What's causing the wobbles on Calcutta Sidings

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Armchair Modeller
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Re: What's causing the wobbles on Calcutta Sidings

Postby Armchair Modeller » Wed Aug 30, 2017 11:00 am

Perfectly flat track just doesn't happen in the real world for all sorts of reasons. There are a number of videos of some very poor American trackwork online. True realism would allow such track to be built and models to successfully run on it. ;)

Engineering perfection vs real atmosphere - a real conflict?

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Noel
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Re: What's causing the wobbles on Calcutta Sidings

Postby Noel » Wed Aug 30, 2017 2:42 pm

Armchair Modeller wrote:Engineering perfection vs real atmosphere - a real conflict?


Just the usual inability to scale the laws of physics...
Regards
Noel

billbedford
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Re: What's causing the wobbles on Calcutta Sidings

Postby billbedford » Wed Aug 30, 2017 6:12 pm

Proto87Stores wrote:That's what I'm trying to fix for P:87. I'm already preparing a new standard for free wheel movement and standard for track flatness. It now seems that Joe Brooksmith was aware of that omission in setting his own idea of the +/- 0.5 mm wheel movement allowance. Note, the otherwise mostly effective NMRA coarse standards don't include those either. But then coarse standard deep flanges do act somewhat as a substitute for both limits in that case.


Joe Brook Smith had nothing to do with this dimension. It was a purely empirical value used by Chris Pendlenton on his early sprung locos and subsequently adopted as a de facto 'standard'.

The 0.5mm is not the wheel movement allowance, whatever that is, but a designed
static deflection of the carrying springs under load. There is also a dynamic deflection which is added to the static deflection and this can be plus or minus. Since this dynamic deflection is closely tied to the rail top, then there is no reason to calculate it.

You have been told this so many times now that repetition is becoming somewhat tedious.
Bill Bedford
Mousa Models
http://www.mousa.biz

Proto87Stores

Re: What's causing the wobbles on Calcutta Sidings

Postby Proto87Stores » Thu Aug 31, 2017 6:02 pm

Terry Bendall wrote:
Proto87Stores wrote:I'm already preparing a new standard for free wheel movement and standard for track flatness.


Preparing such standards is all very well but will they be achievable? What do we mean by flatness of track - across the track or along its length or both and over what distance? What will the tolerances be and can they be achieved and maintained? Can they be achieved on layouts that are taken to exhibitions? Even if this can be done will they eliminate the wobble that started this thread?

Terry Bendall


Flatness can be defined as rail height deviation from the intended gauge centre line on level track or from the intended plane through the centre line on intentionally tilted track. For vertical curves, the centre line should follow the intended vertical curve.

That allows for both a localized point value and a set of maxima over a defined distance. Clearly distances should be chosen that are usefully relevant to the wheel bases of the vehicles used. Twist can be calculted form the results over distance.

And if there is not a wish to set the same limits for mixed operation of rigid wheel base vehicles with suspension fitted vehicles, then it is possible, but probably not desirable, to have two different standards. On inclusive, and one exclusive of rigid wheeled vehicles. Standards are for consistency and interchange. And the latter option would only allow for interchange one way.

That said, actually measuring track flatness would appear to be a difficult task. Some creativity is needed in order to provide simple economic tools to both help construct track within set limits, and to sufficiently accurately measure the results. But given the motivation, necessity is generally the Mother of invention. ;)

None of the above affects sideways deviations, which are apparent in close up views of Calcutta Sidings. And quite likely a major contributor to the vehicle shaking seen. But an associated standard could be written in the same manner for a limit on sideways rail deviation from the intended centre line.

Andy

Philip Hall
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Re: What's causing the wobbles on Calcutta Sidings

Postby Philip Hall » Thu Aug 31, 2017 10:24 pm

I have followed this for a while now and for me the fundamental point is what the trains look like as they move. At a reasonable pace a passenger train waddling along like a duck on speed looks awful. You only have to look at some of the big 00 tail chasers to see that. Maybe I should just say big tail chasers and not mention the gauge but you will get my point.

It is obviously important that the vehicles stay on the rails, but also that they appear to have some kind of weight about them in the way that they move. Springing has been shown to impart that kind of movement, but a passenger train, sprung, compensated or just with movement in the bogies can also sweep along nicely if the vehicles are heavy enough, close coupled with sprung buffers and sprung gangways and coupled in the same way to the engine.

It also does not help if the view of the train is head on with a long lens. Looking at the prototype in this way reveals frightening looking track with bumps and hollows all over the place! Obviously it helps to have track that doesn't look like that...

Philip

allanferguson
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Re: What's causing the wobbles on Calcutta Sidings

Postby allanferguson » Fri Sep 01, 2017 11:50 am

I used to stand on the bridge at Strathbungo (for those who know the Glasgow area) watching trains of empty minerals (mostly wooden, but some steel) heading South. Probably doing about 40 mph, and the dances these wagons were doing made the Charleston look like a Sunday stroll. This was fairly good main line track, and I never heard of a wagon coming off.

Allan F

Proto87stores

Re: What's causing the wobbles on Calcutta Sidings

Postby Proto87stores » Fri Sep 01, 2017 4:31 pm

billbedford wrote:
Proto87Stores wrote:That's what I'm trying to fix for P:87. I'm already preparing a new standard for free wheel movement and standard for track flatness. It now seems that Joe Brooksmith was aware of that omission in setting his own idea of the +/- 0.5 mm wheel movement allowance. Note, the otherwise mostly effective NMRA coarse standards don't include those either. But then coarse standard deep flanges do act somewhat as a substitute for both limits in that case.


Joe Brook Smith had nothing to do with this dimension. It was a purely empirical value used by Chris Pendlenton on his early sprung locos and subsequently adopted as a de facto 'standard'.

The 0.5mm is not the wheel movement allowance, whatever that is, but a designed
static deflection of the carrying springs under load. There is also a dynamic deflection which is added to the static deflection and this can be plus or minus. Since this dynamic deflection is closely tied to the rail top, then there is no reason to calculate it.

You have been told this so many times now that repetition is becoming somewhat tedious.


See earlier post on page 1 by Keith Norgrove http://www.norgrove.me.uk/history_files/Sep72/Sep-72.htm, including

Image
Image

Part of your post is correct. I mistakenly said +/- 0.5mm when Joe wrote 0.5mm as the overall excursion of rail height deviation. (only half as much). Sorry about that. But then I don't lay very flat track, so it provides an extra degree of safety, especially if modelling the earlier mentioned "terrible" US prototype track.

Andy

Proto87stores

Re: What's causing the wobbles on Calcutta Sidings

Postby Proto87stores » Fri Sep 01, 2017 6:30 pm

Here's an excerpt from an interesting Scientific American Article about the Physics of train derailments.

https://www.scientificamerican.com/article/the-physics-of-disaster/

Section 7 does mention certain standards applied to Amtrak NE Corridor track flatness and deviation.

E.g.

QUOTE "Standards are established for maximum distance between rails (gage), maximum dips in each rail (prole), and maximum deviation from straightness (alignment). Higher classes of track require tighter requirements to operate safely at higher speeds. For example, freight trains are limited to 40 mph (64 km/h) on Class 3 track and 60 mph (97 km/h) on Class 4 track. (The track classes are reviewed in Chapter 11.)Although track geometry today is measured automatically with high-speed cars using laser sensors, the standards are based on low-tech methods of measuring the deviation from a 62-foot (18.8-m) string pulled tight. Every 62 feet (18.8 m) of Class 3 track can deviate up to 1.5 inches (3.8 cm) from straight and dip up to 2.25 inches (5.7 cm). The Acela operates at 150 mph (241 km/h) on Class 8 track. Every 31 feet (9.4 m) of Class 8 track can deviate up to 0.5 inch (1.27 cm) from straight and dip up to 1 inch (2.54 cm).

Class 8 track geometry is checked every 30 days. In fact, when Amtrak was preparing to operate Acela at 150 mph, Amtrak’s chief engineer of maintenance, the director of track geometry, and many others rode the geometry car every two weeks for months. They considered it a bonding experience.

The operators will also report any rough or shifted track as it occurs. For all trains operating above 125 mph (201 km/h), at least one train per day has sensors to measure, quantify, and record the location of any rough track.

Concrete, instead of wood, is used for ties on Class 8 track. The concrete is less susceptible to shifting and water damage. At least once annually Class 8 track gage stability is checked with a special car that loads the rail sideways with a force of 10,000 lbs (44.5 kN). Class 8 track is also inspected twice a year with ultrasonic sensors for internal fatigue cracks."

END QUOTE

Proto87Stores

Re: What's causing the wobbles on Calcutta Sidings

Postby Proto87Stores » Wed Sep 06, 2017 6:43 pm

billbedford wrote:
Joe Brook Smith had nothing to do with this dimension. It was a purely empirical value used by Chris Pendlenton on his early sprung locos and subsequently adopted as a de facto 'standard'.

The 0.5mm is not the wheel movement allowance, whatever that is, but a designed
static deflection of the carrying springs under load. There is also a dynamic deflection which is added to the static deflection and this can be plus or minus. Since this dynamic deflection is closely tied to the rail top, then there is no reason to calculate it.

You have been told this so many times now that repetition is becoming somewhat tedious.


Since no-one else has picked up on this, I'll just mention the problematic issues raised here again.

If 0.5 mm is the static deflection of a simple linear Hooke's law sprung wheel bearing, which is carrying some of the weight of vehicle, then as little as as a + or - 0.1mm rail height variation below that wheel will alter the weight carried on that wheel by a whopping + or - 20%. And I doubt even the flattest laid P4 trackwork is with +/- 0.1 mm. So yes, there is every reason to be aware of it's high value.

And since this thread is about discovering and handling vehicle wobble (roll), rather than Flexichas in general, it should be pointed out that springing at the wheel bearings means the center of roll is then the same height above the rail as the wheel axle. Whereas the center of gravity of any unmodified model vehicle is difficult to get below floor level and almost impossible to get to axle level, let alone below.

The problem with a center of roll on level track, that is below the center of gravity, means the vehicle is roll unstable without additional restraints and that even with restraints, will likely have two stable positions, rolled slightly left and rolled slightly right. An extreme analogy would be balancing a broom on its handle.

If the vehicle suspension is equalized at the axle bearing level but sprung at the body to chassis level, then the center of roll is at the body mounting level, which is typically twice as high, and much more likely to be above, or at least much closer to, the center of gravity of a sensibly weighted model vehicle. If the center of roll is above the center of gravity, then the vehicle will be statically roll stable and tend to roll back to a single vertically upright position after any deflective force.

Merely posting a description of a particular mechanical linkage, does not demonstrate or validate the performance of that linkage for a particular purpose, nor does it provide a means of comparison with other linkage implementations. Just guessing or coming up with limited extent anecdotal examples is wishful thinking, not engineering to create and prove an optimal, all cases, design.

Andy

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