Flexi Chassis an Appreciation

[proto87stores]

Since we are playing with analogies, this one works for me.. We're back in the 1920s, and there is a salesman who is telling me that he is very happy with his horse and cart, because it can travel over the tracks on his farm without falling over. It may not be as comfortable a ride on these new-fangled metalled roads, nor pull as much as these 'motor cars' with their difficult-to-maintain rubber tyres, but four wobbly legs and a rigid axle work well enough for him, so we should all be told about it.

That many of us have been there, done that, and worked out why we won't do it again, doesn't mean we shouldn't be reminded.. often.

Not wishing to intervene in a domestic, but analogies are tricky beasts. The logging industry is mostly still wedded to massive motorised machines, but it turns out that if you want to cause less damage to the hydrology, and your ecosystem in general, rather than get the biggest fastest financial hit at the expense of your neighbours downstream and the wildlife, horse drawn logging is best.

A more apropriate example might be the 20 year or so switch to "much cleaner diesel engines" in European automobiles. California more thorough testing has now put that "new fangled improvement" to rest.

Andy

[Will L]

Since we are playing with analogies, this one works for me.. We're back in the 1920s, and there is a salesman who is telling me that he is very happy with his horse and cart, because it can travel over the tracks on his farm without falling over. It may not be as comfortable a ride on these new-fangled metalled roads, nor pull as much as these 'motor cars' with their difficult-to-maintain rubber tyres, but four wobbly legs and a rigid axle work well enough for him, so we should all be told about it.

That many of us have been there, done that, and worked out why we won't do it again, doesn't mean we shouldn't be reminded.. often.

Not wishing to intervene in a domestic, but analogies are tricky beasts. The logging industry is mostly still wedded to massive motorised machines, but it turns out that if you want to cause less damage to the hydrology, and your ecosystem in general, rather than get the biggest fastest financial hit at the expense of your neighbours downstream and the wildlife, horse drawn logging is best.

A more apropriate example might be the 20 year or so switch to "much cleaner diesel engines" in European automobiles. California more thorough testing has now put that "new fangled improvement" to rest.

Andy

Ok Andy, I assume your next set of wheels will be horse drawn.

[proto8stores]

Since we are playing with analogies, this one works for me.. We're back in the 1920s, and there is a salesman who is telling me that he is very happy with his horse and cart, because it can travel over the tracks on his farm without falling over. It may not be as comfortable a ride on these new-fangled metalled roads, nor pull as much as these 'motor cars' with their difficult-to-maintain rubber tyres, but four wobbly legs and a rigid axle work well enough for him, so we should all be told about it.

That many of us have been there, done that, and worked out why we won't do it again, doesn't mean we shouldn't be reminded.. often.

Not wishing to intervene in a domestic, but analogies are tricky beasts. The logging industry is mostly still wedded to massive motorised machines, but it turns out that if you want to cause less damage to the hydrology, and your ecosystem in general, rather than get the biggest fastest financial hit at the expense of your neighbours downstream and the wildlife, horse drawn logging is best.

A more apropriate example might be the 20 year or so switch to "much cleaner diesel engines" in European automobiles. California more thorough testing has now put that "new fangled improvement" to rest.

Andy

Ok Andy, I assume your next set of wheels will be horse drawn.

Could easily be . . .

[andrew jukes]

Will L wrote

At to the truth of the assertion that equal distribution maximises haulage power, we have had this discussion on the forum before, although on the last occasion it was because I was questioned in my assertion that the pulling power of a coupled chassis is limited by the most lightly loaded driving wheel

Yes, I understood all that (and was part of the discussion). The problem is, I haven’t managed to demonstrate it in practice.

Here as an example is one of my best achievers of a high coefficient of friction (defined by me as drawbar pull / adhesive weight):

The loco is an 0-4-0T ‘Pug’, built in the 1960s. It has a rocking front axle and fixed rear driven axle and, incidentally, split frames, double-reduction gearing, a flywheel and a clear cab. It has a 24mm wheelbase.

It weighs 87gm, with 52gm on the front and 35gm on the rear axle. Stability is lousy, and it tips with a little pressure on a front dumb buffer. I suspect it would not be happy at pushing wagons if there was an asymmetric buffer load - so all in all a rubbish design.

On my drawbar pull test rig (consisting of a thread over a wheel hanging down below the baseboard to a little aluminium tray that I can put weights in), it produced the following results:

Going forwards - 15gm lifted without slipping; 20gm with slipping; 25gm with frantic slipping

Going backwards - 15gm lifted without slipping; 20gm with a lot of slipping; 25gm no progress

To test the equal weight distribution theory, allowance needs to be made for weight transfer. The coupling is approx. 14mm from the rails. With a 15gm drawbar pull and a 24mm wheelbase, the weight transfer is 14/24 x 15 = 9gm.

Going forward, the effective axleloads are 52 - 9 = 43gm (front) and 35 + 9 = 44gm (rear)

Going backwards, the effective axleloads are 35 - 9 = 26gm (front) and 52 + 9 = 61gm (rear)

This does give a little support for the equal distribution theory in that the performance, once slipping, is a little better with the more equal effective axleloads - but I’m clutching at straws! Basically, there is not much difference between equal weights and weights in the ratio 2 : 1.

The remarkable thing about these results is how good they are. An achieved coefficient of friction of 15/87 = 0.17 is usefully higher than the 0.15 I've come to think as what you can generally hope for and, with some slipping, it rises to around 0.23. That then brings the awkward question of why I get better results with slipping than the best I can get without slipping? Control, incidentally is through a variable voltage regulator.

I think the problem with the theory may be that it is unrealistic to expect our locos to operate in the narrow band between static and sliding friction - the band exploited by sophisticated diesel locos like Classes 59 and 60 - and no doubt many more since.

Regards Andrew

[andrew jukes]

Correction!

I got front and rear mixed up in the effective axle load calculations. Going backwards, it should have said 26gm (rear) and 61gm (front).

I could also have gone on to demonstrate the drawbar pulls you would have expected if the theory applies.
If a 15gm pull is achieved without slipping with equal axleloads (implying a coefficient of friction of 0.17) then with the lighter axleload reduced to 26gm, you would expect the maximum drawbar pull without slipping to be 0.17 x 26gm x 2 = under 9gm.

Andrew

[billbedford]

At to the truth of the assertion that equal distribution maximises haulage power, we have had this discussion on the forum before, although on the last occasion it was because I was questioned in my assertion that the pulling power of a coupled chassis is limited by the most lightly loaded driving wheel, which is just another way of saying the same thing.

There is a caveat in that this is only true where the weights on the wheels are within about 12% of each other. This is usually true for full-sized practice since there are axle weight restrictions but is often not true for models. Once one axle reaches this magic number it will be in a place where it is permanently subject to sliding friction, rather than static, and the other wheel sets are able to absorb its load within their static friction envelope.

[Will L]

There is a caveat in that this is only true where the weights on the wheels are within about 12% of each other. This is usually true for full-sized practice since there are axle weight restrictions but is often not true for models. Once one axle reaches this magic number it will be in a place where it is permanently subject to sliding friction, rather than static, and the other wheel sets are able to absorb its load within their static friction envelope.

I agree that once the difference between the weights on wheels becomes large, the haulage contribution of the most heavily loaded wheel will predominate. That is the haulage capacity lost to the slipping lightly loaded wheels will be relatively small. However, assuming for a moment there is some weight on the lightly loaded drivers, it will always pull less than a chassis of equal adhesive weight and weight distribution.

That assumption does raise an interesting prospect, given we are not troubled by civil engineers getting difficult about excessive axle loadings on the track, you ought to be able to get quite remarkable pulling performances from single drivers, with no carrying wheels, constructed like a wheel barrow with the handles supported by the tender. Did I hear somebody mumble something about stability?

[proto87stores]

I've never suggested you need to place any beams between the frames. Nor suggested crafting any high tolerance components. Quite the opposite when you consider I'm trying to avoid the need for any hornblocks and the inconvenience of soldering and skilled parts placement and adjustments.

Then I assume you haven't yet considered non-bogies vehicles with more than two or three axles. Even in Sharman original books, while the six-couples designs were relatively easy to follow, the ten coupled ones we're a dog breakfast.

And if you don't know the weight of the vehicle, how can you possibly supply the right wire in the kits?

You can't, just as you can't tell what gauge the kit is going to be built for, so you don't put wheels in the kit.

Actually I'm in the process of designing a replacement for the (rigid) chassis of my "Connoisseur Models" N7 Kit as above. Design goals include the new chassis being fully equalized and easily assembled without having any hornblocks, any construction soldering, or any specialized tools such as axle alignment jigs. The sketch above is an early rough tracing of the original fold-up chassis etch, so I can clean it up and make a substitute that is a drop-in fit with the rest of the kit. (That's the only difficult part).

My larger US locos include a couple of 4-8-8-2's and some 2-10-0's, but the process should be the same. Unfortunately, due to the non-availability of US scale wheels, updating those is not going to happen any time in the near future. The only UK 2-10-0 I possess is an old Airfix kit. But I do have some 4-6-2T's that I already have P4 wheels for.

I'm hoping to have the 12T mineral wagon prototype parts off for etching in the next week or so, but I would really appreciate being pointed to a supplier of the correct sprung buffer parts, so they can be tested for stability during bumpy shunting.

Andy

[proto87stores]

Correction. Make that 2-6-4 Tanks.

[Will L]

Will L wrote

At to the truth of the assertion that equal distribution maximises haulage power, we have had this discussion on the forum before, although on the last occasion it was because I was questioned in my assertion that the pulling power of a coupled chassis is limited by the most lightly loaded driving wheel

Yes, I understood all that (and was part of the discussion). The problem is, I haven’t managed to demonstrate it in practice...

I don't think there is any problem with our understanding of the basis physics, it just that there are always a lots of other factors which can have an influence on the final result as well. Foe instance, I remember Don Roland telling me the problem's they got onto trying to establish the true weight on individual axles, though saying that I doubt you had to much trouble with just two.

[zebedeesknees]

I don't think there is any problem with our understanding of the basis physics, it just that there are always a lots of other factors which can have an influence on the final result as well. Foe instance, I remember Don Roland telling me the problem's they got onto trying to establish the true weight on individual axles, though saying that I doubt you had to much trouble with just two.

He was right! Weighing my 14XX axles produced 36g + 36g + 22g, but the overall came out at 115g... The difference seems to be due to the deflection of the spring of the weighing pad. By adding a factor, it suggests that the axle weights are really 44 + 44 + 26. On the drawbar - 20 lift, 22 slip, 24 thrash slipping, so it must be more than 72g tractive. The chassis can be seen at http://www.clag.org.uk/hl-14xx.html. Beams anyone? No hornguides, just bushes with the lower edge filed away to clear the undersprung CSB wires. More of the drive at the lower half of http://www.clag.org.uk/battery-radio.html

Zeb.

[Russ Elliott]

Interesting and useful figures, Zeb.

The difference seems to be due to the deflection of the spring of the weighing pad.

Yes. Manufacturers of scales (particularly pocket ones) never seem keen to state what their FSD distance is, or realise that the figure might be useful. Typically, as far as I can determine or guess, it could be something like 0.5mm ballpark for a 100g or 200g scale.

Depending on the estimated weight of an axle, a slip of 5 thou or 10 thou plastic, or maybe even a sheet or so of paper, on top of the platform might go some way, albeit crudely, to compensate for the axle deflection, and provide a better value.

fsd-compensation.png

[andrew jukes]

On my single-axle weigh bridge, I shim under the scales so it's easy to do +/- 0.8mm from the adjacent track. Hardly relevant for a compensated 0-4-0, though.

The A4 in the attached poor quality picture has a test chassis that tried to achieve everything in suspension terms that is current P4 conventional wisdom, but was not a success. After quite a long break I will be starting again with some new ideas.

Also worth pointing out that with this loco it doesn't matter what there is between the frames, making the case made some time back for stopping discussion of equalising beams and things completely irrelevant.

Regards Andrew

[proto87stores]

I don't think there is any problem with our understanding of the basis physics, it just that there are always a lots of other factors which can have an influence on the final result as well. Foe instance, I remember Don Roland telling me the problem's they got onto trying to establish the true weight on individual axles, though saying that I doubt you had to much trouble with just two.

He was right! Weighing my 14XX axles produced 36g + 36g + 22g, but the overall came out at 115g... The difference seems to be due to the deflection of the spring of the weighing pad. By adding a factor, it suggests that the axle weights are really 44 + 44 + 26. On the drawbar - 20 lift, 22 slip, 24 thrash slipping, so it must be more than 72g tractive. The chassis can be seen at http://www.clag.org.uk/hl-14xx.html. Beams anyone? No hornguides, just bushes with the lower edge filed away to clear the undersprung CSB wires. More of the drive at the lower half of http://www.clag.org.uk/battery-radio.html

Zeb.

Well, someone noticed that rigid beams save the high accuracy need and cost of hornblocks.

If I'm interpreting the photo correctly, it doesn't look to me as though the CSB is actually continuous. I think that configuration reduces to a rigid equalizing beam for the two driving axles with a virtual sprung fulcrum to the body/chassis and a separately sprung trailing axle.

Not quite the full equalization I've suggested, as the C-ishSB likely adds considerable resistance to the clean, beam rotation, as well as it's up down fulcrum movement. The rotation resistance to the beam is surely non-linear used in that way. Which may be an independent pointer to the likelihood that CSB's are usefully very non linear springs when used vertically.

Andy

[proto87stores]

On my single-axle weigh bridge, I shim under the scales so it's easy to do +/- 0.8mm from the adjacent track. Hardly relevant for a compensated 0-4-0, though.

The A4 in the attached poor quality picture has a test chassis that tried to achieve everything in suspension terms that is current P4 conventional wisdom, but was not a success. After quite a long break I will be starting again with some new ideas.

Also worth pointing out that with this loco it doesn't matter what there is between the frames, making the case made some time back for stopping discussion of equalising beams and things completely irrelevant.

Regards Andrew

Sorry but I'm a little confused. As per my highlighting above. The first statement seems to justify everything I'm trying to pass along that solves such problems. The second I'm not sure I understand, but seems to be suggesting the opposite.

Andy

[andrew jukes]

Yes, Andy, what I wrote was ambiguous. Lindsay on April 11th, backed up by Armchair Modeller ('well said'), seemed to think that trying to improve our understanding of equalising beams and things should be consigned to history. Not so I say. An A4 is an extreme case but large boilered locos with high footplates and large drivers (like A1s and A3s) are almost as unconstrained in what goes between the frames. If you're looking for a chassis that is easy to build, needs little maintenance or adjustment and can produce at least 60gm at the drawbar then I think all options are on the table.

Regards Andrew

[Julian Roberts]

equalising beams and things should be consigned to history. Not so I say.

horse and cart

[Russ Elliott]

If I'm interpreting the photo correctly, it doesn't look to me as though the CSB is actually continuous. I think that configuration reduces to a rigid equalizing beam for the two driving axles with a virtual sprung fulcrum to the body/chassis and a separately sprung trailing axle.

Andy - Ted's 'live beam' serves only to hold the wheelbase. It neither increases nor decreases the degree of equalisation in the CSB.

continuous-livebeam-underslung-mod.gif

[proto87stores]

If I'm interpreting the photo correctly, it doesn't look to me as though the CSB is actually continuous. I think that configuration reduces to a rigid equalizing beam for the two driving axles with a virtual sprung fulcrum to the body/chassis and a separately sprung trailing axle.

Andy - Ted's 'live beam' serves only to hold the wheelbase. It neither increases nor decreases the degree of equalisation in the CSB.

continuous-livebeam-underslung-mod.gif

Not sure if there is a missing fixing point(s) not shown. Otherwise:

If, as per CSB theory and practice, the wire slides, then the wheel base is not fixed LHS to RHS.

Or if however, the wire is fixed to the beam, then the wire cannot as easily slide different amounts in different hoops, to give CSB suspension working the in manner usually claimed. (Expecting the wire length to change while the distance between the two beam points stays fixed is a hugely different spring tension ball game compared to claiming wire sideways bending).

BTW. I'm still wondering how everyone is certain that there is only one static equilibrium position for sliding wire CSB's

Andy

[Will L]

BTW. I'm still wondering how everyone is certain that there is only one static equilibrium position for sliding wire CSB's

Not entirely sure I'm certain what you mean, but I can say we are certain there are a whole range of potential CSB fulcrum solutions for any one Wheel base.

[Proto87stores]

BTW. I'm still wondering how everyone is certain that there is only one static equilibrium position for sliding wire CSB's

Not entirely sure I'm certain what you mean, but I can say we are certain there are a whole range of potential CSB fulcrum solutions for any one Wheel base.

What I'm saying I'm suspecting, is that in any particular example CSB solution, the wire can be slid to the full extent to say the front, with the front loop being the only one fully extended. Or the exact reverse, with the wire slid fully to the rear and the real loop fully extended.
And that after letting the chassis settle back from each case, there could well be different equilibrium states for each. E.g. the wire isn't going to slide fully back to a single central position. And so the settling position of each loop will also be offset to which ever end was stretched last. Basically the sliding action creates an hysteresis in the CSB stability position. Just as in most most other mechanical systems that have reversing.

If such hysteresis exists, it may be very small and not particularly noticeable from a normal viewing position. But then, as Bill said, many P4 layouts have very good flat track, and perturbations from the track are much less that the full +/- 0.5 mm I try to allow for in my designs.

But if no-one looked for it, would it have been noticed, or seen but not recognized?

Andy

[billbedford]

Sometimes I think that until you build a set of CSB frames you are going to come up with more and more irrelevant questions.

Probably just displacement activity.

[Russ Elliott]

Andy - friction between the beam and the fulcrum points, and between hornblocks and hornguides, will influence the attainment of any equilibrium state, but this process does not fall into the phenomenon classed as hysteresis. We simply have a bunch of moving bits, and it is sensible to give them a spot of oil, as one would do with any mechanism. Even when sensibly minimised with a spot of oil, friction does have a beneficial effect in that it contributes to an appropriate degree of damping in the CSB system.

[Will L]

And in any event, the continued movements you would expect from an active suspension system will ensure that they settle back to the designed weight distribution. Russ has always been keen on oiling the fulcrums and while I agree the theory, experience suggest they are never stiff enough without lubrication to material effect performance, particularly as the amount of movement is actually minimal in the first pace..

[martinm]

What I'm saying I'm suspecting, is that in any particular example CSB solution, the wire can be slid to the full extent to say the front, with the front loop being the only one fully extended. Or the exact reverse, with the wire slid fully to the rear and the real loop fully extended.
And that after letting the chassis settle back from each case, there could well be different equilibrium states for each. E.g. the wire isn't going to slide fully back to a single central position. And so the settling position of each loop will also be offset to which ever end was stretched last. Basically the sliding action creates an hysteresis in the CSB stability position. Just as in most most other mechanical systems that have reversing.

Am i missing somethig here?
I will admit to being a CSB virgin, but I am about to tackle one.
I don't recognize this talk of 'loops,' 'stretch' and 'extended' and am worried I'm missing something.
I don't see that there is going to be much movement/sliding back and forth of the suspension wire, nor how that can impact upon the strength/rate of springing, nor can I imagine it stretching.
Nor do I understand the point

Which presumably complicates CSB systems even more if you want the axle boxes to move up and down correctly with the suspension a

since surely that is how the system works. As Bill Bedford put it

Because the wheels are, and should be, fixed to the axles, it is the most practical and economic way of getting wheelset in and out of the frames.

What other system is there, apart from the 'original' one of holes in frames, with a minimal bearing surface to allow twist?

regards,

martin