vehicles on bumpy model railway track.

E.g, for the 4 wheel wagon travelling left with a rocking W iron on the left end, a single rail bump safely raises that body end only half the bump heightand, only symmetrically vertically through the centre of the rocking pivot. For the same wagon travelling right, the same bump has to to tilt the whole body sideways the full bump height on the rigid w iron. Push at least two( or lots) of those wagons coupled together and the inter buffer pushing forces have sudden transverse components that wander asymmetrically all over the map.

There is a lesser but similar asymmetry on bogie vehicles.

4 point suspension averages the L and R bump reaction movements to be the same in both directions and makes the dangerous relative transverse buffer movement less.

Andy

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Re: Flexi Chassis an Appreciation

Postby grovenor-2685 » Mon Jan 22, 2018 11:10 am

That answer just describes problems with simplistic 3 point suspension designs that we are well aware of. It does not answer the question.
Given the long standing requirement for 3 point suspension to keep all of the wheels on the rails what is different that now 4 point suspensions can achieve the same result? And what constraints are needed on the design to achieve this result?
Note that the requirement is for general application to vehicles of any wheel arrangement, not just simplistic 4 wheel wagons.
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Re: Flexi Chassis an Appreciation

Postby Crepello » Mon Jan 22, 2018 12:35 pm

Yes. This repeats a query I posed a while ago: how can 4 non-co-planar wheel contact points be translated into 4 co-planar beam pivot points, merely by taking the mean of adjacent wheel contact points?

proto87stores

Re: Flexi Chassis an Appreciation

Postby proto87stores » Mon Jan 22, 2018 3:22 pm

grovenor-2685 wrote:That answer just describes problems with simplistic 3 point suspension designs that we are well aware of. It does not answer the question.
Given the long standing requirement for 3 point suspension to keep all of the wheels on the rails what is different that now 4 point suspensions can achieve the same result? And what constraints are needed on the design to achieve this result?
Note that the requirement is for general application to vehicles of any wheel arrangement, not just simplistic 4 wheel wagons.
Regards


Nothing has changed. Three point support of a flat plane on a variable non-planar surface merely happens to be the simplest method that is statically stable. And the simplest method frequently ends up as the least expensive, and thus most commonly used, and hence most well known. But 4 point (or increasingly greater points) systems have been around as long and are equally viable, and can handle dynamical displacements from more directions and can have their c of g closer to the edge of their bodies.

For an analogy, in the real world we have unicycles, bicycles, tricycles and quadricycles. Each added support wheel gives an extra dimension in which the cycle is stable, and need not be actively balanced. For the legendary "Little old lady of Pasadena", 4 wheels require the least skill in handling bumpy road conditions safely at otherwise unsafe speeds.

Ditto for thrust lifted aircraft. Helicopters can have 1, 2, 3, or 4 fans. Most new drones have 4 or more and have no stability restrictions on any horizontal direction of travel.

In my financially limited youth, my first powered road vehicle was a motorcycle, my second a souped-up three wheeler with two steering wheels at the front. But my three wheeler could easily out corner any more common "Bond", because my forward direction was far more dynamically stable than a vehicle with a single front wheel. But wisely, it was not fitted with any reverse gear ;).

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. This far exceeds the non-symmetrical triangle shaped area of the three body support points. And is far closer to the size and position of the rectangular c of g stable area provided by 4 point (or greater) sprung wheel support. But does not require the springing or horn blocks to adjust the individual wheel heights.

As to additional restrictions, it does not seem practical to use for mounting long vehicle bodies onto bogies as the long side beams would have to be of tiny cross section to be hidden, yet extremely rigid to not flex. For most non-articulated locomotives, rigid side frames are already present as part of the prototype design.

Hope that helps

Andy

proto87stores

Re: Flexi Chassis an Appreciation

Postby proto87stores » Mon Jan 22, 2018 3:30 pm

Crepello wrote:Yes. This repeats a query I posed a while ago: how can 4 non-co-planar wheel contact points be translated into 4 co-planar beam pivot points, merely by taking the mean of adjacent wheel contact points?


It's geometry. The wheels do start at rest in the parallel plane of flat track. See the partial assembly video posted earlier for the practical demonstration.

Andy

billbedford
Posts: 600
Joined: Tue Oct 02, 2012 2:40 pm

Re: Flexi Chassis an Appreciation

Postby billbedford » Mon Jan 22, 2018 6:40 pm

proto87stores wrote:For an analogy, in the real world we have unicycles, bicycles, tricycles and quadricycles. Each added support wheel gives an extra dimension in which the cycle is stable, and need not be actively balanced. For the legendary "Little old lady of Pasadena", 4 wheels require the least skill in handling bumpy road conditions safely at otherwise unsafe speeds.

Ditto for thrust lifted aircraft. Helicopters can have 1, 2, 3, or 4 fans. Most new drones have 4 or more and have no stability restrictions on any horizontal direction of travel.



Both of these are sprung systems with pneumatic suspensions.....
Bill Bedford
Mousa Models
http://www.mousa.biz


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